Gases evolved during cure and solder reflow of encapsulants and underfills

Encapsulants and underfills are polymeric materials containing some levels of low molecular mass chemicals which turn into volatiles during heat exposure. It is of interest to know what and when the volatiles undergo thermal desorption and are released during high temperature exposure such as solder reflow or environmental testing. Moisture turning to steam is not the only vapor present during solder reflow. With the push towards lead-free and higher reflow temperatures, this outgassing behavior is of particular significance to assist in the improvement of manufacturing yields. Using evolved gas analysis with gas chromatography and mass spectrometry, vapors found from epoxy mold compounds are carbon dioxide, moisture, methyl alcohol, benzene, isobutyl methyl ketone, triphenyl phosphine and occasionally, siloxanes from stress releasing agents. Gases reflecting curing byproducts or unreacted products are observed. For underfills, these gases are either absent or much reduced. Both underfills and encapsulants show some signs of polymer chain decomposition at higher temperatures for lead-free applications. Gas evolution rate increased directly with temperature for both encapsulants and underfills.