I’ve been in the test engineering industry for over a decade and the concept of test coverage has always been of great interest to me. How do test engineers quantify the amount of test coverage they get on a particular board design? We know from experience that every design is different and, therefore, every design needs a slightly different test strategy. That’s what makes our jobs as test engineers ever-changing and fresh. We’re always learning about new test strategies, technologies and methodologies and applying them to the task at hand. So as boards get bigger, denser, faster and more complex, we apply our creativity to keep test coverage apace with the changes.
Sometimes I wonder, is this science or black magic? I believe it’s both. One scientific innovation I’ve seen recently is the adoption of PCOLA/SOQ/FAM by iNEMI. Yeah, I know that’s a lot of letters strung together, but here’s what it means. iNEMI is the International Electronics Manufacturing Initiative and it is spearheading an industry effort to scientifically characterize test coverage. And that’s what PCOLA/SOQ/FAM is. Here’s what the alphabet soup means:
Component Scoring Guidelines
|P||Presence||Does the test determine the presence of the part?|
|C||Correctness||Does the test determine that it’s the correct part?|
|O||Orientation||Is the part oriented properly or is the polarity proper?|
|L||Live||Is the part electrically functional for basic activity?|
|A||Alignment||Can the test determine lateral displacement or minor rotation?|
Interconnect Scoring Guidelines
|S||Shorts||On an interconnect, can shorts within a shorting radius be detected?|
|O||Opens||If there is an open on the pin/trace will there be a test failure?|
|Q||Quality||Is the quality of the solder, wetting, and general structural integrity of the circuit board sufficient?|
Functional Scoring Guidelines
|F||Feature||Can presence or absence of a feature be detected?|
|A||At-speed||Can the pin/interface/feature be tested at min/mid/max speeds?|
|M||Measurement||Can a measurement be taken that confirms performance to a BER, CRC or other requirement?|
The PCOLA/SOQ methodology was pioneered by Agilent in the 1990s, and now iNEMI has added a functional (FAM) component to it. This is innovative because it moves test engineers beyond the limitations of the “shorts-and-opens” fault spectrum and forces us to consider a broader fault spectrum. It also creates a framework for a broader level of defect discovery by introducing additional test technologies as solutions to fill the gaps in test coverage. I’ll get back to this in a later post.
Just curious about the shorts definition; “On an interconnect, can shorts within a shorting radius be detected?” I don’t know what a “shorting radius” is. It seems to me that a short is like being pregnant, you either have one or you don’t.
It seems to me that the iNEMI PCOLASOQFAM way over emphasizes the contribution of PCB assembly level test to the overall test need. If all a product has over being correctly interconnected is features, at-speed and measurement, wouldn’t it be more correct to say that PCOLASOQ=B for built correctly?
Yes Steve, that would be correct. I think that test engineers need to view their craft holistically, in terms of structural (PCOLA/SOQ), functional (FA) and performance (M). Just because a board has been assembled correctly, doesn’t mean it will work. And just because it works (or seems to work!) does not mean it doesn’t have structural defects. Example: a via stub or solder void on a high-speed bus may contribute to a high bit error rate or a reduced margin, even though the bus continues to work. A good video that demonstrates this is here: http://www.asset-intertech.com/NBT_movie.html
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