Issue 46

N. Hiyoshi et alii, Frattura ed Integrità Strutturale, 46 (2018) 25-33; DOI: 10.3221/IGF-ESIS.46.03 26 mechanical properties and fatigue life [1]-[6]. Solder joints in electronic devices undergo cyclic fatigue damage caused by not only mechanical loading but also thermal loading due to the mismatch of thermal expansion coefficient of different connecting parts. The solder joints also have lots of stress concentration parts, so that it is useful for fatigue life estimation of electronic devices to make clear the cyclic crack initiation and propagation behaviour of solders from the stress concentration part at commercial operating temperature. Although Sn-3.0Ag-0.5Cu lead-free solder is widely used solder all over the world, there are some issues for the solder. One of these issues is that high material cost due to it contains Ag element. There are lots of candidate lead-free solders substitute for Sn-3.0Ag-0.5Cu solder in order to reduce the material cost. Sn-low-Ag-Cu solders which is one of the candidate solders have a lower material cost than that of Sn-3.0Ag-0.5Cu solder as reducing Ag element, so that the Sn- low-Ag-Cu solders are useful material for industrial use. The melting point temperature of Sn-low-Ag-Cu solders is 500 K and the material cost is about 4,800JPY (Japanese Yen) per 1kg while Sn-3.0Ag-0.5Cu solder costs about 7,500JPY. Although it is important for commercial safety products design to clarify not only tensile strength and mechanical properties but also fatigue life of the electronic materials, there is little experimental research paper on Sn-low-Ag-Cu solders. There is also little research paper on crack initiation and propagation behaviour of the solders at commercial operating temperatures. The reason for little research paper on the crack investigation of solders might be a difficulty of testing technique for solders which have low strength and small hardness. This study discusses the crack initiation and propagation behaviour of Sn-low-Ag-Cu solders at high temperature. A cyclic push-pull loading tests with a single hole specimen were conducted to investigate the crack initiation and propagation behaviour of the solders. This study also discusses the adaptation of J-integral range parameter for the crack propagation rate evaluation. E XPERIMENTAL PROCEDURE he materials tested in this study are four kinds of low-Ag solders of which chemical composition are listed in Tab. 1. Sn1.0Ag0.7Cu solder (SnAgCu) contains lower Ag element in order to reduce the coarse intermetallic compound forming and to reduce the materials costs. The melting point temperature of SnAgCu is 500 K. Sn1.0Ag0.7CuNiGe solder (SnAgCu+NiGe) is the solder which is added 0.07% Ni and 0.01% Ge to SnAgCu. Sn1.0Ag0.7Cu2.0Bi solder (SnAgCu+Bi) is the solder which is added 2.0% Bi to SnAgCu. Sn1.0Ag0.7Cu2.0BiNiGe solder (SnAgCu+BiNiGe) is the solder which is added 2.0% Bi, 0.07% Ni and 0.01% Ge to SnAgCu. A cyclic push-pull loading tests were conducted with a single hole specimen. Fig, 1 shows shape and dimensions of the specimen, which has a single through hole with 1mm in diameter at a center of the specimen as a stress concentration part. The specimen has a stress concentration factor of K t =2.72, which is calculated by using Eqn. (1) [7]. 2 3 3.00 3.13 3.66 1.53 t d d d K W W W                       (1) where, d and W is diameter of the center thorough hole and specimen width, respectively. The specimen is produced by a mechanical procedure from a low-Ag solders ingot which were casted under controlled temperature and controlled instruction. Figure 1 : Shape and dimensions of the specimen (mm). Fig, 2 is a photograph of an electric hydraulic cyclic push-pull loading apparatus for solders. A 10kN small capacity actuator and a 10kN capacity load cell are adapted in order to conduct the fatigue test for solders. The cyclic push-pull T