PCB Tech - Functional test after PCBA circuit board assembly (FVT/FCT)

The functional test after PCBA circuit board assembly is generally called FVT (Function Verification Test) or FCT (Function Test). The purpose is to catch the poorly assembled board and install it through the simulated circuit board. The full-function test when the whole machine is built is to catch out the circuit assembly boards that may have defects before assembling the whole machine, so as not to discover the defects after the whole machine is assembled, and also to dismantle and reassemble the whole machine, resulting in waste of man-hours and Loss of material.

The appearance of this kind of functional testing machine may vary with the size and shape of the product. In addition, the way of functional testing can also vary depending on the urgency of time, the number of test boards, and the cost. As far as Shenzhen Honglijie knows, the more commonly used function test methods can be roughly divided into the following types.

PCB complete machine plug-in function test:

Usually a complete machine product is composed of more than one circuit board. The so-called actual insertion is to actually assemble all the circuit boards, parts... etc. needed to assemble a complete machine, but do not need to be installed. In the case, because it is necessary to test the function of the circuit board, it is necessary to make the circuit board easy to disassemble and assemble. As for the actual insertion level, it depends on which functions are tested. The ideal situation is of course to be able to test all functions. If not, most of the functions should be tested. The key point should be to be able to catch major pcba functional test. Questions, such as whether it can be started by plugging in the power supply, whether the key functions are normal, whether the screen is displayed normally, etc., otherwise the test is meaningless, just install it as a whole machine and test it.

In addition, before starting the test, a set of functional circuit boards must be prepared as a GOLDEN SAMPLE (GOLDEN SAMPLE). Assume that a set of boards has three boards A, B, and C. When testing A board, Just take out the B and C standard samples as a fixture, only replace the A board for testing, so as to complete the test of the three boards in turn.

The biggest problem with this test method is that it is easy to damage the GOLDEN SAMPLE, because some connectors or cables between the circuit board and the circuit board cannot withstand too many plugs and unplugs, so extension cables are generally used, The advantage is that the extension cable can be assembled easily, so that all the boards are not crowded together. In addition, the extension cable is cheaper and easy to replace. If you use it, you can replace it. It is more cost-effective than using a damaged circuit board. However, the functions of some components have special requirements for signals, and extension cables cannot be used, such as barcode scanners (barcode), touch screens, etc.

The need to spend a lot of time (labor) is also the disadvantage of this test method, so this method is usually only used in the EVT (Engineer Verification Test) stage, where the production quantity is small or the circuit board design has not been finalized.

Needle bed function tester:

This method is to use the layout test point (Test Point) on the circuit board, and then use the needle to lead the signal out for a complete pcba functional test.

When testing, just put the board into the test machine, and then push the probe out to touch the test point of the board to be tested through mechanical action, so that the board to be tested can be simulated as a connection to the whole machine, and finally on the machine. Operate and test all the functions.

If the whole machine is a product with a screen and buttons, the signals of these components will be led out to facilitate the ordering of the board and view the results; if the product does not have a screen and buttons, it must be connected to the computer through a transmission interface. To operate and view the results of the test.

When making this kind of functional test machine, it is generally necessary to have a complete machine that has been determined to have a complete functional test. The fixture manufacturer will disassemble and install the product parts into the test fixture, and then use one end to connect the probe to the other. One-end soldering cable is connected to each board, the probe basically replaces the original connector and cable between the circuit boards in the whole machine, so as to achieve the purpose of quickly loading/unloading the board to be tested., And it is not easy to damage the standard model when using the fixture, so there is no need to change the standard model frequently.

▼The photo below is an ICT test machine, basically FCT/FVT is similar to it, but with external screens and buttons, there will be some FVT photos below.

PCB Needle Bed pcba functional test

▼This picture is a partial enlarged view of the fixture. You can see some test probes. When the board to be tested is placed on the fixture, pressing down will allow the probes to penetrate through the small holes and touch the board to be tested. Test point.

Circuit board function tester probe

▼This is the situation of the internal wiring of the ICT fixture. You can see that the densely packed wiring almost fills the fixture. Usually there are as many probes as there are as many wirings. Generally, there are only more. No less, just knowing which cable is connected to which one is nerve-wracking, so basically every cable will have a number.

Circuit board ICT fixture wiring

▼This is the appearance of the traditional functional test fixture. From the top of the fixture, you can see that some probes are welded with some cables, and the screen and buttons are usually connected to facilitate operation and view the test results.

The appearance of the FVT function test fixture

▼This is the internal cable of the traditional functional test fixture, which is much less than the ICT cable, but the cable is always troublesome, not only difficult to maintain, heat dissipation and signal interference are also problems.

FVT function test fixture internal cable

Wireless needle bed pcba functional test:

This functional testing machine is an improvement on the original needle bed testing machine. It removes the wire originally connected to the back of the test probe and replaces it with a customized circuit board. That is to say, the original probe has only one end and The board to be tested is now in contact with the circuit board at both ends of the probe, but one end is in contact with the board to be tested, and the other end is in contact with the customized circuit board. This circuit board can be routed as needed and then connected to other circuits of the whole machine, or the original connection with other circuit boards can be designed to achieve a complete integration with the product, which is somewhat similar to a Big board, which is helpful Solve the operation problems of some components that are particularly sensitive to signal interference. Because the lines run on the circuit board, some large-area grounding layers can be used to isolate the interference between the lines; in addition, it is easier to repair, because there is not too much complexity You don’t need to find one by one, or worry that the welding wire is broken.

The disadvantage is that the cost is relatively high. Because it needs to make a customized board, the cost of this kind of wireless function test fixture will be about 40% to 70% higher than that of the general needle bed function test fixture, and it is only suitable for a large number of Produce stable products, because as long as any design change of the circuit board is moved to the test point, it is necessary to re-make a customized circuit board.

▼This is a wireless function test machine that improves the traditional function test fixture. The top of the fixture replaces the original cable with a customized circuit board. This fixture also has a product integrated functional fixture on the top.

Circuit board wireless function test machine

(Wireless FVT) This is a wireless function test machine that improves the traditional function test fixture. The top of the fixture replaces the original cable with a customized circuit board. This fixture also has a product integrated functional fixture Above.

▼This is the place where the wireless pcba functional test fixture needs to put the circuit board to be tested. After the board is put on, close the upper cover, and the upper and lower probes will be pushed out to contact the test points of the board to be tested, so as to connect to the whole machine. The purpose of the test.

PCB wireless function test fixture

(Wireless FVT) This is the place where the wireless function test fixture should put the circuit board to be tested. After the board is placed, close the upper cover, and the upper and lower probes will be pushed out to contact the test points of the board to be tested to reach the connection For the purpose of testing the whole machine.

▼This is the internal structure of the wireless function test fixture. There are not too many cables running around in the fixture. These visible cables are used for external connection and communication of the fixture.

PCB unlimited test fixture internal structure

(Wireless FVT) This is the internal structure of the wireless function test fixture. There are not too many cables running around in the fixture. These visible cables are used for the external connection and communication of the fixture.