Failure Analysis for Dummies Explained

Failure Analysis for Dummies
Failure Analysis what it it?
Domenico Fama’ – June 2016 1

2
Introduction
This e.book is an introduction to the basic concepts of the Failure Analysis methodology and some of its practical
applications.
Why this e.book? During my volunteering for young people Orientation it happened that we discussed my CV.
One of the guys was in particular interested to the failure analysis. Then, at home he tried to look for more details
but he was unable to retrieve any introduction , on the web too.
That’s the reason why I decided to reuse some material from my past failure analysis activities to edit this e.book
It is developed into three mains sections, each one with a dedicated chapter:
• In the chapter 1, the basic concepts are presented. They are of wide use, independently for the considered
technology or application. It will be provided too a standard template for the failure analysis report.
• In the chapter 2, a more detailed example is provided of its application to a specific component technology: the
electromechanical micro relay. This electric component is of quite wide use. The analyzed samples specifically
come from TLC applications.
• In the chapter 3, it will be shown how the lessons learned from the failure analysis experiences could be used
to build up a set of criteria to proactively assess the production process of the part ‘s manufacturer, both to
supplier quality assurance and process control purposes.
Index
Chapter 1 - The Structure of a Failure Analysis
Chapter 2 - Example: The Electromechanical micro Relay
Chapter 3 - From Failure to Prevention
Chapter 4 - Glossary
Chapter 5 - Bibliography

Chapter 1
The Structure of a Failure Analysis
Field Failure
Failure
Confirmation
Technological
Cause
Root Cause
3
Reporting

4
Introduction
In the chapter 1, the basic concepts are presented. They are of wide use, independently for the considered
technology or application. It will be provided too a standard template for the failure analysis report.
Sketch 1:
Chapter Index
i) Field Failure or the Level 1 Failure Analysis
ii) Lab Failure Confirmation or the Phase 1 of the Level 2 Failure Analysis
iii) Technological Analysis or the Phase 2 of the Level 2 Failure Analysis
iv) Root Cause Hypothesis or the Phase 3 of Level 2 Failure Analysis
v) Reporting of the Level 2 Failure Analysis
Field Failure
Failure
Confirmation
Technological
Cause
Root Cause Reporting
Phase 1 Phase 2 Phase 3
Level 1 Level 2
Failure Analysis

General Guideline
A component is supposed failed “in field”, after a malfunctioning of a system or a failed manufacturing test for
some of its sub assembly.
The failure analysis that can be done “in field” is quite limited in capacity and generates some hypothesis about
the identity of the component/s supposed to be the cause of the system/ sub assembly failure.
The suspected component/s are more properly “probably failed”: that is the main output of this Level 1 of the
Failure Analysis.
Good Practices
• The majority of these components are analyzed in greater detail, initiating a Level 2 analysis, to have a
confirmation of the fail, in particular when they are discarded with an high frequency.
This can assure that the right cause of the “in field” system/ sub assembly failures are identified, possibly
addressing the right corrective or improvement action (manufacturer action, sub assembly redesign, test
review, etc.).
• Statistics about these probably failed components are maintained, with evidence of their correlation with main
test failed by the system/ sub assembly.
• When a component is submitted to a Level 2 Failure Analysis, it must be assured its handling is adequate (to
avoid to introduce new damages, for example during the dismounting of the component from the sub assembly
and during the transportation to the testing lab).
In addition, an adequate report must be prepared about the sample: how and when was made the hypothesis
of fail, regarding what sub assembly, what where the treatment of the component received before arriving to
the lab (for example: the system/ assembly was subject to reliability o stress test; how the component was
mounted/ dismounted from the sub assembly; etc.)
Field Failure
or the Level 1 Failure Analysis
5

General Guideline
The probably failed component entering the Level 2 of a Failure Analysis, first of all is subject to functional tests to
have a confirmation about the failure.
These confirmation tests usually are electrical in nature and are focused to check the component compliance with
its specifications.
Each possible parameter/ performance non compliant with the specifications must be checked for coherence with
the declared system/ assembly failure. To this purpose could be needed the support of the system/ assembly
design engineers.
If the failure is confirmed, it’s strongly suggested to proceed with the Phase 2 of the Level 2 Failure Analysis: the
Technological Analysis (TA), to the purpose to identify the technological cause of the evidenced failure.
Good Practices
• The confirmation tests it’s opportune that are made following the applicable standards, to assure the alignment
with measure performed in different times, laboratories etc.
• All the measurement instruments (electrical, mechanical, etc.) must be done having assured their metrological
referring chain against golden samples, controlled by certified bodies to this purpose, and with adequate
frequency (usually specified by the applicable standards or by the instrument’s manufacturers).
• In case of custom components, or custom tests, the confirmation tests must be agreed with the supplier/
manufacturer. Off course, this agreement must be met before to initiate the component supply for the
customer production.
• In case of not confirmed failure for the analyzed components, it’s highly suggested to evaluate if proceed any
case to the Phase 2 (TA). As a matter of fact, some functional failure couldn’t be permanent, but the
technological signs still are detectable.
Failure Confirmation
or the Phase 1 of the Level 2 Failure Analysis
6

General Guideline
To the purpose to execute a TA, a set of minimum requirements are applied:
i. less destructive analysis must be preferred. This is true for electrical tests too: too high energies could
destroy the technological evidences;
ii. At each step of the TA, apply with priority the applicable and pertinent not destructive tests;
iii. Assure to have a complete documentation for each step;
iv. In case of found anomaly, first of all verify its pertinence with the confirmed electrical failure before to
evaluate if proceed to another TA step.
v. The TA is terminated once the right technological defect is identified (a defect clearly correlated with the
functional confirmed failure).
In the coming pages each of these requirements will be detailed if opportune.
Good Practices
• If the right technological defect is identified, the TA is terminated to assure the integrity of the remaining
component (in particular in case of joint assessments with component’s manufacturers or with our system/
assembly customers)
Technological Analysis
7

