In the process of PCB subcontracting and material processing, the selection of PCB through tin is very important. In the through-hole plug-in process, if the tin penetration of the PCB board is not good, it is easy to cause problems such as virtual soldering, tin cracking and even parts drop. We should understand these two major points about PCBA tin penetration.
1. Requirements for tin penetration of PCBA contract work and materials
According to the IPC standard, the PCBA tin penetration requirement of through-hole solder joints is generally above 75%, that is to say, the tin penetration standard for the appearance inspection of the panel surface for soldering is not less than 75% of the aperture height (board thickness), PCBA through tin is suitable at 75%-100%. The plated through hole is connected to the heat dissipation layer or the heat dissipation layer for heat dissipation, and the PCBA penetration tin requires more than 50%.
2. Factors Affecting PCBA Tin Penetration
PCBA tin penetration is mainly affected by factors such as material, wave soldering process, flux, manual soldering and so on.
A specific analysis of the factors that affect the tin penetration of PCBA contracting materials:
Tin melted at high temperature has strong permeability, but not all metals to be welded (PCB boards, components) can penetrate into it, such as aluminum metal, its surface generally automatically forms a dense protective layer, and the difference of the inside molecules in structure also makes it difficult for other molecules to penetrate. Second, if there is an oxide layer on the surface of the metal to be welded, it will also prevent the penetration of molecules. We generally treat it with flux or brush it with gauze.
Flux is also an important factor affecting the poor penetration of PCBA tin. Flux mainly plays the role of removing surface oxides on PCB and components and preventing re-oxidation during the soldering process. All will lead to poor tin penetration. You can choose a well-known brand of flux, the activation and wetting effect will be higher, which can effectively remove the oxides that are difficult to remove. Check the flux nozzle, the damaged nozzle needs to be replaced in time, and ensure that the surface of the PCB is coated with an appropriate amount of flux, then it can give full play to the fluxing effect of flux.
3. Wave Soldering
Poor tin penetration of PCBA is naturally directly related to the wave soldering process. Re-optimize welding parameters with poor tin penetration, such as wave height, temperature, welding time or moving speed. First, the track angle should be appropriately reduced, and the height of the wave crest should be increased to increase the amount of contact between the liquid tin and the solder end. Then, the temperature of the wave crest should be increased. Generally speaking, the higher the temperature, the stronger the penetration of tin, but this depends on the temperature tolerance of the components. Finally, the speed of the conveyor belt can be reduced, and the preheating and soldering time can be increased, so that the flux can fully remove oxides, wet the solder ends, and increase the amount of tin.
4. Manual Welding
In the actual plug-in soldering quality inspection, a considerable part of the weldment only formed a cone on the surface of the solder, but no tin penetrated into the via hole. In the functional test, it was confirmed that many of these parts were false soldering, which was mostly caused by manual plug-ins. In welding, the reason is that the temperature of the soldering iron is not appropriate and the welding time is too short. Poor tin penetration of PCBA can easily lead to virtual soldering problems and increase the cost of rework. If the requirements for PCBA penetration are relatively high, and the welding quality requirements are relatively strict, selective wave soldering can be used, which can effectively reduce the problem of poor PCBA penetration.
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