● Micro-chemical stripping
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It happens sometimes that some parts are covered with solder mask, where no solder resist is needed, such as testing points, grounding pads and assembly pins. However, it does not mean that these boards are useless. There are several safe and reliable methods to remove the extra solder mask: scraping, milling, micro-grinding and the chemical stripping. These most common used methods have both advantages and disadvantages. This article will have a simple comparison among these methods.
Solder Mask Removal
Scraping doesn’t require special technology or special settings, but it’s a noisy method. Generally a skilled technician can remove the solder mask from unwanted areas just with the help of a knife, scraper or chisel. However, if the unwanted areas are large, the operator will feel tired. So it is commonly used for the removal of thin soldermask layer. This method can be used with other methods to clear the solder mask in the last step.
Have you ever used milling machine to remove the solder mask? It seems extreme, but it is a very efficient and accurate method for removing solder mask. Because of using sharp cutter, the accuracy must be controlled. The milling system should be equipped with a microscope for visual assistance. Carbide vertical milling cutter is the most common used tool, because it is sharp, and can easily enter the coating and reach the surface of the board. From the opposite direction to turn round cutters is an effective way to control the depth, and the skills and experience of the operator are particularly important.
For removal of solder mask in a large unwanted area on the circuit board, this is a best technology. In the market there are several small desktop systems to remove the solder resist. By means of a pencil-like hand piece abrasive material will be moved forward. The abrasive material just rubs the soldermask layer. The main step of this process is the friction, and thus will produce electrostatic charge. If the circuit boards are equipped with electrostatic sensitive devices, the micro-grinding system must be able to eliminate potential electrostatic damage. In order to remove the abrasive material on circuit board, it must be thoroughly cleaned.
This method is the most effective way to remove the solder resist on the copper surface. The stripping area should be isolated to the other parts of the circuit board surface. And then, apply the chemical stripping agent with a brush or cotton swab. Since the stripping agent is liquid, it is often difficult to control. The chemical agent will erode and decompose the solder mask. Chemical stripping agent generally comprising dichloromethane is a potent solvent. Dichloromethane cannot only quickly remove the solder mask, if the delay time is too long it will also corrode the substrate. For these reasons, the chemical stripping agent must be very carefully used.