Printed circuit boards populated with twenty-five 0.4 mm pitch, 256-pin plastic quad flat packs (QFP) containing no-clean and water-clean solder joints were subjected to thermal cycling in order to induce thermal fatigue failure. QFPs failed from a minimum of 0.27 cycle to a maximum of 5310 cycles. Solder joints in both types of units were examined in the scanning electron microscope, and a relative comparison of the extent of fatigue damage is presented. The failure associated with cracking in the eutectic composition Sn/Pb solder initiated at the stress concentration sites. Crack propagation continued either along the pin-solder interface or solder-pad interface and ultimately resulted in the separation of the pin/pad junctions. In addition, a qualitative and quantitative comparison of the resulting solder micro-structure and the failure mode between the no-clean and the water-clean QFP solder joints was made and discussed.

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