1. Study on Effect of Thermo-Structural loading on
the PCB during Selective Soldering process using
Finite Element Method
Abstract - Electronic components are soldered on to a
Printed Circuit Board (PCB) to form an electronic assembly.
Earlier all solders contained Lead (Pb), but environmental
concerns with Pb have paved the way for development of lead-
free solders to replace the commercial Tin-Lead (Sn-Pb)
solders in electronic packaging systems. Majority of lead-free
solders available exhibit poorer properties and higher surface
tension than traditional Sn-Pb alloys. Therefore lead-free
solders require processing at higher temperatures. In mixed
assembly technology, both Surface Mount Technology (SMT)
and Through Hole Technology (THT) components are placed
on the PCB. During production, SMT components are soldered
initially followed by Selective Soldering process to solder the
THT components. The need for higher processing
temperatures result in higher thermal load on the board,
resulting in complications of unwarranted warpage of PCB
during soldering. The alarming levels of deflection and
bending in PCB in turn endanger the already mounted
temperature sensitive SMT components. This bending also
results in formation of defective solder joints. Consequently
this process requires perfect precision. This paper presents, a
Thermo-Mechanical FE analysis results to elaborate the effects
of various parameters on the PCB strains during the process.
Also discussions on fixture strategy to minimize PCB strain are
explained. The fixture suggested based on the FE analysis can
be implemented during production, resulting in a foolproof
process. To ensure reliable solder joints and safety of the
electronic components, PCB strains are restricted within
certain limits considering safety criteria of critical electronic
components. The FE package ANSYS 15.0 has been used for
numerical simulations. Preliminary results from the FE model
suggest that it is matching with the observations of warpage in
the production samples. Detailed experimental validation has
been planned in the near future.
Keywords - PCB Bending, Selective Soldering, Thermo-
Mechanical, Fixture Strategy, Solder Defects
I. INTRODUCTION
Soldering is a metal joining technique in which a filler
metal (solder) with a melting point not exceeding 450⁰C is
melted and distributed by capillary action between the faying
surfaces of the metal parts being joined [1]. The bonding is
accomplished by melting the solder material, allowing it to
flow among and make contact with the components to be
joined; which do not melt. Finally upon solidification, a
physical bond is formed among these components. Soldering
produces electrically conductive, water and gas tight joints.
As an industrial process, soldering is most closely associated
with electronic products. Here electronic components are
soldered to a PCB to form Electronic Assembly. In mixed
assembly technology, PCB is mounted with both Surface
Mount Technology (SMT) and Through Hole Technology
(THT) components. Initially SMT components are soldered
by reflow soldering, followed by soldering of THT
components. THT components are soldered using Wave
soldering, wherein the entire board is passed over a flowing
wave of molten solder. However this is not feasible because
of already placed SMT components. Selective Soldering
process is employed here to ensure that the already mounted
temperature-sensitive SMT components and the PCB are not
affected from excess thermal loading during wave soldering.
During Selective Soldering, the PCB is selectively exposed
to the molten solder, to reduce the influence on SMT
components. Selective soldering consists of three steps;
fluxing, pre-heating, soldering and finally cooling down. The
final assembly will be inspected for defects using either
manual or automatic inspection technique.
Restrictions on leaded solders because of its hazardous
health concerns have led the way for lead-free solders.
However the introduction of lead-free solders has resulted in
a host of new challenges and concerns. Study carried out by
Klenke [2] and Falinski et al. [3] suggest that majority of
lead-free solders have higher melting points and worse
wettability than conventional leaded solders. This results in
higher temperature requirement during soldering. Therefore
increasing the thermal loads on the board. In Selective
Soldering, this has given rise to excessive, uneven warpage
in the PCB due to localized temperature variations. This
warpage has been the cause for various kinds of defects in
solder joints. The undesirable influence on the various
critical SMT components already mounted on the PCB
should also be considered.
