Surface Mount International Conference Proceedings


Author: Joseph Poole
Company: IBM Corporation
Date Published: 4/28/1997   Conference: Surface Mount International

Abstract: One of the advantages of a no-clean surface mount technology (SMT) assembly proms is the elimination of the defluxing (cleaning)process, which uses significant amounts of water and electricity. Unfortunately, no-clean solder pastes, coupled with tie absence of a cleaning step, can leave solder balls beside discrete components, such as chip resistors and capacitors. Also, wave soldering using no-clean fluxes often leaves solder balls attached to the bottom-side of assemblies. Current industry acceptance criteria permits the presence of solder balls, provided they are firmly attached. Unfortunately, there is no explicit definition of what constitutes firmly attached. Additionally, many customers are concerned that solder balls may become dislodged during the service life of a product potentially moving to other areas of the printed circuit board (PCB) assembly. Dependent on the solder ball size and the spacing of electrically active elements on the assembly, the potential exists for intermittent electrical shorts caused by mobile solder balls. To address this concern, several evaluations were conducted to quantify the potential for movement of solder balls beside discrete components resulting from the reflow of no-clean solder paste. Numerous PCB assembly operations as well as vibration stress testing were examined in an attempt to understand this solderball movement phenomenon. Finally, to determine if the residue was significant to solder ball retention solder balIs resulting from a reduced-residue wave solder flux were subjected to simulated packaging and shipment testing. Keywords: solder ball /no-clean / reliability

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