Solder Mask: A Protective Layer for Copper Traces in PCBs

Whenever you think of making a printed circuit board, it should be a durable component that lasts as long as possible in your device. One of the best ways to maximize board surface protection for maximum durability is to use a solder mask. Custom printed circuit board contain multiple copper pathways for conducting electricity, which can oxidize or short circuit if left exposed. A solder mask forms a thin outer layer that covers and protects all the electrical lines and pads.

Here's all you need to know about this protective layer.

Solder Mask vs. Solder Paste

A solder mask is a thin layer applied to the copper traces of a circuit board to form a lacquer-like covering.

It contains an epoxy-based polymer that protects the electrical lines from oxidation and prevents solder bridges between closely placed solder pads.

On the other hand, solder paste is a creamy mix containing tiny solder spheres held in a solder flux.

The paste helps to connect surface-mount electronic components to the board because it forms mechanical bonds when heated.

It can also be used to attach component pins to the board.

Solder Mask Colors

A solder mask determines the color of the PCB. The green color is the most common because it performs better than the rest, reduces eye strain during the inspection, and is cheap to produce.

However, there are other excellent color options(red, blue, black, white, yellow, and purple) for complete customization.

Red solder mask on a PCB

Solder Mask Materials

The solder mask material used on a board depends on its applications, physical dimensions, components, and hole sizes. Most boards use an epoxy liquid due to its low cost and extreme heat resistance.

Types of Solder Mask

There are four main types categorized based on the application.

Top and Bottom Side Masks

Electronic engineers use top and bottom side masks to spot openings in the green solder mask layer applied.

They add the new layer using epoxy, film, or ink techniques and then solder component pins through the identified holes.

The conductive trace patterns on the upper side of the board form the top traces, and the applied layer is known as the top-side mask.

A similar layer specifies the holes on the lower side of the board, forming the bottom side mask.

Epoxy Liquid Solder Masks

The epoxy liquid is the lowest-cost solder mask type and is a polymer silkscreened to the board surface.

Silkscreening is a printing technique using woven mesh to hold ink-blocking stencils.

The setup allows the ink to drop to specific points to form the pattern, and instead of silk, electronic applications use synthetic fibers.

Thermal curing is the final step in the process, and it helps to attach the new layer permanently.

Liquid Photoimageable Solder Mask (LPI)

A liquid photoimageable solder mask combines two different liquids that get mixed just before application to maximize the shelf life of the resulting ink formulation.

Engineers can silkscreen, curtain coat, or spray the ink to the panel surface, after which the board will not require any surface finish.

However, unlike epoxy liquid, LPI is sensitive to UV light, so after a short tack cure cycle, the board gets exposed to UV using a photolithography process or UV laser.

Before the process starts, the panels must be thoroughly cleaned and monitored for signs of oxidation using an aluminum oxide solution or suspended pumice.

One of the techniques uses contact printers and film tools to expose the board to UV light. The process begins with top and bottom sheet emulsion printing to block the soldering areas.

Next, the film and production panels get fixed in position, followed by simultaneous tooling and UV light exposure.

Such masks are available in multiple colors, including black, yellow, white, red, green, and blue.

Green pcb

Dry Film Photoimageable Solder Masks

The dry film photoimageable solder mask requires vacuum lamination for application, after which the film undergoes exposure and development.

Holes get identified post-development and create a pattern for soldering components to the copper pads.

Electrochemical processing helps layer copper in the holes and trace areas, then applying tin layering protects the copper lines.

Next is the removal of the dry film, followed by the etching of the exposed copper and thermal curing.

This type is ideal for high-density wiring boards because it does not flood the through-holes.

PCB Solder Mask Application Techniques

The four common application techniques are as follows.

Screen Printing

When screen printing, you apply the lacquer-like material using a mesh or stencil and push the mask through the screen’s apertures using a squeegee.

This simple process is cost-effective, but its manual nature makes it unsuitable for large-scale production.

A laser-cut stainless steel stencil

Curtain Coating

This technique is ideal for modern boards that have narrow track widths and higher track densities, and it involves coating the board with an LPI (Liquid PhotoImageable).

The board passes through a sheet of this liquid pouring from a small slit, which reduces material losses.

Spray Coating

The name says it all here. Spray coating involves using a nozzle or spray gun to apply the solder mask on the board.

This technique gives you precise control of the application process, which is handy when coating irregularly shaped PCBs.

But it can be wasteful, and the equipment requires frequent cleaning.

Electrostatic Spray Coating

This spray coating process is similar to the one above but the masking material gets charged with an electric field before spraying.

Once sprayed, this material sticks to the grounded board and forms a precise, uniform coat above the surface.

Standard Thickness of a PCB Solder Mask Layer

The standard solder mask layer thickness is 0.8 mils, but it can be thinner around corners and near the edges of traces, where it can drop to 0.3 mils.

You can avoid these variations by using the regular spray coating or electrostatic spray coating application methods, which create a more uniform solder mask layer thickness.

