1. Are you anywhere in the
supply chain for
manufacturing electronic
equipment?
Are you yet to produce a
plan for transition to Pb
free?
Will increased
obsolescence due to RoHS
affect you?
Are you concerned about
RoHS compliance?
If you answer YES to
any of the above
questions, then you
need to read this
booklet
Price £5 €8Issue 1 2005
Published by the
Component Obsolescence Group
and the
National Physical Laboratory, UK
Web sites: www.cog.org.uk www.npl.co.uk/ei
The Pb-free Minefield
A Guide to Mitigating Risk during the Transition to
RoHS Compliance
3. 2
Introduction
There are two European Union (EU) directives
impacting on producers of EEE (Electrical and
Electronic Equipment). They are sister directives, but
have a different intention;
RoHS (Restriction of Hazardous Substances)
Directive
- This is about banning of specific
substances that may end up in the waste
stream. In others words it is recognised
that some EEE may still go to land fill, so
let’s ensure it is made from safer
materials. This is where the ban on lead
and other materials originates.
- The implementation date - July 2006.
WEEE (Waste Electrical and Electronic Equipment)
Directive
- This is about the prevention and
reduction of waste. In others words trying
to prevent EEE going into landfill by
increased recovery, re-use and recycling.
It imposes labelling requirement on the
producer to make this easier.
- Implementation date - August 2005
(marking of equipment)
- But Jan 2006 for producer responsibility
obligations, & take back by
retailers/distributors.
1
You should also be aware that there are other
directives, which may impact in the near future
2
.
This guide is about the industry change to Pb-
Free brought about by the enactment of the RoHS
directive. (Remember though that there are other
materials covered by the directive (mercury,
cadmium, hexavalent chromium, PBBs and PBDEs
and these should not be forgotten.)
The content of this guide is based upon external documents and
sources, acknowledged in the text wherever possible. In order to
give the guide value beyond this however, the experience and
understanding of NPL and COG representatives has been drawn
on.
At the time of writing statements made are correct to the best of
COG and NPL's knowledge, however readers should refer to
referenced documents before taking any action, and be aware of
the rapidly changing legislative and technical landscape in this
area.
This guide is not intended to provide all the information necessary
for introducing Pb-Free, but to lay down the basics and ensure
appropriate issues are being considered.
4. 3
Legislation
Each member state must adopt the directives into
national law. Up to date information and
exemption information at the EU level
3
and
member state level is available via the NPL
4
or
COG website
5
.
What is the scope of the RoHS directive and who is
affected or exempt?
Producers of the following EEE products covered by
the RoHS directive are:
- Large Household Appliances
- Small Household Appliances
- IT and Telecom Equipment
- Consumer Equipment
- Lighting Equipment
- Electrical and Electronic Tools
- Toys, Leisure & Sports Equipment
- Automatic Dispensers
At the time of writing the EU is reviewing current
exemptions for Medical Devices & Monitoring and
Control Instruments. Military and aerospace are
also outside the scope, but market forces and
supply chain will eventually force the change.
EEE intended for commercial and military/aerospace
markets (termed ‘dual use’) fall into the scope (are
not exempt) from WEEE and RoHS.
You are a ‘producer’ under the directive if you
manufacture EEE and sell your own brand, re-sell
under your own brand, or import or export.
Be clear also that the directive applies per individual
unit put on the market, not per product line. In other
words current existing product lines need to be
converted, not just new ones introduced after the
directive is in force. It is a common misconception
that product lines currently being put on the market
do not need to comply.
‘.. the concept of placing on the market refers to
each individual product, not to a type of product,
and whether it was manufactured as an individual
unit or in a series.
6
’
The definitions are quite broad and there are grey
areas - more guidance is available
6
but absolute
clarification of whether specific products are in scope
is sometimes difficult to get, as there is no body of
case law yet for lawyers to refer to.
There are many niche exemptions (where there is
currently no viable alternative technology) in the
directive and you will need to refer to the document
(or the UK guidance notes)
6
for detailed information.
One common example is for the use of high lead
content solder as an internal contact within electronic
components (hierarchical soldering). There is also a
specific exemption for the rework and repair of EEE.
Even if you believe you are exempt you are still likely
to be affected due to market forces through your
supply chain. It would be a mistake to ignore the
changes and implications.
Does the directive apply to components and printed
circuit boards?
It is important to realise that the directive applies to
EEE (end product, the functional item the customer
sees, e.g. laptop computer, toaster), not to circuit
assemblies, circuit boards, or electronic components
directly. It is the producer of the EEE, who has to
comply (in the first instance), not the producer of
materials or parts that make it up.
However, the EEE producer will passthis
requirement to comply onto everyone throughout
5. 4
the supply chain by specifying RoHS compliant
materials.
Contractual requirements for RoHS compliant
materials imply a legal responsibility on
suppliers.
So in practice the directive impacts everyone in the
supply chain.
Things to think about… now!
What happens when?
The RoHS directive comes into force on 1 July 2006.
However everyone involved in the supply chain for
the production of an EEE product must take some
action to ensure compliance. Even for those involved
with exempt product, careful planning is needed as
market forces, customer demands, and competitors
may mean the move to Pb-Free becomes important
more quickly than expected.
