Authored By:
Burton Carpenter
Freescale Semiconductor, Inc.
Austin, TX, USA
Thomas Koschmieder
Cirrus Logic, Inc.
Austin, TX, USA
Summary
Described is experimental work to improve solder-joint reliability (SJR) of 0.8mm pitch BGA (ball grid array) packages used in automotive under-the-hood applications. Two packages were tested: a 292 IO 17mm MAPBGA and a 512 IO 25mm TEPBGA. Testing was TCoB (temperature cycle on board) in AATS (air-to-air thermal shock) between -40aC and +125aC. The target requirement was to pass 3000 cycles before first failure. For both packages, a three factor eight-cell full factorial DOE was run to study the impact of four variables. The 292MAPBGA experiment varied solder alloy (SAC387 vs. SnAg), die thickness and die size. Solder alloy (SAC387 vs. SnAg), die thickness and PCB (printed circuit board) pad diameter were studied in the 512TEPBGA experiment.
Analyzing each experiment separately, two-parameter Weibull curves were fit to each cell, followed by regression analyses three metrics of lifetime derived from those fits. Both experiments showed packages with SnAg spheres survived longer than those with SAC387 spheres: +921 cycles (17%) for 292MAPBGA and +547 cycles (15%) for 512TEPBGA. Die size was also significant for 292MAPBGA where the smaller die survived +634 cycles (12%) longer. This result was explained by dye-and-pry analysis, which revealed solder-joint cracking progressed most quickly in the die shadow. The other factors were not statistically significant.
The best configurations from each package surpassed the requirement of 3000 cycles in -40aC to +125aC AATS. The 292MAPBGA survived over 5400 cycles and the 512TEPBGA passed 3500 cycles.
Conclusions
In situ electrical monitoring and crack propagation analysis have been completed on two 0.8mm pitch BGA packages, a 17mm 292MAPBGA and a 25mm 512TEPBGA. The primary conclusions of this study are:
- Both packages were able to meet the required 3000 cycles to first failure in -40aC to 125aC AATS.
- SnAg solder spheres result in better solder-joint lifetime than SAC387, +921 cycles (17%) for the 292MAPBGA and +547 cycles (15%) for the 512TEPBGA.
- PCB pad diameter can be designed from 0.4mm to 0.5mm with similar results when using 0.5mm SRO and solder sphere on 0.8mm packages, provided failure occurs on the package side of the solder-joint.
Initially Published in the SMTA Proceedings
|
