What is PCB Soldermask?

PCB soldermask, also known as solder resist or solder stop mask, is a thin layer of polymer applied to the copper traces of a printed circuit board. Its primary functions are:

1. Preventing solder bridges from forming between adjacent traces during assembly
2. Protecting the copper from oxidation and contamination
3. Providing electrical insulation between conductive paths
4. Improving the PCB’s aesthetic appearance with color (commonly green)

The soldermask is applied over the bare copper and developed to leave the areas around pads exposed where components will be soldered.

Why Remove Soldermask?

There are a few scenarios where removing the soldermask from a PCB becomes necessary:

• Repairs: Damaged traces or lifted pads often lie underneath the soldermask and require access for fixing. Removing the coating allows soldering direct to the copper.

• Modifications: Adding new components, cutting traces, or re-routing connections on an existing board design requires selective soldermask removal in those areas.

• Reverse Engineering: Analyzing or duplicating a PCB design starts with uncovering the copper traces to determine the circuit schematic. Removing the soldermask enables tracing the conductive paths.

• Aesthetic Reasons: Some projects call for the unique look of exposed copper on a PCB without the standard green soldermask. Removing the coating achieves this visual effect.

Methods for Removing PCB Soldermask

Several methods exist for stripping away soldermask from printed circuit boards, each with their own advantages and drawbacks. The most common techniques are:

1. Chemical Etching

One effective way to remove soldermask is by chemically dissolving it using solvents. The most popular etchant is sodium hydroxide (NaOH), also known as lye or caustic soda.

Materials Needed

• Sodium hydroxide pellets or concentrated solution
• Glass container
• Plastic tongs or tweezers
• Rubber gloves & safety goggles
• Paper towels
• Isopropyl alcohol

Removal Process

1. Don the rubber gloves and safety goggles to protect yourself from chemical burns. NaOH is very corrosive to skin and eyes.
2. In the glass container, prepare an 8-10% NaOH solution by dissolving the pellets in distilled water.
3. Immerse the PCB into the solution and let it soak for 5-10 minutes. Monitor it regularly.
4. Using the plastic tongs, remove the board once the soldermask starts dissolving and blistering up from the surface.
5. Rinse the PCB thoroughly under running water to neutralize the NaOH.
6. Gently rub the soldermask off with a paper towel. Most of it should come off easily at this point. For stubborn spots, use an old toothbrush.
7. Repeat steps 3-6 if necessary until all the desired soldermask is removed.
8. Rinse the board with isopropyl alcohol to remove any residues.

Pros

• Fast removal over a large area
• Can strip an entire board at once
• Economical consumables (NaOH, water)

Cons

• Requires safety precautions when handling caustics
• Indiscriminate removal, can’t selectively strip small areas
• May damage plastics and lifted soldermask may clog holes

Other suitable etchants for soldermask include methylene chloride, butyl acetate, and various paint strippers. However, these tend to be more expensive or hazardous than NaOH.

2. Mechanical Abrasion

For more controlled and selective soldermask removal, mechanical abrasion using tools offers a practical approach.

Materials Needed

• Rotary tool with abrasive bits (silicon carbide or diamond coated)
• Fiberglass pen or bristle brush
• Flathead screwdriver or dental pick
• Isopropyl alcohol
• Lint-free cloths or cotton swabs

Removal Process

1. Clean the PCB surface with isopropyl alcohol to remove any dirt or oils.
2. For larger areas, use the rotary tool at low-medium speed with a suitable abrasive bit. Gently grind away the soldermask until the copper is exposed. Be careful not to cut into the laminate material or damage components.
3. Switch to the fiberglass pen for finer abrasion around delicate areas like thin traces and SMD pads. The soft fibers will remove the soldermask without gouging the copper.
4. Use the screwdriver or dental pick to scrape off any stubborn bits of coating, especially in tight corners.
5. Wipe away the ablated dust with a cloth or cotton swab dampened with alcohol.
6. Inspect the board closely and repeat steps 2-5 until all the desired soldermask is removed.
7. Clean the PCB thoroughly with fresh alcohol to remove any residual particles.

