Circuit board test design

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Circuit board test design

Undoubtedly, a design that is easy to test is easier to handle in production than a casual design. But engineers often want to load more technology at the lowest cost in the smallest volume, an idea that increases the limits of contact with the board during online and functional testing.

There has also been a reaction to this type of problem market. Software tools have been able to analyze designs, review them according to the rules laid down in assembly and test equipment, and suggest ways to make PCBs easier to produce. If these tools are suitable for your product, it is recommended to analyze each design, at least it can quickly point out where the test contact is found, and the ultimate goal is to make the product easier to manufacture.

Structural configuration that meets high density requirements

The high density can be either a small PCB size, a large number of circuits on the UUT, or both. The above description indicates that the mechanical and electrical structure of the system must be considered to meet the test requirements. The mechanical issues to consider are:

How to support UUT

Test area

Multi-layer board test (Can the tester do parallel test?)

I/O connector

In terms of electrical, if it is a multi-layer board, which one is more economical? Is it a multi-instrument or a switching converter with a small number of instruments? Depending on the UUT structure or the type of instrument required, the answer may not be easy to come by.

Auto or manual?

As the production and speed of each line increase (a major way to achieve economies of scale is to increase the productivity of each test equipment), consideration should be given to whether the test process can be automated. Automated functional testing virtually eliminates the need to load/unload, eliminating the need to add additional test systems, and often does not take into account the increased cost of transport equipment when considering increased throughput.

Disadvantages of test automation include an initial hardware investment, time to integrate with the production line, whether the test system can be synchronized with the line speed, and problems with production if the device fails. The off-line tester does not directly affect the assembly line. If the tester fails, the product can be taken out of the production line and the production line will continue to be produced, so that the production line will not be affected, but processing time and labor are also a problem.

It should be remembered that manual testing may typically use several cables and connectors to connect the UUTs. These cables have a generally lower life than the probes on the needle bed fixtures and should be included in the maintenance plan, which reduces the A malfunction.

Fixture problem

Due to differences in line production, shop floor space and labor rates, fixtures can range from simple plywood with pins and connecting cables to complex automatic needle bed test fixtures that are connected to the assembly line by conveyor belts. Obviously, these factors indicate that there is no fixed solution.

A manually loaded double-sided fixture with a ribbon cable attached to the main I/O connector, the top-mounted probe can access critical test points on the UUT. This is an ideal design for a medium-sized factory. The operator must connect the ribbon cable, close the top plate and start testing. There is no manual exploration for calibration and diagnostics because the top plate has access to all relevant areas. Ribbon cables and top probe connections should be designed for easy replacement because these cables are often bent and subject to wear.

When dealing with fixture suppliers, keep these issues in mind while also thinking about where the product will be manufactured, a place that many test engineers will ignore. For example, we assume that the test engineer is in California, USA, and the product is manufactured in Thailand. Test engineers will consider the product to require expensive automated fixtures because of the high price of the plant in California, requiring as few testers as possible, and the use of automated fixtures to reduce the need to hire high-tech, high-paying operators. But in Thailand, these two problems do not exist, so it is cheaper to solve these problems manually, because the labor cost here is very low, the land price is also very cheap, and the big factory is not a problem. Therefore, sometimes state-of-the-art equipment may not be popular in some countries.

Operator skill level

In high-density UUTs, if calibration or diagnostics are required, it is likely to be manually probed because the needle bed contact is limited and the test is faster (using the probe to test the UUT can quickly acquire data instead of feeding back information to the edge) For reasons such as connectors, it is required that the operator probe the test points on the UUT. Wherever you are, make sure that the test points are clearly marked.

Probe types and general operators should also be aware that issues to consider include:

Is the probe larger than the test point?

Does the probe have the risk of shorting several test points and damaging the UUT?

Is there an electric shock hazard to the operator?

Can every operator quickly find a test point and check it? Is the test point large and easy to identify?

How long does it take for the operator to press the probe on the test point to get an accurate reading? If the time is too long, there will be some trouble in the small test area. If the operator's hand will slide because the test time is too long, it is recommended to expand the test area to avoid this problem.

After considering the above issues, the test engineer should re-evaluate the type of test probe, modify the test file to better identify the location of the test point, or even change the requirements for the operator.

Automatic exploration

In some cases, automatic probing is required, such as when the PCB is difficult to manually explore, or when the skill level of the operator is limited, so that the test speed is greatly reduced, then an automated method should be considered.

Automated probing eliminates human error, reduces the likelihood of shorts at several test points, and speeds up test operations. However, be aware that there may be some limitations to automated profiling, depending on the vendor's design, including:

UUT size

Number of sync probes

How close are the two test points?

Test probe positioning accuracy

Can the system perform two-sided detection of the UUT?

How fast is the probe moved to the next test point?

What is the actual separation required by the probe system? (Generally speaking, it is bigger than the offline functional test system)

Automatic probing usually does not require needle bed clamps to contact other test points, and generally it is slower than the production line, so two steps may be required: if the detector is used only for diagnostics, consider using a traditional functional test system on the production line. The detector is placed on the side of the production line as a diagnostic system; if the purpose of the detector is UUT calibration, the only real solution is to use multiple systems, knowing that this is much faster than manual operation.

How to integrate into the production line is also a key issue that must be studied. Is there still room on the production line? Can the system be connected to the conveyor belt? Fortunately, many new probing systems are compatible with the SMEMA standard, so they can work in an online environment.