The effect of lead free soldering on formation of black pad failure
By this time usable solder alloys for lead-free soldering had already been developed. NEMI (National Electronics Manufacturing Initiative) recommends Sn/3.9Ag/0.6Cu (±0.2%) solder alloy for reflow soldering instead of leaded solder alloys, due to its excellent mechanical properties and other good parameters. The next step is the choosing of suitable pad finish for lead-free soldering. In the age of fine pitch assemblies (QFPs (Quad Flat Pack), BGAs (Ball Grid Array) and CSP (Chip Size Package)) flat and planar plated surfaces are needed. Electroless Nickel Immersion Gold (ENIG) pad finishes are used, when there is a need for aluminum wire bonding besides soldering. At the interface of NiAu plating and the solder a failure can occur, called 'black pad'. Black pad is not only a cosmetic problem, but also a source of weak wire bonding and soldering joints. The question is, whether the lead free soldering affects the formation of black pad failure or not. Experiments were made to answer this question, and this paper will show the results.
An investigation of reliability and solder joint microstructure evolution of select Pb-free FCBGA pad finish and solder ball alloy combinations
As Pb-free packaging technologies evolve, increased emphasis has been placed on the development of robust mechanical and metallurgical solutions. A number of different component and PCB surface finish options are now widely available, as are several different BGA solder ball alloys. Similarly, various different Pb-free C4 bumping alloys are being introduced, compounding the need for clear understanding of how the resulting interconnect systems stand up to the normal rigors of end-user applications. This paper focuses on the component-level reliability and metallurgical evolution of the C4 (flip chip) and BGA interconnect systems formed by the combination of commonly specified surface finishes and BGA solder alloys. The two surface finishes selected were electroless Ni/electroless Pd/immersion Au (ENEPIG), and immersion Sn (IT). The selected BGA alloys were Sn-3Ag-0.5Cu (SAC305) and Sn-3.5Ag. All four of these alloy/pad finish combinations were found to pass the various industry-standard (JEDEC) reliability stress tests without any failures, as measured through open/short testing and CSAM analyses of the packages. However, significant differences in microstructural evolution of the solder/substrate interfaces were observed.
Thermal incompatibility analysis of metal-ceramic systems based on flexural displacement data.
The feasibility of simple tests or analytical methods for prediction of residual stress states in metal-ceramic (MC) prostheses has not been demonstrated. Biomaterial metal-ceramic strips have been proposed to provide sensitive measures of transient and residual stress states through the measurement of midpoint deflection after cooling from the ceramic sintering temperature. The objective of this study was to apply the elastic-viscoelastic analogy to calculate transient and residual midpoint deflections in MC biomaterial strips and to compare these values with deflections measured with a beam-bending viscometer (BBV). Calculations and measurements were made for five MC systems that were found from a clinical study to be "thermally compatible" systems. Metal strips, 64 mm in length, 3 mm wide, and either 0.5 mm, 1.0 mm, or 2.0 mm in thickness, were veneered with four 0.25-mm thick layers of opaque porcelain. Midpoint deflection of the MC strips (ceramic oriented in the posterior position) was measured during cooling from an initial temperature of 700 degrees C. In general, the directions of the measured residual deflections did not agree with the "textbook" convention that negative deflections are associated with positive thermal contraction mismatch (alpha(M) - alpha(c) > 0) regardless of metal thickness. For a metal thickness of 0.5 mm, the residual midpoint deflection for all thermal contraction mismatch cases, except one, was positive (upward deflection) whereas the residual midpoint deflections were all negative when the metal thickness was increased to 1 or 2 mm, independent of the thermal contraction mismatch. The best agreement between calculated and measured values of residual midpoint deflection (+16 microns vs. +14 +/- 2.3 microns, respectively was obtained for MC biomaterial strips with a Ni-Cr alloy (0.5 mm thick) while the largest difference (+346 microns vs. +61 +/- 43.8 microns) was obtained for MC bimaterial strips with a Au-Pd allow (0.5 mm thick). In all but one case, changes in deflection direction as a function of metal thickness were correctly predicted by the viscoelastic analysis. The results of this study indicate that a viscoelastic model is useful for estimating thermal compatibility conditions of MC systems.