Fluxless no-clean assembly of solder bumped flip chips

Abstract

MCNC has developed a radically new fluxless, no-clean process which has shown considerable success with assembly of a variety of flip chip configurations. The process, called PADS (Plasma Assisted Fluxless Soldering) relies on a pretreatment which enables the subsequent solder reflow in inert ambients. Conventional mass production soldering tools can be used, just eliminating the flux dispense and flux cleaning steps, and adding the pretreatment step. Highlights of the applications studies are presented. Examples include high lead (97Pb3Sn) bumped flip chips joined to multilayer ceramic substrates with Mo/Mi/Au microsockets at 350/spl deg/C in nitrogen, eutectic tin/lead solder bumped flip chips joined at 250/spl deg/C to bare copper, 95/5 lead/tin bumped flip chips joined to eutectic dipped FR4 printed circuit boards, joining of 90/10 lead tin bumps to each other at 350/spl deg/C, unique MEMS (Micro Electrical Mechanical Systems) devices joined with the dry fluxless process, and solder bumped flip chips joined to flexible circuits. Other related topics to be discussed include tacking, self alignment and its measurement, balling reflow, and rework operations including hot chip pull and site dress. Other areas of application of the fluxless process are highlighted including hermetic seal band attachment, joining of flexible circuit TAB leads to insulator substrates, and joining to solid solder deposits on FR4.