What next? ….Planning the transition
Will re-qualification be an issue?
Components and materials used will be changing
and, depending upon the final product and
application, certain qualifications might be
invalidated, and may need to be re-assessed. If
relevant, be aware of the time necessary for this.
More details on potential changes to components are
given in “What you order and what you get”.
What about stock?
Products and materials can sit in stock through a
distribution chain so distributors may be unhappy
taking parts containing lead as the deadline
approaches. Technically the directive applies ‘when a
product is transferred from manufacture with the
intention of distribution or use’, so non compliant
product in stock at a distributor can be sold after the
deadline legitimately. Nevertheless human nature
may mean unwillingness to be left with the ‘hot
potatoes’.
Specialist suppliers may continue to supply some
older non-compliant stock to exempt industries.
Not more obsolescence!
Some producers of components, parts and materials
are currently working hard to convert any non-
compliant items to RoHS compliant versions. There
are some situations when this investment is not
justifiable, such as if a part is already nearing the end
of its sales life in 2006, or the cost of alternative
materials for manufacture is too high. It is expected
then that some level of early obsolescence may
occur due to the change by suppliers to Pb-Free
product ranges. Pb-Free enforced obsolescence may
invoke a need to re-design product, involving time
and resource.
Significant levels of obsolescence are likely to
result from RoHS, as some components will not
be converted to compliant versions
RoHS - Do I need to audit?
Companies may need to perform and maintain
internal audits to understand whether what they
already have in stock is RoHS compliant if it is
intended for use in EEE put on the market after the
RoHS deadline. Stock needs to be flushed through
the system. Be aware that many non-compliant
products may end up on the grey market.
6. 5
Who else should I be talking to?
Communication with suppliers and customers is key
here, make sure your plans align where possible, ask
what your customers are expecting and when - do
not wait for them to ask as this may be too late. Again
allow time and resource in your plan. Ownership of
BOM RoHS compliance is a critical issue to be
cleared up as early as possible as there is a new cost
associated with this, try and establish who has the
responsibility for tasks earlier rather than later.
Who else should I be talking to in my supply chain?
Changes to the Assembly Process
The manufacturing process for electronic circuits for
use in EEE will of course change when Pb-Free
materials are introduced. There are many less
fundamental changes, but the main one is the
change to a higher melting point solder alloy used in
manufacture. Main current alternatives alloys are
given in Table 1, but the alloy type likely to be
introduced most widely is SnAgCu which melts at
217°C, compared to 183°C for standard SnPb
eutectic solder. For clarity when this guide refers to
Pb-Free alloy this is SnAgCu unless otherwise
stated.
Table 1. Alloy types (examples)
Solder Composition Melting
Point (ºC)
SnPb 64 Sn / 36 Pb 183
SnPbAg 63Sn / 37SnPb /
2Ag
~179
SnAgCu
(Wave/Reflow)
95.8Sn / 3.5Ag /
0.7Cu
217
SnAgBiCu
(Wave/Reflow)
93.3Sn / 3.7 Ag /
2.1 Bi / 0.8 Cu
210
SnCu (Wave) 99.3 Sn / 0.7 Cu 227
SnCuNi (Wave) 99.x Sn / 0.7Cu /
xNi
227
Note. Elemental symbols are explained in the
glossary.
Since the solder must be melted to form solder joints,
all the materials used to make the circuit assembly
now have to survive higher temperatures. SnCu is a
considered alternative for wave soldering despite its
higher melting point due to its lower cost. The JEDEC
JESD97
7
Pb-Free standard for components now calls
for up to 260°C resistance.
Issues specific to Pb-Free reflow soldering
How do I profile my reflow oven for Pb-Free?
In determining the thermal profile of an assembly line
there are a number of conflicting requirements, which
need to be resolved;
• Solder paste requires a controlled ramp and
preheat to prevent solder balling and to
activate the flux, but too much heat can
cause flux exhaustion
• A high temperature to aid wetting
• A low temperature to prevent component and
PCB damage
7. 6
All these factors become more critical with Pb-Free
soldering.
With such a tightrope to walk, it is imperative that the
engineer is able to accurately measure assembly
temperatures during the soldering process - even
more so when Pb-Free soldering is introduced with
its accompanying higher melting point solders.
Since the maximum soldering temperatures are often
fixed, and the minimum temperature that the solder
joints have to reach has been increased to a few
degrees above the new alloy melting point, the target
window is reduced in size; hence the assembly
requires more careful profiling.
Voiding in Pb-Free processing
8
has been studied and
a guide to profiling is available for Free from NPL
9
.
Will I need to use an inert atmosphere for Pb-Free
reflow?
The use of an inert atmosphere, such as nitrogen, will
always improve wetting but will have an associated
cost. It has been shown that Pb-Free reflow can be
achieved in air if the process is well set up and
controlled. Some engineers may use nitrogen
capability as a ‘safety net’ during initial Pb-Free
production until they can optimise the process,
meaning less risk of high defect rates.
Issue specific to Pb-Free wave soldering
Do I need a new wave soldering machine?
