PCB News - What are the defects of surface mount soldering and their preventive measures

What are the defects of surface mount soldering and their preventive measures
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What are the defects of surface mount soldering and their preventive measures:
1. Poor wetting
Poor wetting refers to the fact that the solder and the solder area of the substrate are wetted during the soldering process, and no metal-to-metal reaction is generated, resulting in missing soldering or less soldering failure. The reason is mostly the surface of the weld zone
It is caused by contamination or welding flux or compound layer formed on the surface of the object to be joined, such as sulfide on the surface of silver, oxide on the surface of tin, etc.
Poor wetting occurs. In addition, when the residual aluminum, zinc, etc. in the solder exceeds 0.005%, the degree of activity is reduced due to the moisture absorption of the flux, and poor wetting may also occur. crest
If there is gas on the surface of the substrate during soldering, this failure is also prone to occur. Therefore, in addition to performing the appropriate soldering process, the surface of the substrate and the surface of the component must be done well.
Anti-pollution measures, choose suitable solder and set reasonable soldering temperature and time.
Two, bridging

The causes of bridging are mostly excessive solder or severe edge collapse after solder printing, or the size of the substrate solder area is out of tolerance. SMD placement offset, etc., in SOP,
The QFP circuit tends to be miniaturized, and the bridge will cause an electrical short circuit and affect the use of the product.
As a corrective measure:
1. To prevent poor solder paste collapse
2. The size of the soldering area of the substrate must meet the design requirements
3. The placement position of the SMD should be within the specified range
4. The substrate wiring gap and the coating accuracy of the flux must meet the specified requirements
5. Formulate appropriate welding process parameters to prevent mechanical vibration of the welding machine conveyor belt
Three, cracks
When the soldered PCB just leaves the soldering zone, due to the difference in thermal expansion between the solder and the bonded part, under the action of rapid cold or rapid heat, due to the effect of solidification stress or shrinkage stress,
SMD will basically produce microcracks, and the impact stress on SMD must be reduced during the punching and transportation of PCB after welding. Bending stress surface mount products are in design
When timing, it should be considered to narrow the gap of thermal expansion, set the conditions such as top heating and cold cut conditions correctly, and select solder with good ductility.
Four, solder ball
The production of solder balls mostly occurs when the solder is scattered due to the rapid heating during the soldering process. In addition, it is also related to the printing, sag, misalignment, and contamination of the solder.
Preventive measures:
1. To avoid over-rapid and poor welding heating, perform welding according to the set heating process.
2. Defects such as sag and misalignment of solder printing should be deleted
3. The use of solder paste should meet the requirements without bad moisture absorption
4. Implement the corresponding preheating process according to the type of welding
5. Suspension Bridge (Manhattan)
Poor suspension bridge means that one end of the component leaves the soldering area and stands upright or obliquely upward. The reason is that the heating speed is too fast, the heating direction is not balanced, and the choice of dry paste
The problem is related to the preheating before welding, the shape of the SMD itself, and the wettability.
Preventive measures:
1. The storage of SMD must meet the requirements
2. The size of the substrate pad length should be properly formulated
3. Reduce the surface tension generated on the end of the SMD when the solder melts
4. The printing thickness of the solder should be set correctly
5. Adopt a reasonable preheating method to achieve uniform heating during welding
iPCB has passed ISO9001:2008, ISO14001, UL, CQC and other quality management system certifications, produces standardized and qualified PCB products, masters complex process technology, and uses professional equipment such as AOI and Flying Probe to control production and X-ray inspection machines. Finally, we will use double FQC inspection of appearance to ensure shipment under IPC II standard or IPC III standard.