Reflow processes surface defects

Assembly requirements also place constraints on the circuit board layout. A very high part density requires a large number of apertures in the solder paste stencil, which can cause the stencil to become locally too flimsy to control the solder paste deposit. A surface-mount circuit board with a very wide range of component sizes and package configurations may require multi-thickness stencils to properly control the paste deposit. Solder paste printing quality is a determining factor in solder-joint defects observed after the reflow process. 

An adjunct to the surface-mount process, this method, sometimes referred to as intrusive reflow, allows the soldering of some through-hole (solder-tail) parts into plated-through holes on the circuit board during SMT oven reflow. This process can eliminate or reduce the need for wave soldering—a step prone to defects. 

Mass-production wave soldering began on one production site in mid-1998 with Sn-5Bi-2Sb-lAg solder on single-sided printed boards with a wide variety of surface-mounted and through-hole-mounted components. The wave soldering process is very stable and has a first-pass defect rate of only 30 ppm. Industrial reflow soldering began on another site in early 1999, with no details available about process stability or yield. 

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