Failure analysis mechanism: analysis of poor performance of PCBA in welding

Date：2021-11-09 13:37:00 Views：2257

PCBA is a system composed of PCB and various electronic components. The main material of PCB is the composite of glass fiber and epoxy resin, which is divided into single panel, double-sided board and multilayer board. In the process of PCBA welding, poor PCBA welding will be caused by the influence of welding data, process, personnel and other factors. Next, we mainly introduce the analysis of poor PCBA performance in welding.

Common bad welding of PCBA

1. Too much residue on PCBA board surface

Excessive plate residue may be due to not preheating before welding or too low preheating temperature and insufficient tin furnace temperature; Walking speed is too fast; Anti oxidant and anti-oxidation oil are added to the tin liquid; Too much flux coating; The component foot is out of proportion to the orifice plate (the hole is too large), causing flux accumulation; In the process of flux application, it is formed without adding diluent and other factors for a long time.

2. Corrosion, green components, black pads

It is mainly due to insufficient preheating, resulting in too much flux residue and too much harmful residue; The flux to be cleaned is used, but it is not cleaned after welding.

3. False soldering

False welding is a very common defect, which is also very harmful to the board. It is mainly related to too little or uneven flux coating; Severe oxidation of some pads or feet; Unreasonable PCB wiring; The foaming pipe is blocked and the foaming is uneven, resulting in uneven flux coating; Improper operation method when dipping tin by hand; The inclination angle of the chain is unreasonable; The wave crest is related to the causes of inequality.

4. Cold welding:

The surface of the solder joint is in the shape of bean curd residue. It is mainly because the temperature of the electric soldering iron is not enough, or the vibration of the weldment before solder condensation, the strength of the bad solder joint is not high, the conductivity is weak, and it is easy to cause the problem of open circuit of components and parts under the action of external force.

5. Solder joint whitening:

Uneven and dull. Generally, it is formed because the temperature of the electric soldering iron is too high or the heating time is too long. The strength of the bad solder joint is not enough, and it is easy to cause the problem of open circuit of components under the action of external force.

6. Pad stripping:

It is mainly because the pad is peeled off from the printed circuit board after being subjected to high temperature. This bad solder joint is very easy to cause the problem of open circuit of components.

7. Tin bead

Process: low preheating temperature (flux solvent is not completely evaporated); The walking speed is fast and does not reach the preheating effect; The inclination angle of the chain is not good, there are bubbles between the liquid tin and the PCB, and tin beads are produced after the bubbles burst; Improper operation during hand tin dipping; Wet working environment; PCB problems: the board surface is wet and moisture is generated; The planning of PCB gas escape hole is unreasonable, resulting in gas entrapment between PCB and molten tin; PCB planning is unreasonable, and the parts are too dense to form air.

There are many reasons for the poor welding of PCBA, among which each process needs to be strictly controlled to reduce the impact of the previous process on the follow-up.

PCB failure analysis steps

1. Visual inspection

Cracks on the substrate indicate the existence of bending stress, and overheated or discolored lines are a sign of overcurrent. Broken solder joints indicate solderability problems or solder contamination. Several solder joints have gray surfaces or too much or too little solder. These are characterized by poor welding technology, pollutants or signs of overheating. Any decolorization may mean overheating. Solder reflow produces pores, which means high temperature.

2. X-ray

Check for any disconnection / shorting or damage to wiring, through holes that are not fully filled, wiring or component pads that are misaligned.

3. Electrical measurement

For the electric leakage caused by pollutants in the solder joint used to confirm cracking and fracture, the voltage can be added to measure the current, but the voltage shall be limited to a reasonable range. Applying some stress during the test may find some intermittent abnormalities.

4. Section analysis

For defects in solder joints and multilayer plates, metallographic samples can be prepared for inspection.

5. SEM and EDX

The pollutants on the substrate surface can be identified by SEM. The pollutants may appear on the surface of the plate or under the conformal coating. Sometimes the conformal coating must be removed without affecting the pollutants. Chlorine, fluorine, sulfur and bromine are concerned elements, and bromine is a flame retardant component in some materials. EDX can be used to check whether there are pollutants in the solder joint. Sulfur, oxygen, copper, aluminum and zinc are pollutants that may cause problems in the solder joint. Solder joint fracture caused by pollutants often occurs in intermetallic compounds at the interface between component pins and solder.

Method for removing conformal layer:

1. Dissolve with solvent. Solvents such as xylene, trichloroethane, methylacetaldehyde ketone and methylene chloride can remove the conformal layer, but do not damage the board or pollutants.

2. Thermal separation: controllable low-temperature heating method is adopted. This method is the best for thick coating. Heat is directly added to the coating to separate it from the base material.

3. Grind off and remove the coatings that cannot be melted by solvent with spraying equipment similar to sand blasting.

4. Plasma corrosion: put the plate in a vacuum chamber and remove the coating with low-temperature plasma. This method is very effective for the removal of poly-p-xylene.

Insulation resistance test:

The decrease of insulation resistance may be the result of dendrite growth, pollutants and other problems. According to the insulation test specification. The insulation of PCB is extremely sensitive to relative humidity, especially those polluted. Generally, the insulation fault is less than 1 megohm. The following are some guidelines for identifying insulation resistance problems:

-Before taking measurements, the PCB should be exposed to the humidity level it is subjected to.

-Some circuits may need to be physically isolated in order to be separated from the whole circuit system, which is to make the measurement of a single line more accurate. Complex components or circuit areas may need to be removed in order to investigate the fault areas of concern.

-Check carefully under the microscope to confirm the actual source of leakage or pollutants.

-Low voltage technology shall be used for measurement to avoid damaging the device.

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