A new experimental tool for analyzing the topography and deformation of electronics components under thermo-mechanical stress is presented. Application examples are shown for a great variety of components, for localizing and quantifying deformations of electronic assemblies. Cooling and heating cycles following JEDEC type thermal profiles have been applied on different components, both before and after assembly. Simultaneously, real time topography and deformation measurements are obtained. These capabilities constitute a powerful tool for failure prediction, risk evaluation, and accelerated development. The high resolution optical setup allows analysis of deformations in the micrometer range, even for very irregularly shaped surfaces

Fayolle, R.

Hertl, M.

Lecomte, J.-C.

Weidmann, D.

I-Revues

  
 
 
 
 
THERMAL STRESS FAILURES: 
A NEW EXPERIMENTAL APPROACH FOR PREDICTION AND PREVENTION 
 
Michael Hertl –– Romain Fayolle – Diane Weidmann – Jean-Claude Lecomte 
 
INSIDIX 
24 rue du Drac – 38180 SEYSSINS – France 
Tel.: 33 (0)4 38 12 42 80 – E-mail: insidix@insidix.com - www.insidix.com  
 
 
ABSTRACT 
 
A new experimental tool for analyzing the topography and 
deformation of electronics components under thermo-
mechanical stress is presented. Application examples are 
shown for a great variety of components, for localizing 
and quantifying deformations of electronic assemblies.
 Cooling and heating cycles following JEDEC type 
thermal profiles have been applied on different 
components, both before and after assembly. 
Simultaneously, real time topography and deformation 
measurements are obtained. These capabilities constitute a 
powerful tool for failure prediction, risk evaluation, and 
accelerated development. 
 The high resolution optical setup allows analysis of 
deformations in the micrometer range, even for very 
irregularly shaped surfaces. 
 
 
 
 
1. INTRODUCTION 
 
 Decreasing the number of component failures 
like delaminations or BGA ball ruptures is a constant 
concern for electronics development engineers. 
Thermal stress induced by CTE mismatch between 
the multiple laminated materials or by increasing 
solder temperatures as consequence of the RoHS 
standard is the physical reason for a great number of 
damages. Additional damage risks are induced by 
hygroscopic stress, which is out of the scope of the 
present paper. 
 Failure analysis is often done by numerical 
modeling [1]. However, the modeling of the thermo-
mechanical behavior of electronic assemblies, 
whether soldered or not on a PCB, encounters many 
difficulties. These difficulties are mainly due to the 
rapid increase of the number of layers and the fact 
that the characteristics of the materials and the 
interfaces under thermal and mechanical stresses 
are partially unknown. 
 
 
 
 
 
Reliable experimental tools are therefore required 
for detection of risks related to thermo-mechanical 
stress of the components [2]. Several conditions 
apply in order to obtain relevant results (Fig. 1): 
 
 The measurement has to be non destructive, in 
order to analyze real components without any 
obligation of cutting, drilling, or mechanically 
deforming them; 
 The possibility to take measurements with a 
large field of view and to permit the 
simultaneous acquisition of all surfaces (S1, 
S2, S3 in Fig. 1) of an electronic board has to 
be given; 
 Measurements have to be done under thermal 
stresses like those encountered by the 
component during its assembly and product 
life time: reflow profiles, on/off cycles, 
external thermal solicitation, and others; 
∆z1 
∆z2 
∆z3 ∆x1 - CTE 1 
∆x2 - 
CTE 2 
Compression 
Tensile 
stress 
z1 
z2 
z3 
0 
S1 
S2 
S3 
z1 + ∆z1 
0
 
∆x3 - CTE 3 
z3 + ∆z3 
Z2 + ∆z2 
Fig. 1: Schematic view of the variation of a BGA’s 
topography with temperature. Top: BGA on PCB at 
room temperature. Bottom: Same assembly at 300°C. 
  Real time measurements simultaneously in plane 
(x, y) and out of plane (z) are necessary [3]; 
 Finally, the setup has to be flexible enough to 
allow analysis for a great number of different 
applications, like package development, CTE 
evaluation, in service stress evaluation, RoHS 
consequences analysis, reliability tests. 
 
