Coupled Thermal-Moisture-Stress Analysis for Electronic Packages

Moisture sensitivity of packages is an area of great concern for the electronics industry. The differential swelling of materials in a non-hermetic package during manufacture, handling, storage, assembly, and then also during its lifetime can cause stresses large enough to damage the package. Of particular concern is the reflow process, during which the stresses due to moisture can be of the same order, or larger, than the thermal stresses. Both the thermal- and moisture-stresses need to be considered to give the total stress in a package, and so to allow realistic stress analysis for prediction of damage and failure. Physical testing is typically used to characterize the moisture sensitivity of packages and these tests can take a significant amount of time - up to a week for sample preparation, after prototype parts have been made. Finite element analyses of electronic packages can be completed in significantly shorter time periods. For finite element analysis, moisture analysis is often overlooked or handled with simplifying assumptions; one common assumption used is to make an analogy between wetness and temperature, such that a pure heat transfer analysis can be used. This paper will briefly discuss these simplified approaches, and then go on to describe a coupled thermal-moisture-stress solution method.