8
Minimum
Requirement
Notes
i /ii – not
destructive tests
The sample under analysis must be preserved as much as possible.
Less destructive tests are those who release to it les energy, who require less dismounting of its parts.
• Less energy released: temperature around normal component operating temperature, lower I*V, limited
shocks, measurement repeated less it’s possible, testing time dynamic near the characteristic time of
the component technology, etc.
• Dismounting: at each TA step identify the test techniques that leave as is the component. For example
X-Ray inspection on closed packages instead of visual inspection of the interior; SEM without carbon
coating (for conductive samples) instead of coated,...
iii - documentation The TA must be implemented with great care, assuring a complete documentation of the component
status at each step of the analysis. The sample we have is unique. The documentation is all what will
remain after a step completion: remember that, from a certain point of the TA, the sample will be
inevitably and progressively dismounted if not destroyed.
A complete documentation is:
• Photo, with evidence of the instruments used and the enlargement amount (this aloud to make
successive measurements on the photo, if needed);
• Measurements, electrical, mechanical, chemical analysis, etc. Highlight the instruments used, the test
circuit, the manual procedures. When ever possible apply standard methods and traceable instruments;
• Personal notes, comments, hypothesis, decisions, evaluation criteria and related actions, who practically
made the test and when, ...;
• Ordered and clean storage of the remaining sample and its debris, avoiding to contaminate them with
foreign materials. In handling, use plastic gloves.
Iv – Technological
anomaly pertinence
To assess the pertinence of a found technological anomaly with the functional failure, it’s possible to use
different sources: specialized bibliography, manufacturer sources, own confirmed experience (previous
case histories, simulation proofing the cause-effect, etc.)
v – TA termination To be sure that the TA could be correctly terminated, a clear Failure Analysis target must be stated (see
also the Phase 3: Reporting) at the beginning of the analysis.
Notes on the General Guidelines

General Guideline
Once identified the highly possible technological cause of the confirmed functional failure, it’s opportune to try to
identify the related root cause that generated the technological defect.
First of all, the root cause could be due to the manufacturer process or to the customer processes (component
assembling, handling, testing).
In addition, in case of root case attributed to the manufacturer, its knowledge could help to assess the adequacy
of the correction/ improvement actions that the manufacturer will propose/ present.
Last but not least, this knowledge will help to design a list of check points for the site visits performed during the
supplier qualification activity (see Chapter 3)
Good Practices
• See Chapter 3
Root Cause
9

General Guideline
Each Failure Analysis (FA) must be closed storing a Report, also if not distributed or if the FA target isn’t meet (in
particular in this case !).
The FA Report has a written and standard form (see an example below)
Good Practices
• The FA target is opportune it’s shared with the people requiring the analysis
• See below an example of the FA Report template:
10
Reporting
of the Level 2 Failure Analysis
FA Level # (1, 2) Report ID (progressive nr) Completion Date (dd/mm/yy)
Who required the analysis Personal References (phone, mail) Request Date (dd/mm/yy)
Request References: (any relevant documentation provided by the people requiring the FA)
Object of the FA: (description of the part: part number, s/n, manufacturer, batch,...)
Target of the FA: (specify the question the report must satisfy to be considered completed)
Supplied Sample: (how many pieces, their marks, ...) Supplier: (trademark)
Analysis: (description of the relevant activities clearly correlated with the Target of the FA)
Conclusions: (clearly state if the FA Target had been meet and how)
(clearly state if the component had been confirmed as fail)
(clearly state if a technological cause of the failure had been identified)
(describe this technological defect: what is it, what is the cause-effect relationship,...)
(document the best available assessed hypothesis about the root cause of the technological defect)
Distributed to: (name) Attachments: (list of the doc.s)
Treated by: (name) Ref.: (phone, email) Signature Control: (name) Signature

Chapter 2
Example: The Electromechanical micro Relay
11

12
A) Contact Erosion
B) Brown Powder Between Contacts
C) Insulating Films on Contacts
D) Contacts’ Sticking
E) Small Melting Area on Contacts
F) Contacts’ Assembling Mechanical Defects
G) Contact Spring Mechanical Defects
H) Armature and Core Sticking
I) Armature/ Core Coating Damage
J) Coil Wire Break
K) Residuals from Laser Welding
L) Environmental Pollution
M) Magnet Assembling
N) Fulcrum
O) Internal Moulded Parts Damage (cracks, burrs)
P) Case Damage
Q) Pins Cleaning
Introduction
In the chapter 2, a more detailed example is provided for the failure analysis application to a specific component
technology: the electromechanical micro relay.
Why the relay? This electric component is of quite wide use. In addition, it’s a quite simple electric component,
easy to be provided and analyzed by a beginner in a self made home lab, with few tools, an optical stereo
microscope, some simply test circuit also for reliability test, possibly an oven.
The analyzed samples specifically come from TLC applications in a ten years of failure analysis activity in 80s/90s.
Chapter Index
The Table 1 (next page) anticipates the experimental relationships found during my activity between
technological defect and electrical failure modes. They have only qualitative value: they haven’t any statistical
validity being related to a limited number of cases (some hundreds of pieces analyzed, not all failed) against
tens of millions of pieces assembled on printed circuit boards (pcb) in the same period.

13
Coil
Electrical failure
mode
(A)
Erosion
(D)
Cold
Sticking
(B)
Brown
Powder
(C )
Insulating
Film
(E)
Melting
Spots
(F)
Assembli
ng
(G1)
Deformati
on
(G2)
Break
(H)
Sticking
(I)
Coating
Damage
(J)
Wire
Break
No Switch X X X X X
Contact Resistance X X X X X
Dropout Voltage X X
Pickup Voltage X X X X
Pickup Time
Coil Resistance X
Bounce Time
Dielectric Strength
Insulation Resistance
Dropout Time
No Electrical Failure X X X X X
Contacts Springs Armature/Core
Magnet Fulcru Pins
Electrical failure
mode
(L)
Environmen
tal Pollution
(K)
Laser
Welding
Residuals
(M)
Assemblin
g
(N)
Mechanic
al Wear
(O1)
Other
Damages
(O2)
Burrs
(O3)
Fractures
(P1)
Venting
Hole
(P2)
Other
Moulding
(P3)
Deformati
on
(Q)
Cleaning
No Switch X X X X
Contact Resistance X X X X
Dropout Voltage X X
Pickup Voltage
Pickup Time
Coil Resistance
Bounce Time
Dielectric Strength
Insulation Resistance
Dropout Time
No Electrical Failure X X X X X X X X
Other Forein CaseInternal Mouldings
Table 1: Anticipation of the found correlations between defined technological defects and electrical failure modes.
It could happens that a defect with clear potential to generate electrical failure don’t appear correlated to
them in practice, due to effective manufacturers controls.

