(A) study on the interfacial reaction between lead-free solders and electroplated Ni or Cu metallization and its effect on mechanical reliability

Abstract

In the microelectronic packaging, Cu and Ni (or Ni(P)) are very common base metals used in lead-free solder interconnection. Owing to good solderability with solders, Cu is utilized in contact with lead-free solders. Ni is often used as UBM for lead-free solders since the reaction rate of Ni with Sn is smaller than that of Cu. The objective of this study is to investigate interfacial reactions and mechanical reliabilities of Ni/Cu (or Cu)/Pb-free solder joints as follows. Copper was supplied to Sn-3.5Ag by electroplating Cu/Ni double UBM, and the amount of Cu was controlled by varying the Cu UBM thickness. Supposed Cu contents in the solder were; 0.2, 0.5, and 1.0 wt% respectively, and the solder joint microstructure was investigated after one, five, and ten reflows. In the case of specimens with 0.2 and 1.0 wt% Cu, only one type of IMC formed, $(Cu,Ni)_6Sn_5$ and $(Ni,Cu)_3Sn_4$ respectively, while two types formed in specimen with 0.5 wt% Cu. No correlation could be formed between the solder joint microstructure and the ball shear test. However, drop test results showed two opposite trends. The drop resistance of 0.2Cu and 1.0 wt% Cu specimens was quite good initially but degraded dramatically with multiple reflows, in contrast to that of the 0.5 wt% Cu specimen which was very poor after one reflow but improved substantially later on. The former was ascribed to thickening of IMC during reflow, while the latter was related to $(Ni,Cu)_3Sn_4$ thickening beneath $(Cu,Ni)_6Sn_5$ and subsequent spalling of $(Cu,Ni)_6Sn_5$ from $(Ni,Cu)_3Sn_4$. Solder joints of Cu/Sn-3.5Ag were prepared using Cu foil or electroplated Cu films with or without using SPS additive. With high level of SPS in the Cu electroplating bath, voids tended to localize at the Cu/ $Cu_3Sn$ interface during subsequent aging at $150^\circ C$, which was truly detrimental to the drop impact resistance of the solder joints. The in-situ AES analysis of fractured joints revealed S segregation on...