Evaluation of Ag wire reliability on fine pitch wire bonding

Abstract

The semiconductor assembly industry has migrated from Au wire to a low-cost alternative, Cu wire, and has recently started increasing the use of Ag alloy wire (Liao et al., 2012). Au wire had long been the standard for manufacturability and reliability but increasing raw material costs has shifted the industry to use Pd-coated Cu (PCC) wire as a new standard. Its use is often seen in the industry's latest fine-pitch wirebond designs. However, the use of Cu wire has some limitations because of its material hardness, especially on die designs requiring a thin Al bond pad thickness (<; 0.7um). In these designs, pad cratering becomes a real concern, and thus Ag wire has found a niche as an alternative low-cost solution to Cu wire. Ag wire is also seen to have similar manufacturability to Au wire. This study focuses on the question of whether 95% Ag alloy wire can be used as an alternative wire interconnection material across a larger portfolio of leading-edge, fine-pitch wirebond designs currently using 18um (0.7mil) Cu wire diameter, with ~46um pad pitch and ~40um bond pad opening (BPO). Two 95% Ag alloy wire compositions were evaluated for reliability performance on both lead frame and laminate-based packages. A dual-row quad-flat no-lead (DRQFN) and BGA package were selected as test vehicles. They both use test die with the same bond pad opening (BPO) size. The results showed that Ag wire could pass thermal cycling (TC) and high temperature storage life (HTSL) tests in both a DRQFN and BGA. However, Ag wire showed a higher failure rate in the BGA than in the DRQFN after highly accelerated stress test (bHAST). A higher Chlorine (Cl) content measured on the laminate substrate was found to correlate with a higher failure rate after bHAST testing on the BGA. Free air ball (FAB) quality was found to be a significant variable on Ag wire bond ability and reliability performance. Poor FAB quality, containing voids and nodes at ball formation, could cause flaws (micro voids) at the Ag-Al bond interface. These flaws at the interface were found to contain high concentrations of Cl, which can result in Al oxide formation and IMC crack post bHAST testing. When comparing the performance of 95% Ag alloy wire from different vendors, it was found that Ag wire with higher Pd content correlated to a reduction of FAB defects and reduced HAST failures. However, for both alloys, no optimized parameters were found that completely eliminated defects below a formed FAB size of 31um. From these results, a clear limitation was found for the use of the 95% Ag alloy wires evaluated, and a minimum bond pad opening for robust Ag wire bond reliability is presented.