Impact of the technological defect on the electrical measurements
Due to the TLC application here considered, the precious metal layer of the contacts is made of a surface flash
(generally a gold alloy) and of a first below noble layer. They are welded on a second layer (base metal: copper
alloy) that is welded on the spring (cupper alloy).
Contact erosion, in my experience could cause the below electrical failure modes:
• high contact resistance (bad contact quality);
• low dropout voltage (non – permanent sticking between normally open contacts);
• loosed switching (permanent sticking between normally open or closed contacts);
• high pick-up voltage (non – permanent sticking between normally closed contacts).
A. Inspection Methods
The erosion is visible by internal visual inspection (destructive analysis) through optical stereo microscope.
Sometimes (extreme cases) through X-ray inspection (non – destructive analysis) .
The phenomena could be confirmed detecting the materials of the second contact layer (base metals), mixed with
those of the precious metal layer, using the EDS/ SEM microanalysis.
B. Technological Causes
The detection of the two layers’ material mix means an electrical/ mechanical wear out.
This wear out is caused by the contacts’ inadequacy (thickness, kind o materials) to fulfil with the requirements of
the expected application (frequency, number, and energy of the switching).
Technological Defect:
A) Erosion of the contacts’ precious metal layer (1/3)
14

Photo 1, 2
C. The photos
Into the Photo 1 it’s clearly visible the contact’s erosion with a
material transport, from the moving contact (upper contact), on the
surface of fixed contact (bottom contact)
The morphology of the transported material is detailed into the
Photo 2.
15

Photo 3, 4
C. The photos [continue]
The resulting lack of material from the surface of the upper contact
(the movable contact) is shown into the Photo 3.
Photo 4 shows the EDS map of the same Photo3 area:
• more clear area – Silver (first under layer);
• darker area - Copper/ Nickel (base metals)
• intermediate grey area – gold flash;
16

Usually the interior of the micro relay is degassed after the packaging to create an internal atmosphere neutral as
much as possible (possibly filled with N2). During the relay operations, electrical arching is generated. The
resulting conditions around the arch could generate chemical reactions between possible uncontrolled
contaminants with resulting carbonate powders (brown powder) able to degrade the contact electrical and
mechanical performances.
The presence of brown powder around the contacts, in my experience could generate the following electrical
failure mode:
• high contact resistance (due to the poor quality of contact resistance impaired by the moisture) .
Electrically the technological defect could be confirmed repeating a certain number of contact resistance
measurements and observing a trend of resistance value decreasing, due to the progressive mechanical cleaning
of the contacts (see Figure 1).
Warning: until black powder isn’t confirmed by internal visual inspection, it’s important to use low voltage
measurement technique to avoid to damage / destroy possible presence of contact surface insulating films (see
section C for details about this technological defect).
B) Brown Powder Between and Around the Contacts (1/3)
17

Photo 5
The brown powder is visible by internal visual inspection
(destructive analysis) through optical stereo microscope.
If the EDS/ SEM microanalysis on the contact surface is used, it
shows a relevant presence of carbon (C ) .
Notwithstanding the degassing, due to the low quality of used
materials (organic molecules in particular), or some process
contaminants (finger traces for examples, or silicon, oil products),
the internal atmosphere could promote the brown powder
formation during the arching produced with high load switching
(breakdown arcs)
C. The photos
Photo 5 shows the presence of powder in the moving contact area:
• on the walls around;
• on the contacts;
• between them.
The EDS microanalysis of the brown powder shows presence of the
cupper (Cu) of the contact springs and carbon (C )
18

Figure 1
Figure 1 the progressive reduction of the contact resistance doing a
sequence of switching that proceed to clean the contacts from the
brown powder residuals.
19

The presence of insulating organic films around the contacts, in my experience could generate the following
electrical failure mode:
• high contact resistance (due to the poor quality of contact resistance impaired by the moisture) .
Electrically the technological defect could be confirmed repeating a certain number of contact resistance
measurements and observing that the resistance values are highly unstable, sometime till 100% change, resulting
in an average high resistance value (see Figure 2).
Warning: the only detection method is electrical (see below), it’s important to use low voltage measurement
technique to avoid to damage / destroy it.
These insulating films aren’t detectable using visual inspection
EDS/SEM analysis is typically is not applicable due to the very thin deposit
The cause of insulating films on the contact surfaces is the internal parts plastic material degassing and its deposit
facilitated by relay operations without high load (the breakdown arching destroy these films)
C. Photo
No photo are available
C) Insulating Film on the Contact Surface (1/2)
20

C) Insulating Film on the Contact Surface (2/2)
Figure 2
Figure 2 the highly unstable values of the contact resistance doing a
sequence of low energy switching
21

The presence of contact sticking, not caused by erosion, also known as cold –sticking, in my experience could
generate the following electrical failure modes:
• loosed switching (due to permanent sticking between normally closed contacts);
• high pick up voltage (non permanent sticking between normally open contacts). .
If present, the cold sticking is detected on new devices (less than 1000 switching).
As a consequence, a possible incoming test could be designed to test supplier materials:
• 20pieces every 100.000 or smaller lots:
• 0 defects allowed;
• Ultrasonic exposure (47 KHz,60W, isopropyl alcohol 25 cc glass breaker, temperature [30; 35]^C, 10 seconds
exposure);
• Electrical test of the pick up voltage (coil voltage manually applied: 0,03 V steps starting from 0 volts)
D) Sticking Between Contacts’ Gold Surface Flash (1/4)
22

The cold sticking is visible by internal visual inspection (destructive analysis) through optical stereo microscope.
It could be detected too by X-ray inspection.
Both cases applying step by step coil voltage. Using X-ray the sticking must be checked through electrical test of the
suspected contacts.
Using the EDS/ SEM microanalysis on the contact surface to confirm this defect, no contact erosion must result (no
base metal).
The causes of cold sticking are mechanical shocks or vibrations. Relevant contacts’ technological parameters tu put
under control this phenomena are:
• surface hardness (higher it’s lower the sticking susceptibility);
• surface morphology (higher roughness, lower susceptibility);
• contact shape (flatness, lower the susceptibility);
• contact force (lower, lower susceptibility).
Some parameter has influence on contract resistance, stability and reliability. Unfortunately isn’t possible to obtain
both at the same time: good contact resistance (low, stable and reliable) and low cold sticking susceptibility
23

