Encapsulants used in flip-chip packages

Low thermal expansion encapsulants have been used in a wide range of IBM flip-chip products to enhance the fatigue life of solder interconnections and to prevent environmental attack. In order to determine whether an encapsulant can satisfy all requirements of a given product or process, the physical behaviors of the encapsulant must be carefully characterized. Typical material properties of interest are: viscosity and flow coverage, thermal expansion, modulus, glass transition temperature, filler size, alpha activity, adhesion, ionics content, fracture toughness, and electrical properties. The sensitivity of these properties to process variables such as encapsulant dispensing, gelling, curing and post curing exposures to environment and reliability experiments need to be explored. An overall evaluation of all these properties is essential for determining the ultimate performance of the encapsulant on specific products. This paper presents a systematic approach to quantify these encapsulant properties. Various test methods and their relevant technical aspects are discussed. Results achieved on a few selected materials are also presented as examples