Effect of TIM Compression Loads on BGA Reliability
This work aims to provide guidance for the potential trade-offs between thermal performances and second level interconnect reliability.
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Authored By:
Nicholas Graziano
Department of Mechanical Engineering, Binghamton University
NY, USA
Harry Schoeller, Ph.D.
Universal Instruments Corporation
NY, USA
Lars Bruno
Ericsson AB
Stockholm, Sweden
Summary
Compressive loads are often applied to Thermal Interface Materials (TIMs) to improve their thermal performance and this stress must be accommodated by the underlying solder joints. The effect of compressive loads on BGA reliability has been analyzed for SAC305 solder in thermal shock (-40/125 °C with 15 minutes dwells).
Assemblies were temperature cycled using two different simulated heat sinks with multiple load magnitudes. One delivered the load directly to the die while the other loaded the outer BGA rows through contact on the substrate. Die loaded samples were tested with and without a backplate. Additionally, two sets of samples were pre-aged for up to 1000 hours at 125 °C under compression prior to solder collapse measurements or thermal shock.
BGA thermomechanical reliability, under different loading configurations and magnitudes, was compared using Weibull failure rate distribution plots. Cross sectioning and dye and pry analyses were performed to identify the location and mode of failure. This work aims to provide guidance for the potential trade-offs between thermal performances and second level interconnect reliability.
Conclusions
TIM related compression loads have a significant effect on BGA reliability. They can lead to changes in both the time to failure as well as failure location.
The worst-case scenario was the 90% loading around the edge of the substrate. The 50% edge loading performed similarly to control case. Loading over the die had a positive effect on reliability. The 90% die loading was the best-case scenario. Using a backplate resulted in better performance by reducing flexure of the PCB. Pre-aging on the other hand had a negligible effect on reliability.
In addition to be the longest lasting sample, the 90% die loaded case was the only one that resulted in a different failure location. While the unloaded and all other cases failed in the die shadow, the 90% die loaded failed instead near the edge of the substrate. Regardless of the location, the failure mode was found to be solder fatigue near the component side pad.
Initially Published in the SMTA Proceedings
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