PCB News - Causes of poor welding in SMT processing

Causes of poor welding in SMT processing and preventive measures
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1. Poor wetting
Poor wetting refers to the fact that the solder and the soldering area of the substrate during the soldering process do not generate a metal-to-metal reaction after being soaked, resulting in missed soldering or less soldering failure. The reason is mostly caused by the contamination of the surface of the solder zone, or the solder resist, or the formation of a metal compound layer on the surface of the joined object. For example, the surface of silver has sulfide, and the surface of tin has oxides, etc., which will cause poor wetting. . In addition, when the residual aluminum, zinc, cadmium, 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. In wave soldering, if there is gas on the surface of the substrate, this failure is also prone to occur. Therefore, in addition to performing appropriate soldering processes, anti-fouling measures should be taken for the surface of the substrate and the surface of the components, suitable solders should be selected, and reasonable soldering temperature and time should be set.
B, Bridge joint
The causes of bridging are mostly caused by 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., when SOP and QFP circuits tend to be miniaturized, bridging will cause Electrical short circuit affects the use of the product.

as a corrective measure:
1. It is necessary to prevent sag during solder paste printing.
2. The size of the soldering area of the substrate must meet the design requirements.
3, SMD placement position must be within the specified range.
4. The wiring gap of the substrate and the coating accuracy of the solder resist must meet the specified requirements.
5. Formulate appropriate welding process parameters to prevent mechanical vibration of the welding machine conveyor belt.
Three, crack
When the soldered PCB just leaves the solder zone, due to the difference in thermal expansion between the solder and the joined parts, under the action of rapid cold or rapid heat, due to the effect of solidification stress or shrinkage stress, the SMD will basically produce microcracks. During punching and transportation, the impact stress on SMD must also be reduced. Bending stress.
Surface mount products should be designed to narrow the gap in thermal expansion and set heating and cooling conditions correctly. Use solder with good ductility.
Four, solder ball
The generation of solder balls is mostly caused by the rapid heating during the soldering process, which causes the solder to fly away. In addition, it is misaligned with the printing of the solder and collapsed. Pollution is also related.
Prevention measures:
1. To avoid over-rapid and bad 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 welding type.
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 cause is that the heating speed is too fast, the heating direction is not balanced, the selection of solder paste, the preheating before soldering, and the size of the soldering area. The shape of SMD itself is related to wettability.
Prevention measures:
1. SMD storage must meet the requirements
2. The size of the length of the substrate pad should be properly formulated.
3. Reduce the surface tension on the end of the SMD when the solder melts.
4. The printing thickness of the solder should be set correctly.
5. Take reasonable preheating method to achieve uniform heating during welding. Paying attention to the trend of environmental protection informatization and the development of various environmental protection technologies,PCB factories can start with big data to monitor the company's pollution discharge and governance results, and find and solve environmental pollution problems in a timely manner. Keep up with the production concept of the new era, continuously improve resource utilization, and realize green production. Strive to make the PCB factory industry realize an efficient, economical and environmentally friendly production model, and actively respond to the country's environmental protection policy.