Test & measurement A tale of automated PCB inspection
Instrumentation Monthly is not normally where you would read a short story but this one is worth telling. In this article, Dieter Swindley of Amfax explains how his company has made the results of PCB inspection unambiguous
innovative minds think differently about solving a problem. It sounds corny but it is true. In this instance the problem that needed solving is automated PCB inspection. The existing tech uses an optical inspection
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approach which has been automated (AOI). Put simply, AOI compares images and gives a result as pass, fail or not sure (aka false call). These ‘spot the difference’ machines rely
on an operator to give their opinion as to whether a PCB passes or fails due to false calls. These exist because visual inspection is an opinion and open to interpretation which is hard to automate. Unfortunately, this interpretative
approach to inspection leads to comments like “AOI is not very good but it’s all that is available” and “AOI is so unreliable that we have contemplated not using it anymore – we need inspection to add value”. Amfax realised that it is important to
remove any ambiguity and interpretation. For inspection to be trusted it is important to know that it is completed in the same way, to the same standard each time. Amfax decided that inspection of PCBs must
involve measurement. It must be against a standard which contains zero ambiguity. The current standard in use is a visual based methodology and is open to ‘interpretation’. It is driven by opinions rather than facts. Worse than that it actually contradicts itself, this makes it hard to apply and also believe in. When dealing with mechanical parts and
assemblies the original design drawings form the standard to which a piece is manufactured. It is checked prior to being released to ensure
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that it meets all the expectations, regulations and ‘general good practice’. Inspection is completed by taking measurements and ensuring that they fall inside the specified tolerances as found on the drawings. It is simple – they are in tolerance or not. Pass or fail. No opinions, no ambiguity. If a PCB is treated as a mechanical piece
part then a more comprehensive and reliable inspection methodology can be
If a PCB is treated
as a mechanical piece part then a more comprehensive and reliable inspection methodology can be used to assess its build quality.
used to assess its build quality Today’s PCB designs are done using CAD.
Designs can be outputted in standardised formats such as ODB++, Gerber, and ‘pick and place’ files. When created correctly they contain all the necessary data required to build from and therefore inspect from. So we have a method of creating a
standard i.e. the CAD, so now we need a method to measure with. Actually there is a
nce upon a time, in a small Dorset town named Blandford, some very talented engineers lived. These
’’ March 2019 Instrumentation Monthly
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