Blog  /  Solder Mask: A Protective Layer for Copper Traces in PCBs

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.

Circuit boards contain multiple copper pathways for conducting electricity, and these 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 on 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.

 

Green solder mask applied on a PCB

Green solder mask applied on a PCB

Source: Wikimedia Commons

 

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. You can also use it to attach component pins to the board.

 

Solder paste viewed under a microscope

Solder paste viewed under a microscope.

Source: Wikimedia Commons

 

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 process, and is cheap to produce. However, there are other excellent color options for complete customization.

 

Red solder mask on a PCB

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 have an epoxy liquid material due to low cost and extreme heat resistance.

 

Types of Solder Mask

 

There are four main types categorized based on the application of solder masks.

 

Top and Bottom Side Masks

 

Top and bottom side masks enable electronic engineers to spot openings on the already applied green solder mask layer. They use the epoxy, film, or ink techniques to add the new layer, 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.

 

Printed circuit board Top and Bottom layer.

Printed circuit board Top and Bottom layer.

Source: Wikimedia Commons.

 

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 that involves using a woven mesh to hold ink-blocking stencils.

The setup allows the ink to drop to the 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.

 

Screen Printing machine

Screen Printing machine

Source: Wikimedia Commons.

 

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 undergo thorough cleaning and monitoring 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 by using 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

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 to layer copper in the holes and trace areas, then an application of tin layering protects the copper lines. Next is the removal of the dry film followed by 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.

 

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 in 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 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.

The purpose of this clearance is to provide enough spacing (solder dam) to 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.

 

SMD circuit (SO case) fitted on a wave soldered printed circuit board

SMD circuit (SO case) fitted on a wave soldered printed circuit board

Source: Wikimedia Commons.

 

Pads

 

A solder mask-defined pad has a smaller mask opening than the 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-resist.

 

Solder Mask Opening

 

As the name suggests, a solder mask opening is a hole within the protective layer that exposes the copper traces to tin soldering. The hole must be at the precise spot to avoid unnecessary exposure of 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 there is no space between the pad and 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

 

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

 

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

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 don't need solder masks
  • 2 PCBs have T or FT mask requirements
  • 3 circuit boards have H or FH solder masks needs

 

Orange flex circuit board with white connector

Orange flex circuit board with white connector

 

Applications of Solder Mask

 

  • Protects the PCB against oxidation and corrosion
  • Prevents shorts from solder bridges by acting as a barrier or insulation

 

Solder Bridges

Solder Bridges

Source: Flickr

 

  • 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 masks 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 sections of the circuit board, 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.

 

Coating thickness measurement system

Coating thickness measurement system

Source: Wikimedia Commons.

 

Pre-hardening

 

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

 

Imaging and Hardening

 

Manufacturers usually use a laser plotted photo film to do the imaging and define the solder masks area. After solder ink coating and tack drying, the film gets aligned to the panel, then exposed to UV light.

The opaque area of the solder masks allows the UV light through, which hardens (polymerizes) 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 also 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 advancements in technology 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 used in 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

Blue solder mask on a PCB

 

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

Therefore, you need to 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 that the chemical only hits the localized spots. It might be easier to order a new board in some instances rather than wasting too much time and effort doing the stripping.

 

Physical Scraping

 

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

 

Knife set

Knife set

 

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