 The scope of the present paper is to introduce 
TDM, a new tool developed for Topography and 
Deformation Measurement under thermo-mechanical 
stress. After a description of the experimental set-up, 
a large section is reserved for application examples 
in thermal management development in various 
fields of today’s electronics.  
 
 
 
2. SET-UP 
 
 The experimental setup is shown in Figure 2. 
The electronics assembly to be studied is illuminated 
by a light source, which projects a structured light 
field on the sample surface. The light’s intensity 
pattern is more or less deformed by the sample’s 
surface structure. The resulting image is captured by 
a CCD camera. 
 
 
 
Fig. 2: Sketch of TDM setup.  
 
 Real thermal stress is available by top and 
bottom heating and cooling elements. The current 
sample temperature is monitored by several 
thermocouples. Thus JEDEC type temperature 
profiles with gradients up to +/-3°C/s may be 
imposed to the component, within a temperature 
range from -40 up to +300°C. 
 This setup allows absolute and relative 
deformation measurements both in plane (xy) and 
out of plane (z). The in plane resolution is 5×10-5 
times the sample length, the out of plane 
resolution is 10-4 times the sample length. 
 In comparison to Moiré interferometry [4], the 
present set-up features two main benefits. First, 
there is no need for a reference grating or any 
other movable part, simplifying noticeably the 
entire alignment and acquisition procedure. 
Second, the measurement procedure works 
independently for each voxel of the measurement 
volume. Therefore, large height variations (∆z 
steps) or even holes in the sample do not affect at 
all the measurement at the surrounding voxels. 
 
 
 
3. RESULTS 
 
3.1. Power device: Al2O3 brazed on Cu plates 
 
When alumina (Al2O3) is deposited on copper 
(Fig. 3), for power components for example, the 
difference in the coefficients of thermal expansion 
between the two materials creates a deformation 
of the assembled parts and results in stress at the 
interface when the component returns to room 
temperature. 
 
 
 
 
 
 
 
 
 
 
 
 
 At the brazing temperature (400–600°C) both 
the alumina and the copper part are plane. During 
cooling, both materials will retract differently due 
to CTE mismatch, and strong deformation occurs. 
At room temperature the alumina part becomes 
convex, while the copper part is concave (Fig. 4). 
The space in between the two parts is filled with 
brazing alloy. 
Bottom 
thermocouple 
Bottom heating 
Top 
heating 
Camera 
Light 
source 
Sample 
holder 
Top 
thermocouple 
Bottom 
Cooling 
Top 
Cooling 
Fig. 3: Video Image of the Alumina part brazed on 
copper.  
Alumina 
Copper 
  However, the interface is subjected to strong 
stress, both in plane (shear stress) and out of plane 
(tensile stress). When submitted to thermal cycling 
this configuration contributes to premature aging and 
significantly increases the failure risk. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig. 4: Characterization of the alumina and the copper 
surface in one single measurement, at room temperature. 
A software zoom is possible on each surface, for a high 
resolution image of the surface topography. 
 
 
Fig. 5: SAM analysis of the alumina – copper interface. 
Crack initiation is observed in all 4 angles of the brazing 
area. 
 
 Additional investigation [5] of the part by 
scanning acoustic microscopy (SAM) confirmed 
crack initiation in the four angles of the brazing area 
(Fig. 5). 
 Introducing TDM measurements in the 
development cycle of this product allowed to 
optimize the characteristics of the part (like the 
exact dimensions and the solder characteristics), in 
order to minimize the effect of opposite bending 
of the alumina and the copper part. 
 
