Authored By:
Rebecca Wheeling, Ph.D., John Laing, Shelley Williams
Sandia National Laboratories
NM, USA
Summary
A castellated via solder joint is a joint configuration where a semi-circular surface area or castellation is soldered to a flat solder pad. The castellated via joint configuration is common in industry, specifically for connecting complex modules to simple board designs to reduce overall board complexity. The reliability and performance of castellated vias as a rigid-flex interconnection method is evaluated here.
This evaluation is split into 2 discreet elements: 1) metallurgical aging of joints and 2) investigating mechanical robustness/integrity of the joints. The metallurgical analysis involved isothermally aging joints and 70 or 100°C for 0, 25, 50, or 100 days. Metallographic cross-sections were cut for several joints per aging condition, and scanning electron microscopy (SEM) imaging was used to evaluate metallurgical reactions within the bulk solder joints and along the joint interfaces.
The mechanical integrity of rigid-flex connections was evaluated by comparing the mechanical performance of as-fabricated joints with that of thermally cycled joints. Two mechanical test methods were employed: 1) Shear; and 2) Peel. Rigid boards soldered to flex cable with this castellated via configuration were cycled from -55°C to 125°C with 10-minute dwells and 10°C/min ramps, between 300-1000 cycles prior to shear and peel testing. Shearing and peeling present different loading conditions to the joints, so different failure modes are observed. Assembly, storage, and service conditions are more likely to induce peel-type loading conditions due to the strain associated with the bent flex cables. Peak failure loads are the metric for comparison. Results to date indicate that the castellated via joint configuration retains high mechanical integrity after 1000 temperature cycles.
Conclusions
- An empirical reliability evaluation was performed on the castellated via interconnection for rigid-flex connections. Shear and peel tests were performed on as-received and thermally cycled units.
- Thermal cycling appears to have a negligible impact on shear performance of the joints.
- A bimodal failure distribution is observed during the peel tests: anchor joints fail at lower loads than the inner joints. Increased thermal cycling appears to decrease the failure loads only for the anchor joints.
- This castellated via joint configuration retains high mechanical integrity after 1000 temperature cycles.
- The current hand-soldering process that produced the castellated via solder joints for this study is not consistent. Neither cable alignment nor solder volume is consistent throughout the samples. These 2 variations alone may have more of an impact on mechanical performance than thermal cycling. More process control and/or developing a semi-automated reflow process is recommended.
- Exposed Au present on nearly every solder pad, both on the PCB side and on the cable side. While evidence of Au embrittlement is lacking, retained Au is not a best practice.
Initially Published in the SMTA Proceedings
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