Influence of intermetallic compound on the stress distribution and fatigue life of halogen-free printed circuit board assembly

A complete halogen-free test vehicle was achieved by assembling five ball grid array (BGA) components with daisy-chain on an 8-layer high density interconnection (HDI) printed circuit board with a low-halide Sn1.0Ag0.5Cu (SAC105) Pb-free solder pastes for reducing the formation of Ag₃Sn. Afterward a board-level cyclic bending test was enforced on the as-reflowed assemblies according to the JESD22-B113 standard to evaluate the reliability of HF PCBA under a low-level load condition. The Weibull analyses showed that the characteristic lives of the assemblies were 99,098 and 130,290 cycles on Cu pad and electroless Ni pad, respectively, and the failure mode was the fracture of Cu trace within the micro-via induced by the crack of resin coated copper (RCC) layer. At the interfaces of solder joints, the intermetallic compound (IMC) formed on both Cu and electroless Ni was (Cu, Ni)₆Sn₅ with various Ni contents of 3.9 and 20.1 at%, respectively, and different thicknesses of 5.6 and 1.1 um. The simulation results pointed out that the factors of composition and thickness of IMC significantly influenced the stress distribution and characteristic life of the HF test vehicle. The stiff (Cu, Ni)₆Sn₅ constrained the deformation of solder joint interconnection, but increased the stress concentrated on soldering pad and RCC layer. Although the (Cu, Ni)₆Sn₅ on the electroless Ni was stiffer than that on the Cu pad, the thicker (Cu, Ni)₆Sn₅ instead increased the stress on the Cu pad due to size effect, and results in a lower characteristic life.