The reliability of fine pitch Sn-3.8Ag-0.7Cu flip chip solder joints with different pads on print circuit boards (PCBs) was studied through thermal cycling. Three different pads were studied, i.e. bare pads, pads with solder masks and pads with micro-via. After assembly, (Au,Ni)Sn4 intermetallics (IMCs) formed both in the bulk solder and at the interfaces due to the immersion-Au finish on the board side. The (Au,Ni)Sn4 IMCs formed in the solder joints on the pads with micro-via were more abundant than those formed in the solder joints on the pads without micro-via. Results showed that the solder joints on the pads with a micro-via had poor reliability due to insufficient solder volume and the formation of large amounts of (Au,Ni)Sn4 IMCs. For the pads with micro-via, the main location of failure was at the (Au,Ni)Sn4/solder interface on the chip side, and for the solder joints on bare pads and pads with solder mask, the possible failure location was in the bulk solder.
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