Effects of non-conductive film (NCF) and substrate materials properties on the reliability of chip-on-board assembly

Abstract

As electronic system integration advances, electro packaging must meet demands such as high I/O capabilities, high speeds, and high densities. 3D packaging is one of the solutions to satisfy these requirements. Specifically through-silicon via (TSV) technology is a promising packaging structure for next-generation devices. It contains a vertical electrical path thorough Si chips. Because of development of TSV technology, new types of materials and methods are needed in chip-on-board (COB) technology. It has a limitation to secure joint stability and reliability of COB assembly when interconnects between devices connected by through-silicon via(TSV) and substrate using conventional reflow bonding method and solder bumps. The first restriction is voids between the chip and substrate which is not filler up with capillary underfill due to difficulty of filling narrow space at fine bump pitch. Although, solder has been used as a bump to form an interconnection during the last few decades, solder bumps are becoming problematic and facing fine solder bump pitch issues like solder bump bridging due to the scaling down of devices. In order to solve these limitations, a b-stage non-conductive film (NCF) technology emerges to solve this reliability issue using thermal compression bonding. In addition, the bump structure should change from solder to Cu-pillar/Sn-Ag micro-bumps. The purpose of this thesis is to investigate the material effects on COB assembly to ensure the joint reliability. In the introduction, electro packaging trend will be introduced and explained the difficulty of COB bonding on the fine bump pitch. Due to the difficulty of COB bonding, new types of materials and technologies should be applied for electro packaging to make fine bump pitch interconnection. The needs of NCF process and humidity effects on electro packaging will be explained. For the bump materials, various bump materials would be compared with conventional solder bump and Cu-pillar/Sn-Ag micro bump in terms of their merits and demerits. Furthermore, the reason of bad interfacial reliability would be explained with IMC ratio on the joint area depending on solder volume when applied Cu-pillar/Sn-Ag micro bumps on COB bonding. Lastly, the levels of electro packaging would be explained, among the packaging levels, functions of level 1, COB assembly was explained. In chapter 2, the reliability of micro solder bump joints on COB assembly was investigated in humidity environment when used b-stage NCF. The adhesion was important to ensure the reliability in humidity environment among the NCF properties. Thus, it is need to enhance the NCF adhesion to secure joint reliability. Silane coupling agent was applied to NCFs to enhance the adhesion. Depending on the amount of silane coupling agent in NCFs, NCF properties were investigated such as mechanical properties, moisture uptake ratio, and adhesion before and after exposed at 121$^oC$, 2atm, and 100% relative humidity (RH) in PCT chamber. By adding silane coupling agent to NCFs, NCF mechanical properties such as modulus and Tg and the adhesion were increased, and moisture gain was prevented. It was due to increase cross linked density. Moisture uptake at OH sites was investigated because it was severely affected at interface which was verified by FT-IR and surface energy. Lastly, the reliability of Cu-pillar micro solder joint was evaluated in humidity environment. In chapter 3, in order to investigate joint reliability on COB packaging, the surface finish of organic substrates was observed. A thermal compression bonding with pre-applied b-stage NCF was adopted to form solder joints between Cu pillar/Sn-Ag micro bump and PCB vehicles with various pad metal finishes. Four different surface finishes (Thin Ni ENEPIG, OSP, ENEPIG, ENIG) were evaluated by thermal cycle(T/C) test up to 2000 cycles at temperatures ranging from -55 to 125 oC and high-temperature storage test (HTST) up to 1000 hours at 150 oC. In chapter 4, the effects of solder resist on COB assembly was investigated. Lack of solder volume at joint area caused bad interfacial reliability at the Cu-pillar/Sn-Ag micro bump on fine bump pitch. Thus, solder volume should be compensated from the solder on pad (SOP) on the substrate. The solder bumps on the pad should be pressured for bonding process by coin process. The stress would cause the deformation of solder bumps and solder resist during coin process. The deformation amount of solder resist was depended on the solder resist mechanical properties. The mechanical properties of solder resist were evaluated. The deformation of solder resist was evaluated at different pressure and temperature of coin conditions. The effects of deformation on the stability of COB assembly were investigated.