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 mum in thickness) of a chemical vapor deposited polymer, Parylene , in enhancing the solder lifetime of a ceramic package containing lar ge-DNP (distance to neutral point) test chips. Both coated and uncoate d (control) packages with chips joined via C4 Pb/Sn solder technology were thermally cycled between near room temperature and liquid nitroge n temperature (-196-degrees-C). At every 50 or 100 cycles, electrical resistances of solder joints were measured at room temperature for the nondestructive detection of solder failures based on a solder electri cal resistance criterion. The thermal cycling experiment and electrica l measurement were continued until solder failure was first noticed in coated packages. The number of cycles to first failure for coated pac kages was found to be twice the corresponding number of uncoated packa ges. To interpret this twofold solder life enhancement associated with Parylene, an elasto-plastic finite-element model was developed and us ed to determine the thermal strain and stress distributions near faile d solder joints for coated and uncoated packages at liquid nitrogen te mperature. Based on the results provided by this model and a low-tempe rature solder lifetime model, we were able to attribute the extended s older life to the modification of the strain and stress fields in the solder joints by the Parylene coating. The model also suggests that th e solder life can be prolonged significantly with a Parylene coating a s thin as 3 mum.