Morphology and Growth of Intermetallics at the Interface of Sn-based Solders and Cu with Different Surface Finishes

Abstract

Several types of surface finishes have been applied on Cu substrates in an effort to facilitate bonding and improve the reliability of lead-free solder joints. In the current research, the effects of printed circuit board surface finishes on the reliability of the solder joints were investigated by examining the morphology and growth behavior of the intermetallic compounds (IMCs) between Sn-based solders and different surface finishes on Cu. Three types of Cu substrates with different surface finishes were fabricated in this study: organic solderability preservative (OSP)/Cu, Ni/Cu, and electroless nickel immersion gold (ENIG)/Cu. Sn-3.5Ag and Sn-3.0Ag-0.5Cu were used as the solders. In the experiment, the solder joint specimens were aged isothermally at 150°C for up to 1000 h. Experimental results revealed that the OSP surface finish promoted the interdiffusion between Cu and Sn during soldering. The composition and morphology of the IMC layer at the solder/Ni/Cu interface were sensitive to the Cu concentration in the solder. Meanwhile, the solder joints with different morphological features of the IMCs exhibited significant differences in shear strengths. The Au-containing ENIG surface finish affected the shear strength of the solder joint significantly at the initial stage of isothermal aging.

Solder Volume Effects on the Microstructure Evolution and Shear Fracture Behavior of Ball Grid Array Structure Sn-3.0Ag-0.5Cu Solder Interconnects

“…23 A comparative study of the formation and growth of IMC at the interface between Sn-based solders (i.e., Sn-3.5Ag and Sn-3.0Ag-0.5Cu) and Cu with different surface finishes of OSP/Cu, Ni/Cu, and electroless Ni immersion gold (ENIG)/Cu showed that the OSP surface finish promoted interdiffusion between Cu and Sn during soldering, that the composition and morphology of the IMC layer at the solder/Ni/Cu interface were sensitive to the Cu concentration in the solder, and that solder joints with different IMC morphological features exhibited significant differences in shear strength. 25 For two different solder joints of Sn-3.0Ag-0.5Cu/Cu and Sn-3.0Ag-0.5Cu/ENIG, the growth rate of the IMC layer for the Cu substrate was more than three times faster than that for the ENIG substrate. 26 Furthermore, it was reported that, for three different board finishes of commercial Cu, Ag-Cu, and Sn-Cu on FR-4 (Photocircuits), the texture of the board finish did not notably influence the growth rate of the layers, and smoother initial board finishes led to stronger and more reliable solder joints.…”

“…17 In addition, although the interfacial intermetallic compound (IMC) layer formed during the soldering process may provide a metallurgical bonding between the solder and substrate, the morphology and thickness of the interfacial IMC have become critically important in determining the reliability of solder interconnects, because of the intrinsic brittleness of the IMC layer and thus the preferential occurrence of crack initiation and propagation at the solder/IMC interface; 18,19 furthermore, the composition and morphology of the interfacial IMC are sensitive to the solder composition, bump size, and geometry. [20][21][22] Moreover, there have been a number of studies [23][24][25][26][27] looking at the influence of the surface finish of the printed circuit board (PCB) and under bump metallurgy (UBM) on the interfacial microstructure and properties of solder interconnects. It has been shown that the thickness of the IMC layer formed initially at the as-soldered small outline J-lead/ Sn-Ag-Cu interface over a Ni/Au PCB surface finish was about 1.7 times that over an organic solderability preservative (OSP) PCB surface finish.…”

Cyclic mechanical behaviour of Sn3.0Ag0.5Cu alloy under high temperature isothermal ageing

“…For thermal ageing of the interface layers, various studies have demonstrated the relation between the changes in the microstructure and in certain mechanical properties [8][9][10][11]. In these cases, ageing laws have been defined for various chemical compositions.…”