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This paper discusses the microstructures of solder joints and the mechanism of thermal fatigue, which is an important source of failure in electronic devices. The solder joints studied were near‐eutectic Pb‐Sn solder contacts on copper. The microstructure of the joints is described. While the fatigue life of near‐eutectic solder joints is strongly dependent on the operating conditions and on the microstructure of the joint, the metallurgical mechanisms of failure are surprisingly constant. When the cyclic load is in shear at temperatures above room temperature the shear strain is inhomogeneous, and induces a rapid coarsening of the eutectic microstructure that concentrates the deformation in well‐defined bands parallel to the joint interface. Fatigue cracks propagate along the Sn‐Sn grain boundaries and join across the Pb‐rich regions to cause ultimate failure. The failure occurs through the bulk solder unless the joint is so thin that the intermetallic layer at the interface is a significant fraction of the joint thickness, in which case failure may be accelerated by cracking through the intermetallic layer. The coarsening and subsequent failure are influenced more strongly by the number of thermal cycles than by the time of exposure to high temperature, at least for hold times up to one hour. Thermal fatigue in tension does not cause well‐defined coarsened bands, but often leads to rapid failure through cracking of the brittle intermetallic layer. Implications are drawn for the design of accelerated fatigue tests and the development of new solders with exceptional fatigue resistance.

Solder joints in electronic packages experience cyclical thermally induced strain when temperature fluctuations are encountered in service. This study investigates three parameters that affect the microstructure and therefore the thermal fatigue behavior of 60Sn-40Pb solder joints. These parameters are: 1) the effect of a tensile component in thermal fatigue, 2) solder joint thickness variations, and 3) hold time variations at the elevated temperature portion of the thermal cycle. Solder joints were thermally fatigued in a tension/compression deformation mode. Cracks developed both in the interfacial intermetallic layer (early in thermal fatigue) and in the coarsened regions of the microstructure of the solder joint (after many more cycles). The effect of joint thickness on solder joints thermally fatigued in shear was also explored. Solder joint thickness was found not to significantly affect fatigue lifetimes. The effect of an increase in the hold time at the elevated temperature portion of the thermal fatigue cycle was also investigated. It was found that time spent at the high temperature end of the fatigue cycle does not determine solder joint lifetime, rather it is the combination of the amount of deformation induced during thermal fatigue in concert with the elevated temperature.

Thin films of Al-l%Si were sputter deposited on Si under a variety of pressures of atmospheric impurity gases. The effect of the impurity gases (oxygen, nitrogen, and water), and deposition temperature (15° and 300° C), on the microstructure and properties of the aluminum thin films were studied. The gas pressures introduced during deposition varied from 5 × 10s−6 Torr (6.7 × 10s−4 Pa) to 1 × 10s-10 Torr (1 × 10s−8 Pa). The thin films were investigated by transmission electron microscopy and were found to have a columnar microstructure with an even distribution of silicon precipitates. Both the grain size and silicon precipitate size increased at the higher deposition temperature. A smaller grain size was found in samples that were deposited under the higher impurity gas pressures tested. The specular reflectance of the films was found to be dependent upon the amount of impurity gases present during deposition, the greater the partial pressure the greater the surface roughness. This study also investigated the possibility of using a Resistivity Ratio (RR) measurement to evaluate the grain size of the Al thin films. It has been previously observed that a small Al grain size has poor electromigration resistance. This study found that the correlation between Al grain size and RR values was good indicating that RR tests may be used as a quick, non-destructive measure of film quality.

Solders of Pb-rich compositions, such as 5Sn-95Pb, are commonly used in electronic packaging applications, and this demanding use necessitates that the microstructure-property-processing relationships of the solder be understood fully. In this study, the microstructure of 5Sn-95Pb solder was characterized using a variety of metallographic techniques. The effect of cooling rate on the precipitation of β-Sn from supersaturated α-Pb was determined. On slow cooling, β-Sn precipitates discontinuously with resultant β-Sn lamella alternating with the α-Pb. Upon rapid cooling, the β-Sn precipitates with a nominally homogeneous distribution. These discrete precipitates were found to have a platelike shape with a (111)Pb habit plane and an orientation relationship of (111)Pb‖(010)Sn and [011]Pb‖ [001]Sn. Regions of the β-Sn precipitates that curved away from this habit plane formed ledges. Upon heating, the precipitates were found to dissolvevia a ledge mechanism. Prolonged aging of both the fast and slow cooled 5Sn-95Pb resulted in a coarsening of the β-Sn precipitates with a resultant decrease in strength. Furthermore, the strength of the aged alloy was observed to be independent of cooling rate.

Aluminum - 2 wt.% copper alloys are commonly used in thin film form as interconnect metallization lines for integrated circuits. Experience has shown that the addition of the Cu to the Al, albeit at a decrease in conductivity, makes the metallizations more resistant to failure by electromigration. However, the mechanism by which Cu increases the resistance to electromigration has never been positively identified. One theory proposes that Cu coats the Al grain boundaries (boundaries are enriched in Cu) and retards grain boundary diffusion thereby reducing electromigration. Another theory suggests that a continuous thin layer of CuAl2 forms along the boundaries also reducing grain boundary transport and therefore the tendency to electromigrate. Recently, Frear et al. have reported on a detailed set of experiments to examine these theories from a microstructural viewpoint. Here, the details of the high spatial resolution microanalysis done to support the study of Fear, et al. are reported.Al- 2wt.% Cu was magnetron sputtered onto a borosilicate glass (BSG) coated (100) Si wafer. The Al-Cu films were sputtered at room temperature from a single source under an argon atmosphere at a deposition rate of 100 nm/min. Films 400 and 800 nm thick were prepared. The films were annealed under a 15% hydrogen forming gas (reducing) at 425°C for 35 min.