PCB Technical - Introduction to functional test after pcb circuit board assembly (FVT FCT)

The functional test after the 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 into The full-function test of the whole machine is to catch all the defective circuit assembly boards before assembling the whole machine,so as to avoid the defects found after the whole machine is assembled, and all the time wasted by dismantling and restructuring.

The appearance of this kind of fct 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. The functional testing methods that I know and are more commonly used can be roughly divided into the following types.

The actual plug-in fct test of the whole circuit board:

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 it is not necessary Install it into the case,because the function of the circuit board is to be tested, 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 functions. Questions, such as whether it can be started by plugging in the power supply, whether the key functions are normal, whether the screen is normally displayed... etc. Otherwise, the test is meaningless, just install it as a complete machine and test it.

In addition,before starting the test, a set of circuit boards with normal functions must be prepared as a standard model (GOLDEN SAMPLE).Assuming that a set of boards has three boards A, B, and C,when you want to test board A,take out the B and C standard samples as a fixture, and replace only 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 flat 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,which 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 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 link 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 the 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 of the flat cable is welded to connect to each board, and the probe basically replaces the original connector and flat 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 the use of jigs is less likely to damage the standard model, so there is no need to replace the standard model frequently.

Wireless needle bed function tester:

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 are not too many complicated Flat cables, you don’t need to find one flat cable, 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 moves to the test point, it is necessary to re-make a customized PCB circuit board.