Effects of Parylene Coating on the Thermal Fatigue Life of Solder Joints in Ceramic Packages

Abstract

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 between near room temperature and liquid nitrogen temperature (−196° 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 electrical resistance criterion. The thermal cycling experiment and electrical measurement were continued 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 of uncoated packages. To interpret this twofold solder life enhancement associated with Parylene, an elasto-plastic finite-element model was developed and used to determine the thermal strain and stress distributions near failed solder joints for coated and uncoated packages at liquid nitrogen temperature. Based on the results provided by this model and a low-temperature solder lifetime model, we were able 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. © 1993 IEEE