Component drop-off caused by gold embrittlement

Abstract

The article also investigates the min. solder volume request for solving Gold embrittlement issue in the limited condition. Gold (Au) embrittlement was known and had been studied for a long time. It will reduce the solder joint toughness not just only in reliability test but in manufacturing process. This kind of failure even occurs just after SMT process, it means we could not apply glue to protect the component from drop-off. The concentration of Gold is the major concern of this failure mode. How to dilute the concentration of Gold from substrate design, component manufacturing, soldering material selection and substrate manufacturing is final goal of forming a robust solder joint. The component drop-off was found in the latter process of SMT. The investigation started from failure mode definition. CSA (Cross Section Analysis) and EDS (Energy Dispersive Spectroscopy) were performed to observe the solder joint structure and alloy composition. Abnormal high concentration of Gold element reveals the Gold embrittlement is the major root cause of solder joint crack. Reducing Gold concentration started from the source of Gold surveying. Soldering related material investigation result shows no abnormality in substrate pad and solder paste but high risk in the soldering terminals of the component. The thicker Gold plating of the component is the major source of high Gold concentration in the solder joint. In general substrate manufacturing process such as desktop motherboard, it might not occur due to the sufficient solder volume provided by enough space and thicker stencil could cover the issue. In this case, the Gold plating thickness is unchangeable, so we only focus on substrate surface treatment and fine tune SMT process parameters to dilute the Gold concentration. The improvement DoEs were conducted for different substrate surface treatment, solder volume and reflow profile with different component batch. For substrate surface treatment, we changed it from ENIG (Electroless Nickel/Immersion Gold) to ENEPIG (Electroless Nickel/Electroless Palladium/Immersion Gold) for better solder joint strength. To increase solder volume, we enlarge the stencil aperture. For reflow profile fine tune, higher and lower peak with longer soaking time were verified for Gold diffusion. CSA and shear test were performed to confirm the solder joint and compare the joint strength. From the DoE results known, the positive factors solder joint strength enhancements are surface treatment and solder volume. ENEPIG solder joint interface is stronger than ENIG solder joint, but it can't dilute the Gold concentration directly. Solder volume increasing is most obvious factor to dilute the Gold concentration. Therefore, in SiP (System in Package) module production, tiny solder joint and high component density restrict the stencil design. The article also investigates the min. solder volume request for solving Gold embrittlement issue in the limited condition.