PCB Tech - Parts drop and PCB gold plating thickness

The relationship between the drop of the part and the thickness of the gold plating of the circuit board

The thickness of the gold plating of the PCB board is specified. After SMT, when the whole machine was assembled, it was found that there was a problem of parts falling. At the beginning, the PCB factory strongly believed that it was caused by the black pad. The soldering pads show the color of black pads, and most of the soldering pads are connected to the feet of the parts as the parts fall. -rich) layer location.

In fact, PCB company’s products are outsourced to professional OEMs, so of course the quality OEMs are responsible for the production, but sometimes there are many unclear points in outsourcing operations, especially issues related to the ownership and compensation of liability When the SMT foundry and the circuit board production plant fought back and forth for several rounds, it is the black pad problem, because the EDX/SEM was applied to the slices, and the phosphorus (P) was considered The content is a bit too high. They said that they also did slices and performed EDX/SEM, but the phosphorus (P) content should be within the normal range; here, the gold-plated layer is too thin and less than 1.0µ". It is said that the gold layer is not very useful in soldering...wait, but no one really does the slicing and analyzes from which level the part is peeled off? Is the IMC not growing well? Is the insufficient temperature heating caused the poor soldering? Is the oxidation of the nickel layer (EN, Electroless Nickel) weakening the welding strength?

After getting the PCB company's goods could not be shipped out, in the end I had to jump down for arbitration and arrest all the people from both sides for a meeting!

First of all, of course, it is necessary to understand the current situation. First, make sure that the parts fall only occur during the box build of the later product, because the parts need to be plugged and unplugged during the assembly of the whole machine. No problem was found in the previous SMT and ICT. Moreover, after checking the circuit board assembly with problems and no problems before, it is found that the good circuit board parts can withstand the thrust of 6～8Kg-f without falling, while the defective circuit board only needs to be pushed to the parts below 2Kg-f. Just dropped.

Therefore, short-term measures can first use thrust to sort (select) good and defective products, but the parts that have been thrust need to be hand welded again to ensure that the parts are not caused by the implementation of the thrust caused by the slight solder joint cracks; as for the complete machine Assembling the finished product is a headache. We finally decided to perform a 100% plug-in test with the last batch of products in the warehouse, and then disassemble the machine according to AQL0.4 to check the thrust of the parts. For other batches, use the pallet. As a unit, 100% plug-in test is performed and 2 sets are selected for thrust test. This is a big project!

Next, we will discuss the true cause of the part falling. In fact, the part falling is nothing more than the several possibilities mentioned above. First check where the part is broken, and you can probably know where the problem is:

▪ If there is no tin on the parts feet at all, it must be caused by oxidation of the parts feet or poor solder paste.

▪ If it does not grow into IMC at all, then the reflow heat should be insufficient.

▪ If the fracture is on the surface of the IMC layer, it depends on whether there is any problem with the growth of the IMC. If there is no problem in the design and the IMC grows poorly, it may be due to insufficient Reflow temperature... etc.

▪ If the fracture occurs between the IMC and the nickel layer, you can check whether the phosphorus-rich layer is obvious. It is recommended to perform elemental analysis to see if the phosphorus content is too much. If the phosphorus-rich layer is obvious and too thick, it will affect the reliability in the future and cause insufficient structure; in addition, it may also be caused by the oxidation of the nickel layer to cause insufficient welding strength.

Take the board with the problem, then slice the solder pads on which the parts have fallen and the solder pads on which the parts have not fallen. In addition, take another board that has no problem produced before, and slice them on the solder pads where the parts are now found to have fallen. .

This is a picture of the problematic board as a sliced part and the solder pad dropped.

It can be clearly seen that the IMC of the parts falling off the pad does not seem to have grown completely, and it seems that the traces of AuSn and AuSn2 will not escape in the future (there is no elemental composition, I am not sure).

A biopsy check where the same part fell, found that the IMC was also normal.

After several days of follow-up and discussion, the truth seems to have gradually improved. We found that the parts fell between the IMC and the nickel layer, and the growth of IMC seems to be a little insufficient. Both parties also found O in the nickel layer. (Oxygen), although one side still insists on the possibility of the black pad of nickel layer corrosion (Ni Erosion), the other side insists that there is no nickel layer corrosion, but it is caused by nickel layer oxidation (Ni oxidization), although it is vaguely felt that the circuit board manufacturer Did not tell the whole truth, but at least the circuit board manufacturer has initially admitted that there are some problems with the manufacturing process of its circuit boards, and found some problems in the management and control of a certain gold slot, and are willing to absorb all the losses, so we are not Continue to scrape manure down.

It's just that Nickel erosion and Ni oxidation seem to be just reversed in the control of the thickness of the gold layer. Perhaps Shenzhen Honglijie's understanding is not enough!

Shenzhen Honglijie's opinions here can be used as a reference. If there are circuit board experts passing by, please feel free to provide opinions. According to the requirements of IPC4552, the thickness of the generalized gold layer is recommended to be 2µ"～5µ", and the thickness of the chemical nickel layer is 3µm (118µ") to 6µm (236µ"). However, the gold layer should be as thin as possible, so as to avoid gold brittleness and reverse corrosion, because "gold" is an unreactive element during the welding process; but if the gold layer is too thin, it will not be able to completely cover the nickel layer. If it takes a long time to solder it out again, it is prone to oxidation and solder rejection. Therefore, the main purpose of "gold" here is to prevent oxidation of the circuit board. As for the purpose of "nickel"? Please refer to this article: What is the purpose of nickel plating on parts or circuit boards in the electronics industry?

Because the price of gold has soared recently, the plating thickness of PCB's ENIG board has been reduced from the original minimum of 2.0µ" to 1.2µ" or more. Sometimes the board will last for three months to six months, and some will be more than a year. I am really worried. To be honest, we are still closely observing whether there will be any side effects of such a gold layer thickness, but since the boss above has already Promise the supplier and decide so, we can only wait and see.

This time the problematic board has been stored for about three months, but the gold layer thickness of the problematic board is only about 1.0µ" or thinner. According to the conclusion of the 8D report that the PCB manufacturer finally answered, it is because the PCB gold layer thickness control is A 2mmx2mm box is used as the measurement base, but the solder pads that have problems this time are actually much larger than this size, so the thickness of the gold layer of the solder pads here is not controlled, resulting in immersion of some boards. Insufficient gold thickness will oxidize the nickel layer of part of the board, resulting in insufficient solder strength. The above is part of the answer from the circuit board supplier. I still have some doubts about it.