The issues to consider are:
• There is a risk that the high tin content Pb-
Free alloys will corrode the stainless steel of
your wave-soldering machine unless it is
relatively new
• You should also consider whether your
system has enough power in the pre-heating
zone to reach the temperatures you will want
in a Pb-Free wave process
• If you are able to convert your existing
system it must be carefully flushed with
batches of molten solder to remove the lead
• The bath will also have to run at higher
temperature which may call into question the
reliability of pumps and other parts
The manufacturers of systems will be able to advise
you on all these points.
You need to think carefully about the logistics of
changing to the new process for your designs,
products and customers.
- If you have only one wave soldering system
you need to allow sufficient down time, and
switch all your products to Pb-Free at once,
making sure your customers understand and
agree to changing at this date
- If you have more than one line it may be
easier to manage switching each product or
customer
Figure 1. Damage to a stainless steel wave solder pot
due to the high Sn content of Pb-Free alloy.
Can I put parts with a Pb lead finish over my Pb-Free
wave?
8. 7
There are two major risks, firstly that the higher heat
will damage parts that are not rated for Pb-Free wave
processing (see section 0) and, secondly, Pb
contamination of the solder bath.
The Pb-Free alloys generally dissolve other metals
faster than SnPb solder, and if SnPb leaded
components or HASL boards are passed over
SnAgCu wave, for example, levels of Pb can build up
quickly.
1% of Pb in a SnAgCu bath can affect the liquidus
point of the solder by 30-40ºC making the process
difficult to control, and there are questions about the
potential effect of this Pb in the final joints.
Generally, a more vigorous and regular bath analysis
regime will be needed in the transition period (maybe
monthly). Again, your solder supplier will be able to
advise but some basic guidelines based on SnAgCu
are:
• Pb – 1% maximum, causes changes in
melting point and process variation (of
course at 0.1% the end product become non-
compliant anyway)
• Cu – 1% maximum, high levels can cause
Cu6Sn5 crystals
• Fe – if present at increasing levels may
suggest stainless steel parts damage
somewhere in the system
Will I still get dross forming on the wave?
Yes. There may be more produced due the higher
temperatures, and the dross will be of higher value
due to its higher silver content. Take care not to mix
SnPb and Pb-Free dross, and do not cross
contaminate by using the same solder recovery
system. This will decrease its recovery value
significantly for any solder supplier.
What is fillet lifting?
Once an assembly comes off the solder wave it
begins to cool and two things happen -
• The solder begins to freeze and contract
slightly as it does so
• The PCB contracts as it cools in the Z-axis,
effectively making the board thinner
The overall effect is tensile stress around the fillet of
any through hole joint, which in some cases is
enough to lift the fillet from the board. This is fillet
lifting as shown in Figure 2 and Figure 3, .
Figure 2. Schematic of fillet lifting
9. 8
Figure 5. Illustration of fillet lifting, pad lifting and fillet tearing
Figure 3. SEM cross section of fillet lifting
In some cases the stress is relieved by the tearing of
the fillet, or by the pad itself lifting from the board.
The risk of fillet lifting is reduced on thinner boards,
by reducing pad size or using resist-defined pads.
NPL considers fillet lifting to be a low risk or benign
defect as a result of thermal cycling studies
10
. Pad
lifting, however, is serious as tracks and leads can be
damaged.
Are there changes for inspection of joints?
For automated optical and X-ray inspection, the NPL
benchmarking study has shown that most existing
equipment can be used on Pb-Free joints without any
reduction in performance provided the threshold
configuration for the system is adjusted
11
.
For manual optical inspection, re-education of
inspectors is required. Well-formed Pb-Free joints do
not look shiny, so inspectors need to understand that
these are not dry joints, and to use the fillet shape as
a guide. Details are well covered in IPC-A-610D
12
,
which includes Pb-Free as well as SnPb joints.
What you order, and what you get
What is the definition of a RoHS compliant part?
The definition given in the directive for the Maximum
Concentration Values (MCV) for the banned
materials is:
• Pb, Hg, Cr(VI), PBB and PBDE = 0.1% by
weight of “homogeneous materials”
• Cd = 0.01% by weight of “homogeneous
materials”
10. 9
In practice this means that, for example for a
particular device:
• The component case must be <0.1w/w Pb
• The component lead must be <0.1w/w Pb
• The marking ink must be <0.1w/w Pb
• The plated finish must be <0.1w/w Pb
• The solder must be <0.1w/w Pb
• …… etc
An example might be a typical QFP device:
–
–
–
–
–
–
–
– The component leads are coated
with a SnPb lead alloy. The level of
lead in this alloy is 15% by weight
of the material
– If the level of lead is calculated as a
percentage by mass of the
component, however, it is only 0.1%
by weight of the device
This component is non-compliant, as the definition
applies to the homogenous materials within the
device not the device as a whole.
How should I specify RoHS compliant components?
The correct specification of electronic components
through the supply chain will be critical in the
transition period to RoHS compliance.
As well as the existing functional and quality
requirements specified for components there are now
two distinct RoHS related properties that must be
clear:
Is the part RoHS compliant?
- Only if there is less than 0.1% by mass of Pb
(or appropriate level of other RoHS banned
substances) in each of the homogenous
materials in the part, or the use of the
substances is covered by an exemption.
Is the part Pb-Free process compatible?
- Only if the component is robust when heated
to the temperatures and for the times
specified in JESD97
7
, and so able to be
soldered via a Pb-Free process without heat
damage.
