SMTA International Conference Proceedings

Thermal Fatigue Reliability And Microstructural Characterization Of A Large, High Density Ball Grid Array With Backward Compatible Assembly

Authors: Richard Coyle, Raiyo Aspandiar, Vasu Vasudevan, Steve Tisdale, Iulia Muntele, Richard Popowich, Peter Read, and Debra Fleming
Company: Alcatel Lucent, Intel Corporation, and Sanmina-SCI Corporation
Date Published: 10/14/2012   Conference: SMTA International

Abstract: Accelerated temperature cycling (ATC) was used to assess the thermal fatigue reliability of a Pb free, 37.5 mm fully populated, 1295 I/O ball grid array (BGA) package assembled with backward compatible or mixed alloy (Pb free BGA/SnPb paste) processing. The surface mount assembly was done using custom SnPb eutectic soldering profiles designed to produce different levels of Pb mixing in the BGA solder balls. The test program included SAC405-SAC405 and SnPb-SnPb assemblies for reliability comparisons.

Failure analysis from the earlier study revealed the data were compromised by a second failure mode in plated through hole vias incorporated into the PCB test vehicle daisy chain. The presence of mixed mode failures (solder joint and via) precluded direct quantitative comparisons among all the test cells. In the current study, the PCB test vehicle was redesigned by increasing the drilled and plated via diameters and by using the absolute minimum number of vias needed for the daisy chains. These changes effectively eliminated via failures at critical solder joint interconnection sites. This paper presents new temperature cycling data that completes the objectives of the original study. The results indicate that mixed assemblies can provide acceptable reliability, even when Pb mixing is not complete or uniform, provided acceptable solder joint quality level is maintained.

Key Words: 

Pb-free solder backward compatibility, mixed alloy assembly, thermal fatigue, and accelerated temperature cycling

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