N. ASRAR, SCHLUMBERGER, SUGAR LAND, TX

Summary: While most of the industries are striving very hard to produce totally lead-free electronic products, many concerns remain regarding lead-free solder joint reliability. One major concern is the robustness of gold metallization of the electronic components for lead-free soldering. Increasing gold content has been known to result in embrittlement and early failure in electronic assemblies. Therefore, information about the lead-free solder/gold metallization interdiffusion at high temperature applications is very important for controlling the technological processes for the reliability of the electronic interconnects. The challenges of solder/gold metallization interdiffusion during high temperature application/test are; gold embrittlement, intermetallics growth, void formation, and also tin-whisker formation. This paper illustrates few case histories of such challenges. After the thermal cycle test of one of the electronic circuit board, a lead-free solder joint fractured between the Oscillator and the pad. Both Oscillator and the pad on the printed circuit boards (PCB) were gold plated. Failure investigations using, microscopic examinations and material characterization by SEM/EDS, revealed numerous voids and high content of gold-tin intermetallic compound (IMC), AuSn4, in the Sn96 solder joints (Figures 1 and 2). These results confirmed that the synergistic effects of void formation, IMCs formation due to the thick gold plating, and coefficient of thermal expansion (CTE) mismatch between organic and ceramic substrates resulted in brittle fracture of the solder joint. Irregular shaped voids are most common in the solder joints of components with thick gold plating. As a result of higher gold dissolution, crystals of AuSn4 is formed, which increases the viscosity of the molten solder and degrades its spreadability. The increased viscosity also restricts the egress of gasses from solder during reflow, resulting in formation of numerous irregular shaped voids, which deteriorates the mechanical strength of the solder joints. Therefore, the plating thickness is the most influential characteristic of the gold deposit as it relates to the ultimate integrity of the solder connections in regards to gold embrittlement. The criticality of gold embrittlement is ensuring uniform dispersion of the gold in the melted solder and a final weight percent that depends on some other factors. The thinner Au deposits minimize Au-Sn intermetallic (AuSn4) formation and its concentration along the reaction interface. Importance of the thickness of the gold termination has been discussed and some parameters to optimizing the thickness of the gold termination have been suggested. In one of the printed circuit board Sn-Au intermetallic formation resulted in tin whisker formation (Figure 3). The circuit board was exposed to 120 h aging cycle at 180°C followed by 10 thermal cycles between - 40oC and 180oC before a failure was noticed. During inspection numerous white whiskers were observed over a lead-free solder surface. During a period of 140 hrs, 0.344 mm long whiskers were formed, which is the fastest growth rate of the whiskers formation reported so far. Scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS) were used for microscopic examination and material characterization of the whiskers, end-cap metallization and the solder materials. The tin whisker formation was attributed to the compressive stress in the tin solder material, which was caused by diffusion of the end-cap metallization, formation of intermetallics, and thermal cycling of the soldered components (Figure 4). The findings contradict the speculation that high temperature annealing may reduce the risk of tin whisker formation. The findings also indicated that the application of conformal coating is not enough to contain the whisker growth within the coat, as has been claimed in the literature. Some remedial measures are suggested to control the lead-free solder-gold metallization diffusion in the electronics interconnects.