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Warpage of Flexible-Board Assemblies with BGAs During Reflow and Post-Assembly



Warpage of Flexible-Board Assemblies with BGAs During Reflow and Post-Assembly
Failure in solder interconnects is a leading cause of reliability failure. Complexity of electronic assembly requires these lead-free interconnects to be reliable.
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Authored By:


Pradeep Lall, Kartik Goyal
Auburn University
NSF-CAVE3 Electronics Research Center, AL, USA

Ben Leever
US Air Force Research Labs, OH, USA

Jason Marsh
NextFlex Manufacturing Institute, CA, USA

Summary


Flexible printed circuit boards lack the structural stiffness of the rigid printed circuit counterparts. Thermo-mechanical deformation in flexible printed circuit assemblies may be very different from that in rigid board assemblies. The double-sided board used for the experiment is of BGA 256-144 combination with dummy components, A-PBGA256-1.0mm-17mm and A-CABGA144-1.0mm-13mm. The three-dimensional measurements of deformation and strain have been visualized on the geometry of the solder joints in the package. Digital volume correlation (DVC) method has been used to find the displacements and strains in interconnects of operational electronics. The x-ray microscopic computed tomography (μCT) system has been used to generate the 16-bit digital volume data. The x-ray detector has ability to image the x-ray attenuation of x-rays through the object. Reliability testing of SAC 305 solder interconnects has been performed on double-sided flexible circuit board using x-ray μCT by heating the package to 100℃. 3D-finite element models have been developed to ascertain the degree of error in the model prediction from non-destructive experimental measurements in reflow and thermal cycling.

Conclusions


In this study, non-destructive DVC have been used to measure the thermo-mechanical deformation of solder joints in flexible and rigid printed circuit assemblies. Measurements from DVC have been compared with FE model predictions to determine the error in strain measurements. The quarter-model is developed for a double-sided assembly with flexible and rigid substrate, to simulate the non-linear thermo-mechanical damage in solder joints using the Anand constitutive model. The MPC approach is also utilized in this paper to glue the uneven mesh obtained due to the offset of the bottom package. This study has been performed on a double-sided test assembly, which consists of BGA packages on two different substrates: rigid and flexible. The top package, PBGA 256 was subjected to thermal load of 100°C, and the effect of it is measured on both, top and bottom package. Both daisy-chain BGA components have pitch of 1mm. For FE simulation, symmetry was enforced, and appropriate boundary conditions were applied. The findings from FE analysis and experimental DVC correlate well with each other as shown in Table 4 above, which establishes DVC, when used in conjunction with x-ray µ-CT as a very reliable technique to determine the deformations or strains and visualize them as they are on the object itself without physically destroying it.

Initially Published in the SMTA Proceedings

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