The effect of globtop process and material condition on voiding

Chip-on-board (COB) packaging is a common IC packaging process in the Hong Kong-South China region. According to the packaging roadmap, the worldwide COB market trend is likely to expand to about 25 billion units per year by 2001 (Hwang, 1998). The popularity of COB is driven by its lower cost and its compatibility with PCB surface mounting processes. COB is now penetrating higher grade products such as telecommunications. There is increasing interest to make COB and globtop encapsulant materials more reliable and able to withstand stringent environmental conditions. This paper investigated possible sources of voiding in the dispensed glob-top material used in COBs. Factors such as substrate temperature, substrate preheat time, dispensing parameters, globtop thawing condition and curing profile were examined. The detection and quantification of voids was performed by C-mode and tomographic acoustic micro imaging (TAMI^TM) mode scanning acoustic microscopy. It was found that voiding behavior was material dependent. Increased pot-life increased void density. It was observed that with optimized substrate temperature and preheat time, the number of voids could be reduced significantly. Also, materials cured at higher than recommended temperatures are more susceptible to voids at the globtop-substrate interface