Both these attributes must be specified. This
becomes clear if one imagines a RoHS compliant
component ordered as such, but which melts or
cracks when actually used in Pb-Free manufacture.
Some RoHS compliant parts sold as ‘compatible with
Pb-Free processes’ actually fall short of the JEDEC
7
thermal requirements, a potential problem for users.
Note: To state that a part is ‘Pb-Free’ does not
necessarily mean it is either RoHS compliant or Pb-
Free process compatible.
Figure 6. Heat
damage to an
electrolytic capacitor
due to Pb-Free
reflow
temperatures.13
11. 10
Device Prepared for reflow as per
JEDEC guidelines, and reflowed
Device Prepared for reflow by end
customer, and reflowed
Figure 7 shows a Scanning Acoustic Microscopy (SAM) image of a SMD semiconductor package when exposed to SnPb
and by contrast Pb-Free reflow conditions. The part exposed to 260C shows significant delamination internally.
Importantly it may not be possible to see this damage with the naked eye.
Are there compatibility issues between Pb-Free
components and processes?
For any product BOM or inventory, the best approach
is to switch all components to RoHS compliant and
Pb-Free process compliant parts before switching to
a Pb-Free manufacturing process. The arrow below
indicates this approach that avoids the risks of heat
damage and Pb contamination of SnAgCu solder.
BGAs and other advanced packaging techniques can
provide an exception.
Solder
Components
OK
- Heat Damaged
Component
- Pb
contaminated
SnAgCu?
OK
- slightly slower
wetting on Pure Sn
- no collapse of BGAs
OKSnPb
SnPb
Solder
Components
OK
- Heat Damaged
Component
- Pb
contaminated
SnAgCu?
OK
- slightly slower
wetting on Pure Sn
- no collapse of BGAs
OKSnPb
SnPb Pb-Free
Pb-Free
Figure 8. The best route to compliance is by changing
components before modifying the soldering process.
Clear
delamination
at die and
encapsulant
interface
12. 11
A potential compatibility issue exists for SnBi plated
parts when used with SnPb solder, or Pb plated parts
with Bi containing solder. Pb and Bi are known to
form a low melting point phase, which may affect joint
reliability
15
.
What are suppliers doing to make parts compliant?
Changes to plastic encapsulants, internal adhesives
and interconnects, and even marking dyes have been
necessary in some cases to ensure none of the
banned materials are present. Even then, changes
are required, as alternative materials do not give the
thermal robustness required for Pb-Free soldering.
The moisture sensitivity of parts is also being re-
assessed in light of Pb-Free. New moisture sensitivity
levels (MSL) are being assigned as the risk of
delamination or ‘popcorning’ is increased. This
increased risk is due to higher process temperatures,
and some component materials are being replaced to
achieve MSL ratings.
Indications from industry are that small packages
such as SOP and TSSOP can withstand Pb-Free
reflow thermal strain without suffering from interfacial
delamination, popcorning, cracking or warping.
However, larger packages may not meet this
requirement. For these parts the MSL classification
could be downgraded by as much as 2 levels. Ref:
IPC/JEDEC J-STD-020C Table 5-1.
Of course one of the main changes is from SnPb
component lead finishes to alternative plated finishes.
The change to these finishes has, to some extent,
taken place ahead of the RoHS requirements in any
case, due to the technical requirements of fine pitch.
The main alternative plated lead finishes are:
• Pd/Ni/Au
• Matte Sn (very common, many passives
already use Sn)
• Bright Sn
• SnAg
• SnCu
• SnBi
• Au
What are the specific issues for the transition to Pb-
Free BGAs?
BGAs provide an exception to the rule (illustrated in
Figure ) that Pb-Free, RoHS compliant parts can be
used in existing SnPb based soldering processes. In
the case of Pb-Free BGAs often there is not enough
heat from a SnPb based reflow profile to allow reflow
of the SnAgCu solder balls. This means the BGA
interconnections will not collapse and the component
may not align well. There is IPC guidance available
on how to try and deal with this process issue
16
based on increasing the temperature of the existing
SnPb reflow process if possible.
13. 12
There are similar issues with other exotic packaging
technologies that rely on multi-level construction and
varying soldering temperatures (e.g. wafer scale
packaging).
What labelling or markings will there be on parts?
While the WEEE directive has detailed requirements
for labelling of EEE items for recycling the RoHS
directive has none. It has essentially been left up to
the industry to prepare itself in this regard.
At the component and circuit board level a standard
for labelling, was released in January 2005:
IPC 1066 – ‘Marking, Symbols and Labels for
Identification of Pb-Free and Other Reportable
Materials in Pb-Free Assemblies, Components and
Devices’
This suggests labelling formats for components,
which stipulate lead levels below RoHS compliance,
and maximum safe reflow temperature. The standard
is a good start, but despite its title it does not deal
with the other banned materials under the RoHS
directive, only lead.
There is a wide range of ‘Pb-Free’ labels being
proposed by individual manufacturers, and it is
important to understand what these actually mean in
relation to other RoHS banned substances, and
reflow process compatibility.
