IMPACT OF LEAD-FREE PCB SURFACE FINISHES ON MICROVOIDING

摘要

Microvoids, also referred to as “champagne voids or interfacial voids”, are tiny voids which are less than 50 μm (2 mils) in diameter and located on the printed circuit board (PCB) side of the solder joint, just above the intermetallic compound (IMC) layer. Since microvoids are continuous and occur at the PCB/solder joint interface, they can lead to cracks and possibly affect the fatigue life of the solder joint. No confirmed root cause for microvoids has been reported thus far. However, the degree of microvoid formation is believed to be affected by PCB surface finish, reflow process, organic surface contamination, solder paste, and the moisture absorption of BGAs and PCBs. A full factorial statistically designed experiment was conducted to determine the main and interaction effects of lead-free surface finishes and peak reflow temperatures on microvoiding. PCBs with Immersion Silver (0.15 micron and 1 micron silver thickness), Electroless-Nickel Immersion Gold (ENIG) and Organic Solder Preservative (OSP) surface finishes were investigated at two different lead-free peak temperatures: 235oC and 250oC. After assembly, electrical resistance test was performed on a daisy chained test vehicle to detect failures on the assembled leadfree packages. The BGAs were then inspected using a high resolution digital X-ray system for microvoids. Microsectioning of the solder joints were later performed to corroborate the X-ray findings. The findings of this work are expected to help understand the root cause for microvoids and to select the optimum lead-free surface finish and peak temperature that minimizes microvoids. This paper discusses, in detail, the important factors which affect microvoids, the experimental setup, the findings of this research, and the recommendations to minimize or eliminate microvoids in a lead-free assembly.