Microstructure Evolution of Sn-Cu Based Solder Paste with Different Cu Concentration Subjected to Multiple Reflows

The evolution in microstructure of Sn-Cu based solder paste with different copper (Cu) content subjected to multiple reflow cycles was investigated. In this study, the Sn-0.7Cu (SC) solder paste was used as based material. The Cu particles were added into SC solder paste to produce new Sn-4Cu and Sn-10Cu solder paste. After that, the solder paste was then reflowed on Cu-OSP surface finished and subjected to six times reflows. Characterization focuses on the bulk solder microstructure, morphology and intermetallic compound (IMC) thickness after multiple reflows. Results reveal that solder composition significantly affect the microstructure formation and growth of IMC.

Optimization of Lead Free Plating for Flip Chip Applications

Traditional flip chip processes have consolidated to a SnAgCu (SAC) solder system. Each company based upon their own needs and application space has come to their own method to achieve the desired final composition of the interconnect. These have included different solder compositions for both the pre-solder and the C4. The various interconnect solutions can range from no solder on one side such as pure Cu or Ni to an interconnect that has identical solder composition for both the substrate and the C4. Decisions for the optimized solution include the need for reliability, cost and yield. Picking the right solution also enables the elimination of defects such as solder voids, interfacial voids, white bumps, micro-solder bumps and non-wets. The optimized solutions are dependent upon many factors that include the fragility of the silicon dielectric, the size of the die, type of flux used at assembly, the assembly process used, method by which SnAg is plated such as various layering techniques, final processes steps in C4, test probe concepts, DSP methods and many more. In order to pick the appropriate scheme for each product and for each industry, it is imperative to know the interaction of all of these factors. This paper provides concepts and data about how to optimize assembly and lead free plating for a particular process. In the plating process, this includes the importance of various layering steps and analysis of incoming chemicals, especially the acids, and in the assembly process, the knowledge and matching of solder hierarchy, warpage, flux characteristics and preparation / cleaning steps prior to underfill. In particular, we will provide the data and the scheme by which it is possible to produce void free solder processes without bleed and feed on SnAg baths that are over 100 amp-hr per liter and over 1 year old.

Microstructure and Strength of Si3N4 Joints Bonded with Sialon-Oxynitride Glass

Si5.4Al0.6O0.6N7.4(β-Sialon)/ Y1.75Si2.625Al1.0O7.5N1.25(oxynitride glass) composite solders with different ratio were designed using SiO2-Al2O3-AlN-Y2O3-Si3N4mixture powders to join Si3N4ceramic to itself. It was found that both solder composition and bonding temperature have strong influence on the microstructure and strength of the joints. As far as using a pure oxynitride glass solder was concerned, the thickness of the seam turned narrow with the increase of temperature, and the strength had a peak value at 1550 °C. When increasing the designed content of β-Sialon, serious composition separation occurred in the seam at low bonding temperatures, and the strength was also lower than as using pure oxynitride glass. Increasing bonding temperature, the strength of the joints was improved and reached the maximum value of near 80 MPa at 1600 °C for the designed 20%β-Sialon. In this instance, large amounts of fine elongated β-Sialon grains were uniformly distributed in the seam. However, for the 60%β-Sialon containing solder, some pores appeared in the seam and its strength was very low at bonding temperatures.

Design and Process Development Concerns for the Assembly of Very Small Solder Joints

This work reports a systematic study that addresses the evolution of the solder and intermetallic bond layer microstructure and resulting properties in microbump based assemblies during reflow and subsequent aging. So far, pure Sn as well as alloys with two different initial concentrations of Ag were considered. Thicknesses down to 5μm were deposited and reflowed on Cu, Ni, and Ni/Au pads of different thicknesses and diameters ranging from 150μm to 11μm. Flip chips with these structures were finally assembled onto different substrate pads using a fine pitch bonder and aged at different elevated temperatures for various lengths of time. Solder and intermetallic layer microstructures were characterized by cross polarizer microscopy and scanning electron microscope (SEM). Different combinations led to size and composition dependent effects including different solder composition and microstructure as well as much faster growth of intermetallic layers, and different ratios of intermetallic layer thicknesses.

Crystal orientation of β-Sn grain in Ni(P)/Sn–0.5Cu/Cu and Ni(P)/Sn–1.8Ag/Cu joints

Electron backscatter diffraction analysis was used to compare the crystal orientation of β-Sn grains in Ni(P)/Sn–0.5Cu/Cu and Ni(P)/Sn–1.8Ag/Cu joints before and after aging. In Ni(P)/solder/Cu joints, the solder composition (Cu versus Ag) significantly affects β-Sn grain orientation. In Ni(P)/Sn–0.5Cu/Cu, there are two types of small columnar grains grown from Ni(P) and Cu under bump metallurgy with a high-angle grain boundary crossing the joint closer to the Ni side; in contrast, Ni(P)/Sn–1.8Ag/Cu has large grains with low-angle boundaries. During thermal aging at 150 °C for 250 h, the Ni(P)/Sn–0.5Cu/Cu joints undergo a more significant microstructural change than the Ni(P)/Sn–1.8Ag/Cu joint. Additionally, obvious ledges developed along the high-angle grain boundary between the upper and lower areas in the Sn–0.5Cu joint.