Chung et al. [4] carried out a Thermo-mechanical FE
analysis to predict the warpage of PCB during Reflow
soldering, in which they considered detailed modelling of
PCB in order to accurately predict the effect of CTE (Co-
Subraya Krishna Bhat1
, Raghavendra Deshpande2
, Peter Beck3
, Sudarshan Hegde2
,
Y. S. Upadhyaya1
, Chandan Kumar Ghosh2
1
Dept. of Mech. and Mfg. Engg., Manipal Institute of Technology, Manipal University, Manipal, India
2
Robert Bosch Engineering and Business Solutions Pvt. Ltd. (RBEI), Bengaluru, India
3
Robert Bosch Kft. (RBHU), Budapest, Hungary
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2. efficient of Thermal Expansion) and Elastic Modulus
mismatch between the intermediate layers of the PCB.
Zukowski et al. [5] presented a novel approach of material
homogenization to accurately predict the dynamic response
of PCB. It was observed that simulated response is in good
agreement with the experimental results. This approach has
been extended in this study to suit for a Thermo-Structural
analysis.
Few frequently observed defects during Selective
Soldering are:
i. Solder overflow (Fig.1)
ii. Improper vertical hole filling (Fig.2)
iii. Solder bridging
These defects can be attributed to the various soldering
process parameters such as inadequate fluxing, non-
homogeneous pre-heating and clearance between solder pot
and PCB. However the influence of PCB bending on these
defects cannot be ignored, which has not been addressed in
detail in the literature.
Fig.1: Solder overflow
Fig.2: Poor Vertical Hole Filling
II. OBJECTIVE
Fig.3 depicts the schematic representaion of PCB response
during Selective Soldering. Uneven warpage of PCB under
the thermal loads from Soldering nozzles is illustrated here.
The paper deals with a simulation based approach to tackle
this issue of PCB bending so as to ensure the safety of the
SMT components and formation of reliable solder joints in
Electronic products. To tackle the issue of PCB warpage
during soldering process, a fixturing strategy has been
proposed in this paper.
Fig.3: Schematic representation of Observed bending in PCB
To evaluate the PCB deformation during the soldering
process, analysis is required. Therefore, the objective is this
study is to analyze the effect of Thermo-Structural loading
on the PCB during Selective Soldering process.
III. METHODOLOGY
A. Approach Adopted
To constrain the PCB, we introduce supports at different
locations. The space available for placing support mounts
(open space) can be obtained from the Electronics layout
side. After placing support mounts in the open space
available on the PCB surface, the appropriate loads and
boundary conditions are considered in the FE analysis to
evaluate the PCB strains. A limiting condition for PCB
strain has been defined considering failure criteria of critical
SMT components. The simulation thus helps in determining
number of support mounts and their location to limit the
PCB strains within acceptable limits. This concept has been
depicted in Fig.4 below.
Fig.4: Introduction of supports to constrain PCB warpage
B. PCB Modelling Strategy
PCB is usually made up of intermediate layers of Cu
traces and dielectric substrate (called Prepreg), which is a
composite of glass fiber and epoxy resin. A PCB may be
single sided (one copper layer), double sided (two copper
layer) or multi layered (outer, inner copper interior copper
layers). It provides electrical connectivity between different
electronic components by using Conductive copper traces,
pads and other features etched from copper sheets laminated
into a non-conductive substrate. Fig.5 shows the detailed
constructional features of a PCB.
Solder Nozzle
Initial PCB Configuration Observed PCB Warpage
Support Mounts
Solder Nozzle
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3. Fig.5: PCB Constructional features [6]
The accuracy of any FE model depends upon several fronts
such as Geometry, Boundary conditions and Material data.
Usually for simulations, PCB is described with equivalent
isotropic or orthotropic material models for computational
efficiency. However this assumption cannot account for the
interlayer stress due to the CTE and Elastic Modulus
mismatch between each layer. In this paper detailed model
of PCB is considered for the analysis with different
intermediate layers of Copper and Prepreg of appropriate
thickness.
