Solder paste is an indispensable electronic auxiliary material in the SMT industry. The placement and operation requirements of solder paste are very professional. If the operation is not done properly, it will lead to some adverse phenomena. Below we will discuss four common defects in solder paste during the soldering process. The symptoms and corresponding solutions are summarized as follows:
1. Short circuit: The cause of short circuit is mostly caused by excessive solder paste printing or edge collapse after solder paste printing, or the PCB pad size is not standard, and the fine-pitch QFP.SOP.PLCC is mounted caused by time shift.
The countermeasures for short circuit are as follows:
1. The PAD size setting on the PCB should conform to the international standard;
2. The accuracy of SMD components should be within the specified range when mounting;
3. Select the appropriate stencil thickness (a stencil with a thickness of 0.15MM is used if the minimum spacing is above 0.5MM, and a stencil with a thickness of 0.13MM is used if the spacing is below 0.5MM);
4. The PCB wiring gap and the coating accuracy of the solder resist paint must meet the specified requirements;
5. Establish correct welding process parameters.
2. Solder balls: The main reason for solder balls is that the preheating temperature is too high, the solder paste printing is offset, the solder paste collapses, the amount of solder paste printing is too much and the SMD mounting height is too low.
Its improvement measures:
1. The preheating temperature of the reflow soldering equipment is too high, and the heating slope does not exceed 2 degrees per second;
2. PCB positioning adopts PIN method, and the printing accuracy is high to avoid printing offset;
3. The storage and use requirements of solder paste must meet the standards;
4. The thickness of the stencil and the shape of the opening must meet the standards;
5. The mounting height of SMD should be 1/2 of the thickness of solder paste.
3. Standing parts: Standing parts are the phenomenon that one end of the SMD component leaves the PAD and stands up. The reason for this phenomenon is that the PROFILE of the heating equipment does not meet the standard, the problem of solder paste printing, the design size of the PAD and the oxidation of the SMD soldering end.
Improve strategy:
1. The storage environment of SMD components must meet the requirements;
2. The temperature profile must meet the standard requirements;
3. PAD spacing must meet international standards;
4. Solder paste printing should be uniform;
5. SMD must be accurate when mounting.
4. Cracks: The reason for this phenomenon is that when the PCB leaves the welding area, due to the difference in thermal expansion between the components to be welded and the welding material, under the action of rapid cooling, the impact of tin solidification tension and microcracks at the welding end of the components.
In view of this phenomenon, the soldered PCB should be packed well during transportation, and shockproof cotton or special packaging box should be added to reduce the vibration of components during the transfer process, and the influence of external force on SMD should be paid attention to; another point is to set Correct heating and cooling conditions, do not use rapid heating and cooling methods; the last is to choose good quality solder paste suitable for component welding.
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