PCB Tech - how to test circuit board assembly function test (FVT)

In electronic assembly plants,circuit board assembly function testing is one of the important project processes.Here we will briefly introduce what is FVT/FCT:Function Verification Tester (FVT) or Function Test (FCT)?

How to test circuit board assembly function test (FVT)

FVT/FCT is basically a full function test when the simulated circuit board is actually installed into the whole machine.Its purpose is to grab out any defective circuit assembly boards before assembling into the whole machine, so as not to find defective boards after being assembled into the whole machine.But also to be completely dismantled and reorganized. The machine and method of this kind of test may vary greatly depending on the product. According to the urgency of time, demand, and cost, the actual application can be divided into the following ways:

The whole circuit board is plugged in:

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.used to assemble a complete machine, but it does not need to be installed. 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.The degree of actual insertion depends on the degree of testing.The ideal situation is of course to be able to test all functions,if not,most of the functions should be tested,and the focus should be to catch major problems. For example, whether it can be started by plugging in the power source, whether the key functions are normal, whether the screen is normally displayed, 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 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 cord can be assembled easily,so that all the boards are not crowded together. In addition, the extension cord is cheaper and easy to replace. If it is worn out, it is more cost effective to replace it with a new circuit board.However,some components that are particularly sensitive to signal requirements cannot use extension cords, such as barcode scanners,touch screens,etc.

Labor is also a disadvantage of this test method,so it is usually only used when the number of stages like EVT (Engineer Verification Test) is small.

Needle bed testing machine:

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

When testing, just put the board into the test machine, and then push the needle point 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 and linked to the appearance of 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,they will be brought out,otherwise,it will be additionally connected to the computer through the transmission interface to operate and view the test results.

When making this kind of machine, it is generally necessary to have a complete machine that has been determined to have a complete test function. The fixture manufacturer will disassemble the product and install it into the test fixture, and then connect it with one end of the thimble and the other end of the welding wire. Each board and thimble basically replace the connectors and flat cables that connect the circuit boards in the original machine, so as to achieve the purpose of fast loading/unloading the board to be tested, and the use of jigs is less likely to damage to the standard Template.

Wireless needle bed testing machine:

This test machine is an improvement of the original needle bed tester. It removes the wire originally connected to the test thimble and replaces it with a customized circuit board, which means that the original thimble only has one end in contact with the board to be tested. Now one end of the thimble is in contact with the board to be tested, but the other end is in contact with the customized circuit board. This circuit board can run circuits and then connect to other circuits of the whole machine, which helps to solve some problems. Signal interference is particularly problematic for the operation of sensitive components. 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.

The disadvantage is that the cost is relatively high.Because of the need to make customized boards, the cost of this type of wireless test fixture may be about 50%-65% higher than that of the general needle bed fixture, and it is only suitable for mass production and stable production. Product, because as long as there is any modification on the board to be tested and moved to the test point, it is necessary to re-make a customized circuit board.

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