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.
A new technique for decalcifying thin ground specimens of adult human enamel.
Abstract A new decalcification technique is described for the recovery of the organic matrix from thin ground specimens of adult human enamel. An aqueous solution of chromium sulphate which has both fixing and decalcifying properties is used. The results are compared with those obtained when an adjacent part of the same ground specimen was decalcified with the hydrochloric acid solution of Brain (1962). Dependent on slight differences in pH of the chromium solutions, two different appearances of the recovered matrix are obtained: above pH $ 3.2 a structural matrix which is positively birefringent at refractive index 1.55, whereas below this pH a matrix which is homogeneous and non-birefringent and almost similar to the matrix obtained after hydrochloric acid decalcification. The two types of matrices are discussed and the one favoured on morphological grounds, the chromium sulphate structural matrix, is related to present concepts of the structure of the native enamel matrix.