Wire Bonding and Soldering Study of Thin Ni-P ENEPIG Surface Finish

Electroless nickel electroless palladium immersion gold (ENPIG) is widely used for the surface finish of integrated circuit (IC) package substrate. The Ni-P coating has magnetic effects and it will affect the signal integrity of IC device, and the thinner Ni thickness will be better for the improvement of high frequency signal integrity of the IC. So in the recently years, ENEPIG with thin Ni-P (0.1~0.3μm) layer were developed for the surface finish of substrate and is applied in high frequency radio frequency module. In this work, ENEPIG with 0.17μm, 0.27μm and 0.32μm Ni-P, 0.08~0.1μm Pd and Au is prepared for the wire bonding and soldering performance study. 0.7mil PdCu wire bonding and pull test shows the thin Ni-P ENEPIG surface finish have good wire bonding-ability with pull force over 4g and wire edge residue on the bonding finger after pull test. With the increasing of the Ni-P thickness, the pull force shows slight increasing. The Pb-free reflow of the substrate before wire bonding will lead to the obvious decrease of the pull force. Sn3.0Ag0.5Cu solder ball mounted on the 350μm ENEPIG pad with 0.1~0.3μm Ni-P layer. Cross section analysis shows that the Ni-P layer was consumed just after one cycle Pb-free reflow. EDX results shows (Cu,Ni)₃Sn intermetallic compound (IMC) formed between the Sn-3.0Ag-0.5Cu solder and ENEPIG with thin Ni layer. This is an interesting finding as there is no previous report. With the increasing of the reflow, the content of Cu element in the IMC layer increased as more copper on the package substrate will be consumed. As the conclusion, the ENEPIG with 0.1~0.3μm Ni-P layer is suitable for wire bonding and Pb-free soldering.