Silicone resins and gels have been used extensively for IC protection. Reduction of interfacial moisture and ionic contamination is considered crucial to device reliability.
The present research has revealed an interesting phenomenon. Prolonged lifetime was obtained for comb pattern samples using a non-conventional encapsulation method. In order to increase specimen sensitivity to interfacial moisture, samples were contaminated prior to encapsulation. Three groups of contaminated samples encapsulated with polysiloxane were produced. Group I was encapsulated with a polymer/CaCl2/polymer/substrate structure. The structure of group II consisted of only the polymer on the contaminated substrates. Group III was encapsulated with a CaCl2 embedded polymer/polymer /substrate structure. Following exposure at 85°C, 100−10 amperes after 2,000 hrs, with no visible corrosion. Group II and III samples showed visible corrosipn after 390 to 470 hrs with leakage current of the order of 10−5 amperes. Direct measurement of RH at the polymer/substrate interface and water sorption experiments were done to explain the mechanisms of these packaging structures.
The experimental results suggest that a desiccating multilayer coating can maintain low interfacial water concentrations. During exposure to high external RH, water from the surroundings diffuses through the top layer, producing an interlayer consisting of a solid CaCl2 and a CaCl2-saturated solution. Until all of the salt dissolves, the water chemical potential of the saturated solution defines the water content at the polymer/substrate interface. Hence the RH at the polymer/substrate interface was maintained below that required to dissolve the residual surface contamination for a longer time than by conventional encapsulation. This technique could be useful to reduce the dependence of IC reliability upon pre-encapsulation cleaning.