Component counterfeiters will see clear potential in
the re-marking of non-compliant components during
the transition period, potentially making the problem
worse. There is a COG guide ‘The Date Coding
Minefield’ which covers counterfeiting issues.
Enforcement and Compliance
How can I test or validate RoHS compliance at goods
in?
There is no quick, low cost test for lead that is reliable
at the detection levels required for RoHS. Proposed
options for analysis range from Atomic Absorption
Spectroscopy to simple ‘litmus’ tests. A full
assessment of possible test methods and
approaches to RoHS compliance testing is available
from DTI
17
.
The way the level of banned substances is defined in
the legislation means that compliance testing of
components may mean disassembly to each
individual component material making the overall cost
of analysis considerably higher. To apply analytical
techniques to analyse the hundreds of individual
materials for RoHS compliance in, for example a
laptop, costs might begin to approach several
hundreds of thousand pounds.
So, in practice, the use of analytical techniques for
compliance testing is likely to be limited to situations
of high potential risk and lack of trust with the supply
route. Testing of all components and materials is
probably not feasible.
If we cannot test all parts, how do we show
compliance to the enforcement body?
At the time of writing the enforcement body for the
UK has been named as the National Weight and
Measures Laboratory, and a preliminary statement
has been released. It is expected that a helpline run
by this body will be in place by the third quarter of
2005.
18
14. 13
In the majority of cases ‘due diligence’ will be shown
by the use of supplier declarations stating the RoHS
compliant status of materials and parts you wish to
use. You are required to keep this documentation for
up to four years after EEE product was placed on the
market.
If you have received counterfeit or ‘out of
specification’ parts from a supplier before, it may be
that some analysis of parts beyond a supplier
declaration is expected. There is no case law for
RoHS yet, which means that the exact level to which
evidence will be required is difficult to judge.
Phrases such as ‘what more could we have done?’
and ‘available technology approaches not involving
excessive cost…’ are useful in considering if you
have shown due diligence, but ultimately in the
absence of a clearer definition, lawyers will admit it is
a matter of opinion and will remain so until cases
have reached court.
Reliability Issues Influenced by Pb-
Free
Solder is the fundamental building block of electronic
assemblies, and as such the change to a new
materials poses a challenge to the electronics
industry probably greater than that seen with the
elimination of CFCs, or the change to no-clean flux
technology for example. Concerns about how reliable
alternative materials and processes have been and
will continue to be a focus for research and studies
for several years, but we do not yet have the history,
experience and understanding taken for granted with
SnPb.
Are Pb-Free solder joints as reliable as traditional
SnPb solder?
The main universal failure mode for electronic solder
joints is low fatigue cycling, caused by thermal
mismatch as the materials present expand and
contract with changes in temperature. In the typical
surface mount construction shown in , the different
expansion co-efficient of the ceramic component and
epoxy-glass PCB result in a stress on the solder as
the electronics heats up or cools down. The solder
creeps to accommodate this stress and is damaged
over time until potentially a crack forms; ultimately
resulting is a loss of conductance.
In the majority of cases SnAgCu Pb-Free joints will
provide a greater resistance to thermal cycling fatigue
than SnPb. However, in some more extreme cases
where strain levels are very high, cracks are seen
earlier in Pb-Free solder joints.
Such examples can be created by the component
geometry itself, materials used and end use
environment. Typically these would be where some
of the following apply:
15. 14
- There is a non-compliant lead on the
component (the solder joint must
accommodate all the strain)
- The component is ceramic and the substrate
is not expansion matched
- The temperature range in use is large (e.g.
avionics, specific industrial control
applications)
- The components are large in size
Guidance on the effects of thermal changes on SnPb
and Pb-Free solder joints is available from NPL
19
- all
reports are free.
What happens if the joint is a mixture of Pb and Pb-
Free alloys - possible in repair or when Pb parts
contaminate a Pb-Free process?
Mixed alloy creation in rework and repair.
There is likely to be haphazard introduction of
alternative alloys across the industry and with board
assemblers utilising a range of alloys it may be
difficult to determine the alloy of manufacture during
the transition period. Imagine situations where sub-
assemblies are manufactured, reworked and repaired
at various locations - clearly there is the potential for
solder mixtures to be created.
NPL research has been conducted in this area,
where solder pastes with tin-lead and Pb-Free alloys
were mixed to simulate alloy mixes in reworked
joints
21,22
. The impact on reliability was assessed
using thermal cycling, shear testing and
microsectioning. No adverse effects related to long-
term joint reliability were found, however in
processing a large ‘pasty range’ is exhibited by the
mixtures. So while the risk appears low, NPL
recommends using the original alloy of manufacture
wherever possible (a strong argument for clearly
labelling circuits with this information if possible).
Global
Strain
LifetimeFatiguePerformance
SnAgCu
performs
best SnPb
performs
best
small devices
plastic package
small in use delta T
Compliant leads
large standoff
large devices
ceramic package
large in use delta T
direct attach
low standoff
Figure 10. Illustrative Schematic of Thermal Cycling
Performance
Using SnPb plated surface mount parts in a Pb-
Free reflow process.
