7 - SMT Problems At the Post-reflow Stage

Abstract

This chapter discusses the major problems related to solder paste applications in the surface mount technology (SMT) post-reflow stage, with a primary emphasis on the impact of flux residue on reliability as well as on subsequent manufacturing operations. A brief explanation of the white and charred residues is also given in the chapter. White residues are flux residues remaining on the boards after post-soldering cleaning. Here, the cleaners used may be aqueous or organic solvent systems. The composition of the white residues is fairly complicated. It may be flux itself; charred flux ingredients; or reaction products of flux with metals, cleaners, board laminates, or solder masks. One of the reasons for the appearance of the white color for the insoluble flux residue is the “light scattering” effect. Before cleaning, the flux residues generally appear as clear or translucent solids. During cleaning, the cleaner may extract and remove only some soluble ingredients of the residue and may leave behind the insoluble part as a foamy, loose texture. Charred residues are caused by overheating and may or may not be cleanable. As charting involves excessive heating and oxidation, the thinner the flux film, the worse the charring will be.