C. Simulation Model
Selective Soldering setup consists of the following
systems:
 Single board PCB assembled with SMT components
 Fixture Setup to support the PCB
 Pre-heating Chamber
 Soldering Chamber
 Cooling Stage
 Inspection Stage
PCB assembled with SMT components in panel form is
milled to separate each single board. These single boards are
sent to Selective Soldering machine. Here, the THT
components are placed on board by inserting the lead wires
through holes in the PCB. Finally this assembly held by a
fixture setup is sent into the soldering machine. The fixture
setup consists of setup to support the PCB and the THT
components (Fig.6). In this paper we evaluate the response
of PCB considering the molten solder temperature as
thermal loading and fixture setup in the form of structural
loads and boundary conditions.
Fig.6 (a): A support mount for THT components [Courtesy: Kurtz Ersa Co.]
Fig.6 (b): Fixture Plate with support mount [Courtesy: Kurtz Ersa Co.]
To model the reality in FE domain, few assumptions
have been considered:
 Bare PCB without any electronic components has been
considered for this analysis. Considering bare board
will be the worst case scenario since, the stiffness of the
board will be minimum.
 Initial model is considered to be stress free (Pre-strain
is not considered for the analysis)
 PCB is heated to a uniform homogeneous temperature
during pre-heating.
 PCB is modelled in detail considering the individual
Copper and Prepreg layers. Orthotropic material
property is assumed for the PCB.
 All other materials are assumed to be Homogeneous,
Linear and Isotropic.
The solder temperature is applied as thermal loading on the
PCB in the region of selective soldering. Transient thermal
loading is applied for the duration of soldering. Fig. 7(a) and
7(b) show the typical temperature distribution in the PCB on
component side and solder side respectively at the end of
soldering time. These figures clearly demonstrate
temperature difference across the thickness of the PCB (out
of plane direction of PCB) which is the source of CTE
mismatch. The CTE mismatch across the intermediate
layers induce the uneven warpage in the PCB. Further, the
fixture setup is considered by introducing structural
boundary conditions in the FE Model.
Fig.7 (a): Temperature distribution in PCB (Component Side)
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4. Fig.7 (b): Temperature distribution in PCB (Solder Side)
IV. RESULTS AND DISCUSSIONS
A. Simulation Results
Initially to estimate the detrimental effect of thermal
load, PCB warpage is evaluated under pure thermal load,
and free-free boundary condition (no constrains). The
warpage observed across the diagonal of the PCB is shown
below (Fig.8). Fig.9 shows the PCB deflection profile
across the two diagonals.
Fig.8: Free-Free Warpage of PCB across diagonal (only thermal load)
According to study carried out by Keimasi et al. [7] Ceramic
Capacitors are highly susceptible to PCB flexing. It was
found that these components fail around 1700 micron strain.
The Normal Elastic strains under pure thermal loading are
exceeding this limit of 1700 microns. Hence it is evident
that PCB bending under thermal loading is a cause of
concern. Fig.9 shows the deflection profile of PCB across
the two diagonals under pure thermal loading.
Fig.9: Deflection across two diagonals of the PCB with only thermal loads
To reduce this warpage, supports are added on the free
space available on the surface of PCB (Fig.10). This
imposes restriction against lateral deflection thus resulting
in reduction of this warpage. Currently, these supports are
placed based on intuition and thumb rules. In the described
approach, simulations are performed with various
combinations of number and location of supports to
optimize the PCB strain and deflection. Further iterations
lead to reduction of PCB deflection. This process is
continued until the deflection and thus the Board strains are
within 1700 microns.
Fig.10: PCB supported by Fixture Setup
Fig.11 shows the fixture setup considered in the model.
PCB is carried on a Carrier plate along the assembly line.
PCB deflection reduces after placement of supports
(Fig.12). The resulting deflection profile across the
diagonals is shown in Fig.13. In this case, the maximum
deflection in the board is reduced by a factor of 100 after the
placement of supports. Implementing these support
locations therefore ensure that PCB deflection is within the
safety limits.