Using SnPb plated component leads with Pb-Free
solder paste poses potential risks:
- Risk of heat damage to the part
- The formation of a Pb rich low melting point
phase in the SnAgCu joint (see Fig 11)
Figure 9. CTE Mismatch – Low Fatigue Cycling
16. 15
White areas show Pb rich veins
Figure 11. SEM digimap shows presence of Pb
contamination from component plating in a SnAgCu
surface mount joint
The effects of the low levels of lead (less than 5%)
that occur in this scenario on the reliability of SAC
joints are still under debate. At the time of writing this
is the subject of a current NPL project
23
.
Using SnPb plated parts in a Pb-Free wave
soldering process.
When Pb plated parts come into contact with a Pb-
Free solder wave, Pb can dissolve into the solder
bath. This has immediate implications because with
around 1% Pb present, the eutectic point of SnAgCu
can be effected by 40-50ºC making control of the
process difficult.
There are also implications for subcontractors in the
cross-contamination between product lines.
Monitoring of impurities in the solder bath is therefore
now more important, especially in the transition
period (See Changes to the Assembly Process).
Bismuth and Lead
There are potential reliability risks caused by the
combination of Bi and Pb in a solder joint. For this
reason the use of a Bi containing alloy is not
recommended, especially in the transition period
when Pb plated parts might find their way into
product.
The reduction in reliability is probably due to the
formation of a low melting 16Sn32Pb52Bi phase
15
.
What is the effect on circuit laminates when
processed with Pb-Free solder?
These and other issues are covered in more depth in
the full NPL study
24
.
Solderability degradation
When circuit boards are exposed to soldering
processes, the heat results in a degradation of the
solderability of the board surface finish. This has
always been the case, and in fact is rarely a problem
except for aged boards or for processes involving
many heating processes.
Due to the higher temperatures required for Pb-Free
manufacture, solderability of laminates can be
degraded more during each step potentially resulting
in a decreased yield of acceptable joints.
Increased Z-axis expansion
Epoxy-glass FR4 will expand with heating in the z-
axis to a far greater extent than in the plane of the
circuit board. This is due to the constraining
properties of the glass reinforcement fibres.
Z-axis expansion can be problematic for copper
plated via through holes if the copper is thin or of
poor quality. Risk of copper cracking or damage is
increased with Pb-Free processes as the
temperature change is greater, and the laminate is
higher above its Tg (Glass Transition) temperature
where expansion is greater.
17. 16
Board Sagging
At reflow temperature, FR4 laminates are softened
and quite flexible and rely of their glass fibre
reinforcement to help retain shape. For heavy circuits
board support has always been an issue, with edge
supports allowing too much board sag. Board sag
often results in components falling off the underside
of the circuit during soldering.
With the increase temperature associated with Pb-
Free, boards will sag more. The solutions are as they
are now; design the board for support with a central
wire, use pallets, or change to a different laminate
material. It is better to support the board properly.
This is not an issue that should cause a general
move from FR4 as the main laminate type.
Moisture in laminates
Multilayer PCBs are sometimes baked prior to
processing to remove entrapped moisture. Whilst
baked will slowly degrade solderability it has the
advantage of reducing the risk of any delamination in
laminate layers due to the presence of moisture at
processing temperatures. With lead free the risk of
delamination is increased due to higher reflow
temperatures.
Conductive Anodic Filamentation
Conductive Anodic Filamentation (CAF) is a
subsurface failure mode for woven glass-reinforced
laminate (FR4) materials, in which a copper salt
filament grows and results in a consequential
electrical short between plated through-hole (PTH)
walls or adjacent copper planes. In an NPL study
FR4 multi-layer circuits, were exposed to different
manufacturing conditions and studied for resistance
to CAF initiation and growth
25
.
The study suggests that peak temperatures of 250ºC
in reflow are potentially harmful to the CAF
performance of some standard FR4 laminates. It
should be said that laminate suppliers understand the
mechanisms involved and can offer CAF resistant
laminates, and that other material and processing
factors can have a greater effect than the influence of
Pb-Free reflow. Risk of CAF is higher if you have
smaller via wall-to-wall gaps, the presence of
moisture and higher voltages. One must look at
design and end use to assess risk.
Figure 12. Z-axis expansion will increase with Pb-Free processing.
Crack in
copper
plating
Crack in
copper
plating
18. 17
Route for
CAF Growth
Anode + Cathode -
Production
of H+ ions
Production
of OH- ions
Low
pH
High
pH
Route for
CAF Growth
Anode + Cathode -
Production
of H+ ions
Production
of OH- ions
Low
pH
High
pH
Figure 13.Schematic of CAF growth between two vias.
Can I still use no-clean fluxes with Pb-Free and have
reliable non-corrosive residues?
In a no-clean soldering process, the acids and
activators in the flux or paste medium react as follows:
- Pre-heating allows reaction with joint surface,
removing and floating oxides away
- At peak reflow, good wetting occurs and
enough heat is supplied to either volatilise or
decompose the active flux components
- Upon cooling there is minimal acidic residue left
This balancing act works in the same way for Pb-Free
fluxes except at the higher reflow temperature
conditions. Pb-Free fluxes and paste tend to have
different acids and solvents to cope with heat, but to be
sufficiently exhausted after reflow to become benign.
Is there an increased risk of tin whiskers with Pb-Free?
