Review of SMT reflow welding defects and SMT process

Review of SMT reflow welding defects and SMT process
Cause of defects encountered Manufacturer during SMT wafer processing reflow soldering
The skills to solve reflow soldering defects in SMT chip processing plants. During reflow soldering, we often encounter welding defects. Aiming at this problem, Guangzhou SMT patch editor combed and found 13 reasons for this situation., Today, I will list them in detail with you.
1. Poor wettability
2. Insufficient solder, open circuit of faulty soldering
3. Suspension bridge and gear shift
4. Welding spot bridging or short circuit
5. Walk the welding ball near the welding spot
6. Porosity distributed on the surface or inside of the solder joint
7. The height of solder joint contacts or exceeds the part body (material absorption phenomenon)
8. Tiny tin wire between welding ends of components, between pins, between welding ends or between pins and through-hole
9. The electroplating layer at the end of the module peels off to varying degrees, exposing the data of the module body
10. Component surface inversion
11. There are cracks and defects of varying degrees on the body or end of the component

12. Cold welding, also known as fuzzy solder joints
13. Some defects are invisible to the naked eye, such as the minuteness of the solder joint, the internal stress of the solder joint, and the internal crack of the solder joint These can only be detected by X-ray, solder joint fatigue test and other methods These defects are mainly related to temperature distribution For example, if the cooling rate is too slow, large crystalline particles will be formed, resulting in poor fatigue resistance of solder joints However, if the cooling speed is too fast, it is easy to produce cracks on the part body and the solder joint For example, if the peak temperature is too low or the reflow time is too short, solder will be produced Insufficient melting and cold welding, but too high peak temperature or too long reflow time will increase the formation of co interface metal compound, which will make the solder joint brittle and affect the strength of the solder joint If the temperature exceeds 235 ℃, it will also cause skin itching on PCB Resin carbonization affects PCB efficiency and life
Focus of SMT process audit
raw material:
Electronic components and MSD components shall be stored in designated and clearly marked positions, and the storage area can ensure effective and reasonable FIFO;
All components and MSD components shall be reasonably distinguished and clearly marked;
Whether the quality assurance certificate and warehousing inspection report of chemical raw materials can effectively monitor the chemical composition of aluminum ingots, including the management and labeling of MSDS;
Whether the traceability system of components is complete and whether the FIFO is embedded in the traceability system
Whether the ESD system is effectively implemented, whether it is certified, and whether the records are complete
Solder paste printing:
Solder paste storage and storage management
Service life and cleanliness of wire mesh
Solder paste printing parameter management, SPC application
Printer maintenance
Cmk evaluation of printing press
Certification and evaluation of solder paste material composition, efficacy certification and evaluation
Management and traceability of solder paste printing program
SMT process:
Are the SMT process parameters reasonable and effective? How to alarm and correct abnormal process parameters?
SMT program management and traceability
Confirmation of the first sample (size and efficiency)
Whether operators and quality personnel are qualified for the following test operation evaluation:
X-ray, special inspection of key parts, leakage inspection, appearance inspection, quality or density inspection
SMT placement accuracy and Cmk
SMT machine level feeder maintenance
SMT refueling and feeding management and simple management of errors and missing data are in place
Whether the traceability of SMT process and components can be traced to the batch of each component
reflow welding process:
Whether the contour is measured regularly
Maintenance of reflux furnace
Is the Cmk target of the reflow furnace achieved?
Field detection and SPC monitoring of key parameters of reflux furnace
Evaluate and analyze the ability of PCB welding effect
SMT appearance inspection:
Whether the inspection standard is clear
Check the effectiveness of the method and whether it is equipped with auxiliary equipment of advanced equipment, such as AOI, AVI, etc.
Whether the inspection rate is evaluated SMT appearance inspection and MSA is evaluated
Whether FPY and DPMO have statistical data, and whether the traceability system is effective in real time?
Whether the quality anomaly early warning mechanism is timely and effective, and whether the traceability system is embedded

The above is the explanation given by the editor of pcb circuit board company.
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