 
3.2. Lead free JEDEC thermal profile effect on 
BGA deformation and damage 
 
 New JEDEC type temperature profiles for 
soldering under lead free conditions attain 
sensibly higher peak temperatures than classical 
reflow profiles, with strong incidence on damage 
risks due to CTE mismatch or other stress induced 
deformations. 
 If such a high temperature profile is used for 
soldering a BGA, then both the BGA and the PCB 
will strongly bend during the soldering process. 
The final form of the balls will be fixed at more 
than 200°C, when the solder becomes solid. 
However, the final geometric form of BGA and 
PCB is determined only when the assembly is 
back at room temperature. Therefore, some balls 
will see compressive, others tensile stress (Fig. 1). 
 TDM allows following in detail the 
topography of the parts to be soldered during a 
JEDEC type reflow profile (Figures 6 and 7). 
 The topography images show several 
incidents: 
• The initial topography at room 
temperature is slightly concave. Up to 
150°C, the BGA changes its topography 
only slightly, to become flat at this 
temperature. Between 150 and 200°C, the 
topography changes slowly, and the BGA 
becomes convex. 
• Above 200°C, a dramatic topography 
change takes place, with a final amplitude 
of the convex deformation of about 220 
µm. Fast changes of topography, 
especially from concave or flat towards 
convex, are in most cases related to 
delamination. Indeed, for this BGA 
delamination has been confirmed by 
SAM. By multiplying the number of 
images between 200 and 245°C, it is 
therefore possible to determine the exact 
temperature where delamination occurs. 
B 
B’ 
B 
A Length = 36.6 mm Pt = 0.403 mm A’ 
B’ 
A 
A’ 
 • After cooling down to room temperature, 
some convex deformation remains. This 
hysteresis effect is the second characteristic 
observation when delamination occurred. 
 
 This kind of measurement has several direct 
benefits for the thermal management developer: The 
temperature where delamination occurs can be 
determined, and if possible the applied reflow profile 
adapted accordingly. The amplitude of deformation 
allows an estimation of the failure risk of the 
component in future applications, even if no direct 
delamination occurs. And successive measurements 
with the same temperature profile allow an 
evaluation of the componenet behavior under future 
cyclic load, like on/off cycles or for fatigue control. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
3.3. Accuracy control on very small 
components 
 
 With increasing miniaturization, quality 
control on very small components becomes an 
issue in all fields of electronics. Parts 
manufactured by subcontractors needs to be 
controlled before being integrated in complex 
assemblies. 
 Figure 8 shows the photograph of a standard 
module for SIM cards in mobile phones. Critical 
dimensions are the height and the parallelism of 
those parts of the module which will be soldered 
onto the PCB. 
360 720 Time (s) 
Temperature (°C) 
50 
100 
150 
200 
Tmax 
ini 
2 
3 
4 
 5 6 7 8 9 
10 
11 
12 final 
Fig. 6: JEDEC type reflow profile generated and monitored by TDM. The arrows indicate temperatures where 
deformation measurements are automatically triggered. These temperatures may be defined by the user. 
170°C * 225°C * 
25°C 
165°C 
183°C 
200°C * 157°C * 
Tmax 
25°C 
25°C 
Heating 
245°C 
150°C 200°C 
173°C 
143°C * 
25°C 
Cooling 
Fig. 7: Deformation measurements performed on a BGA at different temperatures during the temperature profile shown 
in Figure 6. Strong deformation occurs between 200 and 245°C. Note the hysteresis in deformation when the 
component is back at room temperature. 
  
 
 
 
 
 
 
 
 
 
Fig. 8: Photography of a SIM card module. Dimensions of 
the component: 25 x 15 x 2 mm ( l × w × h). 
 
 Figure 9 shows the topography of the bottom 
side of the component, measured with TDM. The 
height and the parallelism of the parts to be soldered 
are clearly identified, and may be compared to 
manufacturing specifications. Note that TDM is able 
to obtain high resolution topography even for 
components with very strongly structured surfaces, 
even in presence of multiple holes. 
 
 
 
Fig. 9: Height profile of the bottom side of the SIM 
module. The most prominent parts (green) are used for 
soldering the component onto the PCB. 
 
