PCB Tech - PCB process The relationship between the drop of parts and the thickness of gold plating on the circuit board

PCB process The relationship between the drop of parts and the thickness of gold plating on the circuit board

Recently, our company’s SMT factory and circuit board manufacturers have been discussing the thickness of ENIG (Electroless Nickel Immersion Gold) board. The problem of parts falling was initially determined by the SMT factory to be caused by black pads, because from the appearance, the solder pads where the parts were dropped showed the color of black pads, and most of the solder pads They are also connected to the feet of the parts as the parts are dropped. It is assumed that they may fall on the Electroless Nickel layer or the P-rich layer.

In fact, our products are produced in our own professional factory, so the quality factory of the production is of course responsible. This is the problem of the black pad. Because the slices are made and the EDX/SEM is applied, the phosphorus (P) content is considered to be a bit high. They said that they also did slices and EDX/SEM, but the phosphorus (P) content should be within the normal range; here, the gold plating layer is too thin and less than 1.0μ", and the other side argued that the gold layer is in the solder There is not much use in the middle... Wait, but no one really takes care to slice and analyze the part from which layer is peeled? Is the IMC not growing well? Is the temperature insufficient heating caused the solder bad? Is the nickel layer (EN, Electroless Nickel) Oxidation causes the welding strength to weaken?

After we got the goods from our company couldn't get out, in the end we 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 part drop only occurs when the product is assembled in the later stage (Box Build), 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., And 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 below 2Kg-f The parts fall off.

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!

Then, we will clarify 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, 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 a problem with the growth of the IMC. If there is no problem in the design and the IMC is not grown well, it may be the problem of 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 the phenomenon of insufficient structure; in addition, the oxidation of the nickel layer may cause insufficient welding strength.

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 black matting 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 The whole truth has not been stated, but at least the circuit board manufacturer has initially admitted that there is a problem with the manufacturing process of its circuit board, and has found some problems in the management and control of a certain gold slot, and is willing to absorb all the losses.

It's just that Nickel erosion and Ni oxidation seem to be reversed in the control of the thickness of the gold layer. Perhaps my understanding is not enough! Working bear’s opinions here are good for your 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")～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, and it will be stored for a long time. If you want to solder it out again, it will be easy to oxidize and resist solder. Therefore, the main purpose of gold here is to prevent oxidation of the circuit board.

Because the price of gold has soared recently, the coating thickness of our company’s ENIG board can be reduced from the original minimum of 2.0μ" to 1.2μ" or more. That is to say, the thickness of the gold layer is too thin to be thinner, plus the board. Sometimes it takes three months to six months after release, and some are more than a year later. 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 above boss has agreed The supplier decides so, we can only wait and see.

This time the problematic board has been left for about three months, but the thickness of the gold layer of the board in question is only about 1.0μ" or thinner. According to the conclusion of the 8D report that the circuit board manufacturer finally answered, it is because of the gold layer of the circuit board. The thickness control is based on a 2mmx2mm box as the measurement benchmark, 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 some boards. The thickness of the immersion gold is insufficient, so that part of the nickel layer of the board is oxidized, and finally the welding strength is insufficient.