Helen L. Yeh

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. >

Fabrication and performance studies of multilayer polymer/metal interconnect structures for packaging applications

Multilayer copper/polyimide interconnect structures were fabricated using a reactive-ion-etching-based lift-off technique. Conductor cross-sectional area control, planarity, and a gap-free structure were made possible by the use of a novel siloxane-polyimide. The resultant structure consisted of two signal wiring layers between two ground planes with a nominal impedance of 40 Omega . Although redundant metallization processes were found to repair open lines, they resulted in an increase of the number of processing steps and could result in an increase of defects. Stud chain structures were found to survive cooling to 77 K with very little change in their characteristics, while heating of the copper interconnections to 350 degrees C in a reducing environment reduced their resistance by 3%.<<ETX>>

Factors affecting the interconnection resistance and yield in multilayer polyimide/copper structures

The use of a lift-off technique to fabricate a high-density structure consisting of multiple layers of metal/polyimide thin-film structures on a silicon substrate is described. To achieve better performance and high yield, the process design, the processing parameters, the thickness of the Cr/Cu/Cr metallurgy, and the use of suitable polyimide dielectrics, were evaluated. The plasma processing conditions, the types of passivation metals on Cu, and the use of a siloxane-polyimide as the gap-fill/etch-stop material were all shown to play a critical role in affecting the interconnection resistance and yield of the multilayer thin-film structures. By optimizing these parameters the feasibility of fabricating high-density thin-film wiring layers with good yield is demonstrated. >

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>>

Ambient dependence of metal diffusion through a less‐electronegative metal layer

The diffusion of metals through a less‐electronegative metal layer is studied for its dependence on ambient gases. When the diffusion is essentially one directional with little interdiffusion, the observed ambient dependence is in agreement with that described by the surface potential model proposed earlier. An enhanced diffusion is expected when the ambient increases the work function of the less‐electronegative metal through which the diffusion of the more‐electronegative one is studied. This has been observed for the diffusion of Au and Cu across the intermediate Ni layer in the Au/Ni/Cu structure under both oxygen and hydrogen, and for the outdiffusion of Cu through Ni in the Ni/Cu structure by oxygen.

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. >

Factors affecting the interconnection resistance and yield in the fabrication of multilayer polyimide/metal thin film structures

The use of a lift-off technique to fabricate a high-density structure consisting of multiple layers of metal/polyimide thin film structure on a silicon substrate is described. To achieve better performance and high yield, the authors evaluated the process design, the processing parameters, and the thickness of the Cr/Cu/Cr metallurgy, along with the use of suitable polyimide dielectrics. The plasma processing conditions, the types of passivation metals on Cu, and the use of a siloxane-polyimide as the gap-fill etch-stop material were all shown to play a very critical role in affecting the interconnection resistance and yield of the multilayer thin film structures. By optimizing these parameters the feasibility of fabricating high-density thin film wiring layers with good yield is demonstrated.<<ETX>>

Palladium enhanced dry soldering process

The authors report an enhancement technique for promoting a dry soldering process without the use of the wet chemical flux. A thin layer of Pd (about 1000 AA) on the surface of the areas to be soldered was found to be effective in promoting the solder wetting and joining. The effectiveness of the PD coating was studied at temperatures from 215 to 350 degrees C in hydrogen and forming gas ambient. Both a low-temperature solder (eutectic PbSn) and a high-temperature solder (Pb5Sn) were tested. The structure and the effectiveness of palladium coatings prepared by the immersion plating and the evaporation were compared. Applications would be in the SMT (surface mounting technology), pin brazing, pin soldering, and other soldering/bonding operations.<<ETX>>

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>>