Effects of pad surface finish on interfacial reliabilities of Cu-pillar/Sn-Ag bumps of 2.5D TSV-enterposer on PCB applications

Abstract

As technology advances, electronic device requires package with higher I/O, faster speed, and higher density. 3D-packaging is one of the solutions for such these needs. The key technology which make 3D-packaging more powerful is the through silicon via (TSV) technology. It is the method of forming vertical electrical path through Si chip, and it is one of the most promising packaging structures for next generation. 2.5D TSV interposer on BGA substrates is one of the most commonly used structure using TSV technology, and advanced packaging companies such as Samsung and Qualcomm are currently working on 2.5D TSV interposer on BGA package structure. However, conventional flip chip process using solder balls is not suitable for 2.5D TSV-interposer to BGA substrate interconnection, since the aspect ratio of solder ball cannot withstand fine-pitch of TSV interconnection. In order to solve this problem, interconnection structure should be replaced to Cu pillar/Sn-Ag bumps on Cu pad of BGA substrates. However, two major problems occurs using Cu-pillar/Sn-Ag bump structure in 2.5D TSV interposer on BGA substrates: undefill voids problem due to fine pitch and narrow gap and bad interfacial reliability due to decreased solder volume. To solve underfill voids problem, non-conductive film(NCF) bonding process, a pre-applied type underfill film is used. This allows fluxing, bonding, and underfilling at single bonding process eliminating voids issue in the underfill. The second major problem, bad interfacial reliabilities of Cu-pillar/Sn-Ag on Cu pad joint, is often induced by Cu pad oxidation. And this problem can be solved using pad surface finishes such as OSP, ENIG, and ENEPIG. By use of the NCF bonding process and the surface finishes on Cu pad, highly reliable joints as well as voidless underfill can be obtained. In this research, effects of Cu pad surface finishes on interfacial reliabilities of Cu-pillar/Sn-Ag bump on Cu pad of BGA substrates using NCF bonding process are investigated using three types of surface finish (OSP, ENIG, ENEPIG). All the bonding in this experiments was performed using NCF bonding process with lamination temperature of 80°C and bonding temperature of 250 °C. As test vehicle, top chip with Cu-pillar/Sn-Ag bump and BGA board with three different types of surface finish (OSP, ENIG, ENEPIG) were used. At OSP surface finish, two commonly known Cu6Sn5 and Cu3Sn IMCs were formed at Sn-Ag bump/Cu pad interface. In the case of ENIG and ENEPIG surface finish, chunky type IMC phase of (Cu, Ni) <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</inf> Sn <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</inf> and needle type IMC phase of (Ni, Cu) <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Sn <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</inf> were formed respectively. Bonded test vehicles were tested under thermal cycle test up to 1000 cycles, and the tendency in contact resistance reliability was observed: OSP>ENEPIG>ENIG. Formation of different IMC phases depending on pad finishes are presumably the main reasons for the failures. Additionally, further analysis on the reliabilities of joints will be achieved by high temperature storage test (HTST) at 150°C for 1000 hours.