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PCB Layout and Test Must Be Under One Roof

PCB layout and test go together, especially today when OEMs are packing their sub-systems with virtually every electronic technology and component packaging currently available to satisfy market demands. Putting those two critical stages of a blossoming new product as close together as possible makes all the sense in the world.

Fig 1 - Functional TestBut in a lot of cases, while keeping PCB layout and test together makes good technical sense, the business side of dollars and cents butts in, and some OEMs decide to separate the two by sending layout to one vendor and test to another – just to save some extra dollars.

All kinds of things go wrong, and there’s a long list. Let’s name a few. An OEM’s timeline is usually extended simply because project control is lost. The design is late to the fab house, further extending the timeline. Deliverables are not complete and/or critical data is missing such as drill chart callouts. Bad blood between layout vendor and fab vendor follows further jeopardizing the OEM project.

By taking this route, the bottom line is the OEM fails to gain the benefits of a highly coordinated resource expertise.

Check out our article on this subject to get a more comprehensive understanding. Meanwhile, here are some tips you can take to the bank.

  • Assure yourself PCB layout and test are under one roof.
  • Find out how much interaction there is between layout and test engineers. It benefits you greatly when test engineering passes on valuable info to in-house layout engineering.
  • Check out the PCB layout engineer on your project to see what he/she knows about flying probe and ICT test guidelines and limitations. Find out what he/she knows about functional test, Fig. 1. (See image above)
  • Often test coverage is increased after PCB layout engineering confers with test engineering to get new insights, new data.
  • Find out how often your PCB layout engineer steps out of his or her role to investigate and learn more about test-related practices.
  • For RF designs, ground shields between signal lines improve trace-to-trace impedance control. If not properly done, test results return the layout back to design.

These are just a few examples of what the seasoned PCB layout engineer must know to successfully prepare a layout for it to achieve high-test coverage. Further, it’s important he or she fully understand the significance of such test procedures and characteristics as smoke test, gain and frequency response test, analog signature analysis (ASA), propagation velocity test, probing techniques, launch edge versus reflection edge, filtered TDR option, and others.