Photo 6,7,8
C. The photos
Photo 6 shows a fixed contact with the presence of a cold sticking
area (left side)
Photo 7 is the detail of the area in Photo 6: there is some attached
material, coming from the surface of the above movable contact
(EDS/SEM microanalysis done along the black line into the picture).
Also notes the smoothness of the contact surface outside the
sticking area.
Photo 8 shows the attached material coming fro, the movable
contacts (left side)
24

Photo 9, 10
C. The photos
A technique to avoid the cold sticking is to increase the roughness of
the contact surfaces.
Photo 9 shows a smooth contact surface.
Photo 10 a rough surface
25

The presence of small melting surfaces on the contact area (average 100 µm2), in my experience isn’t cause of any
electrical failure modes. And similarly apply for reliability issues.
It’s possible to reproduce small melting areas by the application of a capacitor discharge during the switching of
the contacts:
• at least 10- 12 V:
• peak time 3 µ seconds minimum;
• more than 20 discharges.
Possible small melting area is visible by internal visual inspection (destructive analysis) through optical stereo
microscope, but confirmed by SEM (morphologic analysis of the single melting spots) .
The cause of small melting area on contact surface is the application of electrical discharges to burn possible
insulating films, due to the degassing of internal plastic parts after the relay package sealing. If the discharge
energy level is sufficiently low, it’s possible to burn the film without to deeply melt the surfaces (no base metal).
This “low cost” on-line process is concurrent with the more reliable and generally used off-line “backing “ process
(degassing of internal plastic parts, before or after the assembling, always before the packaging, at [120; 139]^C,
in controlled atmosphere, minimum 8 hours).
E) Small Surface Melting Area (1/4)
26

Technogical Defect:
Photo 11
C. The photos [follow]
Photo 11 shows the presence of an original small melting area on
each contact
Photo 12 shows an enlargement of one of melting area in Photo 11:
the diameter is around 100 µm.
27

Technogical Defect:
Photo 13, 14
Photo 13 shows another original melting spot. This is 10 µm only of
diameter.
Photo 14 shows a melted area produced in the laboratory by a
capacitor discharge circuit during the contacts’ switching: to be
compared with the previous photos (11, 12 e 13).
[follow]
28

Technogical Defect:
Photo 15, 16
Photo 15 shows the detail of one of these area. It is a 100 µm of
diameter (to compare with Photo 12)
Photo 15 shows a further detail (7000 x enlargement), highlighting
the structure of the overall micro area, made out of single spots,
averagely 10 µm of diameter, due the single capacitor discharges (to
compare with Photo 13). Into the white circle a single melting spot.
29

The presence of mechanical defects generated during the contacts’ assembling process, in my experience isn’t
cause of any electrical failure modes. And similarly apply for reliability issues.
This kind of defects is generally visible by internal visual inspection through optical stereo microscope . In some
special case, the SEM could be needed
Contacts’ assembling process not completely under control
F) Contacts’ Assembling Mechanical Defects (1/3)
30

Technogical Defect:
Photo 17, 18
C. The photos
Defective assembly phase: contact welding
Photo 17 shows two movable contacts with two kind of assembling
defects:
• welding excess (it appears like balls protrusions on the left side of
the contacts);
• punching (see below Photo 18 for a detail)
Photo 18 shows the detail of a punching defect: it appears like a
crack on the spring contact, may be due an excess of pressure during
the contact welding on the spring.
[follow]
31

Technogical Defect:
Photo 19, 20
Defective assembly phase: contact punching
Photo 19 shows a movable contact with signs of indentation all
along its length (along and above the white line)
Photo 20 shows an enlargement of Photo 19.
32

The presence of contact spring mechanical defects, in my experience could generate electrical failure. The specific
failure mode is related to the specific mechanical defect. Below some details related to the specific defects I
highlighted during my past lab activity.
G1) Spring Deformation - it could generate the following electrical failure mode:
• high contact resistance (poor quality of contact caused by lower contact force).
G2) Spring fracture - it could generate the following electrical failure modes:
• loosed switching (air gap so high to avoid any electrical contact);
• high pick up voltage (higher air gap between fixed and movable normally open contacts );
• high contact resistance (poor quality of the contact caused by lower contact force);
• low drop out voltage (higher air gap between fixed and movable normally closed contacts) .
G) Contact Spring Mechanical Defects (1/6)
33

Technogical Defect:
G1) Spring Deformation
Photo 21
The spring deformation is visible by internal visual inspection
through the optical stereo microscope (sometimes by non
destructive X-ray inspection: see for example Photo 22).
The cause of the spring deformation are a not adequate supplier’s
assembly process (handling during pins bending, case marking, etc.)
or the customer’s assembly process (mechanical shocks during
electrical test or part pick up and place): generally speaking, any
mechanical shock due to a not adequate handling. To this purpose,
some manufacturer suggest to avoid any shock equivalent to a
vertical fall of more than 0,3 meters.
In Photo 23 a very unusual case !
C. The photos
Photo 21 shows a clear air gap between two movable contacts, due
to their spring deformation.
[follow]
34

Technogical Defect:
Photo 22
Photo 22 shows a spring deformation (circled area: the near left
movable contact spring is bended left. To compare with the not
deformed spring to the right of the picture
[follow]
35

Technogical Defect:
Photo 23C. The photos [continue]
Photo 23 shows a spring deformation (fixed spring) due to a very
unusual case. The relay failed the electrical test (loosed switching)
after 1000 hours at 60^C.
Note the additional rubber spring on the left: this is a not acceptable
technological solution to implement the spring function, due to the
unreliable elasticity characteristics of the rubber, in particular if the
relay operate at the extremes of specified temperature range.
36

Technogical Defect:
G2) Spring Fracture
Photo 24, 25
The spring fracture visible is visible by internal visual inspection
through the optical stereo microscope (sometimes through X-ray
inspection: see Photo 25).
The cause of the spring fracture is the mechanical wear (usually
after more than 1 million switches) or poor springs’ laser welding
process.
C. The photos
Photo 24 shows the detail of a spring fracture.
Photo 25 show a detail of the fracture surface: “beach marks” are
evident, this kind of parallel lines, characteristic of the mechanical
fatigue.
[follow]
37

Technogical Defect:
G2) Spring Fracture
Photo 26, 27
Photo 26 shows a X-ray panoramic view of the contact springs and
the armature.
The circle (bottom) highlights the break between the spring and its
support. The break was caused by poor welding process (laser
welding).
Photo 27 shows a detail of this break (circled area)
38

