Microstructure Of Lead-free Solder Joints Research Articles

Fatigue performance and microstructure of lead-free solder joints in BGA assembly at room temperature

One of the crucial factors in evaluating the reliability of an electronic appliance is fatigue failure of the interconnecting solder joints. In most situations, large bulk samples are used to investigate the fatigue characteristics of the solder materials. Only a few studies have examined the solder joints frequently observed in ball grid array (BGA) components. This study implemented a specialized test vehicle in BGA assembly connecting two substrates with nine identical solder joints. Four SAC-based solder alloys were tested using an Instron micromechanical tester in the stress-controlled and strain-controlled methods at room temperature and constant strain rate. At two distinctive stress and strain levels, the fatigue performance of alloys featuring organic solderability preservative (OSP) and electroless nickel-immersion gold (ENIG) surface finishes was investigated. The fatigue results obtained from the stress-controlled and strain-controlled tests were compared. Microstructural analysis was performed using scanning Electron Microscopy (SEM) and energy dispersive spectroscopy (EDS) to identify the precipitates in the bulk solder and the morphologies of IMC layers. The thickness of intermetallic compound (IMC) layers and the failure mode were examined in each case. The results demonstrated that the OSP surface finish significantly outperformed the ENIG surface finish regardless of the testing process and solder alloy. Various failure modes were observed, depending on the solder alloy and IMC layer.

Impact of crystalline orientation of lead-free solder joints on thermomechanical response and reliability of ball grid array components

The microstructure of lead-free solder joints often consists of only one or a few randomly oriented tin grains as a result of cooling conditions and reactions that take place during the solidification. Due to severe anisotropy of the tin phase and this complex microstructure, the stress state of each joint will be unique, and lead to a dispersion in times to fail. This study aims at evaluating the impact of orientations of tin grains on the stress state of a BGA component. Different combinations of solder joint grain orientations are studied through thermo-mechanical simulations in order to assess the stress state of each joint. A significant effect of neighbouring joints has been observed for the different cases.

Influence of the fluxes properties on quality and the microstructure of lead-free solder joints executed by selective soldering

Purpose – The purpose of this paper is to investigate the influence of the properties of new compositions of fluxes for selective soldering on lead-free solder joints quality and microstructures as well as showing which flux properties are the most important. Design/methodology/approach – The three different types of fluxes were tested, which differed in composition, solids content, amount and type of activators added. The selective soldering process was done with the use of lead-free solder SAC 305. The test boards had two coatings SnCu (HASL) or Au/Ni. Basic chemical and physical properties of fluxes were examined according to the relevant standards. Different types of components from the bulky ones, demanding more heat, to the smaller ones were used during the assembly process. AOI and X-ray analyses as well as cross-sections and SEM analyses were utilized to deeply assess the quality and microstructure of the investigated solder joints. Findings – The results showed that information about density or static activity of flux is not enough for correct flux assessment. The dynamic activity of flux measured by wetting balance method is the best for this, especially in the case when there is short soldering time and heat transfer is hindered. The quality and the microstructure of lead-free solder joints are related not only with wetting properties of the flux used for soldering but also with other properties like solids content in a flux. Research limitations/implications – It is suggested that further studies are necessary for the confirmation of the practical application, especially of the reliability properties of the joints obtained with the use of the elaborated fluxes. Originality/value – The results showed that type of flux (ORL or ROL) as well as minor changes in their dynamic activity and solids content might have significant influence on quality of solder joints and their microstructure. It was noted that selective soldering is demanding technique and optimization of soldering process for different type of components and fluxes is important.

Microstructure evolution observation for SAC solder joint: Comparison between thermal cycling and thermal storage

This paper describes a reliable and reproducible method to reveal the microstructure of lead-free solder joints. This method is then used to follow the aging of lead free assembled chip resistors and BGA, with either ENIG or immersion Sn finishes. Two types of aging were applied: accelerated thermal cycles (ATC), and iso-thermal storage at three different temperatures. The associated failure mechanisms are compared and discussed. During ATC, microstructural changes under thermo-mechanical fatigue are evidenced, whereas thermal storage only induces intermetallic evolution, depending on the finish type.

Microstructural development of Sn-Ag-Cu solder joints

The effects of Cu as pad material and of the metallization of pad (with Sn) and component (with Ni) on the evolving microstructure of lead-free solder joints were studied. A solder paste with composition 95.5wt.%Sn-4.0wt.%Ag-0.5wt.%Cu was used. Partial dissolution of the Cu substrate led to a change in the overall composition of the solder, which caused a precipitation morphology different from the one expected regarding the initial composition. Kinetics of growth of the Cu6Sn5 phase, as particles in the bulk of the solder and as a reaction layer adjacent to the Cu pad, was studied in the temperature range 125–175°C.