Flip Chip and Heat Spreader Attachment Development

Abstract

Flip chip packages offer many advantages over traditional wire bonding based packages. Flip chip packages have high input/output (I/O) handling capability, better electrical performance and smaller size. Proper package design, assembly materials and process integration are needed to build a reliable flip chip package. To reduce package cost, solders are typically used for flip chip interconnection, thus solder joint reliability is critical for flip chip package product application. Part one of this dissertation was to study the impact of a small amount of Ni on lead free flip chip solder joint thermal shock reliability. Two groups of substrates were used in this study: solder (Sn/1%Ag/0.5%Cu) on pad (as reference) and Sn finished pad bumped with a Ni containing solder (Sn/1%Ag/0.5%Cu/0.05%Ni). Flip chip die bumped with SnAg eutectic solder was used for assembly on the two groups of substrates. The bumped die assembly on bumped (dome shape) substrate was successfully demonstrated. Assembled package thermal shock reliability test and failure analysis were performed. It was found that the small amount of Ni from the bumping solder paste was concentrated at the substrate site intermetallic (IMC) layer forming , but this did not impact the solder joint thermal shock reliability significantly. Part two of this dissertation was to study metallization and indium solder based heat spreader attach for flip chip in package applications. For metallization stack selection, it is commonly believed that during soldering and the following solder joint service life time, the solder materials should not consume the underling metallization, otherwise, dewetting or severe solder joint reliability degradation will typically occur, thus a minimum thickness of a barrier metal beneath top anti-oxidization layer (Au typically) is typically used. Ti/Ni/Au flip chip die backside metallization was evaluated in our team before. The Ni layer was used as a barrier metal and the resulting indium solder joint had good reliability. In this study, Ti/Au thin film metallization without the Ni barrier was studied. It was found that the Au thin film was converted to IMC completely during soldering and there was no IMC formation between the In and Ti, however, the indium solder attachment had significant shear and pull strength. The attachment strength was not degraded by multiple lead free reflow or thermal aging testing. Ti/Au (2000 Å) die based heat spreader attach (24mm x 24mm Cu on 22mm x 22mm Si) showed early delamination compared with Ti/Ni/Au die based assembly after thermal shock cycle testing. The Au thin film thickness effect was further evaluated. The next round assembly with Ti/Au (3000 Å) die did not show early delamination and had similar multiple reflow, thermal aging and thermal shock cycle reliability with Ti/Ni/Au die. The lower shear strength for Ti/Au (2000 Å) based assembly was correlated to its early failure, since during thermal shock cycle testing, the joint was in shear stress. Part three of this dissertation was to evaluate adhesive material based heat spreader attach for medium power application. A flat heat spreader was selected for cost reduction. A thermally conductive silicone was used as the thermal interface material. Another co-cure-able non-thermally conductive silicone was applied between the substrate and the heat spreader as a mechanical reinforcement. The manufacturing process was developed and the resulting structure was subjected to sequential assembly and environmental reliability tests. There was no interfacial delamination and no significant pull strength degradation after sequential stress testing.