Search

We examined the interfacial morphology and shear deformation of flip chip solder joints on an organic substrate (chip-on-board). The large differences in the coefficients of thermal expansion between the board and the chip resulted in bending of the 1-cm2 chip with a curvature of 57 ± 12 cm. The corner bump pads on the chip registered a relative misalignment of 10 μm with respect to those on the board, resulting in shear deformation of the solder joints. The mechanical properties of these solder joints were tested on samples made by sandwiching two Si chips with electroless Ni(P) as the under-bump metallization and 25 solder interconnects. Joints were sheared to failure. Fracture was found to occur along the solder/Ni3Sn4 interface. In addition, cracking and peeling damages of the SiO2 dielectric layer were observed in the layer around the solder balls, indicating that damage to the dielectric layer may have occurred prior to the fracture of the solder joints due to a large normal stress. The failure behavior of the solder joints is characterized by an approximate stress analysis.

We report the soldering behaviors of high-temperature (300 °C) lead-free SnSb alloys on Cu foils and phased-in Cu–Cr thin films. By increasing the Sb content from 5 to 15 wt%, the solder surface became rougher and the wetting angle decreased from 50° to 20° on the Cu foils. Interfacial compounds were found to be Cu6Sn5 and Cu3Sn. The Cu6Sn5 showed a scallop-type morphology, whereas the Cu3Sn had a layer-type morphology. The growth of the latter was found to be diffusion limited. On phased-in Cu–Cr thin films, the solders showed much lower wetting angles than on the Cu foils, but the dewetting phenomenon was observed after 1 min of reflow time in the 85Sn15Sb alloy. In comparison, we found no dewetting of the high-temperature 95Pb5Sn solder.

Solder reaction-assisted crystallization of electroless Ni–P under bump metallization in the Si/SiO2/Al/Ni–P/63Sn–37Pb multilayer structure was analyzed using transmission electron microscopy, scanning electron microscopy, energy dispersive x-ray, and electron probe microanalyzer. The electroless Ni–P had an amorphous structure and a composition of Ni85P15 in the as-plated condition. Upon reflow, the electroless Ni–P transformed to Ni3Sn4 and Ni3P. The crystallization of electroless Ni–P to Ni3P was induced by the depletion of Ni from electroless Ni–P to form Ni3Sn4. The interface between electroless Ni–P and Ni3P layer was planar. From the Ni3P thickness-time relationship, the kinetics of crystallization was found to be diffusion controlled. Conservation of P occurs between electroless Ni–P and Ni3P, meaning that little or no P diffuses into the molten solder. Combining the growth rates of Ni3Sn4 and Ni3P, the consumption rate of electroless Ni–P was determined. Based upon microstructural and diffusion r...

We have studied the wetting behaviors of eutectic SnPb solder on Cu leadframes plated with Pd/Ni and Au/Pd/Ni thin films. For the Pd/Ni, we used two thicknesses of Pd of 760 and 2500 A to study the thicknesses effect. For the Au/Pd/Ni, we used either immersion Au (50 A) or acid Au (50 A) flashed on 1000 A Pd to study the effect and method of adding Au to the Pd surface. In addition, we investigated the wetting on a PdSn3 compound surface made by plating Sn or eutectic SnPb on Pd/Ni samples and then stripping the Sn or SnPb layer off. In all these samples, we found the wetting behavior to be time and temperature dependent. No stable wetting angles could be observed; the diameter of the solder caps increased with wetting time. Yet rather surprisingly, the diameter decreased with increasing wetting temperature from 200 °C to 240 °C. The largest cap diameter was observed at the lower temperature of 200 °C for Pd/Ni and stripped Pd/Ni, but at the intermediate temperature of 220 °C for Au/Pd/Ni. The smallest cap diameter was obtained at the higher temperature of 240 °C for all substrates. In examining the interfaces, we found the reaction products to be a ternary Pd–Ni–Sn compound and Ni3Sn4 for all substrates. The ternary compound grains were broken off from the interface, and scattered into the molten solder as soon as the Ni3Sn4 was formed. The latter forms a rather uniform layer consisting of small scallop-type grains on the unreacted Ni. We found that the growth rate of Ni3Sn4 is much slower, by a factor of about 10, than that of Cu6Sn5 in the reaction between Cu and eutectic SnPb.

