Package size and epoxy mass effects on package hermeticity requirements

Hermetic packages are required to protect sensitive electronic components from atmospheric constituents. The primary constituent of concern is water. Hermeticity requirements for large sealed assemblies are somewhat difficult to address because for normal leak rates, long bomb times are required in order to get enough helium into the package so that during leak rate measurement, a detectable concentration of helium comes out. Additionally, because many packaging houses seal in an atmosphere containing a significant partial pressure of helium, application of the current specification inadvertently requires larger packages to be more hermetic than smaller ones. Logically, small packages should have more stringent leak rate requirements than large packages, since for an equivalent leak rate the time to reach a given concentration of moisture in a larger package is longer. The trend in hermetically sealed assemblies has been to larger sizes. Examples include multi-chip modules (MCMs) and chip-on-board assemblies. This paper illustrates the inconsistencies which can occur when performing leak measurements on large packages. Also, the effect of gettering by moisture absorbing substances is evaluated both analytically and by examining residual gas analysis test results. It is found that moisture gettering by epoxies delays considerably the time for moisture build-up.