John W. Osenbach

Sn Corrosion and Its Influence on Whisker Growth

The microstructure and crystal structure of condensation-induced corrosion products, vapor phase induced oxidation products, Cu-Sn intermetallics, and Sn whiskers that formed on electroplated matte Sn on Cu-alloy after exposure 2500 h in a 60 degC/93%RH ambient were characterized with scanning electron microscopy, (SEM), focused ion beam (FIB) microscopy, energy dispersive spectroscopy (EDS), transmission electron microscopy (TEM), and selected area electron diffraction (SAD). The corrosion product was identified as crystalline SnO2. The oxidation of Sn in condensed water was at least four orders of magnitude larger than that in moist vapor at 60 degC. All Sn whiskers were found to be within 125 mum of the corrosion product. Based on these observations, a theory was developed. The theory assumes that oxidation leads to the displacement of Sn atoms within the film. Because the grain boundaries and free surfaces of the film are pinned, the oxidation-induced excess Sn atoms are constrained within the original volume of the Sn-film. The trapped excess Sn atoms create localized stress, excess strain energy, in the Sn-film. If and when the pinning constraint is relaxed, as for example would occur when the surface oxide on the film cracks, then the Sn atoms can diffuse to lower energy configurations. When this occurs, whisker nucleation and growth begins. The theory was tested by detailed measurements and comparison of the corrosion volume and the whisker volume in two different samples. The volume comparisons were consistent with the theory

Lead free packaging and Sn-whiskers

Replacement of Pb/Sn terminations on electronic devices with pure Sn has proven to be much more difficult than expected. The main problem is Sn whisker formation. Sn whiskers are single crystal, mechanically strong, metallic filaments that can nucleate and grow over time in such a way as to lead to device failure in the field. In this study, we present the results of Sn-whisker formation and growth studies for plated matte-Sn on copper lead frames employing stress tests as outlined in the proposed JEDEC Sn-whisker specification. The results indicate that the propensity for whisker formation decreases with increasing Sn-thickness, and with a post plate heat treatment. The propensity for whisker growth is found to dramatically change once the devices are subjected to a melting of the matte-Sn plating. A reflow preconditioning is found to be a major variable influencing the formation and growth of Sn-whiskers on bare Cu lead frames which even negates the potential benefits for thicker Sn-plates and a post-plate anneal. Only the application of a Ni-underplate was found to produce a truly robust matte-Sn whisker solution resistant to excessive whisker growth in both the as-made and post-reflowed states.

Infrared light induced degradation of polymeric materials used in optical devices

This paper summarizes the current understanding of laser-induced damage in polymeric material systems. A detailed literature review including macroscopic and microscopic models for high optical intensity induced degradation and damage of polymeric systems is given. This is followed with experimental results and discussion on degradation of two different commonly used polymeric systems (acrylic and epoxy) due to exposure to medium optical intensity infrared light. We show that in acrylic systems, degradation occurs slowly over time at infrared wavelengths as high as 1480 nm and optical power densities as low as 5/spl times/10/sup 4/ W/cm/sup 2/ via a multi-photon absorption process. We show that in bisphenol based epoxy systems (bisphenol based epoxies are widely used as structural adhesives in many optical devices), degradation also occurs via a multi-photon absorption process. The degradation process in both cases is very sensitive to both wavelength and optical power density. Finally, we give preliminary design guidelines for the use of bisphenol-based epoxies in high optical power density applications.

Degradation of Pb/Sn solder balls on electrolytic Ni/Au substrates as a result of post assembly heat treatments

The electrolytic Ni/Au surface finish is widely used in the electronic industry to provide both a highly solderable and wire bondable surface for BGA packages. We found that the robustness of the solder ball/substrate joint could be compromised by exposure to post attachment thermal aging such as dry-bake processing after assembly. The degree of degradation is dependent upon both the substrate Au thickness and the post assembly exposure time. We were able to identify using cross sectional SEM analysis coupled with EDX that the degradation of the solder joint was resultant from the formation of brittle Au-Sn IMC at the solder to Ni interface. This occurred even though the Au concentration in the solder joint was less than 0.7wt.%, significantly less than the 3-5wt% needed for brittle fracture in bulk materials. The brittle Au-Sn IMC was not present in the as assembled solder joint, however it began to form after only 2 hours of exposure to 125/spl deg/C. The brittle IMC effectively reduces the fracture toughness of the joint. Additional thermal exposure of the package lead to further degradation of the solder toughness.