Temporary masking material is required when you need to protect PCB, plated thru-holes, contacts, pins, posts, terminals, and gold fingers during manufacturing process. For example, they provide short-term high-temperature protection from molten solder to 268ºC. Reliable solder masks are nonflammable, non-contaminating and will not leave ionic or corrosive residue.
Common uses of temporary masking materials
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Masking open PCB thru-holes in wave soldering process:
Circuit boards are often designed to work within multiple different product models, so different feature sets may require some areas to be populated and others unpopulated. To make sure these open vias stay open (and not soldered in the wave soldering process) mask is applied over the open areas. The wave soldering process isn’t discriminating, and will solder anything that is metal and fluxed. Solder will not stick to mask, however, so the area underneath will be unaffected.
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Masking contact areas or solvent-sensitive components:
When applying conformal coating to a circuit board, it is important to avoid contact areas. Coatings are generally insulative, so will impede the function of contacts, switches, and anything that relies on metal-to-metal contact. Components that are solvent-sensitive or which have moving elements may also need to be masked. This is less necessary in selective spray systems, but if applying coating from an aerosol can, or through a hand sprayer or dipping process, masking is often required.
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Damping around low stand-off components:
SMT (surface mount technology) and BGA (ball grid array) components often have very low stand-off areas– the space between the component and the surface of the board. Even when applying conformal coating with a selective spray system, you run the risk of material being drawn beneath the component through capillary action. For some designs, this may be problematic, and one solution is to mask surrounding the component to create a dam.
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Temporary adhesive for soldering double-sided SMT PCBs:
A significant challenge of assembling double-sided SMT circuit boards is how to solder components on the bottom-side of the board. Running the top-side of the board through the reflow oven isn’t a problem because gravity helps hold the components in place as the solder melts and then solidifies. When the board is flipped to run the bottom side though, you are counting on the surface tension of the solder to hold components in place. Although that may work for smaller components, larger components and BGA corners may need spot mask as an adhesive.
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Protecting temperature sensitive component in the reflow process:
Because latex mask is thermally insulative, creative engineers use it to protect heat-sensitive components in the reflow process. However, since this goes far beyond the intended purpose of temporary solder mask, solder mask manufacturers will probably be of little help qualifying the product for this application.
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Masking areas in painting, powder coating or plating process:
If you picture temporary solder mask as liquid masking tape, it opens up a world of potential applications. Again, it is well outside the intended purpose, so you will need to test and qualify for yourself.
Type of solder masks
There are various types of solder masks used for different manufacturing processes with different requirements. Based on your specific application, you can rely on the advantages and disadvantages of each masking material to choose the suitable type at a reasonable cost.
Solder masks can be divided into the following categories:
Masking Tape: Masking tapes are made of thin paper or plastic film with an adhesive backing. Masking tapes come in various widths and adhesive strengths, making them suitable for different applications.
Application methods
For small production, the main methods to apply temporary solder mask are by hand. Just simply squeeze the bottle or place the tape to apply mask to the areas that need protection.
For large scale production operation, solder mask can be sceen printed over large areas of the circuit board, similar to solder paste printing process. Of course, this method only applies to liquid solder masks. Screen printing has the potential to drastically increase production throughput, the printing screen can be re-used many times before it needs to be completely cleaned.
Screen printing with the solder mask will involve testing with a number of screen mesh sizes to determine the best mesh size for the degree of detail required for the specific application, while taking into consideration the viscosity of the mask to be used. If the mesh size is too small the viscosity of the mask will prevent its movement through the holes in the screen. If the mesh size is too large, too much mask will be applied, which may result in spreading of the mask into areas of the board that should not be masked.
In addition, you can consider an automatic dispensing method. Normally, a dispensing system using compressed air will be used. In a pneumatic dispensing system, the mask is pressurized with either compressed air or nitrogen and forced out of a dispensing tip or needle. The areas of concern when using this methods are the potential of clog the tip or needle and the ability of curing solder mask in the tank.The solder mask used must be the material with a low shear cure. In the case that the solder mask cures in the container, the cured material should first be removed before continuing to use.
How to remove temporary solder mask?
After the masking material has performed its function, for example after a soldering or conformal coating process, it is then removed.
Peelable mask is usually removed by hand or with tweezers.
Washable mask is engineered to be removed in an aqueous inline or batch wash system. If you are using a closed-loop system, which filters and recirculates wash water, make sure you use a mask that is compatible with your physical filters and ion beds. If you experience foaming, it is an indication that either too much mask material is being passed back into your cleaning system or it is interacting with dissolved flux in the same wash water. This is generally solved by replacing your wash water with fresh DI water, but a defoaming agent can be added as a stopgap solution.
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