Effect of Multiple Reflow Cycles on Solder Joint Formation and Reliability

In a typical electronic assembly, many of the solder joints undergo multiple reflow cycles during the course of a complete manufacturing process from wafer bumping to the board level assembly. Effect of these reflows on the solder joint formation and its evolution during these cycles was investigated. In the first part of the study, small size BGA’s were assembled with 12mil spheres of a number of lowsilver lead-free solder alloys. Substrate surface finish used was Cu-OSP. Components were subjected to up to six Pbfree reflow cycles. After reflows, joints were cross sectioned and the thickness of the IMC layers formed was measured. Also, SEM images of the cross sections were recorded to see any change in the interfacial IMC morphology as well any change in the bulk alloy microstructure. The mechanical integrity of the solder joints was evaluated using high-speed ball pull test. In the second part of study solder joints were formed using Pb-free solder pastes and reflowed two times under three different reflow profiles. The main measure of solder joints used was the voids formed in the joints. Effect of the reflows on the short term and long term solder reliability is discussed. INTRODUCTION Reflow soldering is the most common technique used in the formation of solder interconnects in electronics at the component and board level assemblies. The increasing complexity of the components such as stacked Package-onPackage (POP) or System-in-Package (SiP) and circuit board assembly (e.g., stacked, double sided boards, etc.) makes it impossible to form all the solder joints in a single reflow. More often than not, there will be some interconnects that will be forced to go through more than one reflow cycle. Following the first reflow, sometimes the subsequent reflow profiles are the same as the first while in other circumstances a hierarchy of reflows with decreasing temperatures is necessary. Solder joints formed in the first reflow further evolve during the subsequent reflows. Therefore, it is important to understand the impact of multiple reflows on solder joint reliability. Similar to the complexity of the electronic components, PWB assembly has become more complicated. It is common these days to have double sided boards. That means the solder joints on the first assembled side undergo one additional reflow cycle. Therefore, even for SMT assembly, it is important to understand the effect of multiple reflows on the solder joint formation, growth and reliability. Solder joint formation is basically a reaction between the solder and the pad material. In the process a fraction of the pad material is dissolved into the solder and forms an intermetallic layer at the interface. A thin uniform layer is an essential requirement for the solder joints. However, a thicker IMC layer is undesirable because of the brittle nature of the IMC’s. A thick IMC layer is associated with early failure in mechanical shock tests. Growth of intermetallic layers during aging at high temperature was studied by Zribi et. al [1]. In this study, Sn4.7Ag1.7Cu and Sn3.5Ag1.0Cu alloys were used on Cu and NiAu substrates. It appears that the presence of Cu and Ni together alters the nature of interfacial IMCs. Zhang et al [2] reported the effect of different substrate metallization under multiple reflows soldered with Sn3.5Ag1.0Cu. Effect of different substrate metallization used in soldering to Sn3.5Ag and subjected to multiple reflow was also reported by Koo and Jung[3]. It was demonstrated that the solder bumps with Electroless Nickel Immersion Gold (ENIG) finish showed a dramatic decrease in the shear force after 4-5 reflows. Change in shear force was negligible for Cu and Electrolytic Nickel Gold (NiAu) pad metallizations. However, a recent study by Zhang et al [4] on ENIG finish showed that solder joints formed with Sn3.0Ag0.5Cu8.0In, Sn3.0Ag0.5Cu and SnPb alloys do not show any decrease in the shear force up to 12 reflows. Rather the results presented in this paper show a small increase in the shear force during the initial first few reflows. The effect of multiple reflows on Sn4.0Ag0.5Cu spheres soldered to immersion Sn coated pads was reported by Lai et al [5]. A change in the morphology of the interfacial IMC layer during multiple reflows has been reported. This report also shows results of the “Ball Impact Test”, which appears to be just another name for high-speed ball shear test. The results show a degradation of solder joints undergoing multiple reflows. In SMT assembly, usually a solder paste is used to attach the BGA, QFP, LGA etc. type of components to the PWB using a solder paste. Solder paste consists of a solder alloy in the powder form mixed with a flux that has a solvent, a thickener, an activator package and a tackifier as some of