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Ultimate Guide to Temporary Solder Mask

What is Temporary Solder Mask?

Temporary solder mask (also called “spot mask”) is used every day in the assembly of electronic circuit boards. Solder mask is engineered to protect thru-holes or “vias” in wave soldering. It has to withstand extreme soldering temperatures and still be removed easily and efficiently at the end of the PCB assembly process.

If you are spraying or dipping conformal coating, mask can be used to protect connectors and other areas that can’t be coated.

Not to be confused with “permanent” solder mask, commonly known as solder resist, temporary solder mask is engineered to be removed. Temporary solder mask is an essential tool for automated soldering processes.

Chemask® Solder Mask products protect PC boards, plated thru-holes, contacts, pins, posts, terminals and gold fingers during wave soldering. They provide short-term high-temperature protection from molten solder. All Chemtronics solder masks are nonflammable, non-contaminating and will not leave ionic or corrosive residue. Chemask® Solder Masks are available in convenient squeeze bottles for a precision manual application or in gallons for automated applications.

 

What are The Common Uses for Temporary Solder Mask?

  1. Masking open PCB thru-holes in wave soldering process – This is the most common use for spot mask. 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.
  2. 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.
  3. Damming 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.
  4. 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.
  5. Protecting temperature sensitive-components 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.
  6. 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.

 

Why Should I Use Spot Mask Instead of Kapton tape or a masking boot?

Besides temporary solder mask, Kapton tape (popular E.I. du Pont de Nemours brand name of heat resistant polyimide tape) or pre-formed silicone masking boots are commonly used to cover select areas of circuit boards. Polyimide tape is popular because it is readily available and applies quickly, without worrying about cure time. A boot simply snaps onto the area to be masked. The main advantage of solder mask over these other methods is flexibility. Tape generally comes in rolls or die-cut shapes, which may work perfectly for most areas, but there are always those special circumstances. In addition, the adhesive on tape can leave residue behind that can create wetting issues for conformal coating. Masking boots are preformed, which requires more planning and lead time than many contract manufacturers can afford. Boots also need to be cleaned as they can be coated with baked-on flux, or discarded increasing costs.

 

How Do You Apply Temporary Solder Mask?

The main methods to apply temporary solder mask are by hand (with a squeeze bottle or tube), with a pneumatic system, or stenciled.

Manually / hand application

This application method is as simple as it sounds. You squeeze the bottle to apply mask to the areas that need protection. However, believe it or not, the way you hold the bottle can make a big difference on how easily the cured mask is removed.

Many operators hold the bottle at a 90° angle as they drag the dispensing tip across. This often forces the mask through the vias, causing three potential problems:

  1. The mask tends to stick to the inside of the thru-holes, and may mushroom out the far side. This creates a plug on the other side, which increases the chance of breakage when peeling off the cured mask.
  2. The material inside the thru-holes are less accessible if washing off mask in an aqueous cleaning system.
  3. Holding the bottle at 90° results in a thinner bead. When using a peelable mask, this bead acts as a draw-string, so a thinner strip has more of a tendency to become brittle in high soldering temperatures, and is more likely to break during removal.

Hold the bottle at 60° to have precise and light placement, with a bead thick enough to prevent breakage during removal.

Pneumatic Dispensing

In a pneumatic dispensing system, the mask is pressurized with either compressed air or nitrogen and forced out of a dispensing tip or needle. Take care that you use a mask with a low shear cure. A high shear curing mask experiences accelerated curing when pressure is applied. As the mask is forced through the dispenser, pressure will increase, which will tend to clog the tip or needle.

Another potential area of concern is the mask curing in the tank. If the mask skins over in the tank, and those cured pieces are drawn through, it could also clog the dispensing tip. Pressurizing with nitrogen can help alleviate this problem. If there is skinning in the tank or in the original mask container before the material is transferred, it is important to remove the cured material, and not mix it into the liquid mask.

Most solder masks can be thinned with DI (deionized) water to reach the optimal viscosity. Mix carefully to avoid whipping, introducing air bubbles into the mix. You’ll regret it as any air bubbles in applied mask will cause issues at soldering temperatures.

Stencil Printing

Mask can be applied with a stencil in a similar process as printing solder paste in a surface mount assembly process. Of course, this process is only feasible when there is a flat surface. When selecting a mask for stencil printing, make sure the viscosity is high enough to be controlled and not drawn beneath the stencil through capillary action. Also consider how the stencil will be cleaned. Choosing a washable mask makes this a more straightforward process. A peelable mask can be used, but you will either have to clean the stencil while the mask is still wet, or you will have to physically peel off the cured material. This is especially important when printing through a screen (i.e. screen printing). Screen printing should be avoided with very fine mesh screens because of the cleaning challenges.

 

How Do You Use Temporary Solder Mask for Conformal Coating?

If you think of spot mask as basically liquid masking tape, the application process becomes more intuitive:

  1. Apply the mask to the contact areas to protect. Allow the mask to fully cure.
  2. Apply the conformal coating.
  3. After the coating is dry to the touch, but before it is fully cured, peel off the mask.