Diagonals
across PCB
Y
XZ
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5. Fig.11: Support placed on PCB
B. Experimental Results
To validate the FE simulation results, measurements were
carried out for the PCB layout. Strain measurements were
carried out at 9 different locations on the surface of PCB
(Fig14). However these measurements were not carried out
at soldering temperatures, but at lower temperatures. The
PCB was heated to a uniform homogeneous temperature and
measurements were carried out. This is due to the
complicacies of resistance based strain measurements at
high temperature. Nevertheless, actual measurements at
actual soldering temperature has been planned in the near
future. The observations from these preliminary
measurements have been analyzed here. Fig. 15 (a) and (b)
show the comparison with the simulation results for Elastic
Strains in X and Y directions respectively. Further the
comparison of simulation and measurement results have
been tabulated in Table 1(a) and 1(b) respectively.
Fig12: PCB deflection after placing supports
Fig.13: PCB deflection across two diagonals after placing supports
Here X and Y indicates measurement values and Simu(x)
and Simu(y) indicates the results from simulation along X
and Y directions respectively. As shown in Table I and II,
although there is a high degree of correlation between the
simulation and the measurement results in most locations, at
few of the locations large difference is observed with the
measurement data. A maximum difference of 191.3% has
been observed in a particular location. This can be attributed
to the assumptions made in the analysis. In this analysis,
Pre-warpage of the PCB has not been considered. Pre-
warpage may occur due to various reasons including the
preceding assembly operations. As discussed in the
introduction, before Selective Soldering of THT
components, the SMT components are assembled on the
PCB by Reflow soldering. The warpage occurring due to
thermal loads during Reflow soldering has not been
considered here. Further the board undergoes various
processes such as In-Circuit Test and Pre-milling of Panel.
The strain induced due to these processes are not included in
this analysis.
Fig.14: Strain Gauge mounting locations for measurement
Support
PCB
PCB Carrier Plate
X
Y
Z
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6. Fig.15 (a): Comparison between Simulation and Experimental Results:
Normal Strains in X - direction
Fig.15 (b): Comparison between Simulation and Experimental Results:
Normal Strains in Y – direction
TABLE I. NORMAL STRAINS IN ‘X’ – DIRECTION
1 2 3 4 5 6 7 8 9
Experimental
Results
[mm/m]
235.8 240 250.9 186.6 188.4 218.7 122.5 191.4 105.3
Simulation
Results
[mm/m]
246.7 280.9 220.7 201.4 222.6 153.7 121.9 157.9 112.2
Variation (%) 4.62 17.04 13.68 7.93 18.15 42.29 0.49 21.21 6.55
Strain Measurement PointsNormal Strain in
'x' direction
TABLE II. NORMAL STRAIN IN ‘Y’ – DIRECTION
1 2 3 4 5 6 7 8 9
Experimental
Results
[mm/m]
23.44 48.65 12.55 24.82 24.05 22.36 1.61 35.28 6.31
Simulation
Results
[mm/m]
53 46.09 28.32 21.12 26.19 25.83 4.69 41.09 3.25
Variation (%) 126.1 5.55 125.7 17.52 8.89 15.52 191.3 16.47 94.15
Normal Strain in
'y' direction
Strain Measurement Points
V. CONCLUSIONS
This paper discusses a novel approach of simulation
based fixture strategy to support the PCB during Selective
Soldering process. This approach aids in the quantitative
evaluation of PCB warpage under the Thermo-Structural
loading conditions involved in the process. FE Simulation
driven fixture placement results in a foolproof operation. By
limiting of PCB bending, concern of component failure will
be avoided. This approach also ensures the quality and
reliability of solder joints formed during Selective Soldering
process. Experimental measurements have been carried out,
which validate the accuracy of the developed FE Model.
VI. FUTURE SCOPE
The described methodology has been developed based
on the practical observations during the Soldering process.
As explained in the discussions, preliminary results are
matching with production samples. However, detailed
experimental validation at soldering temperatures is being
planned in the near future.
ACKNOWLEDGMENTS
The authors express grateful appreciation for all the
support, encouragement and computational facilities offered
by Robert Bosch Engineering and Business Solutions Pvt.
Ltd. (India). In particular, the colleagues from Robert Bosch
Kft. (Budapest), for sharing valuable information on the
practical observations in the plant during production and for
their insight on the actual process.
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[3] Wojciech Falinski, Halina Hackiewicz, Grazyna Koziol and Janusz
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