Effect of Ni(P) thickness of ultrathin ENEPIG on the interfacial reaction and board level reliability of solder joints

The amorphous Ni(P) layer in traditional Electroless Ni/Electroless Pd/Immersion Au (ENEPIG) surface finish has an adverse effect on its performance in high speed circuits due to the relatively high electrical resistance which will cause impact to the signal integrity. By limiting the thickness of Ni(P)layerless than 1μm in ultrathin ENEPIG, the impedance can be significantly reduced and its solderability and corrosion resistance remain as good as those of traditional ENEPIG. In this article, two kinds of ultrathin ENEPIG with Ni(P) layer thicknesses of 0.112 and 0.185μm respectively were reflowed with commercial Sn-3.0Ag-0.5Cu solder. After that the morphology and growth of interfacial IMC were investigated using SEM and EDS analysis. Solder ball shear test, drop test and thermal cycle test (TCT) were then conducted to evaluate their bonding strength as well as performance in board level reliability tests. Results showed that although Ni(P) layer was depleted soon in ultrathin ENEPIG samples, the Ni-Sn-P layer could still act as a diffusion barrier and hinder the growth of brittle IMC, which contributed a lot to the bonding strength. Ultrathin ENEPIG samples with 0.185μm Ni(P) layer survived both TCT and drop test and showed better reliability than electroplated Ni/Au in drop test and better resistance to thermal fatigue than OSP in TCT, indicating that ultrathin ENEPIG could satisfy the reliability requirement for actual application.