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The reliability of fine pitch Sn-3.8Ag-0.7Cu flip chip solder joints with three different pads, i.e., bare pads, pads with solder masks, and pads with microvia, on printed circuit boards (PCBs) was studied through thermal cycling. After assembly, (Au, Ni)Sn(4) inter-metallics (IMCs) formed both in the bulk solder and at the interfaces due to the im...
Contexts in source publication
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... samples used for the thermal cycling testing were ASH chips assembled with MT030 boards as shown in Fig. 1. The geometrical and functional details of these ASH chips can be found in [1], which have a peripheral array of Al-1wt%Cubond pads with small pitch sizes of 225 μm and 300 μm and a pad opening of 75 μm diameter. The under bump metallization (UBM) for ASH chips is electroless Ni (EN), and the surface finish on the boards is electroless ...
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... pad opening of 75 μm diameter. The under bump metallization (UBM) for ASH chips is electroless Ni (EN), and the surface finish on the boards is electroless Ni immersion gold (ENIG). The thickness of electroless Ni on both chips and PCBs is 6 μ m. The pads on the boards are defined by different methods and have different features, as indicated in Fig. 1, i.e. pads with solder mask definition, bare pads and pads with microvias. Bare pads and pads with microvias do not have a solder mask definition. The pads on the boards are formed as a peripheral daisy chain with test pads that can be used to assess the electrical resistance of the whole chain and individual connects. Isolated pads ...
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... possible failure mode for the solder joints formed on bare pads and the pads with solder mask was crack propagation in bulk solder as shown in Fig. 9. For a few solder joints, cracks propagated along the grain boundaries of β-Sn as shown in Fig. 10, and grain boundary sliding is a possible mechanism for this crack propagation. The Ag3Sn/solder interface can be a weak position for a crack to initiate and propagate [14]; however, sometimes the Ag3Sn IMC can change the direction of crack propagation and increase the path length that cracks have to travel during thermal cycling, as ...
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... 10, and grain boundary sliding is a possible mechanism for this crack propagation. The Ag3Sn/solder interface can be a weak position for a crack to initiate and propagate [14]; however, sometimes the Ag3Sn IMC can change the direction of crack propagation and increase the path length that cracks have to travel during thermal cycling, as shown in Fig. 11. It seems that in some cases the existence of Ag3Sn IMC can increase the reliability of solder joints. Delamination at the underfill/chip and underfill/solder interfaces might happen during thermal excursions with large ramp rates, and some research has found this phenomena in thermal shock tests [2,4,13,15]. The reason is that the CTE ...
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Citations
Cyanide-free electroless plating was used to deposit nickel/gold coating on the patterned circuits with fine-pitch. The relationship among the controlled process parameters, the skip plating, over-plating and pit forming were investigated. Results show that due to the uniform discharge sputtering, the copper layer is failed to be deposited around BVHs and lines of Al2O3 samples. The skip plating is mainly caused by the self-oxide and the leakage sputtering with copper. Excess Pd activation may result in over-plating during plating nickel. With enhancing the uniformity of the substrate materials and raising the annealing temperature, the number of pits decreases significantly. The thickness of the Au coating can reach up to 2 µm with the smooth coating surface and without the spillover. The electroless plating Au progress can be used to fine-pitch selective area plating.
