Heads Up To DFT - Can Be Costly If Not Monitored - Nexlogic
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How often do you get the bad news that your PCBs yielded a low 30 percent or so test coverage when you were expecting 70 to 80 percent test probe accessibility? It often comes as a shock since you were dead certain your PCB design was indeed manufacturable. At least, that’s what your system designers led you to believe.

Long story short, the PCB designer didn’t exactly communicate effectively with manufacturing and test. In instances like this, the design engineer is more interested in functionality testing than actual manufacture and test. The bottom line is going back to square one to figure out what went wrong, and at the same time, incurring those extra costs you didn’t factor in.

dft-flying-probe-testerWhen manufacturing personnel test the assembly of that particular board, then it becomes difficult to near impossible. The reason is all the critical access or test points that are supposed to be on that board are not there. DFT’s main requirement is to give the probes of a flying probe tester the necessary contact points they can make contact to on the circuitry for measurement purposes, as shown in Fig. 1.

By not giving assembly and manufacturing those critical access points, the result is low test coverage and in worst cases, it can miss all critical tests.

Check out our article on DFT and some of the critical issues you can encounter when it comes to PCB manufacturing. Meanwhile, here are some tips and hints that’ll give you a heads up to help avoid this costly problem.

  • First and foremost work with a one-stop PCB design and assembly EMS Provider so that design and manufacturing engineers effectively collaborate on your project to get to that 70 to 80 percent testing level.
  • Find out for sure your PCB designer is taking advantage of all the advanced DFT features built into high-end CAD tools.
  • How much experience does your PCB designer have with high-speed designs? Find out because placing a test point on a high-speed signal, for example, can deteriorate its signal integrity. Impedance is mismatched at that point.
  • Can your PCB designer make intelligent decisions when designing in test points while keeping signal integrity intact?
  • Know if your high-speed PCB designs have passive components installed properly so they can be correctly tested.
  • Do some checking on how innovative your PCB designer is when it comes to making critical design adjustments to open up areas for test probes.

Without question, the primary consideration is for the designer to provide the test engineer all the necessary access points so he or she can properly write the test associated with a particular design. Probe points are the key areas that need to be considered in a design. Special attention has to be given to those areas where it’s not possible to have test points for every net. However assurance should be made that the critical traces that must be tested have probe points.