10 - Residual Gas Analysis (RGA)

Abstract

Residual gas analysis (RGA) is performed on sealed electronic packages by means of a mass spectrometer. A mass spectrometer is an instrument that converts a gas sample into ions, separates the ions in accordance with their mass to charge ratio, and then collects the ions so as to report their relative quantity. All mass spectrometers have three common features: a source of electrons to impact the sample gas, thereby creating ions; a mechanism to sort the produced ions according to their mass to charge ratio; and a method of deflecting/collecting these ions. There are several types of mass spectrometers, but the type used for RGA is the quadrupole mass spectrometer. The test is performed by first preconditioning the sample a 100°C for 12–24 hr. The package is then mounted in the inlet port of the mass spectrometer. An O-ring seal is made between the package cover and the mass spectrometer. The mounted sample is then enclosed in an oven, which maintains the sample at 100°C. The inlet port of the spectrometer contains a mechanism that can pierce the cover of the package. The sample and spectrometer are pumped down to a vacuum level to insure no interference from the residual gas of the spectrometer. The cover is then punctured and the gas in the package escapes into the spectrometer, where it is analyzed. The original purpose of this test was to measure the moisture in packages. This is still the primary purpose, as Method 1018 has a maximum moisture requirement of 5,000 ppm. Leak rate calculations must be based on the partial pressure of the leaking gas. The RGA report is given in percentages. The partial pressure of the gases in the package only agrees with the percentages when there is one atmosphere in the package at room temperature.