PCB process Why the product execution burn-in (BI) also can not intercept the DDR false solder problem

The company recently has a The product encountered the problem of DDR memory chip (chip) soldering, and when we actually dispatched personnel to the location used by the client for maintenance, it was found that the product could not be turned on. Just press and hold the DDR IC to turn on the product, but after releasing the pressed DDR The product does not turn on again.

The products have been 100% tested in the factory, and there is a 12H burn-in (B/I) program, how can there still be Bad products flow into the hands of customers, what is going on?

From the description of this problem, this should be a typical HIP (Head-In-Pillow, pillow effect) double ball soldering problem, which is usually caused by IC chips or The FR4 of the PCB is bent and deformed when it flows through the high temperature area of ​​the reflow (reflow), and the solder ball of the BGA and the solder paste printed on the PCB cannot be in contact with each other after melting.

According to experience, 99% of HIPs generally occur on the outermost row of solder balls around the BGA, and the reason is almost all BGA loading The board or PCB is deformed and warped when the reflow is high temperature, and the deformation is reduced after the board is returned to temperature, but the molten tin has cooled and solidified, thus forming the appearance of double balls leaning together.

HIP is actually a serious BGA solder defect, although this kind of defect rate is not high, but because it is easy to pass the factory's internal test program and It flows to the customer, but after the end customer uses it for a period of time, the product will be sent back for repair due to poor contact, which seriously affects the company's reputation and the user's experience.

However, it is obvious that all products have burn-in (Burn/In) operations, and the production lines are 100% electrically tested, why can't they intercept DDR What about the soldering problem? This is actually a very interesting question. The following is just the personal experience of the work bear, and it does not mean that the real situation must be like this.

First imagine under what circumstances will HIP appear open? In most cases, it should happen when the board is heated and begins to deform, that is, if the product is just turned on and is still in the cooling stage, the double balls of the HIP may show a false contact state, so there is no problem with normal booting, when the product is turned on. After a period of time, the product began to heat up, and gradually the plate began to deform slightly due to the heat, so the phenomenon of open circuit appeared.

So the electronic assembly factory (EMS, Electronics Manufacturing Service) The possible reasons for not detecting DDR air welding are:

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When the product was burned in (B/I), the power was not turned on for testing. It is possible that the product is only placed at a certain temperature and for a certain time, and the B/I is not turned on at all. Of course, no problem can be detected. This most often happens when only Factory that produces circuit board assembly (PCA).

The product is plugged in and powered on for burn-in (B/I), but there is no program designed to run the DDR memory test. Some solder joints of DDR virtual soldering may not affect the booting action of the product, and there will be problems only when the program runs to certain memory addresses.

Assuming that the product is plugged in and the DDR memory test is performed when the product is burned in, some errors will disappear as long as it is restarted. Recording at any time during the process, it is very likely that there is no way to catch such DDR problems. Therefore, when the product performs burn-in, it is best to perform self-test and record whether there has been any error or failure, so as to know whether there is a real burn-in problem during the burn-in process.