Facilitating The Reduction Of Head-In-Pillow Defects And Improving Assembly Reliability & In-Line Productivity Using Nitrogen Reflow
Authors: Maëva S. Tureau, Ph.D., Gregory K. Arslanian, Christine C. Dong, Ph.D. Company: Air Products and Chemicals, Inc. Date Published: 10/13/2013
Abstract: With unprecedented market and technological challenges associated with RoHS compliance and increased assembly functionality and miniaturization, many solder joint issues in ball-grid array (BGA), chip-scale package (CSP), and package-on-package (PoP) have led to assembly defects in the electronic manufacturing industry, such as head-inpillow (HIP) defects. HIP defects are typically occurring due to poor wetting (random HIP defects) and/or warping (edge or center HIP defects) of the component. As a result of using devices with finer pitch and size and in some cases finer powder size in solder pastes, higher oxidation level will occur at the solder ball surface and within the solder paste due to the higher surface to volume ratio of the solder balls and particles, and decreased flux activity. Formulating the appropriate solder paste rheology and flux chemistry along with using proper component and materials storage to reduce oxidation and suitable assembly processes (printing, placement, and reflow) are necessary to reduce and prevent solder joint defects. While current industry trends tend to prioritize solder paste/flux dipping processes and the use of solder paste flux formulations with a higher metal oxide reducing potential, a straightforward reflow technique that uses a nitrogen inert reflow atmosphere during the heating stage can be employed to improve assembly reliability by 1) preventing the oxidation of the metal surfaces and 2) alleviating the need for aggressive flux chemistry (cleaner and more corrosion-resistant joints). In this paper, we will show evidence of the benefit of reflowing 0.8 mm pitch BGA and 0.5 mm pitch CSP components under a reduced oxygen environment (~1400 ppm O2 level) by capturing air vs. nitrogen live recordings of the reflow process. The comparative videos and snapshot pictures show that N2 reflow resulted in faster formation of cleaner solder joints with reduced tendency of forming HIP defects, providing improved assembly reliability and in-line productivity, and facilitating the self-alignment of poorly aligned components upon reflow.