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Why does poor tin welding always occur in PCB

December 2, 2019

PCB Image:Why does poor tin welding always occur in PCB

In the process of PCB Prototype design and production, do you usually meet the problems that poor tin welding always occur in PCB? For engineers, once poor tin welding always occur in one PCB, it means re-welding or re-production is needed, so the effect is rather painful. So, what are the major reasons for poor tin welding in PCB? How can we avoid this problem?

Generally, the reason for poor tin welding in PCB is that some parts of the PCB surface is not coated with tin, so there are many PCBs with poor tin welding. There are many reasons for poor tin welding in the PCB, which can be concluded in the following parts:

Debris such as grease, impurities, may stick to the board of PCB, or grinding particles may stay in the circuit surface in the process of process of board production, or silicone oil may remain in it, which will all contribute to poor tin welding. If the above problems occur in the checking process, solvent can be used to clean them. But if it is silicone oil, special cleaning solvent is needed, or it will be difficult to be cleaned out.

There is another cause for poor tin welding in the PCB. When the PCB is stored for too long time or when the storing condition is too wet, or workers are not rigorous in the production process, serous oxidation in the tin surface or darkness may occur in the copper surface for the board or the parts on it. Under these circumstances, replacing soldering flux can no longer solve the problem, so technicians need to re-solder them again, which will improve tin welding effect.

In the process of PCB soldering, if the soldering time is not enough or if the soldering temperature is not high enough, or the wrong soldering flux is chosen, poor tin soldering may also occur. Generally, the working temperature for soldering tin is 55~80℃ higher than its fusion point and poor tin soldering also occurs when preheating time is not enough. Besides, the distribution of soldering flux amount on the circuit surface is also affected by the proportion, so checking the proportion will also dismiss possibilities of using wrong soldering flux due to wrong stickers or poor storage conditions.

In the process of soldering, both the quality of tin soldering materials and the cleanliness of the ends are closely related to the soldering effect. If there are too many impurities in the soldering tin or if there is too much dirt in the ends, poor tin soldering may also occur on the PCB. Therefore, in the process of soldering, impurities in the soldering tin should be measured in time to ensure every end is clean enough. For the disqualified soldering tin, a standard one needs to be replaced.

In addition to the above conditions, there is another circumstance which is similar to poor soldering tin in PCB, which is called tin soldering stripping. It usually occurs in the tin-lead plating board, which is rather similar to poor tin soldering conditions. Different from poor tin soldering, most of the soldered tin which are already plated on the board is stripped back to the tin solder machine when the soldering tin surface is separated from the tin wave. Compared to poor tin soldering, tin soldering stripping is more serious, because once it occurs, it may not necessarily improve the board even by re-soldering it. Therefore, engineers must return the PCB board back to the factory for repairing once the problem occurs.