Voiding/Drop Test for Mixed Alloy BGA Assembly

Yan Liu, Joanna Keck, Erin Page, and Ning-Cheng Lee
Indium Corporation
Clinton, NY

Low melting 57Bi42Sn1Ag (BiSnAg) was explored for replacing SAC solders as a low-cost solution. In this study, BGAs with SAC105, SAC305, and BiSnAg balls were assembled with SAC105, SAC305 or 57Bi42Sn1Ag solder paste. Joint mechanical strength, drop test performance, and voiding performance were evaluated against the reflow profile. SnPb was included as a control. The findings are as follows:

(1) The microstructure of solder joints showed that, among all of the combinations, only BiSnAg-105 LT and BiSnAg-305 LT exhibited well-distinguishable alloy regions. For SAC-BiSnAg systems, Sn-dendrites were noticeable at LT, while Ag3Sn needles developed at HT. The joints were homogeneous for the rest of the combinations.

(2) In the shear test, combinations involving BiSnAg solder were brittle, regardless of the Bi alloy location and reflow profile, as evidenced by stress-strain curves and morphology of the ruptured surface. The strong influence of Bi on the rupture site may have been caused by the stiffening effect of solder due to the homogenized presence of Bi in the joint. With the stiffened solder, the brittle IMC interface became the weakest link upon shearing, although the brittle BiSn crystalline structure also contributed to the rupture.

(3) In the drop test, all Bi-containing solder joints performed poorly compared with Bi-free systems, which was consistent with shear test results. Drop numbers increased with increasing elongation at break of solder bumps as measured in the shear test.

(4) Voiding was affected by flux chemistry and reduced by low alloy homogenization temperatures and solid top factors, but was increased by low surface tension factor, melting sequence factor, overheating factor and wide pasty range factor. Compared to SAC or SnPb systems, the BiSnAg system is low in voiding if reflowed at LT. In this study, voiding had an insignificant effect on shear strength and drop test performance.

Low melting 57Bi42Sn1Ag (BiSnAg) was explored for replacing SAC solders as a low-cost solution. In this study, BGAs with SAC105, SAC305, or BiSnAg balls were assembled with SAC105, SAC305 or 57Bi42Sn1Ag solder paste, and the joint mechanical strength, drop test performance, and voiding performance were evaluated against the reflow profile. SnPb was included as control. The findings are:

1. The microstructure of solder joints showed that, among all of the combinations, only BiSnAg-105 LT and BiSnAg-305 LT exhibited well-distinguishable alloy regions. For SAC-BiSnAg systems, Sn-dendrites were noticeable at LT, while Ag3Sn needles developed at HT. The joints were homogeneous for the rest of the combinations.

2. In the shear test, all combinations involving BiSnAg solder were brittle, regardless of the Bi alloy location and reflow profile, as evidenced by stress-strain curves and morphology of the ruptured surface. The strong influence of Bi on the rupture site may be caused by the stiffening effect of solder due to the homogenized presence of Bi in the joint. With the stiffened solder body, the brittle IMC interface became the weakest link upon shearing, although the brittle BiSn crystalline structure also contributed to the rupture.

3. In the drop test, consistent with shear test results, all Bi-containing solder joints performed poorly compared to Bi-free systems. Drop number increased with increasing elongation at break of solder bumps measured at shear test.

4. Voiding was affected by flux chemistry. It was reduced by a low alloy homogenization temperature and solid top factor, but was increased by a low surface tension factor, melting sequence factor, overheating factor, and a wide pasty range factor. Compared to SAC or SnPb systems, BiSnAg system is low in voiding if reflowed at LT. In this study, voiding had an insignificant effect on shear strength and drop test performance.