High Reliability Solder Resist with Strong Adhesion and High Resolution for High Density Packaging

This paper describes material analysis and material design of high reliability solder resist (SR) with excellent performance for high density packaging. There is growing demand for higher speed and higher data bandwidth signal transmission for many applications, such as 5G communication, artificial intelligence (AI), and advanced driver-assistance systems (ADAS). Such applications require high density and high performance IC packaging with fine Cu wiring and high frequency signal transmission. Solder resist materials for such packaging need to satisfy many special properties, such as high resistance to Cu electrochemical migration, strong adhesion to low profile Cu layers, and accurate photo-lithography resolution. However, development of a solder resist material that has all the excellent properties above is highly challenging, because many of these properties are trade-off. Indeed, adhesion of conventional SR to low profile Cu layer dropped more than 80% after high temperature and moisture HAST condition. Additionally, photolithography resolution below 50 μm was highly challenging due to light scattering. To overcome the trade-offs, this research began with the detail material analysis of the organic and inorganic components in SR materials. First analysis in polymer structures showed that resin with less shrinkage and less hydrolysis increased the initial adhesion, as well as adhesion after high temperature and high moisture condition. Next study on filler type and surface treatment revealed that the organic and inorganic surface treatment were effective to improve adhesion stability and resolution. This can be explained by the higher electrical affinity and less light scattering. By integrating the fundamental analyses, a new SR with excellent adhesion stability (85% of initial adhesion), high photolithography resolution below 40 μm, and excellent Cu migration resistance below 8 μm L/S.