The goal is to make this layer at least 0.5 mils thick to prevent copper trace exposure and SMT issues.

However, it should not be too thick, as well, because it can misalign the electronic components and compromise the quality of the solder joint.

Solder Mask Design Guidelines

Here are a few parameters involved in the design of solder masks.

Tented Vias

A tented via is a hole covered with a solder mask to prevent exposure. However, unlike filling, the via only has covered ends on the annular ring, so there is a cavity.

Otherwise, it would be a mask plugged or filled via, depending on the technique used to seal it.

The purpose of tenting is to:

• Cover and protect as many conductive pads as possible to reduce the chances of bridging during assembly
• Minimize the chances of via(hole) damage in the operating environment
• Reduce the chances of paste migration in dog bone standard BGA patterns or when vias are on the edges

Tenting vias are the most preferred option for protecting circuit boards due to their low cost.

Since it has different methods, the LPI (Liquid Photoimageable) solder mask is the most popular because it is the cheapest.

Solder Mask Clearance

The space between the paste mask and the circuit board surface components is the solder mask clearance.

You can also define it as a tolerance to determine how close the two layers should be to each other.

This clearance provides enough spacing (solder dam) between the surface features so that the mask can prevent solder bridge formation.

In most cases, solder mask dams should be half the width of the conductor spacing, such as 50µm for conductive layers measuring 100µm in width.

Pads

A solder mask-defined pad has a smaller mask opening than a copper pad. In comparison, the non-solder mask-defined type has a space between the mask and pillow.

Therefore, the mask pad clearance determines the size of the copper pad used in ball grid arrays in the former.

The solder mask process must consider fitting tolerances to ensure the mask clearance is always more expansive than the solder pad.

This design will enable an optimal soldering process and a place free of solder resistance.

Solder Mask Opening

As the name suggests, a solder mask opening is a protective layer hole exposing the copper traces to tin soldering.

The hole must be at the precise spot to avoid unnecessary exposure to the copper, which might lead to damage, corrosion, or shorting.

Most fabricators leave a 1:1 opening with the copper pad to easily alter the appropriate manufacturing process.

Solder Mask Coverage or Expansion

Solder mask coverage or expansion is a specification that allows you to adjust the gap between the solder mask and surface features using the design software. It can be zero, positive, or negative.

Zero Solder Mask Expansion

Zero expansion means no space between the pad and the solder mask.

Positive Solder Mask Expansion

If there is a gap between the outer uncovered pad circumference and the end of the solder mask, it defines the positive expansion.

Negative Solder Mask Expansion

Negative expansion implies the solder mask overlaps a section of the pad.

IPC Standards for Solder Masks

The application of the solder mask falls into these categories as per the IPC-SM-840 performance and qualification specification of permanent solder mask and flexible cover materials.

Telecommunications - T

Computers, phones, and telecommunication devices fall into this category.

The overlays and solder masks used in these devices are ideal for high-performance commercial and industrial applications.

However, any breakdown will not lead to life-threatening situations.

Telecommunication devises board

Military/High Reliability - H

The devices and equipment that fall into this category are critical, and interrupted service can lead to life-threatening situations.

Thus, these devices have tough overlays and solder masks to withstand vital and demanding applications.

Flexible Printed Board Applications (Telecommunications) - FT

A similar application to telecommunication devices as described above, but the solder masks used is for the flexible boards.

Flexible Printed Board Applications (Military/High Reliability) - FH

This standard applies to the solder mask used in the flexible boards found in critical military equipment and devices.

The solder mask material requirements fit three classes when dealing with bare boards.

• One board doesn't need solder masks
• 2 PCBs have T or FT mask requirements
• 3 circuit boards have H or FH solder masks need

Orange flex circuit board with white connector

Common Solder Mask Issues With PCBs

Four issues can arise if you don’t apply the mask correctly, the first being solder bridging.

A solder bridge is an unintended electrical connection between two solder joints, which can short the board or components.

Bridging can occur if you fail to leave enough space between the solder mask and the SMT pad or if the components are misaligned due to a thick mask layer.

Solder balling can also occur when a tiny piece of solder pins itself to the board’s surface when soldering.

If the mask layer is too thick, it can hamper the flow of solder during reflow soldering, causing these solder balls to form on the surface.

A reflow oven machine

Solder can also build up excessively above the pad if the SMT electronic components placed above them are misaligned.

Remember, if the mask is too thick, it can misalign the components.

Lastly, if the mask layer is too thick, it forms a cavity around the pad that traps flux vapor to form a bubble.

When soldering, the molten metal cannot displace this vapor easily to form an electrical joint between the pad and the component, resulting in a solder skip.

How To Distinguish Solder Mask, Surface Finishes, and Conformal Coating

The difference between the three is simple. Solder mask gives the PCB its color and protects the entire circuitry from oxidation or corrosion. Additionally, it prevents the formation of solder bridges on the exposed circuit sections.

A PCB with an ENIG surface finish

On the other hand, surface finishes are metal or organic coatings layered above the bare copper metal on the solderable areas to prevent oxidation during storage (before assembly).