 A 3D representation of the component is given 
in Figure 10. This view, which is calculated on the 
basis of the 2D bottom view acquisition shown in 
Figure 9, allows detailed verification of the 
dimensions of further critical parts, like the springs 
in the centre of the module. 
 Overall, application of TDM measurements to 
this part allowed a big step forward towards total 
quality control of pieces delivered from 
subcontractors, manufactured following 
specifications defined in the order. 
 
 
Fig. 10: 3D view of the SIM module, calculated from 
the image given in Fig. 9. Note that one single 2D 
bottom side measurement of the component is sufficient 
for generating this view.  
 
 
3.4. Quantitative deformation measurement on 
BGA after assembly on PCB 
 
 In this application, we compare, for the same 
component/PCB set, the assembly process with a 
Pb/Sn profile, Figure 11, and with a lead free 
profile, Figure 12. These Figures show diagonal 
warpage profiles measured on the BGA while 
soldering on a PCB. In each Figure, the top 
diagram corresponds to the time interval where 
temperature increases, the lower diagram to the 
interval where temperature decreases. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig. 11: Diagonal warpage profile on a BGA during 
soldering on a PCB. Analysis for a Pb/Sn temperature 
profile (maximum temperature 230°C) 
 
Heating T [°C] 
Cooling 
25 50 75 100 125 150 175 200 230 
 230 225 200 175 150 125 100 75 50 25
  During the two different reflow profiles, the 
topography changes with temperature strongly differ. 
This situation induces different effects on the 
interface integrity, which might be seen afterwards 
using acoustic microscopy, and ball interfaces, which 
might be seen by X-ray tomography. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig. 12: Diagonal warpage profile on a BGA during 
soldering on a PCB. Analysis for a lead-free temperature 
profile (maximum temperature 255°C) 
 
 After assembly on the PCB, the same BGA 
component has different remaining warpages 
depending on the reflow profile. 
 
 
 
Fig. 13: SAM image of the BGA soldered on PCB using a 
lead-free reflow profile. White circles: crack initiation 
 
 
 
 
Additional measurements by scanning acoustic 
microscopy confirmed the different behaviors of 
the BGA under Pb/Sn and lead free profiles: 
Figure 12 shows the initiation of cracks in the case 
of the lead free profile. No cracks are detected in 
the BGA soldered with the Pb/Sn profile. 
 
 
4. CONCLUSIONS 
 
 A new experimental tool, TDM, has been 
developed for topography and deformation 
measurement under thermo-mechanical stress. 
This tool enables high resolution absolute 3D 
imaging of warpage (z) and in plane (xy) 
deformation. 
 Four examples have been discussed for 
application of TDM in prediction and prevention 
of failures in electronics. In each example, 
analysis of the different components in an early 
stage of the development and production cycle 
allowed an early failure risk analysis, successive 
design changes, and thus shorter time to market as 
well as lower risk of customer returns. 
 In future, the results obtained by TDM will be 
coupled with modeling, which will allow easier 
definition of boundary conditions and validation 
of modeling results by experimental values.  
 
 
 
5. REFERENCES 
 
[1] Suter et al., Proc. of the  EuroSimE 2005 conf., 25-30  
[2]
 Eurosime May 2004 – Brussels Belgium "Topography 
and Deformation Measurement under thermo-
mechanical solicitations" TDM Equipment - Static and 
dynamic measurements. 
[3]
  PC2A – September 2004 – Grenoble France 
"Topography and Deformation Measurement under 
thermo-mechanical solicitations" Static and dynamic 
measurements: A new approach for reliability 
improvements in electronic. 
[4] Stellrecht et al., IEEE Trans. Comp. and Packaging 
Technologies 27(3), 499-506 (2004) 
[5] Symposium Brasage sans plomb –2002 – Grenoble 
France Méthodologie d'évaluation de la Fiabilité des 
Interfaces d'Assemblage – Les techniques non 
destructives (CND) appliquées au monitoring de la 
cinétique de dégradation. 
 
25 50 75 100 125 150 175 200 225 255 
255 225 200 175 150 125 100 75 50 25    
Heating T [°C] 
Cooling 


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