H) Armature and Core Sticking (1/1)
39
The presence of armature and core sticking, in my experience could electrical failure modes:
• loosed switching (sticking avoid that switching happens);
• high pick up time(1...5 seconds, when sticking force is lower than the magnetic force: initially there is a lock of
the armature movement and later the switching happens).
The armature and core sticking generally isn’t permanent and the electrical test could destroy it.
It could be confirmed through FTIR analysis of the contact area armature/ core surfaces.
The defect is due to the activation at temperatures higher tan 50^C of the washing process residuals.
This washing compound is generally made of paraffin (used as lubricant for moulded parts or for coil wire during
winding), carboxylic acid salt (residual of the chemical nickel coating process or the armature and the core),
mechanical wear out residuals of the armature and core. There is evidence that this kind of compound generates
sticking also without paraffin.
Usual corrective actions are to avoid paraffin and to improve washing process, for example using ultrasonic
equipment for nickel plated parts.
C. Photos
No photos are available

The presence of armature/ core coating mechanical defects, in my experience isn’t cause of any kind of electrical
failure.
The armature/core coating damage is visible by internal visual inspection through optical stereo microscope
It could be confirmed through EDS/ SEM microanalysis.
The defect is due to the mechanical wear caused by the interference between armature and core.
It can generate an erosion of the plating or “smear” of the organic coating (parylen), used by the manufacturer.
The function of this coating is to avoid that the erosion residuals move free inside the relay impairing the
electrical characteristics.
I) Armature/ Core Coating Mechanical Damage (1/2)
40

I) Armature/ Core Coating Mechanical Damage (2/2)
Photo 28
C. The photos
Photo 28 shows one of the core poles of a relay that operated 2
millions of times. The picture shows two area (circled) with evidence
of the core coating damage. They are the contact points with the
moving armature (not showed in the picture).
41

The presence of wire coil break, in my experience could electrical failure modes:
• loosed switching (no magnetic field is generated by the coil);
• high coil resistance.
The defect generally could be detected by internal visual inspection through optical stereo microscope.
It could be confirmed also by EDS/ SEM micro analysis, detecting the cloride (Cl) of the aggressive chemicals used
by the welding / washing processes.
The defect is due mechanical stress during winding/ welding of the coil.
In alternative it could be caused by internal corrosion.
The aggressive chemicals could come from the residuals of the internal coil welding.
Or they are due to lack of package sealing, with possibility that the residuals come in from the washing process,
after the component mounting on the printed circuit board.
J) Wire Coil Break (1/2)
42

J) Wire Coil Break (2/2)
Photo 29, 30
C. The photos
Photo 29 shows a damage of the coil wire (circled area).
The relay resulted to pass the sealing test (gross leak test).
Photo 30 is a detail of the damaged wire. It highlights that the wire
is interrupted due to corrosion.
The genesis could be a sequence of wire coating mechanical damage
then corrosion due to aggressive chemicals (from manufacturer
process) accessing the metal material of the coil wire.
43

The presence of residuals from the laser welding process, in my experience could electrical the failure mode:
• low drop out voltage (lack of armature mechanical move).
It could be confirmed also by the EDS/ SEM micro analysis of the laser welding residuals.
The defect is due to the poor cleaning after the laser welding.
C. Photos
No photos are available
K) Residuals from Laser Welding (1/1)
44

45
L) Environmental Pollution (1/2)
The presence of pollution from the manufacturing environment inside a relay, in my experience could generate
these electrical failure modes:
• loosed switching (no magnetic field is generated by the coil);
• high contact resistance;
• low drop out (due to pollution impairment of the armature movement);
• loosed switching (pollution between contacts that avoid electrical contact).
It could be confirmed also by EDS/ SEM micro analysis.
The pollution is due to the poor cleaning of the relay’s manufacturing environment (here we are considering
environmental pollutants, different from welding residual, internal plastic parts degassing etc.)

L) Environmental Pollution (2/2)
Photo 29, 30
C. The photos
Photo 29 shows the oxidization of the area between the two
movable contacts
Photo 30 shows the EDS microanalysis of the highlighted
area.
The CuNiAl alloy of the springs evidences oxidization
(Oxygen peak at the beginning of the spectrum).
Please note that there isn’t any Cl traces.
46

47
M) Magnet Assembling (1/1)
The presence of magnet assembling defects, in my experience could generate the electrical failure modes:
• loosed switching (magnet circuit defect which cause not sufficient magnetic field).
For example in case of lack of adhesion due to gluing defects, lack of mechanical stability due to moulding slot
defects)
The defect is due to the poor manufacturing process or lack of incoming materials control-
C. Photos
No photo available

48
N) Fulcrum Wear (1/4)
The presence of fulcrum wear, in my experience don’t causes electrical failures.
The defect is due to the not optimized design of the fulcrum, that for example aloud excess of friction during the
move due to wipe.
To reduce the friction, sometime a lubricant is used (generally teflon).
Other causes could be: fulcrum assembling, or mechanical wrong handling of the component (for example during
the relay assembling on the PCB).

Photo 31, 32
C. The photos
Photo 31 shows the body block, the coil block and the magnet.
The circle area highlights two spots of residuals.
The relay switched 2 millions times.
Photo 32 shows the detail of the circled area
[follow]
49

Photo 33, 34
Photo 33 shows one of deposits. There is evidence of fibber glass,
used as filler of the material used into the moulding
armature/springs around the fulcrum
Photo 34 shows the armature block with the movable contacts, the
moulding that join them and how the moulding leave free the
fulcrum area .
The circle highlights the fulcrum area
[follow]
50

Photo 35, 36
Photo 35 shows a detail of the fulcrum The arrows indicate the two
wear points corresponding to the deposits on the magnet (Photo 31
and 32).
Photo 36 shows a detail of the left point: the original round profile is
flattened and eroded due to the mechanical wear.
51

The presence of internal moulded part defects, in my experience could generate electrical failure. The specific
failure mode is related to the specific defect. Below some details related to the specific defects I highlighted
during my past lab activity.
O1) other Damages- The internal moulded parts could be damaged by many different processes during the relay
assembling. Below (O2 and O3) will be detailed the main moulding process defects (burrs and fractures). Here are
gathered all other causes. In my ten years experience it happened only one time, generating an electrical failure
mode:
• loosed switching (due to the armature lock).
O2) Burrs- In my experience I haven’t any evidence of correlated electrical failure.
O3) Fractures- In my experience I haven’t any evidence of correlated electrical failure
The moulded part defects are visible by internal visual inspection through the optical stereo microscope.
O) Internal Mouldings Defects (1/4)
52

