Thermal Fatigue Assessment of Lead-Free Solder Joints

In this paper the authors have investigated the thermal fatigue reliability of lead-free solder joints. They have focused their attention to the formation of the intermetallic compound and its effect on the initiation and propagation behaviors of fatigue cracks. Furthermore, they also studied the effect of voids in the solder joints on the fatigue reliability. An isothermal fatigue test method was used in this study to improve the efficiency of fatigue study, and several different lead-free solder alloys, Sn-Ag-Cu, Sn-Ag-Cu-Bi, Sn-Cu and Sn-Zn-Bi were investigated. There are two kinds of fracture mode in lead-free solder joints, one is solder fatigue mode, and the other is an interface fatigue mode. Based upon the experimental results, it was found that the mode transition of the fatigue crack is affected by not only the properties of the intermetallic layer but also the tension strength of the solder material. If the tension strength is lower than a critical value, the fatigue cracks in the solder joints appear within the solder domain, that is the solder fatigue mode, and their fatigue life can be assessed by Manson-Coffin’s law of the bulk solder material. On the other hand, if the tensile strength is higher than that value, the interface between solder and Cu pad breaks much earlier than the solder fatigue life, that is the interface mode. The authors have found if Sn-Zn-Bi is used with another Pbfree solder material, a kind of composite structure can be built during the reflowing processes. In this study, the mechanical fatigue strength of this kind of Sn-Zn-Bi solder joint was studied. Based upon the results of mechanical shear fatigue test and FEM(Finite Element Method) analysis, it was found that if Sn-Zn-Bi was used as reflow solder with Sn-Ag-Cu ball, the CSP solder joints are as reliable as the pure Sn-Ag-Cu CSP. This is because Sn-Zn-Bi solder paste and Sn-Ag-Cu solder ball did not melt together completely and formed two-layer structure, and this two-layer structure reduces the stress concentration at the joint corners, and prevents successfully the occurrence of the interface cracks. As a result the fatigue life of Sn-Zn-Bi/Sn-Ag-Cu CSP is equivalent to that of SnAg-Cu joints. The other new problem is that voids are very easily formed in lead-free solder joints during the reflow process, and the effect of the voids on the fatigue strength of solder joints has attracted attention. In this study, the relationship between the voids and fatigue strength of solder joints was examined using mechanical shear fatigue test. Using the mechanical shear fatigue test, the effect of the position and size of voids on fatigue crack initiation and crack propagation has been investigated. It was found the fatigue life of solder joints is affected not only by the size of the void but also by its location. But if the size of the void is lower than a limit its effect will become negligible. INTRODUCTION Recently surface mount technology such as, BGA (Ball Grid Array), CSP (Chip Size Package), and flip chip, has been adopted in many electric and electronic devices such as computers, household electric appliances, and so forth. The tendency of recent developments in electronic components is characterized by miniaturization, performance enhancement, and the high pin count of the package. At the same time, there have been serious debates about the environmental problem where Pb should be removed from the solder joints. Debates are now developing to a remarkable movement to establish regulations for the removal of Pb, especially in European countries and Japan. Therefore, many studies have been aggressively promoted to develop technologies for replacing Sn-Pb solder with lead-free ones. The authors have studied and established the evaluation method of thermal fatigue strength for the BGA structure of the Sn-Pb eutectic solder [1]-[10]. With recognition growing about lead-free solder materials, it became clearer that conventional Sn-Pb eutectic solder has many merits such as a low melting point, good wettability and high electric and mechanical reliability, and it is very difficult to find a lead-free material that has superior or comparative characteristics compared with the eutectic solder. At the moment, the most favorable candidates for the lead-free solder materials are SnCu for the flow soldering, and Sn-Ag-Cu, Sn-Ag-Bi and SnZn-Bi for the reflow soldering, because using these kinds of lead-free solders can achieve almost as close a reliability level as using Sn-Pb eutectic solder. However, it is anticipated that the intermetallic compound formed by the thermal fatigue cycle will seriously affect the thermal fatigue strength under some special conditions [11]-[12]. The authors have paid attention to the intermetallic compound formation between the solder bulk and the Cu-pad due to thermal cycling, in Sn-Ag-Cu (Bi), Sn-Cu and Sn-ZnBi lead-free solder materials [11]-[15]. They have discussed the relation between the intermetallic compound and the thermal fatigue strength and the relation between the mechanical properties of solder materials and the fatigue failure modes of the solder joints. There is the serious problem that voids are easily generated in lead-free solder joints during the reflow process as compared with Sn-Pb eutectic solder. The reason why the voids are formed in the lead-free solder joints seems to be related to the rise and fall time of temperature during the reflow process, but the effect of voids on thermal fatigue reliability of lead-free solder joints is still unknown. Therefore, the mechanical fatigue tests of CSPs (Chip Size Packages) were carried out in order to compare the fatigue strength of solder joints in which voids exist and solder joints without voids. The authors examined the relation between the formation of voids and the fatigue strength of solder joints based on the results. INTERMETALLIC COMPOUND AT THE INTERFACE SODLER OF SOLDER JOINTS Fig.1 Structure of solder joint Figure 1 shows an enlarged view of the microstructure around the interface of a solder bulk material and the Cu-pad. The intermetallic compound is formed at the interface if the Cu-pad is not plated by Ni, and the components of the compound are Cu-Sn (Cu6-Sn5 or Cu3-Sn) for the case of eutectic solder and Sn-Ag-Cu (Bi). The fatigue fracture mode of the solder