This finish also provides a solderable surface during assembly.

But a conformal coating is paint protection that covers the PCB in its final production processes to insulate the circuit’s function and mounted components from environmental conditions.

Applications of Solder Mask

• Protects the PCB against oxidation and corrosion
• Prevents shorts from solder bridges by acting as a barrier or insulation
• Protects the PCB from contaminants
• Prolongs the shelf life of the circuit board
• Reduces the amount of solder paste required for soldering
• Stops the development of metal whiskers, which can cause malfunctions or shorts
• Raises the breakdown voltage of the dielectric material

How Is A Solder Mask Applied?

The following steps describe the solder mask application process on a PCB.

Board Cleaning

Cleaning removes dirt and contaminants, after which the surface gets dried.

Solder Masks Ink Coating

Factors like the PCB reliability requirements and application area determine the coating thickness.

The coating process occurs on a vertical coater, and the thickness might vary on different circuit board sections, such as on the foils, substrate, and traces.

The thickness also depends on the PCB manufacturer's capabilities and the equipment used in the process.

Pre-hardening

Pre-hardening makes the solder masks coat relatively solid, which enables the removal of the unwanted mask layer. This unwanted coat is easily removed from the board in the developing stage.

Imaging and Hardening

Manufacturers usually use a laser-plotted photo film to visualize the solder mask area. After solder ink coating and tack drying, the film is aligned to the panel and exposed to UV light.

The solder masks' opaque area allows UV light through, hardening (polymerizing) the ink underneath.

If using Laser Direct Imaging (LDI), you don't require photo films because the UV light will harden the areas retaining the solder ink directly.

Developing

This process involves dipping the PCB in a developer to remove any unwanted solder masks while ensuring the openings expose the copper foil in the required areas.

Final Hardening and Cleaning

The last step is hardening and cleaning. Final hardening is vital because it makes the solder mask ink visible after mounting to the panel surface.

Next, the solder masks-covered board goes through a cleaning process before applying the surface finish.

Solder Masks Processing With Inkjet

Circuit board manufacturing traditionally used inkjet systems to imprint the markings, but recent technological advancements brought in Direct Jetting.

DJ is a printing technique that enables manufacturers to print the solder masks directly on the board according to the design input.

The process uses a piezoelectric head and eliminates most of the steps in the inkjet's photolithography, leading to these advantages.

• Environmentally friendly processing
• Economical use of solder masks
• Reduced equipment and process variables
• Minimal material usage

How Do I Remove Solder Mask From My Board?

Blue solder mask on a PCB

Mistakes can happen when layering the mask, such as covering parts that should remain exposed.

Therefore, you must be thorough before sending your design files for manufacturing because removing the mask is difficult after curing. However, it is possible. You can use one of these methods to remove the layer.

Chemical Stripping

A chemical like methylene chloride can corrode the solder mask's layer but will damage the laminate and copper layers below if it goes past the covers. Therefore, you should not expose the circuit board for an extended period.

Also, tape the areas that don't need stripping so the chemical only hits the localized spots. In some instances, it might be easier to order a new board rather than wasting too much time and effort on stripping.

Physical Scraping

Physical scraping is risky because it is easier to scrape than the solder mask layer. But it is possible, and you can use a Dremel rotary tool, X-Acto knife, or razor for the task.

Knife set

Alternative Solder Mask Materials or Methods for PCBs

The epoxy liquid is the traditional mask material, but you can also use these alternatives.

Peelable Solder Mask

This temporary mask is applied as a liquid before soldering, cured to dry, and then peeled off after soldering.

Its purpose is to protect areas like via-holes and connectors during the soldering process and prevent issues like solder bridging and splashing.

But this mask is not suitable for dense boards with fine pitches between components and it can damage the surface beneath or leave marks on removal.

Solder Paste Stencil

Using solder paste stencil as a mask is rather unconventional but can work. A stencil is a plastic or metal screen with holes cut out to match the board’s solder pads for solder paste application.

Its primary purpose is to help with solder paste application while avoiding excesses or insufficient application.

Since the screen is reusable, you can repurpose it to cover and protect the entire circuitry, except the pads, which get a surface finish.

However, the copper can corrode and oxidize since the stencil does not form an airtight seal on the PCB surface. So, this solution is not permanent, but it can protect dense, fine-pitch PCBs for a short time.

No Solder Mask (NSM)

You can forego mask application when making cheap, simple circuit boards for testing and prototyping. These PCBs will be vulnerable to issues like oxidation, corrosion, mechanical damage, and electrical faults (shorts), but these factors are not critical if you’re using them for testing or prototyping.

In most cases, these NSM boards have few connections and components to mitigate any electrical and reliability issues.

Summary

As you can see, solder masks form vital protective layers for the copper traces and conductive lines in a circuit board and are essential in the PCB manufacturing process.

Therefore, contact us today to get started if you want a professionally made circuit board with this protective layer applied correctly per your designs.