A study on interfacial reactions between electroless Ni-P under bump metallization and 95.5Sn-4.0Ag-0.5Cu alloy

Abstract

Even though electroless Ni-P and Sn-Ag-Cu solders are widely used materials in flip-chip bumping technologies, interfacial reactions of the ternary Cu-NiSn system are not well understood. The growth of intermetallic compounds (IMCs) at the under bump metallization (UBM)/solder interface can affect solder-joint reliability, so analysis of IMC phases and understanding their growth kinetics are important. In this study, interfacial reactions between electroless Ni-P UBM and the 95.5Sn-4.0Ag-0.5Cu alloy were investigated, focusing on identification of IMC phases and IMC growth kinetics at various reflowing and aging temperatures and times. The stable ternary IMC initially formed at the interface after reflowing was the (Cu,Ni) 6 Sn 5 phase. However, during aging, the (Cu,Ni) 6 Sn 5 phase slowly changed into the quaternary IMC composed of Cu, Ni, Sn, and a small amount of Au. The Au atoms in the quaternary IMC originated from immersion Au plated on electroless Ni-P UBM. During further reflowing or aging, the (Ni,Cu) 3 Sn 4 IMC started forming because of the limited Cu content in the solder. Morphology, composition, and crystal structure of each IMC were identified using transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Small amounts of Cu in the solder affect the types of IMC phases and the amount of the IMC. The activation energies of (Cu,Ni) 6 Sn 5 and (Ni,Cu) 3 Sn 4 IMCs were used to estimate the growth kinetics of IMCs. The growth of IMCs formed in aging was very slow and temperature-dependent compared to IMCs formed in reflow because of the higher activation energies of IMCs in aging. Comparing activation energies of each IMC, growth mechanism of IMCs at electroless Ni-P/ SnAgCu solder interface will be discussed.

Effect of Pd Addition in ENIG Surface Finish on Drop Reliability of Sn-Ag-Cu Solder Joint

“…Jeon et al reported the formation of (Cu,Ni) 6 Sn 5 and (Ni,Cu) 3 Sn 4 IMCs at the interface between the Sn-4.0Ag-0.5Cu solder and electroless Ni-P layer. 15) Ho et al investigated the effect of a small perturbation in the Cu concentration in the reaction between Sn-3.9Ag-xCu solders (x ¼ 0:2 $ 3:0 in mass%) and Ni layer during a liquid-solid reaction at 250 C. 16) They reported that the slight variation in Cu concentration produced completely different reaction products. Generally, the thickness of the IMC layer at the interface between the solder and substrate is very important in determining the reliability of the whole package because an excessively thick IMC layer is sensitive to stress and sometimes provides initiation sites and paths for the propagation of cracks.…”

Failure mechanisms of lead-free chip scale package interconnections under fast mechanical loading

“…Especially, the reactions between Ni(P)|Au-coated soldering pads and solders have been studied extensively. [8][9][10][11][12][13][14][15][16][17] Likewise, the cracking of interconnections soldered on the Ni(P) metallization has also been reported in these papers and the occurrence of the failure is typically explained by the oxidation or contamination of the plating bath that affects the quality of the metallization.…”

Mechanical Property Evaluation of Sn-3.0A-0.5Cu BGA Solder Joints Using High-Speed Ball Shear Test

“…As shown in Fig. 2c 15 Ho et al investigated the effect of a small perturbation in the Cu concentration on the reaction between four Sn-3.9Ag-xCu solders (x = 0.2 wt.% to 3.0 wt.%) and a Ni layer during a liquid-solid reaction at 523 K. 16 They reported that a slight variation in the Cu concentration produced completely different reaction products. Generally, the thickness of the IMC layer at the interface between the solder and substrate is very important in determining the reliability of the whole package, because an excessively thick IMC layer is sensitive to stress and sometimes provides initiation sites and paths for crack propagation.…”