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72 SMT Magazine • October 2017 LGAs were used for assembly onto PCBs that also added additional manufacturing challeng- es. A printed circuit board was designed to ac- commodate a plastic LGA1156 package, as well other packages that are beyond the scope of this article. The ENEPIG thicknesses were defined per IPC-4556, Jan. 2013 release: electroless Ni, 118−236 μin; electroless Pd, 2−12 μin; and im- mersion Au, 1.2 μin minimum. The LGA1156 had daisy-chain patterns for checking opens after assembly and for opens during the reliability evaluation. High I/O LGA daisy-chain patterns on ENEPIG PCB finish not only enabled solder joint reliability evaluation, it also provided another verification method for the condition of interconnections after as- sembly. The PCB daisy-chain patterns were de- signed to match LGA designed packages to make a complete resistance loop after the package was assembled onto the PCB. Three key parameters were evaluated before being ready to commit to a larger number of assemblies for reliability evaluation. Generally, after LGA1156 and PCB were baked for moisture removal, tin-lead eutectic solder paste was applied on LGA1156 pads and reflowed to form solder domes on their pad. Ceramic packages were required to repeat the process to achieve the desired height of solder dome. After solder paste application on the PCB pads, the LGA1156 was placed onto the PCB and prepared for assembly. Figure 3 shows a photo of the bumped plastic and ceramic LGAs (1156 and 1272 I/Os) and the final assembly. The test RELIABILITY OF ENEPIG BY SEQUENTIAL THERMAL CYCLING AND AGING Figure 3: ENEPIG PCB surface finish for the assembly of plastic LGA with 1156 pads (left top) and ceramic LGA (right top). LGA1156 with ceramic version were assembled onto ENEPIG PCB finish (bottom).