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64 SMT Magazine • August 2015 high Tg laminate, the failure would occur more easily. Close examination of the broken track, the quality of the original plated connection points and analysis of the operating environ- ment would be required. Solder Squeeze/Compression Solder paste squeeze-out is a process issue which largely relates to the paste volume and placement force. Each of the X-ray images show paste that has been displaced then re- flowed leaving what UK engineers call "solder sausages." If solder paste is displaced due to incorrect process parameters or poor control it will not flow back across the solder mask. This is due to the downward pressure of the pack- age as other terminations start to reflow and wet to the pad surfaces. X-ray of components that have been placed in production prior to reflow is a good way of seeing any variation on placement force without disturbing any packages. PCB Outgassing and Blowholes New lead-free processing and materials lead to an old problem, same causes. Outgassing from a plated through-hole board is caused by moisture in the printed board expanding dur- ing soldering. The gas comes out of the hole as water vapour while the solder is in a liquid state. Voids are mostly seen on the base of the board because the solder has solidified first on the topside, hence the expanding vapour has only one way to escape. The size of the voids, blowholes or pinholes is related to the amount of escaping gas and the point at which the sol- der starts to solidify. Testing boards is easy with the oil outgassing test: old problem, old test and same old solution. Conducting the test shows where and how the gas escapes from holes and can show if the position is random or on se- lected areas of the barrel. Baking boards can eliminate the moisture from the board but it does wonders to the solderability of new surface finishes, and it does not necessarily get to the root cause of the problem. The most common reason is the thickness of copper in the plated through-hole which may not be able to evenly cover poor drilling. This can also be impacted by the great- er dissolution rates in lead-free assembly. The lead-free alloy also has an impact on what the joint looks like depending on whether it's non eutectic or not. SMT figure 12: outgassing from a plated through-hole board. Bob Willis is the owner of He will be at SMTA international in chicago offering free advice during the exhibition. To view a free webinar on process defects, causes and cures click here. FeAture CASE STuDY: DETECTING PROCESS DEFECTS continues

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