Technogical Defect:
O1) Other Damages
Photo #
The only case of moulded parts defect different from burrs/
fractures that I experienced was due to bad soldering process of the
coil wire with consequent moulded parts thermal deformation till
the complete blocking of the armature move.
C. The photos
No available photos.
53

Technogical Defect:
O2) Burrs
Photo 37, 38
Burrs are due to poor moulding process control and can cause
electrical defects by mechanical interference with moving parts and
after some wear to generate residuals that can impair electrical
contacts (permanently or temporary).
C. The photos
Photo 37 shows the armature block with the movable part of
armature moulded with the movable springs.
Some burrs are evidently covering the fulcrum (white arrows).
Photo 38 shows the details of one of burrs.
In case of fulcrum wear, this burr can produce isolated residuals
risky for the electrical contacts.
54

Technogical Defect:
O3) Fractures
Photo 39, 40
Moulding fractures are due to poor moulding process control and
can cause electrical defects if not properly cleaned generating
residuals that can impair electrical contacts (permanently or
temporary).
C. The photos
Photo 39 shows a detail of the armature block near the fulcrum: the
white arrows indicate a fracture of the moulding.
Photo 40 shows the details of one of the fractures. Note how it is
surrounded of fractured plastic particles that can impair electrical
contacts.
55

The presence of case defects, in my experience could generate electrical failure, generally due to corrosion,
allowing to external corrosive moisture to enter the relay. The related failure modes are:
• loosed switching (due to chemically deposited materials, for example by corrosion);
• High contact resistance (see above).
P1) Venting Hole Defects – The venting hole is case moulding injection point and the last access to the internal
of the relay before to be sealed. It’s also used to inject a neutral atmosphere (usually N2). A process not
completely under control could leave at it a hole.
P2) Other Moulding Defects - Usually they are located near the pins or the case bottom corners.
P3) Deformation - In my experience, for this kind of defect I have evidence of a correlation with electrical failure
only in case of deformation till case fracture.
Deformation could happen due a not correct soldering on PCB process (thermal shock that cause dilatation of the
internal N2 atmosphere without a proper cooling profile). If present on new pieces coming from the supplier, the
deformation could be due to a not correct sealing process. No photos are available.
The moulded part defects are visible by internal visual inspection through the optical stereo microscope.
The sealing defects (O1 and O2) are usually detected through the standard Gross Leak Test (bubble test).
P) Case Defects (1/4)
56

Technogical Defect:
P1) Venting Hole
Photo 41, 42
Venting hole defects are due to poor moulding process. They are
controlled through the Gross Leak Test applied 100% to every
production batch.
C. The photos
Photo 41 shows a bottom of a relay. The arrows highlight the
venting hole that appears not sealed.
Photo 42 shows the detail of the venting hole, definitively not
sealed.
57

Technogical Defect:
P2) Other Moulding Defects
Photo 45
All the moulding defects are due to poor moulding process. They
are controlled through the Gross Leak Test applied 100% to every
production batch.
C. The photos
Photo 43 shows a bottom of a relay. The arrows highlight the sealing
defect at the case corner (white square)
Photo 44 shows the detail of the moulding defect, then confirmed
by the bubble test.
[follow]
58

Technogical Defect:
P2) Other Moulding Defects
Photo 45, 46
Photo 45 shows the consequence of the lack of sealing: coil wire
corrosion and then open circuit. The cause was the entering of
water plus flux residuals during washing process (see the below
Photo 46)
Photo 46 shows the detail of the moulding
defect, then confirmed by the bubble test:
note the highest peak of Cl that caused the
cupper corrosion
59

60
Q) Pin Cleaning (1/2)
The presence of pin cleaning defects, in my experience isn’t correlated with electrical failure.
The nature of the poor cleaning could be analyzed with SEM/EDS microanalysis.
The defect is due to the poor manufacturing process (cleaning of finished parts, moulding process, etc.)

Q) Pin Cleaning (2/2)
Photo 47, 48
C. The photos
Photo 47 shows a bottom of a relay. The circle highlights one of the
not cleaned pins, in this case due to a poor moulding process.
Photo 48 shows the detail of the excess of moulding on the pin.
61

Chapter 3
From Failure to Prevention
62

63
Introduction
In the chapter 3, it will be shown how the lessons learned from the failure analysis experiences could be used
to build up a set of criteria to proactively assess the production process of the part ‘s manufacturer, both to
supplier quality assurance and process control purposes.
From Failure to Prevention
On the base of the acquired experience during the Failure Analysis (see Table 1), it’s possible to design a map
of the criticalities and controls for the relay manufacturing process.
This map is very useful to the purpose to implement the supplier qualification visit, in particular to support the
site visit.
Practically, the first step is to ask the relay process flow to the manufacturer.
Then we can place the site visit check points on it, focusing the process phases that are resulting to be the root
cause of the technological anomalies we found during the failure analysis.
Priority check points will be those for technological criticalities that are resulting to be correlated with the
confirmed functional failures (mainly electrical in our case).
In Table 2 the priority 1 check points correspond to the upper rows.
With lower priority, will placed for site assessment the check points related to technological defects not
resulting to us correlated with confirmed functional failure (Table 2, lower rows).
During the site visit, as a minimum we’ll verify:
• If/ how the defect is prevented and in what phase/s;
• How it’s controlled that the defect really isn’t in place in the semi finished/ finished relay.

