Solderability of electroless deposited Ni-P coatings with Sn-3.8Ag-0.7Cu and Sn-3.5Ag lead-free solder alloys

摘要

Purpose - To study the influence of storage time and environment on the solderability of electroless nickel plated samples with Sn-3.8Ag-0.7Cu and Sn-3.5Ag lead-free solders and to provide criteria for the use of an electroless nickel (Ni-P) under bump metallization (UBM) without immersion gold protection. Design/methodology/approach - Electroless nickel coatings were deposited onto pure aluminium foil through a procedure developed for the UBM of wafers prior to flip chip bumping. Their solderability with lead-free solders was studied using the wetting balance technique. Samples stored in different environments for different periods of time were tested to study the dependence of the solderability of Ni-P coatings on the storage time and temperature. The degree of oxidation of the Ni-P coatings was examined by means of X-ray photoelectron spectroscopy and the surface microstructure and roughness of the coatings were analyzed by scanning electron microscopy and atomic force microscopy. Findings - It was found that the Ni-P coatings were unacceptable for direct soldering without the assistance of a flux, due to poor wettability, even when using a freshly prepared Ni-P coating. Therefore, a suitable flux with nitrogen inerting had to be applied to assist the soldering process. The results also show that the solderability of Ni-P coatings was affected by the phosphorus content, and the Ni-P coating with high phosphorus content had a good solderability. The storage time and temperature did not influence the wettability significantly with the assistance of strong flux. Research limitations/implications - The stability of the plating solution and the consistence of the phosphorus content in the coating are not easily controlled. This has resulted in implications for surface analysis and wetting testing. Ni-P coatings with different levels of phosphorus content are being investigated in detail. Originality/value - The value of the paper lies in its study on the solderability of lead-free solders to Ni-P coating after storage in different environments and for different periods, which can provide some criteria for the use of Ni-P UBM without immersion gold protection.