Effect of Process Thermal History on the Microstructure of Copper Pillar Sn-Ag Solder Joints
Authors: Mohammed Genanu, Jim Wilcox, Eric Cotts, Jae Joon Choi, Ki Seok Kim Company: Physics and Materials Science at Binghamton University, Universal Instruments Corporation, CrucialTec Date Published: 9/17/2017
Abstract: Two extremes of reflow time scale for copper pillar flip chip solder joints were explored in this study. Sn-2.5Ag solder capped pillars were joined to laminate substrates using either conventional forced convection reflow or the controlled impingement of a defocused infrared laser. The laser reflow joining process was accomplished with an order of magnitude reduction in time above liquidus and a similar increase in solidification cooling rate. The brief reflow time and rapid cooling of a laser impingement reflow necessarily affects all time and temperature dependent phenomena characteristic of reflowed molten solder. These include second phase precipitate dissolution, base metal (copper) dissolution, and the extent of surface wetting. This study examines the reflow dependent microstructural aspects of flip chip Sn-Ag joints on samples of two different size scales, the first with copper pillars of 70µm diameter on 120µm pitch and the second with 23µm diameter pillars on a 40µm pitch. The length scale of Pb-free solder joints is known to affect the Sn grain solidification structure; Sn grain morphology will be noted across both reflow time and joint length scales. Sn grain morphology was further found to be dependent on the extent of surface wetting when such wetting circumvented the copper diffusion barrier layer. Microstructural analysis also will include a comparison of intermetallic structures formed; including the size and number density of second phase Ag3Sn precipitates in the joint and the morphology and thickness of the interfacial intermetallics formed on the pillar and substrate surfaces.
Cu pillars, Laser Reflow, Mass Reflow, LeadFree Solders, Microstructure, Cooling Rate.