Volume 7, Issue 6 (June 2010)
Ball Grid Array Lead-Free Solder Joint Strength under Monotonic Flexural Load
This work compared ball grid array (BGA) lead-free solder joint strengths to eutectic lead (Sn–Pb) solder joint strengths under monotonic bend load at room temperature. Flexural test methodologies for evaluating solder joint strength are presented. Various effects on solder joint strength were summarized systematically into three parts. The first part focused on the effect of solder joint geometries. BGAs with Sn-4Ag-0.5Cu and 63Sn-37Pb solders were tested, respectively. The effects of package side solder resist opening sizes, solder ball diameters, and board side metal defined/solder mask defined pads were investigated with 0.062 in printed circuit board (PCB). The results showed that the solder joint strength of Sn–Ag–Cu solder is lower than that of the traditional Sn–Pb solder under room temperature board flexural load and similar dynamic load. The second part investigated the effects of type of package (plastic BGA (PBGA) versus ceramic BGA (CBGA)), board thickness (0.093 in. versus 0.135 in.), and the effect of rework (reworked versus non-reworked) with Sn-3.9Ag-0.7Cu and 63Sn-37Pb solders. The joint strength of Sn–Ag–Cu solder is consistently lower than that of eutectic Sn–Pb solder for both board thicknesses, both CBGA and PBGA packages, and both non-reworked and reworked packages. The third part explored the feasibility of alternative low temperature solders as board-level interconnects. In addition to the traditional 63Sn-37Pb solder and the lead-free Sn-4Ag-0.5Cu solder, four other lead-free solders (Sn-52In, Sn-58Bi, Sn-57Bi-1Ag, and Sn-9Zn-0.006Al) were tested with 0.044 in PCB. Effects of board surface finishes with immersion silver (ImAg) or organic solderability preservatives, and pads with via-in-pad (VIP) or non-VIP pads were investigated. Test results showed that most of the BGAs with non-VIP pads performed better than those with VIP pads, except Sn–In solder with ImAg surface finish. The Sn–In solder showed the lowest performance, while Sn–Bi and Sn–Bi–Ag solder compositions showed better performance. Sn–Zn–Al solder joint strength performs better than others.