Rotary Tool Bits	Description
Silicon carbide	Ideal for bulk removal
Diamond coated	Precise control
Bristle brush	Non-abrasive, for cleanup

Pros

• Precise control over removal area
• Preserves surface finish of copper
• Usable on populated boards
• No chemicals needed

Cons

• Time-consuming for large areas
• Requires steady hands to avoid damage
• Abrasive dust may be difficult to clean

Mechanical abrasion offers the highest degree of control when removing soldermask, but it sacrifices speed. This makes it best suited for small jobs or working around delicate components.

3. Laser Ablation

For quick, precise, and non-contact soldermask removal, laser ablation systems provide a high-tech solution. These use focused laser light to vaporize the coating.

Materials Needed

• Laser ablation machine with suitable wavelength (ex: 355nm UV)
• Fume extraction unit
• Cleaning swabs and isopropyl alcohol

Removal Process

1. Place the PCB into the laser ablation machine and focus the beam.
2. Program the machine with the desired ablation pattern for selective removal. This is typically done in software that converts drawings to machine paths.
3. Activate the laser and let it vaporize the soldermask in the selected areas. The high energy breaks the molecular bonds.
4. Remove the PCB once the process is complete.
5. Clean any ash residue from the surface using a swab and alcohol.

Pros

• Fast removal of complex patterns
• No physical contact with PCB surface
• Leaves a clean, undamaged copper surface
• Easily automated for batches

Cons

• High equipment costs
• Requires technical skills to operate
• Generates vapor fumes that need extraction

Laser ablation offers unmatched speed and precision for soldermask removal, but the expensive equipment makes it only practical for high-volume production or specialized applications.

Choosing a Removal Method

With multiple ways to strip soldermask from PCBs, it’s important to select the right method for your needs. Consider these factors when deciding:

• Scope of removal: Is it the whole board or just a small area? Chemical etching is best for bulk stripping, while mechanical and laser methods offer precise local removal.

• Component presence: Are there parts already soldered on the PCB? Mechanical abrasion is safest for working around delicate components. Chemicals and lasers risk damaging them.

• Equipment availability: What tools do you have access to? Chemical etching requires minimal equipment, mechanical methods need abrasives, and laser ablation uses specialized machines.

• Time and budget: How quickly do you need results and what’s the project budget? Chemical stripping is quick and cheap, mechanical is slow but economical, laser is fast but costly.

No single soldermask removal technique is perfect for every situation. It’s up to you to weigh the pros and cons and choose the best approach for your PCB needs.

FAQs

1. Is it safe to handle sodium hydroxide for chemical stripping?

When used carefully with proper protective gear (gloves, goggles, ventilation), NaOH is safe for soldermask removal. However, always exercise caution as it is very corrosive and can cause chemical burns. If any gets on your skin, rinse it off immediately with plenty of water.

2. Can I use a hobby knife to scrape off soldermask?

While it’s possible to remove soldermask with a sharp hobby knife, this risks gouging or cutting the delicate copper traces underneath. Stick to softer abrasives like fiberglass pens that are less likely to damage the PCB surface.

3. Will removing the soldermask affect the PCB’s performance?

No, removing soldermask doesn’t impact the electrical functionality of a PCB. Its main purpose is to prevent solder bridges and protect against the environment. As long as you don’t damage the copper traces in the removal process, the circuit will still work as designed.

4. How can I remove soldermask from between tightly spaced IC pins?

For tight areas around dense pins, chemical etching or laser ablation are the most effective methods. Mechanical abrasion is very difficult to do without bridging the pins. If you must use abrasion, try a soft fiberglass pen with the finest tip and work very carefully under magnification.

5. Can I re-apply new soldermask after stripping the old coating?

Yes, once you’ve cleaned off all the old soldermask, it’s possible to re-apply a fresh coating. This is commonly done after PCB repairs and modifications. You’ll need to use a screen printing process or a photoimageable soldermask and UV exposure unit to selectively coat the areas you want masked. Allow it to fully cure before handling.

Conclusion

Soldermask removal is a useful skill to have when working with PCBs. Whether you need to repair a damaged board, modify an existing design, or reverse engineer a circuit, knowing how to strip away the protective coating is essential. The three main methods – chemical etching, mechanical abrasion, and laser ablation – each have their strengths and weaknesses. By understanding these techniques and when to apply them, you’ll be well-equipped to handle any soldermask removal challenge that comes your way. Just remember to always work carefully, use the proper safety precautions, and take your time for the best results.