SMT007 Magazine

SMT-Aug2015

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58 SMT Magazine • August 2015 website. Each month, we focus on a different defect type and provide photographic examples or videos showing the defects happening in real time. You can visit the IPC website or my You- Tube channel to see many of the 40 clips pro- duced showing solder balling, dendrite forma- tion, popcorning, PCB delamination and head- in-pillow failures. There are also some technical book review clips included that may be a useful reference source. Selected Process Defects from the Industry Now let's look at some of the most common process issues faced in the industry and some typical examples. However, it's worth looking at what actual industry experiences. The fol- lowing three graphs show the results of short online surveys from engineers attending online webinars regarding the most common prob- lems or process issues with PCBs and assembly (Figure 3). PCB Delamination Delamination is a blister, or air gap, formed when water vapour forms and ex- pands during heating, not like a blister on your skin which is filled with fluid. Delamina- tion may occur when moisture in the board expands during soldering or rework. With the higher temperatures of lead-free there is more energy placed on the materials. When and if moisture accumulates in specific areas of the board more energy is focused in those partic- ular areas. Figure 4b shows a lack of proper adhesion of the epoxy to the surface of the inner layer which may be related to the cop- per surface preparation, the prepreg or the press cycle. Cutting the surface of the blister can often tell you more than a microsection and you don't need a lab. A full paper on the work we have done at NPL on PCB and moisture content is available from the Defect Database. figure 2: a sample screenshot from the defects database search screen (left), search result (center), and close up of the defect (right). figure 3. What engineers consider the most common problems or process issues with pcbs and assembly. FeAture CASE STuDY: DETECTING PROCESS DEFECTS continues

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