Effect of pinhole Au/Ni/Cu substrate on self-alignment of advanced packages

The self-alignment of advanced packages (μBGA) on both non-pinhole and pinhole Au/Ni/Cu pads has been discussed. It is found that a slight reduction of self-alignment of the packages using pinhole pads occurs. Rutherford backscattering spectrometer (RBS) results suggest that this reduction should not be attributed to the oxide formation of the surface or interface layer in the Au/Ni/Cu pads. The solder wetting experiments show that slow spreading of molten solder on pinhole pads may result in a reduction of effective board pad surface area that can be wetted. This will reduce the restoring force of the solder joints, and thus causing a less better self-alignment of the packages using pinhole pads. Oxidation of nickel at the exposed area and Au/Ni interface is observed to occur by direct exposure of substrate pads through pinholes during aging. The solder wetting of the aged pads has been described. For flux reflow soldering, the aging of the pads seems to have no serious effect on the self-alignment of the package. However, it is found from the peel-off test that a few solder joints of the samples after reflow have weak adhesion strength at the solder and aged pinhole pad interface. The mechanism for this weak adhesion strength has been proposed.