Coupling effect in Pt/Sn/Cu sandwich solder joint structures
ABSTRACT The interaction between Sn/Cu and Sn/Pt interfacial reactions in Pt/Sn/Cu sandwich joint structures was studied. We found the interfacial Sn/Pt reaction to be greatly influenced by the opposite Sn/Cu reaction. The PtSn4 interfacial compound formation rate was very sluggish compared with that of the single Sn/Pt reaction case. On the other hand, the growth rate of the Cu6Sn5 compound at the Sn/Cu interface was not affected by the opposite Sn/Pt reaction, which has a rate similar to that of the single Sn/Cu reaction case. However, the morphology of the Cu6Sn5 grains was different than in the single Sn/Cu reaction case (i.e. it had the conventional scallop-type shape). In the sandwich case, the Cu6Sn5 grains had a column-like appearance. The column-like morphology of the Cu6Sn5 grains is due to the small interfacial energy, γsolder/Cu6Sn5, caused by the dissolution of Pt from the molten solder. Also, we found that the Pt dissolution would also cause a reduction in the solubility of Cu in the molten solder. The above two parameter changes lead to a diminishing of the ripening flux among Cu6Sn5 grains. Hence, smaller Cu6Sn5 grains would not be depleted and the separation distance between Cu6Sn5 grains would not be widened.
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ABSTRACT: Solder is widely used to connect chips to their packaging substrates in flip chip technology as well as in surface mount technology. At present, the electronic packaging industry is actively searching for Pb-free solders due to environmental concern of Pb-based solders. Concerning the reliability of Pb-free solders, some electronic companies are reluctant to adopt them into their high-end products. Hence, a review of the reliability behavior of Pb-free solders is timely. We use the format of “case study” to review six reliability problems of Pb-free solders in electronic packaging technology. We conducted analysis of these cases on the basis of thermodynamic driving force, time-dependent kinetic processes, and morphology and microstructure changes. We made a direct comparison to the similar problem in SnPb solder whenever it is available. Specifically, we reviewed: (1) interfacial reactions between Pb-free solder and thick metalliztion of bond-pad on the substrate-side, (2) interfacial reactions between Pb-free solder and thin-film under-bump metallization on the chip-side, (3) the growth of a layered intermetallic compound (IMC) by ripening in solid state aging of solder joints, (4) a long range interaction between chip-side and substrate-side metallizations across a solder joint, (5) electromigration in flip chip solder joints, and finally (6) Sn whisker growth on Pb-free finish on Cu leadframe. Perhaps, these cases may serve as helpful references to the understanding of other reliability behaviors of Pb-free solders.Materials Science & Engineering R-reports - MAT SCI ENG R. 01/2002; 38(2):55-105.
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ABSTRACT: The growth and size distribution of scallop-type Cu6Sn5 intermetallic compound (IMC) at the interface between molten SnPb solder and Cu was investigated, along with a systematic study of morphology change of Cu6Sn5 morphology change as a function of SnPb solder composition. When SnPb solder composition was changed from eutectic (63Sn37Pb) to about 40Sn60Pb, Cu6Sn5 with round scallop-type morphology was found. In other compositions, the Cu6Sn5 scallops showed faceted scallop-type morphology. This morphological change is due to variation of interfacial energy between Cu6Sn5 and solder with a change of solder composition. The growth rate of Cu6Sn5 layer was proportional to cube root of time, and size distribution was in good agreement with the Flux-Driven-Ripening (FDR) theory. The pre-exponent factor k obtained by the measurement was 2.10×10-14 cm3/sec. Based on the k value, the calculated channel width with was about 2 nm, which was in good agreement with experimental observation by transmission electron microscopy.