Functional Testing During PCB Processing
Denser PCB, higher bus speeds, and analog RF circuits, to name a few, present unprecedented test challenges. Functional testing in this environment requires careful design, well-thought-out test methods, and the right tools to provide credible results.
While keeping these issues in mind when dealing with fixture suppliers, it is also important to think about where the product will be manufactured, an area that many test engineers overlook. For example, let's assume that the test engineer is in California, USA, and the product is manufactured in Thailand. The test engineer would think that the product would require expensive automated fixtures because of the high cost of the plant in California, the requirement for as few testers as possible, and the need for automated fixtures to minimize the need to hire high-skilled, high-wage operators. But in Thailand, neither of these problems exist, and it is cheaper to let labor solve these problems because labor costs are so low here, and land prices are so cheap, that large plants are not an issue. So sometimes first-class equipment may not always be popular in some countries.
A, Technology Level
In a high density UUT, if calibration or diagnostics are required it is likely that manual probing will be required, due to reasons such as limited access to the needle bed and faster testing (testing the UUT with probes allows data to be captured quickly rather than feeding information back to the edge connector), and so requires that test points on the UUT are probed by an operator. Wherever possible, make sure that the test points are clearly labeled.
Probe types and general operators should also be aware that issues to consider include:
- a, Is the probe larger than the test point? Is there a danger of the probe shorting out several test points and damaging the UUT? Is there an electrical shock hazard to the operator?
- b, Can each operator quickly locate and inspect the test points? Are the test points large enough to be easily recognized?
- c, How long does it take the operator to press the probe against the test point to get an accurate reading? If the time is too long, there will be some trouble in the small test area, for example, the operator's hand will slide due to the long test time, so it is recommended to enlarge the test area to avoid this problem.
After considering the above issues test engineers should re-evaluate the type of test probes, modify the test files to better identify the test point locations, or even change the requirements for operators.
B, Automated Probing
In some cases will require the use of automated probing, for example, in the PCB is difficult to probe manually, or the operator's skill level limitations that make the test speed is greatly reduced, then we should consider using automated methods.
Automated probing can eliminate human error, reduce the possibility of short-circuiting several test points, and make the test operation faster. However, it is important to realize that automated probing may also have some limitations that vary depending on the vendor's design, including:
- a, the size of the UUT;
- b, number of synchronized probes;
- c, how close are the two test points to each other?
- d, the positioning accuracy of the test probes;
- e, Can the system probe both sides of the UUT?
- f, How fast does the probe move to the next test point?
- g, What is the actual interval required by the probe system? (Generally it is greater than off-line functional test systems).
Automated probing usually does not use a needle bed fixture to contact other test points, and generally it is slower than a production line, so two steps may be necessary: if the probes are for diagnostic purposes only, consider a traditional functional test system on the production line and put the probes as a diagnostic system at the side of the production line, or if the probes are intended to be UUT calibrated, the only real solution is to use more than one system , realizing that this is still much faster than manual operation.How to integrate into the production line is also a key question that must be examined; is there room on the line? Can the system be connected to a conveyor belt? The good news is that many of the new detection systems are compatible with the SMEMA standard, so they can work in an in-line environment.
C, Boundary Scanning
This technology should be discussed as early as the product design stage, as it requires specialized components to perform this task. In UUTs with predominantly digital circuits, devices with IEEE 1194 (boundary-scan) support can be purchased so that most diagnostic problems can be solved with little or no probing. Boundary scan reduces the overall functionality of the UUT because it increases the area of each compatible device (4 to 5 additional pins per chip and some wiring), so the principle of choosing this technique is that the cost spent should result in improved diagnostic results. It should be remembered that boundary-scan can be used to program flash memory and PLD devices on the UUT, which further adds to the rationale for selecting this test method.
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