Effect of intermetallic compounds on the shear fatigue of Cu/63Sn-37Pb solder joints

Abstract

The effect of Cu-Sn intermetallic compounds (IMC) on the fatigue failure of solder joints has been studied by means of shear cycling. The samples consist of leadless ceramic chip carriers (LCCC) soldered onto FR-4 printed circuit boards (PCB), and are prepared by conventional reflow soldering using a 63Sn-37Pb solder paste and then aged at 150 C for 1, 4, 9, 16, 25, 36, and 49 days. The specimens are subjected to low cycle fatigue shear tests controlled by the displacement. The results indicate that the fatigue lifetime of the solder joints depends on the thickness of the IMC layer between the Cu-pad and bulk solder, and the quantitative relationship between the lifetime and thickness can be described as a monotonically decreasing curve. The greatest decrease is over the thickness range up to 2.8 m, when the IMC/bulk solder interface becomes flat, corresponding to a lifetime decrease to 62% of the as assembled value. For further increase in IMC thickness the lifetime decreases more slowly. Evidently, the effect of the Cu-Sn intermetallic compounds on the joint fatigue lifetime is not only concerned with the IMC thickness but also the interface morphology. A thick and flat IMC layer has a deleterious effect. The results of X-ray diffraction and metallographic analysis show that cracks initiate underneath the component metallization, and propagate along the IMC/solder interface, then toward the fillet. The Cu 3 Sn ("-phase) is formed between the Cu-pad and -phase, and grows more quickly than thephase during storage and long term operation or aging tests. However, the Cu 3 Sn makes only a small direct contribution toward fatigue failure.

Effect of Zn Addition on Interfacial Reactions Between Sn-Bi Solder and Cu Substrate

“…In addition, the average approximate concentration of Sn, Cu, and Zn in the IMC layer adjacent to the Cu is also obtained which are 24, 56, and 20, respectively. These results which are consistent with the elemental-mapping analysis suggest that the IMC layer adjacent to the solder is a Cu 5 (Zn,Sn) 8 IMC layer while the IMC layer adjacent to the Cu substrate a Cu 6 (Sn,Zn) 5 IMC layer. The formation of Cu 6 (Sn,Zn) 5 IMC between Cu 5 (Zn,Sn) 8 IMC layer and the Cu substrate may be attributed to the penetration of Sn atoms through the Cu 5 (Zn,Sn) 8 IMC layer during the long solid-state reaction to form Cu 6 (Sn,Zn) 5 IMC.…”

“…The average approximate concentration of Sn, Cu, and Zn in the IMC layer of ve different spots were obtained which are 21, 33, and 46 at.%, respectively. By taking to consideration these results, the binary phase diagram of Cu-Zn 20) and other studies on Sn-BiZn solder on Cu substrate 18) , the Cu-Zn IMC is deduced to be Cu 5 (Zn,Sn) 8 .…”

“…The elemental analysis of the thermally aged solder joints revealed that a layer of Cu 6 (Sn,Zn) 5 IMC was also formed between Cu 5 (Zn,Sn) 8 and the Cu substrate after thermal aging for 1008 h.…”

“…• The interfacial IMC layer of the as-re owed Sn-Bi-Zn solder joint was determined to be Cu 5 (Zn,Sn) 8 . The elemental analysis of the thermally aged solder joints revealed that a layer of Cu 6 (Sn,Zn) 5 IMC was also formed between Cu 5 (Zn,Sn) 8 and the Cu substrate after thermal aging for 1008 h.…”

“…Although the formation of interfacial intermetallic compounds (IMCs) is crucial to create the solder joint, the growth rate of interfacial IMC must be controlled because, with increasing the thickness of these IMC layers, tensile strength 6) , thermal fatigue life 7) , and isothermal shear fatigue life 8) will decrease. For instance, authors 9) reported the deteriorating effect of interfacial IMCs with high thicknesses on the shear strength of solder joints.…”