Parylene encapsulation of ceramic packages for liquid nitrogen application

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 IBM ceramic packages containing large-DNP (distance to neutral point) test chips during liquid-nitrogen operation. Coated and uncoated (control) packages with chips joined using C4 (controlled collapse chip connection) Pb/Sn solder technology were thermally cycled between near room temperature and liquid-nitrogen temperature. At every 50 or 100 cycles, the 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 was twice the corresponding number for uncoated packages. To help interpret this two-fold solder-life enhancement associated with parylene, an elastoplastic finite-element model was developed and used to determine the thermal strain and stress distributions near failed solder joints for coated and uncoated packages during thermal cycling. Based on the results provided by this model and a low-temperature solder lifetime model, the extended solder life was attributed to the ability of Parylene to modify the strain and stress fields in the solder joint as well as to its barrier and conformal-coating properties