PCB Tech - Discussion on AOI/MDA/ICT/FVT/FCT of several test methods after circuit board assembly

Discussion on AOI/MDA/ICT/FVT/FCT of several test methods after circuit board assembly

At present, the industry's testing methods for PCBA boards can be roughly divided into three major parts: AOI, ICT/MDA, and FVT/FCT. In addition, some people use X-Ray for on-line full inspection, but it is not common, so this article will not be included in the discussion.

Below we will discuss the capabilities of these three test methods in general, and because the current three methods have their own advantages and disadvantages, it is difficult to replace the other two with only one method, unless someone thinks that the risk is small and can be ignored.

AOI (Automated-Optical-Inspection):

With the advancement and maturity of imaging technology, AOI is gradually adopted by many SMT production lines. Its inspection method is to use image comparison, so there must be a golden sample that is considered to be a good product and record its image. Then the other boards are compared with the images of the standard model to judge whether they are good or bad.

Therefore, AOI can basically determine whether there are missing parts, tombstones, wrong parts, offset, bridges, empty soldering, etc. on the assembled circuit board; but it cannot identify the soldering properties directly under the parts, such as BGA IC or QFN IC, as for fake welding and cold welding, it is difficult to judge by AOI. In addition, if the characteristics of the parts have changed or there are micro cracks, it is difficult to be identified by AOI.

Generally, the misjudgment rate of AOI is very high, and it takes experienced engineers to adjust the machine for a period of time to stabilize. Therefore, during the initial introduction of the new board, a lot of manpower input is required to re-judge whether the problematic board laid down by AOI is really problematic.

ICT/MDA (In-Circuit-Test/Manufacturing-Defect-Analyzer):

Traditional testing methods. The electrical characteristics of all passive components can be tested through test points. Some advanced test machines can even run the program on the circuit board to be tested, and do some functional tests that can be run by the program. If most of the functions can be completed through the program, you can consider canceling the following FVT (functional test).

It can catch missing parts, tombstones, wrong parts, bridges, reverse polarity, and can roughly measure the weldability of active parts (IC, BGA, QFN), but it is not suitable for empty welding, false welding, and cold welding. Certainly, because this kind of solderability problem is intermittent, if it happens to be touched during the test, it will pass.

Its disadvantage is that there must be enough space on the circuit board to place the test points. If the fixture is not designed properly, the electronic parts on the circuit board and even the traces in the circuit board will be damaged due to mechanical actions.
The more advanced the test fixture, the more expensive it is, some even as high as NT$1 million.

FVT/FCT (Function Verification Test):

Traditional functional testing methods are usually combined with ICT or MDA. The reason for the need to match ICT or MDA is that the functional test needs to actually be electrically connected to the circuit board. If there is a short circuit on the circuit on some power supplies, it is prone to damage to the board under test. In severe cases, the circuit board may even be burned. Ann's concerns.

The functional test also cannot know whether the characteristics of the electronic parts meet the original requirements, that is to say, the performance of the product cannot be measured; in addition, some by pass circuits cannot be measured by the general function test, which needs to be considered.

The functional test should be able to catch all the parts' weldability, faulty parts, bridges, short circuits, etc., except for the By pass circuit, and the problems of empty, false, and cold welding may not be completely detectable.

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