With the move to RoHS compliant components more
suppliers are choosing to use pure tin as a component
lead finish. Tin whiskering is not a Pb-Free specific
failure mode, but because of this increased use of tin
there it is a related issue. Tin whiskers are single
crystals of tin, commonly 1-3 microns in diameter and
several millimetres in length; their growth rates are
highly variable and unpredictable. They are problematic
because they can cause electrical shorts between
leads, pads etc, either directly or if they break off.
The main mechanism for tin whiskers is the increase in
compressive stress due to intermetallic growth between
a copper lead and the tin plating. Components suppliers
use barrier layers (Ni), and additional organics with the
tin to reduce the risk of whisker growth, and the
performance of recent tin plated finishes offer a
significant improvements in resistance. NPL have done
studies in the area
27
, but there is no evidence to
suggest that Pb-Free processing or solder has an effect
on whisker growth.
Figure 14. SEM image of tin whisker growing from a Tin-
plated lead
There are mitigation strategies to reduce the risks of tin
whiskers based upon baking to relieve stresses and
avoiding bright tin finishes
28
.
19. 18
DEFINITIONS/GLOSSARY
Bi - Bismuth, chemical element
BGAs - Ball Grid Arrays, component type
BOM - Bill of Materials
Cd - Cadmium, chemical element
Cr - Chromium, chemical element
Cu6Sn5 - An inter-metallic compound formed from
copper and tin
Directive - A European Union (formerly EC-European
Community) legal instruction, binding on all Member
States but which must be implemented through national
legislation within a prescribed time-scale.
Dross – Oxidised solder that collects in a wave
soldering bath over time. Significant build up can
prevent proper operation of the soldering wave so dross
must be periodically removed. Dual Use - EEE Intended
for an end-use that is exempt (e.g. military), or out of
scope, but also for an end-use in scope (e.g.
commercial use)
EDX - Energy Dispersive X-ray, a method for detecting
elements present using an electron microscope
EEE - Electronic and Electrical Equipment
ELV - End of Life Vehicles (Directive)
EU - European Union (formerly EC-European
Community)
EuP - Energy Using Products (Directive)
Homogeneous - For the purpose of the RoHS directive
'Homogeneous material' means a material that cannot
be mechanically disjointed into different materials
HASL – Hot Air Solder Levelled, a printed circuit board
solderable finish based on molten solder levelled using
hot air knives. Standard HASL uses SnPb, but Pb-Free
versions can use SnCu, SnAgCu or other alternative
alloys.
MCV - Minimum Concentration Value
Member States - The European Union or EU is, an
inter-governmental organisation, made up of European
countries, which currently has 25 member states.
MSL - Moisture Sensitivity Level
NPL - National Physical Laboratory
Pastey Range - A temperature range over which the
solder behaves like a paste, rather than a liquid or solid,
exhibited by non-eutectic solders
Pb - Lead, chemical element
PBB - polybrominated biphenyl (flame retardant)
PBDE - polybrominated diphenyl ether (flame retardant)
Pb-Free - Containing no lead, or less that 0.1% by
mass of any homogeneous materials if the RoHS
definition is applied
PCB - Printed Circuit Board
Popcorning - Component damage due to delamination,
caused by heat and moisture
QFP - Quad Flat Pack, component type
REACH - Registration, Evaluation, and Authorisation of
Chemicals (Directive)
RoHS - Restriction of Hazardous Substances
(Directive)
SAC - Popular term for the most common Pb-Free alloy
composed of Tin (Sn), Silver (Ag) and Copper (Cu).
SAM - Scanning Acoustic Microscopy, a non-
destructive technique for detecting internal physical
damage to components
SEM - Scanning Electron Microscopy
SnAgCu - The most common Pb-Free alloy, composed
of Tin (Sn), Silver (Ag) and Copper (Cu)
SnPb - Standard Tin Lead solder
SnPbAg - Standard Tin Lead Silver solder
SOP - Small Outline Packages, component type
TSSOP - Thin Shrink Small Outline Packages,
component type
WEEE - Waste Electronic and Electrical Equipment
XRF - X-Ray Fluorescence, an analytical technique for
the detection of elements
Z-axis - In the context of a PCB, the vertical direction
perpendicular to the board surface (x and y plane)
20. 19
Component Obsolescence Group (COG) is a special
interest group of like-minded professionals, from all levels
of the supply chain and across all industries and relevant
Government agencies, concerned with addressing and
mitigating the effects of obsolescence. COG promotes a
pro-active approach to the management of obsolescence
and the development of processes for dealing with it,
through the mutual, voluntary sharing of obsolescence
issues, strategies and solutions.
The Group operates as aself financing body to:
• Discuss the criteria which affect product obsolescence
• Identify and analyse various approaches for addressing
obsolescence
• Promote the advantages of pro0active management of
obsolescence
• Arrange training courses on subjects in the field of
obsolescence management
• Organise conferences and trade exhibitions
• Encourage additional parties to join the Group
• Liaise and co-operate with other National and
International bodies
• Enable data sharing and provide links to available
information
A series of meetings are held during the year to allow
discussion of topics concerning component obsolescence and to
hear presentations on possible obsolescence solutions.
A Web Site (www.cog.org.uk) is available including links to
obsolescence information around the world. Company name
changes are tracked by theWho are they now? feature. Part of the
web site is available to members only. Regular conferences,
exhibitions and training sessions are also arranged. Membership is
based on Company Sites.
