We investigated Intermetallic compound formation mechanisms and their effect on the integrity of ball grid array Cu/Ni/Au/solder joints integrity were investigated. Substrates with three types of Au plating, and thus three different thicknesses [ Electrolytic (2.6 and 0.75pm). Immersion (0.25pm), and Selective (0.02pm) ] were used. After solder reflow, the solder joints were annealed for up to 1000 hrs at 150°C. Optical and electronic metallography together with Energy Dispersive Spectroscopy were used to locate and identify phases present in the joint for different annealing times. Brittle failure of solder joints was ascribed to the formation of a ternary intermetallic (Auo.5Ni0.5)Snd at the interface solderhbstrate. In the absence of post-reflow thermal aging, only NiaSnd was observed at the interface and it did not decrease the mechanical reliability of the joint. Tensile-shear stress tests were performed on unaged samples as well as samples aged for 1 hr, 4 hrs and 450 hrs.

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With the increasing focus on Ball Grid Array (BGA) technology as a high performance semiconductor package, there is an urgent need to develop industry wide standards for package quality and reliability. A significant factor in BGA package quality is the robustness of the attachment of the solder ball to the package. Currently, ball shear testing is used as a destructive test method to assess solder joint integrity. For a given shear rate, a common recognized acceptance criterion for this test is a minimum shear force of 1Kg for connections on packages with 0.75 mm solder ball diameter and 0.64 mm pads. A shear force of 1Kg alone, however, does not guarantee a quality solder joint. The solder ball failure mechanism can not be determined from shear force data alone but it is critical to understanding solder joint quality. A failure mode analysis, along with a fundamental understanding of the shear test process and results is needed to accurately assess solder joint quality.A common pad finish on BGA packages and PWB substrates is nickellgold, using either electrolytic or electroless deposition processes. There are reports of brittle fracture in BGA and PWB interconnects with both types of nickellgold surface finishes. During post assembly test and manufacturing, the solder to pad interconnection has been shown to separate under certain conditions. In this study, ball shear testing is performed in conjunction with thermal preconditioning to evaluate brittle fractures on a thermally enhanced BGA with electroless Ni/Au bond pads and on a plastic BGA with electrolytic nickellgold bond pads. Cross sectioning and fractography reveal interfacial failure in both types of packages. However, detailed metallography and scanning electron microscopy with energy dispersive x-ray analysis confirm two distinct interfacial failure mechanisms. The failure analysis is interpreted in terms of the implications on package quality and performance. Although this evaluation is performed on BGA packages, the results of the failure analysis are applicable to brittle fractures on PWBs that use a NVAu surface finish.0-7803-5502499 $10.00 01999 IEEE 1999 IEEVCPMT lnrl Electronics Manufacturing Technology Symposium 16 IntroductionAs the demand for smaller, faster and lighter electronic products with increased functionality continues to escalate, the semiconductor packaging industry has experienced a shift from traditional peripheral leaded devices to area array technology. More specifically, Ball Grid Array (BGA) technology has become one of the high performance semiconductor packages of choice for advanced applications. One of the primary factors contributing to the quality of a BGA package is the robustness of the solder bail attachment to the package. Currently, ball shear testing is used throughout the

Purpose -The purpose of this paper is to assess the long-term reliability of lead-free Sn96.5Ag3.0Cu0.5 (SAC305) under accelerated temperature cycling (ATC) conditions. Test vehicles consisted of commercial 2512 ceramic chip resistors soldered to printed circuit boards (PCBs) using three different Pb-free surface finishes: organic solderability preservative (OSP), immersion silver (IAg) and electroless nickel immersion gold (ENIG). Design/methodology/approach -Two populations of solder joints were monitored continuously during a thermal cycle of 08C to 1008C with a ramp rate of 108C/min and a 30 min dwell at the temperature extremes. One population was cycled to 2,500 cycles, the other population was cycled to 8,250 cycles. Failures were defined in accordance with the IPC-9701A industry test guidelines and failure data are reported as characteristic life, h. Microstructural evolution was characterised using metallographic techniques and back-scattered scanning electron microscopy (SEM). Fractography was performed on post-cycled resistors to determine failure mechanisms. Findings -The results showed that the lifetime of the chip resistors could be ranked as follows: OSP . ENIG . IAg after 8,250 thermal cycles, when . 73 per cent of the population had failed. It was found that the relative performance of OSP finishes improved with longer periods of cycling. This was attributed to diffusion of copper into the bulk solder and which subsequently, over time, acted as a dispersion strengthening mechanism. The relatively poor performance of IAg finishes was attributed to void formation. Originality/value -The paper provides new and valuable information to end-users about the long-term reliability of lead-free solder on three commonly used lead-free surface finishes. The performance of each surface finish is explained in terms of microstructural evolution and void formation.

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