Assuring reliable PEMs by statistical monitoring of thermal and mechanical properties of molded plastic parts sampled from production

Plastic encapsulated microcircuits (PEMs) are assembled on high-volume production lines. Reliability expectations for such products are increasingly demanding. Thermal and mechanical properties of plastic encapsulants used to manufacture PEMs, and alterations to these by molding processes, are among the factors which influence assembled product reliability. Important properties include the filler content (typically 65-85% by weight) of the material, glass transition temperature (T_g), and coefficients of thermal expansion (CTEs; alpha 1, measured below T_g, and alpha 2, measured above T_g). These properties and the changes that occur in them during processing, can alter mechanical stresses and other factors influencing performance of assembled parts. While filler content is a raw material lot-oriented variable, T_g and CTEs of the finished molded units are influenced both by the manufacturer´s formulation of the raw material lot and by assembly process variation. This paper describes the variability of T_g, CTE, and filler content observed for more than one thousand assembled lots of PEMs. Product molded in six different plastic mold compounds was (and still is) sampled on a weekly basis over several years. Statistical control charts quantifying the variability observed in the material properties are presented. Trending and out-of-control occurrences are discussed. This database has proved useful not only in guiding plastic assembly operations in process control and improvement, but also in communicating the extent of control of the PEM assembly process to customers