Ho-Ming Tong

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 mu m 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 degrees 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 mu m. >

Parylene encapsulation of ceramic packages for liquid nitrogen application

A study was undertaken to determine the effectiveness of a thin layer (9.4 mu m 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.<<ETX>>

Ceramic packages for liquid-nitrogen operation

To evaluate their compatibility for use in a liquid-nitrogen computer, metallized ceramic packages with test chips using controlled-collapse solder (Pb-Sn) technology were cycled between 30 degrees C and liquid-nitrogen temperature. Room-temperature electrical resistance measurements were made at regular intervals of cycles to determine whether solder failure accompanied by a significant resistance increase had occurred. For the failed solder joints characterized by the highest thermal shear strain amplitude of 3.3%, it was possible to estimate the number of liquid-nitrogen cycles needed to produce the corresponding failure rate using a room-temperature solder lifetime model. Cross-sectional examination of the failed solder joints using scanning electron microscopy (SEM) and energy-dispersive X-ray analysis indicated solder cracking occurring at the solder-ceramic interface. Chip-pull tests on cycled packages yielded strengths far exceeding the minimal requirement. Mechanisms involving the formation of intermetallics are proposed to account for the observed solder fracture modes after liquid-nitrogen cycling and after chip pull. SEM examination of pulled chips in cycled packages found no apparent sign of cracking in quartz and polyimide for chip insulation. >

Bending of Flex Leads During Thermal Cycling

A three dimensional finite-element stress model was used to simulate the lead bending incurred during thermal cycling of a Flex connector joined to a silicon carrier via an eutectic solder. Three different coating conditions (no coating, with a polyimide coating, and with a silicone coating) were simulated. With the polyimide coating, lead bending was found to occur at the corner inner leads of the Flex as a result of the plastic strains accumulated there during thermal cycling. In the case of silicone, the corresponding plastic strains grew during the first thermal cycle but saturated subsequently, confirming the negligible lead bending observed experimentally. For the case of no coating, the highest plastic strain was found to be borne in part by the polyimide film at its edge and thus no significant bending was observed. In all cases, the results provided by the model agree well with the experimental observations.

Integrity of ceramic packages upon liquid nitrogen cycling

The integrity of two types of test packages (A and B) during cycling between 30 degrees C and liquid nitrogen temperature (-196 degrees C) is discussed. Type-A packages had polyimide-insulated chips and were characterized by higher thermal shear strain amplitudes (<or=3.3%) than type-B packages, which contained quartz-insulated chips. For either type of packages, the chips were joined to the metallized ceramic substrate using the C4 solder (Pb-Sn) technology. Solder lifetime modeling and the solder fracture/cracking mechanism are discussed. No sign of fracture or delamination of the chip insulation was observed for either package type. The important features and conclusions of the study are summarized. The results indicate the potential of the packages for liquid nitrogen operation.<<ETX>>