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SMT-Oct2017

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66 SMT Magazine • October 2017 by Reza Ghaffarian, Ph.D. JET PROPULSION LABORATORY CALIFORNIA INSTITUTE OF TECHNOLOGY Abstract Electroless nickel/electroless palladium/im- mersion gold (ENEPIG) surface finish for PCBs has now become a key surface finish that is used for both tin-lead and lead-free solder as- semblies. This paper presents the reliability of land grid array (LGA) component packages with 1156 pads assembled with tin-lead solder onto PCBs with an ENEPIG finish and then subjected to thermal cycling and then isothermal aging. To determine thermal cycle reliability, daisy- chain LGA1156 packages were used to enable the monitoring of solder joint failures. The as- semblies were built with a vapor phase reflow machine or using a rework station. Then, they were subjected to thermal cycling ranging from -55°C to 125°C. After the completion of 200 thermal cycles, the assemblies were isothermally aged for 324 hours at 125°C to determine the ef- fect of isothermal aging on intermetallic forma- tion and growth, which is one of the concerns for tin-lead solder assemblies. To determine the effect of exposure at temperatures higher than 125°C, the aged samples were subjected to 100 thermal shock cycles between -65°C and 150°C. Several characterization methods were used to ensure the integrity of solder joints. These included nondestructive evaluation by X-ray, daisy-chain monitoring at thermal cycle/aging intervals, and destructive characterization by cross-sectioning. The cycled/aged samples were cross-sectioned and characterized by optical and scanning electron microscopy (SEM). Assembly processes and SEM photomicrographs showing damage progression and IMC/microstructural changes, as well as elemental analyses by X-ray energy dispersive spectroscopy (EDS), were also presented. Introduction The IPC standards team has helped in easing the implementation of ENEPIG surface finish for PCBs by releasing a standard with a large char- acterization database 1,2 . This data and that gen- erated by industry 3 generally concentrated only on the basic testing approaches and mostly for FEATURE

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