Influence of Sb on IMC growth in Sn-Ag-Cu-SbPb-free solder joints in reflow process

The influence of Sb on the soldering reaction and growth kinetics of Intermetallic compound (IMC) in Sn-3.5Ag-0.7Cu-xSb (x=0, 0.5, 1.0, and 1.5) lead-free solder joints is investigated in this study. Scanning electron microscope (SEM) is used to observe microstructure evolution of solder joint and to estimate the thickness and the grain size of the intermetallic layers. IMC phases are identified by energy dispersive X-ray (EDX) and X-ray diffractometer (XRD). Results show that some of the Sb powders are dissolved in the beta-Sn matrix (Sn-rich phase), some of them participate in the form of Ag-3(Sn, Sb), and the rest dissolves in the Cu6Sn5 IMC layer. Both thickness and grain size of IMC decrease when Sb is added. The growth exponents for both IMC layer and grains are determined by curve-fitting. The results reveal that Sn-3.5Ag-0.7Cu with about 1.0 wt.% Sb solder system exhibits the smallest growth rate and gives the most prominent effect in retarding IMC growth and refining IMC grain size. Based on the thermodynamic and phase diagram analysis, Sb has higher affinity to Sn element, and it will reduce the activity of Sit by forming SnSb compound, resulting in a decreased driving force for Cu-Sn IMC formation. A heterogeneous nucleation effect for retarding the IMC growth due to Sb addition is proposed. (C) 2004 Elsevier B.V. All rights reserved.