joints in which the intermetallic compound is remarkably formed depends upon the solder materials, processing conditions, testing conditions, and so forth. The fracture modes can be roughly classified into the following three types: (1) The first case is a fatigue fracture in the solder bulk layer, it is called solder fatigue failure mode in this study. It is caused by the low cycle fatigue due to the repeated nonlinear strain range. For this case, the fatigue life assessment can be carried out by using Manson-Coffin’s law as a basis [3]. (2) The second is a fatigue failure mode in the intermetallic compound layer or at the interface of the intermetallic compound and the solder material. It is called interface fatigue mode. This failure mode is caused by the decrease of ductility due to the growth of the intermetallic compound layer. (3) The third is a peeling failure mode of the Cu-pad from the printed circuit board. This is caused by the stress concentration. The criterion for the strength assessment is now under investigation. Furthermore, for the mode (2), the factors affecting to these fracture mode can be estimated as follows: (1) The reasons for failure in the intermetallic compound layer are the followings ּ Growth of the intermetallic compound under the reflow process; ּ Growth of the intermetallic compound due to the dwell time at high temperature during the thermal fatigue test; ּ Repeated stress concentration at the intermetallic compound layer at low temperature during the thermal fatigue test. (2) The reasons for failure at the interface of intermetallic compound and the solder layer are the followings ּ The growth of the intermetallic compound and the repeated stress concentration; ּ The coherency of the grain boundaries between the intermetallic compound and the solder bulk. It was found by the experimental results that it scarcely happens that the different fracture modes arise simultaneously. In almost all cases, only one mode of fracture is dominant. Therefore, it is important to investigate the conditions under which each fracture mode dominantly occurs. It has been found that the failure modes can be controlled and then a good guideline can be acquired to develop a new solder material and soldering process. In this paper the authors have paid attention to the difference of fracture mechanism of the solder joints, and they investigated the initiation criteria of each fracture mode and the effect of the mechanical properties of solder material on these failure modes. MECHANICAL FATIGUE TESTING EQUIPMENT Until now, the isothermal mechanical fatigue test was proposed to evaluate the fatigue reliability of the Sn-Pb solder joints instead of the thermal cyclic test by the author’s group and the other groups [7],[8], [10], [11],[16]. It has been confirmed that the fatigue life of solder joints under power cycling can be predicted by the same Manson-Coffin’s curve given by the isothermal mechanical fatigue tests. Furthermore, the shift of the failure modes can be checked by comparing the experimental results of the fatigue life with the value predicted from the fatigue life curve for the solder failure mode. If the fatigue life of a solder joint is much shorter than the predicted value, that means the failure mode has changed from the good solder failure mode to the interface failure mode or the other bad mode. The consistency of these two kinds of test methods has sufficiently been taken for the Sn-Pb solder joints case, where the failure mode was almost always the solder fatigue mode. However, the growth of the intermetallic compound in high-temperature dwell time must be considered, because the interface failure mode may become one of the major modes, Pad Layer Plating Layer Intermetallic Compound Solder when a lead free solder material is used. In this study in order to investigate the interface fatigue behavior using the isothermal fatigue test method, a specimen with lead-free solder was heat-treated before the test to accelerate the growth of intermetallic compound. And then the cyclic mechanical deformation was directly given to the specimen to measure its fatigue life. Figure 2 shows the schematic illustrations of isothermal mechanical fatigue test equipment used in this study. The specimen was fixed to the jigs by bonding both upper and lower surfaces. Then cyclic shear displacement was applied repeatedly to the upper jig. A displacement controlled fatigue test was carried out by applying triangular waves with a constant displacement rate. The cross sections of the solder joints were observed through a microscope with high magnifying power (maximum to 3000 magnifications on the display) during the whole fatigue cycles. And the relative displacement on the upper and lower surfaces of the solder ball, ∆δre, was measured directly and automatically by the digital microscope system. Although, there could be some different definitions for the number to failure Nf, in this study, it was defined as the number of cycles when the load measured by a piezo type load cell drops about 10% from the initial level of the reaction load. And in order to investigate the stress and strain behavior in the solder joint under the mechanical cyclic loading, the analytical code ANSYS was used to carry out the elasto-plastic analyses. Fig. 2 Isothermal fatigue-testing equipment RESULTS OF MECHANICAL FATIGUE TEST In this study, shear type mechanical fatigue tests were used to examine the failure modes and fatigue strength of the lead free solder joints. Figure 3 shows the structure of a shearing type solder joint. Because the width is much longer than the height of the solder layer, this kind of solder joint could prevent the stress and strain concentrations at the edge of the solder layer. The lead-free solder materials used in this study are Sn-Cu-Ni, Sn-2.9Ag-0.5Cu-3Bi, and Sn-8Zn-3Bi. These specimens were heat-treated before the test by holding at 150oC for 200 hours to accelerate the growth of intermetallic compound as in the thermal cycle test. And the specimen of Sn-8Zn-3Bi was heat-treated by holding at 110oC for 300 hours. The fatigue life of each solder joint was Fig.3 Structure of shearing type specimen Fig.4 S-N curves of Sn-Cu-Ni solder joints Fig. 5 S-N curves of Sn-2.9Ag-0.5Cu-3Bi solder joints Fig. 6 S-N curves of Sn-8Zn-3Bi solder joints Computer Spring Displacement Theoretical position resolution:0.05