CBGA结构热循环条件下无铅焊点的显微组织和断裂

MICROSTRUCTURE AND FRACTURE OF Pb-FREE SOLDER INTERCONNECTS IN CBGA PACKAGES UNDER THERMAL CYCLING

用热循环实验、扫描电镜观察焊点横截面和有限元模拟的方法研究了陶瓷球栅阵列封装(CBGA)结构中无铅焊点的组织和热疲劳行为． CBGA结构中,在焊料与铜焊盘和银焊盘的界面处分别形成了Cu6Sn5和Ag3Sn．在热循环过程中,铜焊盘处Cu6Sn5层增厚,并出现Cu3Sn;镀银陶瓷芯片一侧,Ag3Sn层也增厚,焊球中靠近界面处Ag3Sn的形态从条状向球状过渡．增加热循环周次,疲劳裂纹最先出现在芯片与焊球界面处焊球的边角位置上,有限元模拟表明此处具有最大的剪切应力．在印刷线路板处,裂纹沿Cu6Sn5和焊料的界面扩展;在陶瓷芯片处,裂纹沿Ag3Sn界面层附近的焊球内部扩展．

Microstructure and thermal fatigue behavior of Pb-free solder interconnects in ceramic ball grid array （CBGA） packages were examined by cross-sectional microscopy, thermal cycling experiments and finite element modeling. In the as-reflowed condition, Cu6Sn5 and Ag3Sn formed at the solder interfaces with Cu and Ag metallizations respectively. Following thermal cycling, the Cu6Sn5 layer grew thicker and Cu3Sn was found at the interface with Cu metallization. On the ceramic side, visible thickening of the Ag3Sn layer was also observed. The morphology of Ag3Sn in the solder near the interface changed from strip-like to spherical shape. As a result of repeatedly thermal cycling, the fatigue crack appeared first at the corner of the solder ball with the chip, where the maximum shear stress was found by the finite element analysis. The cracks preferred to propagate along the Cu6Sn5/solder interface on the side of the print circuit board and in the solder joint near the interfacial Ag3Sn layer on the ceramic side.