Challenges of super high thermal performance adhesive in power device application

Abstract

In power device package development, super high thermal performance adhesive either solder or polymeric adhesive are highly desired for better thermal resistance and RDSON. High thermal polymeric adhesive consists of high silver loading, unique thermoset resin, and solvent. Solvent evaporates during oven curing, silver flakes will be compacted to generate high thermal and electrical conductivity. Thermal and electrical properties increase directly with degree of compactness. This paper reveals 23.5W/K.m adhesive, applying average thin bondline, minimize creation of voids, good interfacial adhesion are essential to meet TO and SOIC package thermal resistance. Furthermore, proper selection of leadframe finishing is essential to prevent package delamination and also improves package thermal resistance by 12%. Working principle of high thermal adhesive increases the challenges of processability. Solvent raises the concern of voids, high silver loading increases the risk of dispensability and reliability test, lower basic resin content may reduce the adhesion strength. Solvent is added in to improve dispensability, however solvent evaporation in oven may causing channeling voids. Thus, optimizing curing profile by isothermal TGA is essential to ensure curability and elastic modulus properties are maintained. Isothermal TGA shows that higher hold temperature and longer hold time is needed to drive out all the solvent before the adhesive is fully cured. Staging time (open time) is another crucial control to minimize void formation. Two types of staging time, which are duration between epoxy dispensing to die attach & duration between die attach to curing control are established to ensure good dispensability, glue coverage, void formation and die shear strength. Higher filler loading in adhesive increases the probability of needle clogging, `missing dot' and inconsistency of dispensing. Thus resulting insufficient glue coverage, voids and high yield loss. Proper selection of nozzle size improves `missing dot' and dispensing consistency. The internal design of shower head is important factor to improve dispensability. Shower head dispensing and writing methodology are studies, result reveals that writing method on high filler loading adhesive give comparable result with shower head dispensing method. Despites the challenges of die bond process on high filler loading adhesives, high thermal adhesive has its advantage to achieve thicker bond line thickness due to high silver loading. This enhances reliability performance and process yield.