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12 The PCB Magazine • July 2014 ELECTRONIC ASSEMBLY WITH SOLDER continues Clearly there is a great deal of nuance in the detection and identification of solder related defects; numerous books have been written over the last few decades that both characterize and suggest methods for eliminating or mitigat- ing them (the devil is also "in the details" as another aphorism attests). It is not within the scope of this brief commentary to provide de- tail on all of the various types of solder relat- ed defects which can extend from the macro to the micro but for the benefit of the reader the following figure is offered providing represen- tative examples of a number of the defects described above. The "Devil's" Impact on the PCB The importance of managing the soldering process is clear, but making a good solder joint is also just part of the story and there are a number of defects that can be generated within a printed circuit assembly because of the soldering process, including: a) Corner Cracking: A crack that forms at the interface between the whole and the land that sur- rounds. It is normally the result of the Z-axis expansion of the PCB during the thermal excursions such as soldering. b) Barrel Cracking: Another phenomenon associated with the soldering process; it is similar in some ways to a corner crack except that it is manifest near the center of the hole. c) Post Separation: A separa- tion of the plating in the through hole from an innerlayer connection d) Hole-Wall Pull Away: Hole-wall pull away is manifest as a bulge in a plated through hole, which reduces its diameter. e) Resin Recession: Roughly, the opposite of hole-wall pull away wherein a small gap is formed be- tween the plated hole wall and a resin rich area of a plated through-hole f) Delamination: A separation of the lay- ers of a multilayer circuit. It is normally seen in cases where the glass transition temperature of the resins used in the multilayer structure is exceeded. g) Pad Cratering: Another phenomenon unseen before the introduction of lead-free sol- Figure 1: representative images of some of the solder related defect found on printed circuit assemblies. top row – left to right: head in pillow, graping, misregistration short and over- heated flux. Second row, left to right: tombstoning, insufficient solder with open, popcorning. third row, left to right: dewet- ting, blowhole, shorting between adjacent leads. Fourth row, left to right: cracked solder joints, voids, excessive solder. Bot- tom row, left to right: cracked component, tin whisker, open (lifted lead).