64
Table 2
[follow]
Electrical
failure mode
(A)
Erosion
(D)
Cold Sticking
(B)
Brown
Powder
(C )
Insulating
Film
(E)
Melting
Spots
(F)
Assembling
YES (a) Change
Specification
(d) Contact
design to
secification
(b ) Plastic
parts
degassing
(b ) Plastic
parts
degassing
NO NA1 Contact
"assembling"
NA 1: Melting Spots are produced during a procedure to eliminate possible Insulating
Films on contacts. In this case check if this is the only method the manufacturer
uses to avoid Insulating Films
Contacts
Coil Magnet
Electrical
failure mode
(G1)
Deformation
(G2)
Break
(H)
Sticking
(I)
Coating
Damage
(J)
Wire Break
(L)
Environmental
Pollution
(K)
Laser Welding
Residuals
(M)
Assembling
YES (g1) Spring
Assembling
NA2 (h) Washing
process of
the relay
parts
* (j1) Coil
wire
welding
* (j2) Coil
assembling
(l)
Manufacturing
area general
cleaning
(k)Cleaning of
Laser Welding
(m) Magnet
assembing
NO Interference
control
Armature/
Core
NA 2: The root cause, in our experience, ies related to the wear out during component
functional aging (after 2 millions switches)
Springs Armature/Core Other Forein Material

65
[continue]
Just for an example, the priority check points are placed into the theoretical simplified process flow of
the next page (Sketch 2)
Fulcrum Pins
Electrical
failure mode
(N)
Mechanical
Wear
(O1)
Other
Damages
(O2)
Burrs
(O3)
Fractures
(P1)
Venting
Hole
(P2) Other
Moulding
(P3)
Deformation
(Q)
Cleaning
YES (p1) Closing
ofthe
venting hole
*(p2a) Case
Moulding *
(p2b) Component
handling during and
after sealing
(p3) Component
handling during
and after
sealing
NO Shock
control
during
habdling
(after
fulcrum
assembly)
Controls to
avoid that
every
process
don't causes
damages to
near parts
Moulding
Armature/
Springs
Moulding
Armature/
Springs
*case
moulding
*Pin
cleaning
control
Internal Mouldings Case

66
Sketch 2: Theoretical simplified relay manufacturing process flow, with priority check points mapping (red labels,
see also Table 2)
Contacts
Movable
Spring k) g1)
Armature g1)
Contact
Fixed
Spring g1), k) g1), k) g1), j2), k)
Pins
Case Base b) h) j1), k), m)
p2)
Coil b) , h), j2)
Core
h), j2),
m)
Case top
cover b)
p3) p3) p3) p3) p1)
l) on the overall manufacturing area
Coil/ core
welded on pins &
fixed on base
Moulding
Armature /
Springs
Contact
welding
Movable contacts
moulded with
armature
Contact
welding
Fixed Contacts
Pins
moulded
to base
Base with Pins
Fixed Contacts
welding on
pins on base
Coil/ core
assembly
Armature with movable
contact mounted on base
and welded to pins
Top
moulding
Venting
hole
closing
Degassing
+ N2
Relay
Marking
Bubble
test
Electrical
testPackagingStorage

1. Parts of a relay
68
The Sketch 3 shows the basic principles of the functioning of an electromechanical relay, like those
analyzed into Chapter 2. The coil is energized through the pins # 1, 10 (left).
When the coil is at rest, the movable spring is in the position shown into the sketch, and the left
movable/ fixed contact (pin #9) are closed. This is one of the two Normally Closed (NC) contacts. The
second, not showed into the sketch, is at pin #1.
The second contact of the spring, at pin #7, is open: this is one of the two Normally Open contacts. The
second is at pin # 4.
Sketch 3

69
When the coil is energized, it generate a magnetic field that attract the armature (not shown into the
Sketch 3: see Sketch 4). The armature is moulded with the spring of the movable contacts: its
movement, due to the energized coil, pulls the spring that, rotating around the fulcrum (pin# 8), open
the NC contact (pin #9) and close the NO contact (pin #7).
This movement and the related change of closed contact (from #9 to # 7) is the so called switching.
The voltage sufficient to terminate this contact change is the so called Pick up voltage.
The same, in parallel happens for the contacts at pins #2 and #4 (not showed into the sketch).
When the coil return at rest (applied voltage below the so called Drop out voltage), the armature is
released and it returns to its original at rest position, moving in the same way the spring of the movable
contacts. In this way the NO contact return to open status, and the NC contact to its original closed
status.
Spring of the movable contacts
Core
Movable Armature
Sketch 4
Coil
Moulding Armature - Spring

70
2. Electrical Parameters of a relay and their measurement methods
Pick up and Dropout voltage
The voltage applied to the coil has a triangular shape: profile named A into the Sketch 5.
The relay output voltage profile is named B into the same sketch.
The Pick up voltage is the voltage applied to the coil when the NO contact voltage appears permanently
closed.
The Drop out voltage is the voltage applied to the coil when the NC contact voltage appears to return
closed.
The step shown in the left of the B voltage profile is due to a relevant difference (0,3 ... 0,5 Volts)
between the voltage required to open the NC contact and the voltage needed to close the NO contact. A
relay with this characteristic is named “tri-state-rely”. This step, sometimes it’s also shown during the
decrease of the coil voltage.
Sketch 5

71
To perform these measurements, I used the circuit specified by the standard IEC 255/ 7 . Of course it’s required
the availability of an oscilloscope..
Some manufacturer prefers to apply a pulse wave to the coil (height equal to the maximum pick up and
minimum drop out voltage): this is fine for a “Go – No Go” (Pass/ Fail) test, but it’s not sufficient to perform the
component performance characterization during a failure analysis.
Pick up, Dropout and Bounce times
I used the same IEC 257/ 7 circuit to measure these time parameters.
Now, the voltage applied to the coil is squared in shape, with the maximum voltage level equal to the nominal
coil voltage (printed on the relay package). The Sketch 6 shows the IEC 257/ 7 output waveform for time
measurements.
The Pick up time is measured as the time between the coil energized (T1) and the NO closed after any possible
bounce (T2).
The Drop out time is measured as the time between the coil de-energized (T3) and the NC newly closed (T4),
after possible bounces.
The Bounce Pick up time is measured as the time between the first NO closure (T5) and the last NO closure
(T2). Bounces can happen both during Pick up and Drop out: the Sketch 5 shows only bounces during the Pick
up.
The sketch also shows coil voltage spikes: respectively higher than the nominal coil voltage and lower than the
ground level.

