Reliability of Lead-Free Solder Connections for Area-Array Packages

Pb free solder for electronics assemblies is fast becoming a reality primarily because of market driven forces. This impacts the entire electronic manufacturing industry, from component supplier to equipment manufacturer. While the industry has identified possible alternates to Sn/Pb solder alloy, much work still needs to be done, especially in the area of component & board level reliability and assembly process development. One of the key factors in selecting the replacement for Sn/Pb solder is the board level reliability with an alternate alloy system used for a wide range of electronic components in the industry. While the initial selection is based on reliability evaluations using leadframe components, data is still lacking for BGA and CSP type components to finalize the selection. This paper seeks to fill this gap by providing the board level reliability comparison of Pb free alloys for area array (BGA & CSPs) type of components. Nine different Pb free alloy systems including three compositions of Sn/Ag/Cu alloy systems are considered, and their reliability is compared with Sn/Pb eutectic alloy for two BGA type packages. The packages were assembled with Pb free balls and were mounted on printed circuit boards in order to evaluate the board reliability during temperature cycling (using three cycling conditions). The data presented here suggests that the Pb free alloys being considered are as good or better in reliability compared to the Sn/Pb eutectic alloy currently in use. The data also indicates that the acceleration factor for the alloy systems from different cyclic conditions are significantly different compared to Sn/Pb eutectic solder. Introduction Finding a viable alternate to Sn/Pb solder alloy requires R&D effort involving materials and processes development and reliability evaluation. A number of industry groups and consortia [IDEALS, NEMI, JEIDA, IPC, NCMS] had and are working in these areas to identify the possible solder alloy replacement. During the last year or so, the industry has converged towards a Sn/Ag/Cu based alloy, although a consensus has not yet been reached on the exact composition. A variety of Sn/Ag/Cu alloy compositions have been recommended by the industry groups and consortia , with only a slight difference in actual composition. Although board level reliability is a major part of these evaluations, little data has been published comparing the reliability of this alloy with the Sn/Pb alloy, especially for area array packages such as BGAs and CSPs. Further, the effect of different Sn/Ag/Cu compositions and temperature cycle conditions on reliability is not clear at this point. Almost all of the on going activities on finding the viable and reliable replacement for Sn/Pb solder use some kind of accelerated test condition. For it to be considered as a potential candidate, it is generally assumed that the new alloy must perform as good or better than the Sn/Pb alloy in a reliability test. Certainly this would be a correct approach if all the solder alloys behaved the same during temperature cycling and the acceleration factor from the field level conditions were the same for all alloys. This, however, is highly unlikely as each alloy has its own high temperature deformation mechanisms and behavior, and therefore the acceleration factors from field to accelerated level can be significantly different. Since field condition experiments cannot be run in a reasonable amount of time, it is imperative that different accelerated conditions be used to determine the acceleration factors from one to another accelerated condition. This will help not only in understanding the expected behavior at field level conditions, but also in determining if a certain alloy can be used for the wide range of applications. With the above in mind, the work reported here had the following objectives: 1) Collect and compare board level reliability data for BGA type packages, 2) Investigate the effect of Sn/Ag/Cu composition on reliability, 3) Investigate the acceleration factors for different temperature cycle conditions, and 4) Select the Pb free Solder ball alloys for BGA type packages. The above objectives were met by partnering with industry groups such as NCMS and by conducting internal evaluations. Two different types of BGA type packages were selected for this evaluation in the initial phases. The packages were assembled with different solder ball alloys and were mounted on test boards for Presented at IPC SMEMA Council APEX 2001 For additional information, please email marketing@amkor.com