MOISTURE ABSORPTION BY ENCAPSULATED MICROCHIP PACKAGES

Abstract

This paper describes an experimental study that was designed to measure specific mass increases (due to the absorption of moisture) that occurred in selected pre-dried plastic encapsulated microchip packages (PEMs) exposed to moist air. Three different types of PEM packages were tested during the course of this work. Two of these PEM types were "dummy" ball grid arrays (BGAs) and plastic leaded chip carriers (PLCCs). The other PEMs tested were "real" metric plastic quad flatpack packages (MQFPs). Moisture absorption isotherms for all of the PEM types were measured at ambient temperatures and at relative humidity (RH) levels very near 50% and 100%. A theoretical method, based upon our experimental results, that could be used to predict moisture absorption behavior between 0 and 100% RH levels (at ambient temperatures) was also described.One result of this study was that nearly 50% of the moisture absorbed by selected pre-dried PEM specimens, during a 50 hr time period, occurred within the first 5 hr of the total exposure time. This fact indicates that even relatively short PEM exposure times, in very high RH environments, can be very serious with regard to the "popcorning" effect that may occur when moisture laden PEMs are reflow soldered onto printed wiring boards. The need to protect pre-dried, moisture sensitive, PEMs in dry air or in dry inert gas environments, prior to any high temperature assembly process, is clearly justified.