SMTA International Conference Proceedings


Author: Adam Zbrzezny et al.
Company: Celestica Inc.
Date Published: 9/25/2005   Conference: SMTA International

Abstract: The effects of different degrees of solder alloys mixing on the reliability of lead-free (LF) Sn-3.0Ag-0.5Cu (wt.%) (SAC 305) ball grid array (BGA) devices assembled with a Sn-Pb eutectic paste were investigated. The first case involved eight memory modules in a back-to-back configuration reflowed on standard graphic cards.

In the second case, one LF BGA ASIC component per board was assembled on the graphics cards of different design. Depending on the reflow conditions, the solder interconnects displayed varied degrees of Sn-Pb and LF solders intermixing. It was established that in order to receive a homogeneous solder alloy, the reflow peak temperature had to be in the 218°C-222°C range.

The reliability of solder interconnects of memory and ASIC modules was assessed by subjecting the cards to up to 1500 cycles of accelerated thermal-cycling with a profile from 0°C to 100°C. In the first case, the control Sn-Pb/Sn-Pb assemblies were more reliable than the mixed assemblies. Regardless of a degree of homogeneity of the BGA balls, the predominant failure mode of the mixed solder joints was interfacial cracking through a Pb-rich phase near the intermetallic layer.

In contrast, only partial cracks propagating diagonally through the bulk solder were present on the control boards. In the second case, despite only partial mixing, no fatal cracks were observed in the solder joints. It was concluded that a combination of state of stress and segregation of the Pb-rich phase at the interface was responsible for the shortened thermal-mechanical fatigue life of the mixed solder interconnects.

Key words: reliability, backward compatibility, solder joints, mixed solders, Sn-Pb, Sn-Ag-Cu

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