Here are a few things to consider:

  • Peelable mask is recommended. Since conformal coating is generally hydrophobic (repels water), it will be difficult to wash off mask under a coating layer.
  • Natural latex is recommended, because synthetic mask can sometimes interact with the harsh solvents used in conformal coating.
  • For the cleanest masked edge, peel the mask before the coating is fully cured. Otherwise it could create a cracked, ragged edge, or even pull up the coating.

 

How Do You Remove Spot Mask?

Peelable mask is usually removed by hand or with tweezers. Resourceful engineers have come up with devices with rotating brushes and other creative methods, but there is no commercially available equipment for this purpose.

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.

 

Can Solder Mask be Left on the Circuit Board?

Temporary solder mask is intended to be just that – temporary. Manufacturers of solder masks, like Chemtronics, always recommend removing the mask because there is no testing to support doing otherwise. If you decide to leave the solder mask on as a permanent part of the PCB, you will have to do your own functional and reliability testing to ensure it does not cause problems.

 

How Long Should I Cure Temporary Solder Mask?

The curing step is the bottleneck of the masking process, so it will always be tempting to push the PCB through sooner rather than later. Mask cures from the outside-in, so it skins over, and then cures on the inside. This can take anywhere from 30 minutes to 24 hours depending on the type of mask and the thickness of the bead.

When uncured mask hits molten solder, the liquid element instantly vaporizes and, if the cured skin is not thick enough, will forcefully exit. This will create voids in the mask and, in extreme cases, lift the board out of the solder and throw solder beads all over the PCB. Not good!

The wave solder pre-heat zone does provide a margin of error, but it is not recommended for the total cure effort. The mask may not need to be fully cured when it is put on the wave conveyor, but it should be cured to the touch with a firm skin. Beyond that, you will need to test within your own process because it depends on the mask material, the thickness of the bead, and the thermal profile.

How High of a Temperature Can Temporary Solder Mask Withstand?

Solder mask is engineered to withstand molten solder temperature for the very brief time (about a second) it is exposed in a typical wave soldering process. Most commercially available masks should be able to withstand melting point temperatures of lead-based and lead-free solder, up to 650°F (343°C). As exposure time is lengthened, like in a reflow oven, there is more potential the mask will bubble, char, and become brittle. How long a mask can withstand extreme temperatures depends on the thickness of the bead and the type of mask.

 

Can Spot Mask Run Through SMT Reflow?

When mask is run through a reflow soldering process, it is not only exposed to extreme temperatures, but also for a longer period of time compared to wave solder. This increases the risk the mask will bubble, char, and become brittle. How long a mask can withstand extreme temperatures depends on the thickness of the bead and the type of mask. Natural latex is generally recommended for reflow. Synthetic materials tend to become brittle, and washable masks will be very difficult to remove.

 

How Do I Select The Best Temporary Solder Mask for My Application?

There are several factors to consider when selecting a solder mask:

Removal Method

The first decision when selecting a solder mask is the last step-- how it will be removed.

If you currently run your PCBs through a batch or in-line wash system, you have the option to use a water-washable mask. Chemask® WF can be removed in a batch or in-line cleaning system, and will not clog filters or damage ion filter beds.

Chemask® WF Solder Masking Agent is a high-temperature temporary spot mask that protects component-free areas from molten solder during wave soldering. It is water soluble, designed to be removed with open and closed loop aqueous cleaning systems. Chemask WF is low foaming and has no effect on deionized water (DI) system resin beds. This water-soluble formulation is stable to rosin, organic and inorganic fluxes.

If you use solvent cleaners, or don’t clean your boards at all, peelable masks are your best option.

Peelable solder masks are commonly used in no-clean soldering environments or by PCB assemblers that clean with solvents. Peelable masks are commonly used for masking contact areas and over chemically-sensitive components in conformal coating processes.

Sensitive Metals

Chemask® Non-Ammoniated Solder Masking Agent is a latex, ammonia-free, fast curing, peelable temporary spot mask formulated for safe use on sensitive metals. It has low odor and is safest on sensitive metals like copper. It contains high-temperature resistant compounds that protect component-free areas during wave soldering. Chemask NA may be used to protect pins, posts, contacts and edge connections in the solder reflow oven or during conformal coating processes.

If you are looking for a strong peelable mask that is the easiest to remove, check out our natural latex masks like Chemask Peelable, our best-selling mask.

Masking for Conformal Coating

Chemask® Peelable is ideal for masking areas from conformal coating. It is a fast-curing peelable solder masking agent. It contains natural latex formulated with high-temperature resistant compounds that protect component-free areas during wave soldering. Chemask may be used to protect pins, posts, contacts and edge connections during conformal coating processes.

Masking Large Vias

Chemask® HV Solder Masking Agent is a temporary, high viscosity, fast curing, peelable solder masking agent that works well for masking large vias. It is a high-temperature resistant compound that protects component-free areas during wave soldering. Chemask HV can be introduced to the wave solder preheat within four minutes without adverse effects. Chemask HV may be used to protect pins, posts, contacts and edge connections during conformal coating processes.

Contact us at 770-424-4888 or info@itwcce.com and we can help you find the best temporary solder mask for your requirements.

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