Improving Yields Through Design For Testability (DFT)
In our industry, design for test (DFT) and design-for-manufacturing (DFM) aren’t always copacetic, as you may have experienced with your previous PCB projects. Put another way, PCB test engineering isn’t always aligned with PCB manufacturing. Someone or some ones drop the ball along the line.
When manufacturing personnel test the assembly of a particular board, then it becomes difficult to near impossible to properly test it. 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 so they can make contact to on the circuitry for measurement purposes. Absence of those critical test points translates into low-test coverage. For instance, a PCB achieves 30 percent test coverage when it should be 70 to 80 percent accessible with testing probes.
In some highly complex, high-speed PCB designs, finding and squeezing out minuscule areas for test probes to hit the circuitry can be challenging. One approach to designing in those special test points is to change a circular ring around a via to a square shape, as shown in Fig. 1. There is less area in the circular pad compared to a square pad of the same size. But with the square pad, the test engineer can gain additional copper areas at the corners, allowing proper probing sites.
There are a number of key considerations that must be taken into account for high-test coverage and for it to comply with DFM requirements. Check out our EE Times article on this topic to get more details.
In the meantime, here are some tips and guidelines to get you more in tune with DFT and DFM:
- High-speed PCB designs must have properly installed passive components for correct testing. Any minor change in inductance, capacitance, or resistance, meaning small impedance change triggers a big signal integrity issue.
- All nets should have test points accessible from one PCB side, ideally the less populated side.
- Effective testing at PCB manufacturing assures the functional test people there are minimal to no defects in the manufactured board.
- In-circuit test (ICT) or flying probe test is applied to give assurance that all components are present at assigned locations with desired values.
- Flying probe or ICT can also detect any one or several significant PCB defects or failures.
- It’s important for the PCB designer to take advantage of all the advanced DFT features built into high-end CAD tools.
It’s also important to note that at times placing a test point on a high-speed signal can deteriorate its signal integrity because it’s a different feature, and impedance is mismatched at that point. In cases like this, the designer is prone to lean more toward maintaining signal integrity, rather than meeting manufacturing and testing requirements for that design.
There can be any number of different tradeoffs like this one depending on a PCB design such as cross talk versus cost of manufacturing. Therefore, it is important for effective DFT to be based on a close partnership and working relationship between PCB design and test engineering.