PCB Technical - How to analyze and judge the PCB parts falling?

[PCB parts falling] seems to be the dream of many process and quality control engineers, but the problems encountered by everyone are different. In view of the fact that many novices encounter such problems, most of them don’t know where to start their analysis. So here I will share my own methods and steps for your reference.

Generally, if the parts on the circuit board are dropped, most of the problems cannot be separated from the "welding quality", and the final answer seems to be one of the following, or a combination of two or more results:

1. There is a problem with the surface treatment of the circuit board.

2. There is a problem with the surface treatment of the solder foot of the part.

3. Poor storage conditions of boards or parts cause oxidation.

4. There is a problem with the reflow temperature process.

5. The welding strength cannot withstand the influence of external force actually used.

Several steps of failure analysis of circuit board parts falling:

The following describes the analysis steps for the circuit board parts falling.

The first step is to obtain information

This is very important. If the source is wrong, no matter how exciting it is, it will be in vain.

Please confirm the description of the undesirable phenomenon to the respondent first, and try to inquire about the following information first:

1. What happened? Please try to describe the undesirable phenomenon clearly. Under what conditions did the parts fall? Has the product been dropped? Under what environment did it happen (gas station, outdoor, indoor, air-conditioning)? Has it undergone any special tests (high and low temperature)?

2. The problem occurs on the client side? Is it in the production process? At which step of the process did the problem occur or be discovered?

3. When did the problem occur? Was it discovered during the production process? Or was it discovered during finished product testing? Are defective products concentrated in the same Date-code?

4. What is the surface treatment of the board? ENIG? OSP? HASL? ENIG will have the problem of black nickel, HASL will have the problem of poor tin eating after the second side, and OSP will have the problem of poor tin eating after expiration.

5. What is the thickness of the board? 0.8mm? 1.0mm? 1.2mm? 1.6mm? The thinner the board, the greater the chance of deformation and bending, and the greater the problem of tin cracking.

6. What is the surface treatment of the welding foot of the part? Matte Tin? Gilded?

The main ingredient of solder paste? SAC305 (tin, silver and copper)? SCN (tin copper nickel)? The melting point of different solder pastes will be different.

7. It is best if you can call up the reflow measurement curve at that time.

The second step is to obtain defective products and retain evidence for subsequent analysis

Please obtain the actual PCB board of the defective product. If the parts have been completely dropped, it is best to obtain the dropped parts as well, so that a control group can be used for a complete analysis. If there is more than one defective product, the more you can get, the better.

The third step is to check the solderability of the circuit board

After getting the defective product, check the solderability of the circuit board and the component feet at the same time, and observe the difference between them.

When checking the solderability, it is recommended to observe under a microscope, so that you can see some subtle problems.

How to analyze, judge and sort out the problem when the circuit board parts are dropped

It is necessary to check whether the solder is on the solder pad (disk) of the circuit board for defects such as solder rejection or de-wetting. Such problems usually come from the poor surface treatment of the circuit board or the poor storage environment of the circuit board As a result, the solder pad is oxidized.

Of course, sometimes there is a problem that the temperature of the reflow furnace is not enough to cause the welding to fail. At this time, you can use a soldering iron to try to see if the soldering pad can eat tin. If even the soldering iron can't eat tin, it can almost be judged as a problem with the PCB itself.

Please note: Some spray tin plates use the "tin, copper, nickel (SCN)" composition, which has a melting point 10°C higher than SAC305. The melting point of SAC305 is 217°C; the melting point of SCN is 227°C.

If the oxidation caused by poor storage conditions of the circuit board can be further eliminated, you can ask the PCB supplier to come over and look at the product directly, or return the PCB to the supplier for analysis and processing.

If there is a dispute, the thickness of the surface treatment can be measured first. Generally, ENIG needs to check the thickness of the gold layer and nickel layer; while HASL needs to check the thickness of the tin spray, OSP directly sees whether there is oxidation.

If there is still a dispute, it must be sliced for detailed analysis.

The fourth step is to check the solderability of the feet of the dropped parts

It is recommended to observe the solderability of the parts under a microscope, so that you can see some subtle phenomena that are invisible to the naked eye.

To check whether the tin is good on the part feet, it is recommended to check the composition of the plating layer of the part feet to see if the melting tin temperature matches the temperature of the reflow furnace. For some parts that use silver plating sputtering, the sputtered silver is only attached to the surface of the part, and its silver content is easily eaten by the SAC solder paste, causing the problem of reduced soldering strength.

Please note that the cut surface of some parts will have areas where copper is exposed and not electroplated. This place is usually not easy to eat tin, but it is generally designed in a place that does not need to eat tin or is not important. It is not necessary to eat tin on the side of QFN.

The fifth step, check whether the feet of the dropped parts are brought up with the solder pads

If there is no problem with the solderability of the circuit board and the component feet, check whether the solder pads/pads on the circuit board are also detached or connected to the dropped component feet. If so, you can further confirm the component and PCB The welding is good, and it can be proved that there is no problem with reflow.

If the soldering pad is not taken away by the dropped parts, you can first check whether the reflow temperature curve meets the requirements of solder paste. If there are excess defective products, it is best to use a soldering iron to test whether it can be removed. The dropped parts are soldered back to the circuit board. If it can be soldered back, it means that the temperature or solder paste can be strengthened to overcome this problem, but it is recommended to do a thrust test of the parts, take the board that is confirmed to be no problem, and the board that is now re-adjusting the solder paste and temperature curve. Whether there is a difference in thrust comparison, if there is a difference, it is recommended to check the surface treatment of the PCB. Sometimes the surface treatment is poor, which will cause local pad oxidation. The surface treatment of ENIG may have a black pad problem, and the second side of HASL may have IMC The problem has been generated.

The sixth step is to check the section where the part fell

Please observe the peeling surface of the circuit board and the component feet under a microscope to see if the section is rough or smooth. The rough surface is usually caused by a one-time external force to cause the parts to peel off; the smooth surface is usually broken under long-term vibration. If it is an ENIG PCB, it may also be black nickel, which may cause peeling on the nickel layer.

The seventh step, biopsy check IMC and play EDX

If none of the above steps can determine the problem of falling parts, destructive slicing will be done at the end. It is recommended that both the circuit board and the dropped parts be made when slicing.

The purpose of slicing is twofold:

1. Check whether there is IMC generation, whether the IMC generation is uniform, and you must type EDX to see what kind of IMC component it is. Regardless of the thickness of the IMC, if the IMC grows unevenly or locally, the strength of the solder will be reduced, and the thrust of the part will be reduced. Poor IMC growth may be due to oxidation or insufficient temperature.

Extended reading: The relationship between PCB welding strength and IMC

2. Jinghuai confirms on which floor the fracture occurred.

If the breaking point is in the IMC layer, it usually means that there is no problem with solderability, but the solder strength is not enough to cope with the impact of external force on it. This is generally a problem that must be solved by the company. Only some RDs will require BGA or parts to be reinforced with Underfill or dispensing.

If the fracture surface is not on the IMC layer but on the PCB end, it is more of a PCB question.

Conversely, if the fracture surface is at the end of the part, it is more biased towards the problem of the part.