Feasibility of Pb Flakes Reduction Post Reflow Cleaning Process

Formation of Pb (Lead) flakes after die attach reflow process is a common concern when soldering packages with high temperature and soldering material with high in Lead components. Pb flakes are visually manifested on the chip bonding pad after the post Reflow cleaning process, general term as “flux cleaning process”. Small Pb solder spheres can leave behind stains on bonding pad and could reduce wire-bonding yields, with the failure mode of non-stick-on-pad. This Pb flakes formation was investigated and the findings from this phenomenon will be discussed in this paper since very few studies found in current literature. Hypothesis for Pb flakes formation could be attributed during solder Reflow process. Rapid temperature excursions caused the flux component within the solder paste to rapidly expand. Some of the ingredients within the flux component will reach their boiling temperature, at which small solder spheres will experience “popcorn effect” and re-deposited themselves to the bond pad. Hence, this paper also described the feasibility studies of different flux cleaning methods on how to effectively remove this Pb flakes. Process/Equipment and Materials were considered on these studies to validate the applicable solutions for Pb flakes removal on bonding pad. Considerations are the following for the flux cleaning evaluations; Equipment: Spray-in-Air (Water-based chemical), Centrifugal type (both Water-based & Solvent-based chemicals) and Ultrasonic (both Water-based & Solvent-based chemicals). Due to implications or risks identified from these feasibility considerations, the current Ultrasonic process + new cleaning Chemistry with better results were selected to precede further evaluation, including Reliability assessment.