72
A test equipment without control, particularly on spikes lower than ground level, can cause serious
measurements misinterpretations and problems about test correlation with the manufacturer.
For example, spikes lower than the ground level, with very high voltage values (100... 200 volts), also if very
short in time (few µsec), cause measurement of Drop out Time lower about 1,5 ... 2 msec than the correct
value. To avoid the spikes I suggest to use a “fast diode” in parallel with the coil, or to use a pulse generator to
energize the coil.
Sketch 6

73
Contact Resistance
The contact resistance measurement is a “four points” measurement.
The most used methods are two: low voltage ac; 100 mA dc.
Low voltage method use instruments that assure (IEC 257/7) that voltage peak is minor than 20 mV. This
is the method to be used when there is the suspect of non conductive films formation between
contacts.
“Four points” measurement is done applying a dc test current fixed at 100 mA directly to the socket pins
of the relay.
With reference to the Sketch 3, whatever the methods used, the resistances are measured at the pins:
C1, C2, C3, C4.
Sketch 3

74
3. Instruments
Optical stereo microscope
The optical stereo microscope use two independent
optical objectives to observe the sample, having
different angles and optical paths so to return a
stereoscopic view to the observer (“tri dimensional”
View).
For detail about the functioning, see at Wikipedia
EDS/SEM microanalysis
Energy Dispersive X-ray Spectrometry (EDS), or Energy
Dispersive X-ray Analysis (EDX) is an analytical method
that employ the X ray emissions of an electron excited
sample to identify the sample’s atomic species.
It’s very useful for solid samples.
Each atomic specie, once excited, return to a not-excited
state releasing photons of energies very specific for this
kind of atom. The on board SW of the EDS detector has a
data base of these energy emissions and in this way it
identify the kind of atoms on the surface of the sample.
The source of the high energy electrons is a Scanning
Electron Microscope (SEM)

75
In the figure on the right the basic SEM’s functioning principles
For detail about the functioning of the SEM and SEM/ EDS,
see at Wikipedia.
FTIR
The Infrared (IR) Spectroscopy in Fourier Spectrum is an
analytical thecnique wide used to identify mainly organic
materials.
For detail about the functioning see at Wikipedia

76
Bubble Test (Leak Test)
To have an idea how it could function a bubble test, see the
video at https://www.youtube.com/watch?v=ta53DwOJdhw
Please consider that, for failure analysis purposes, you don’t
have to add colorants to the test liquid.
In this case it’s opportune to have a lateral view of the
vessel containing the liquid and the sample to better identify
possible bubbles emerging from the component under test (sign
of a leaking and of its position on the component’s body).
In addition the liquid test can’t be water. It has to not corrosive
liquids.
The test could be done also without vacuum application (gross
Leak).
Oscilloscope
There are different kind of oscilloscope, from portable to highly sophisticated (their price is proportional to
their functional complexity and completeness). See at Wikipedia for details about its functioning.

Chapter 2
1. “Physical Processes in Contact Erosion”, LH Germer, July 58, Journal of All.Ph., Vol 29, nr.7, pg. 1067
2. “Fundamental Processes of Fast Arc”, JL Smith, WS Boyle, March 59, BSTJ, pg 537
3. “Arching of Electrical Contacts in Telephone Switching Circuits”, MM Atolla, BSTJ, Parts I, II, III, IV, ,
September 57 pg 1231, November pg 1493, May 54 pg 535, January 55 pg 203
4. “Dynamic Model of Stationary Contacts Based on Random Variations of Surface Features”, RF Malucci, June
92, IEEE Trans. CHMT 15, nr 3, pg 339
5. The Formation of Insulating Silicon Compounds on Switching Contacts”, A Eskes, March 88, IEEE Trans.
CHMT 11, nr 1, pg 78
6. “A Reliability Study of Relay Suitable for Surface Mount Process Usinh High- Temperature– Resistant
Plastics”, M. Ohba, K. Kuzukawa, K Ozawa, March 88, IEEE Trans CHMT 11, nr 1, pg 85
7. “Organic Deposits on Precious Metal Contacts”, HW Hermance, May 58, BSTJ pg 739
8. “Wear and Contamination of Electroplated Gold Films in Line Contact”, Zhuan-ke Chen, May 92, IEEE Trans
CHMT 15, nr 3, pg 378
9. “Extraneous Metal Deposits from Production Processes on Contact Materials”, CHA Haque, July 54, BSTJ, pg
807
10. “Fundamental Relay technology”, Phoenix Contact Application Note 105396-en-00, 2012, at
https://www.phoenixcontact.com/assets/downloads_ed/global/web_dwl_technical_info/105396_en_00.pdf
Chapter 4
1. “Effect of Measurement Conditions on Low-level Contact Resistance”, J Muniesa, JY Mousson, March 84,
IEEE Trans CHMT 7, nr 1, pg 81
78

79
L’autore
Domenico Famà è HR Business Partner dal 2004 ed ha ricoperto questo ruolo in diverse aziende multinazionali, dopo aver cominciato a
lavorare nella funzione HR nel 2000. In precedenza ha svolto attività tecniche, dopo una laurea in Fisica all’Università di Pisa nel 1986.
Domenico è un utente appassionato di social networks, di cui cerca di esplorare le potenzialità, compatibilmente coi limiti di tempo di una
vita sola.
Appassionato dei temi del lavoro nelle organizzazioni e nella società, cerca di seguire affannosamente la sua evoluzione sempre più veloce.
In particolare dedicandosi ad alcune tematiche: etica ed innovazione della cultura manageriale; inclusione delle diversità e relazioni
interculturali; auto-orientamento nelle transizioni di vita-lavoro.
Con lo scopo di diffondere una cultura manageriale innovativa ed etica, supporta attivamente alcune associazioni no profit, ad esempio
l’European Foundation for Quality Management: www.efqm.org per cui svolge opera volontaria di Assessor per le aziende che desiderano
impegnarsi in un percorso per l’Eccellenza.
Sulle tematiche dell’inclusione delle differenze e delle relazioni interculturali, interviene volentieri ad incontri e seminari ed ha pubblicato,
con A.Cilona, il capitolo sulla gestione della Diversity (“Diversity: Prospettive e Criticità”) in “Persone, Organizzazioni e Lavoro”, Franco
Angeli 2009, curato dal Prof. A Cocozza della LUISS.
In tema di orientamento nelle transizioni di vita-carriera, per giovani in fase di inserimento nel mondo del lavoro o per profili più senior, ha
spesso collaborato in partnership con Fondazioni come quelle di Adecco o ATM, nell’ambito delle attività di Citizenship delle aziende in cui
lavora.
Coltiva (nel senso più proprio del termine) un sogno: avviare una attività di trasformazione agroalimentare, sviluppando un piccolo fondo di
proprietà seguendo criteri di sostenibilità (ambientale, sociale), di biodiversità (salvaguardia di specie native), di innovazione tecnologica
(micro agricoltura) e di accoglienza di eccellenza, dando corpo nel suo piccolo al detto “dalle stelle alle stalle” (dalla fisica all’agricoltura)

Failure Analysis for Dummies Explained

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