Effects of parylene coating on the thermal fatigue life of solder joints in ceramic packages

A study was undertaken to determine the effectiveness of a thin layer (9.4 μm in thickness) of a chemical vapor deposited polymer, parylene, in enhancing the solder lifetime of a ceramic package containing large-DNP (distance to neutral point) test chips. Both coated and uncoated (control) packages with chips joined via C4 Pb/Sn solder technology were thermally cycled near room temperature and liquid nitrogen temperature (-196°C) until solder failure was first noticed in coated packages. The number of cycles to first failure for coated packages was found to be twice the corresponding number for uncoated packages. To interpret this twofold solder life enhancement, an elasto-plastic finite-element model was developed. Based on the results provided by this model and a low-temperature solder lifetime model, it was possible to attribute the extended solder life to the modification of the strain and stress fields in the solder joints by the parylene coating. The model also suggests that the solder life can be prolonged significantly with a parylene coating as thin as 3 μm