An information pack is available from:
Component Obsolescence Group
PO Box 314
Harpenden
Herts AL5 4XL, UK
Tel: +44 (0) 1582 762934 Fax: +44 (0) 1582 461928
e-mail:info@cog.org.uk
Component Obsolescence Group
21. 20
1
Open letter for Chris Tollady, DTI WEEE Implementation, March 2005
2
Examples are ELV directive (Automotive Electronics), Batteries Directive, EuP, REACH
3
http://europa.eu.int/comm/environment/waste/rohs_consult.htm
4
www.npl.co.uk/ei
5
www.cog.org.uk
6
DTI RoHS REGULATIONS, Government Guidance Notes, Consultation Draft- July 2004
7
JESD97, MAY 2004, Marking, Symbols, and Labels for Identification of Lead (Pb) Free Assemblies, Components, and Devices
8
NPL report due for publication Sep 2005, www.npl.co.uk/ei/publications/
9
http://www.npl.co.uk/ei/publications/codeofpractice.html
10
NPL Report MATC (A) 89, ‘Compatibility of Lead-free Solders with PCB Materials’, Miloš Dušek, Jaspal Nottay and Christopher Hunt, August
2001
11
NPL Report MATC (A) 119, ‘Comparison Of Automatic Optical Inspection Systems For Use With Lead-Free Surface Mount Assemblies’, M
Wickham & C Hunt, July 2002
12
IPC-A-610D, Acceptability of Electronic Assemblies. February 2005.
13
NPL Report MATC(A)79, ‘Stability of Electronic Components in Lead-Free Processing’, Alan Brewin, Ling Zou & Christopher Hunt, December
2001
14
NPL Report MATC(A)79, ‘Stability of Electronic Components in Lead-Free Processing’, Alan Brewin, Ling Zou & Christopher Hunt, December
2001
15
‘The Effects of Trace Amounts of Lead on the Reliability of Six Lead-Free Solders’, Thomas A. Woodrow, Ph.D., Boeing, IPC/JEDEC 8th
International Lead Free Conference 2003, San Jose, CA
16
IPC-7095, Design and Assembly Process Implementation for BGAs, (section 8.4.5.6)
17
‘Possible Compliance Approaches for Directive 2002/95/EC (The RoHS Directive)’, Dr P Goodman, Dr C Robertson, Mr R Skipper, Mr J Allen
(www.dti.gov.uk/sustainability/weee/ROHS_Compliance_Full_Report.pdf)
18
www.nwml.gov.uk/rohs.pdf
19
Free downloads from www.npl.co.uk/ei/publications/, Or, as a short -cut, if you know the reference for any NPL report you can download it directly
from: http://libsvr.npl.co.uk/npl_web/search.htm
20
Free downloads from www.npl.co.uk/ei/publications/, Or, as a short -cut, if you know the reference for any NPL report you can download it directly
from: http://libsvr.npl.co.uk/npl_web/search.htm
21
NPL Report MATC(A)85, ‘Reliability of Joints Formed with Mixed Alloy Solders’, Alan Brewin, Christopher Hunt, Milos Dusek & Jaspal
Nottay, April 2002
22
NPL Report MATC(A)106, ‘Rework of Mixed Lead-Free Alloys - A Guide’, Martin Wickham, Alan Brewin & Christopher Hunt, April 2002
23
NPL Studio Project, ‘Impact of Unknown Component Alloy Finish During the Lead-free Transition Period’, publication expected Oct 2005,
www.npl.co.uk/ei/studio/transition/
24
NPL Report MATC(A)91, ‘An Assessment of the Suitability of Current PCB Laminates to Withstand Lead-free Reflow Profiles’, Martin
Wickham, Ling Zou & Christopher Hunt, April 2002
25
NPL Report MATC(A)155, ‘Susceptibility of Glass- Reinforced Epoxy Laminates to Conductive Anodic Filamentation’, Alan Brewin, Ling Zou &
Christopher Hunt, January 2004
26
NPL Report MATC(A)155, ‘Susceptibility of Glass- Reinforced Epoxy Laminates to Conductive Anodic Filamentation’, Alan Brewin, Ling Zou &
Christopher Hunt, January 2004
27
NPL Report MATC(A)148, ‘Developing a test to characterise internal stress in tin coatings’, Wickham, M, Fry, A T, Hunt, C, October 2003
28
iNEMI TIN WHISKER USER GROUP, ‘Recommendations on Lead-Free Finishes for Components Used in High-Reliability Products’, Version 3
(Updated May 2005)
22. The Pb-Free Minefield
Published by the Component Obsolescence Group
P.O. Box 314, Harpenden, Hertfordshire AL5 4XL UK
Tel: +44 (0)1582 762934 Fax: +44 (0)1582 461928
e-mail: info@cog.org.uk Web: www.cog.org.uk
This booklet has been sponsored by the following organisation
ELFNET , Unit 3, Curo Park, Frogmore, St Albans, Hertfordshire AL2 2DD, UK
Tel: +44 (0)1727 871311, Fax: +44 (0)1727 871345, info@europeanleadfree.net
www.europeanleadfree.net