Thin films of Au have been widely used as surface passivation layers and soldering layers to enhance the wetting of molten solders in the electronic packaging industry. A unique feature of Au is that it has a high dissolution rate in most Pband Sn-based solder alloys. We have studied the wetting behavior and interfacial reactions of two Pb-containing (63Sn-37Pb, 95Pb-5Sn) and four Pb-free solders (pure Sn, 96Sn4Ag, 57Bi-43Sn, 77.2Sn-20In-2.8Ag) on Au foils, using scanning electron microscopy and energy dispersive X-ray analysis. The 95Pb5Sn, having the highest dissolution rate of Au among them, formed sunken interfaces which show a double curvature (positive and negative) near the wetting tip. The formation of Au2Pb is everywhere within the solder cap. Pure Sn/Au showed less compound formation compared to 96Sn-4Ag/Au, but both formed a smaller sunken interface compared to 95Pb-5Sn/Au. The SnInAg showed an inhomogeneous interfacial reaction initially with two different types of interface (flat and curved or sunken interfaces). However, the flat interfaces disappeared after a slightly longer reflow. The only solder which formed a relatively flat interface with Au is the eutectic SnBi, which also showed a very slow rate of intermetallic compound formation. Owing to the sunken interface, we measured the wetting angles from the cross-sections.

We have studied the wetting behaviors of eutectic SnPb solder caps on pure Au foils at 200 °C. Surface morphology, wetting angle, and wetting tip stability were examined by scanning electron microscopy and energy dispersion x‐ray analysis. In addition, interfacial reaction in the bulk diffusion couple of the solder and Au was studied. Intermetallic compound (AuSn4) was observed on the surface of the solder cap, often called the cold joint, as early as 2 s of reflow at 200 °C. The wetting angle decreased with reflow time but remained constant after 180 s of reflow. However, the side‐band width and the diameter of the solder cap continuously increase with the reflow time to 5 min when the entire eutectic SnPb solder cap is fully consumed by intermetallic compound formation. In the bulk diffusion couple of eutectic SnPb and Au, the growth and morphology of the intermetallic compound (AuSn4) was influenced by Au dissolution. Since Au, showing excellent wetting behavior, forms the platelet‐type intermetallic c...

In the soldering reaction at 200 °C between eutectic SnPb and plated Pd/Ni on Cu leadframes, two thicknesses of Pd of 760 and 2500 A were used. Even very thin Pd can lead to continuous spreading of the solder on the leadframe; no stable wetting angle can be observed. Anisotropic spreading and spike formation were found due to the mechanical effect of rolling of the leadframe, and they are more pronounced in the thinner Pd samples. In the interfacial reaction, we have observed the formation of a ternary Pd–Ni–Sn compound and Ni3Sn4. While spalling of the ternary compound occurs, no spalling of the Ni3Sn4 was found. The latter forms a rather uniform layer consisting of small scallop-type grains. We found that the growth rate of Ni3Sn4 is much slower, by a factor of about 10, than that of Cu6Sn5 in the reaction between Cu and eutectic SnPb.

We have studied the wetting behaviors and interfacial reactions of Pb‐containing (63Sn‐37Pb, 95Pb‐5Sn) and Pb‐free solders (pure Sn, 96Sn‐4Ag, 57Bi‐43Sn, 77.2Sn‐20In‐2.8Ag) on Au foils in order to understand the role of Pb in Pb‐based solders. Surface morphology, wetting angle, and interfacial reaction of the solders were studied by Scanning Electron Microscopy (SEM) and Energy Dispersive X‐ray Analysis (EDX). Pb‐containing and Pb‐free solders (pure Sn, 96Sn‐4Ag, 77.2Sn‐20In‐2.8Ag) showed rough surfaces due to the precipitation of intermetallic compounds on the surface of the solder caps. The eutectic SnBi (57Bi‐43Sn) solder, however, showed a smoother surface. The wetting angle of the eutectic SnPb (63Sn‐37Pb), pure Sn, 96Sn‐4Ag, and 77.2Sn‐20In‐2.8Ag solders decreased significantly with reflow time, while the eutectic SnBi and 95Pb‐5Sn solders showed a much smaller decrease. A large amount of intermetallic compounds was formed throughout the entire region of the solder cap for the eutectic SnPb, 96Sn‐4Ag, 95Pb‐5Sn, and 77.2Sn‐20In‐2.8Ag, mainly due to the high solubility of Au in these solders. Slow intermetallic compound growth was observed for the eutectic SnBi solder.