What is Solder Mask Layer in PCB Manufacturing?

Do you need your PCBs to function well? Of course, you do! That’s why you want to know more about solder masks – essential layers of PCBs.

Soldering mask is a complex process that requires a complete understanding of the processes, materials, and pre and post-procedures. Without such knowledge – fabricating PCBs and any other electronics is barely possible. You will learn the basics of soldering, its methods, applications, and many more from this post.

WHAT IS A SOLDER MASK LAYER

For starters, printed circuit boards are sandwich, laminated structures comprising multiple layers. These include a substrate, copper, solder mask, and silkscreen ones.

Basically, a solder mask is a protective layer of a photoimageable polymer applied onto copper surfaces/traces of a PCB.

This lacquer-like thin coating ensures that copper pads are exposed and can transfer electrical signals. Also, a solder mask should prevent copper from deteriorating over time.

TYPES OF SOLDER MASK

Soldering can be performed using four basic types of solder masks. Let’s review them in detail and specify the ways they can be applied to PCBs. We will return to a standard application procedure and innovative ink-jetting at the end of the post.

Top and Bottom side Solder Masks

Top and bottom-side masks are typically used to cover detected openings in the previously applied soldering layer. In other words, such a method helps to fix deteriorations in a PCB’s soldering.

Unprotected traces can be identified visually – by the sign that they are not covered with a colored (usually green) layer. Alternatively, PCBs may pass a quality testing procedure. It will help to find out that components are malfunctioning and require them to be covered.

Traces on the top side of the board can be covered by using a top-side mask. Ones on the bottom sides are processed with a bottom-side mask, respectively.

The application process is basically adding a molten solder mask through the epoxy or film methods. All of them are discussed further. Engineers may either coat the entire PCB one more time. Or, it is better to add adjustments in the solder mask design to coated only unprotected traces.

Epoxy Liquid Solder Masks

It is the most straightforward and the most affordable method of soldering a PCB. It is done with the use of silkscreen printing.

In this case, the epoxy, or thermoplastic polymer, is pushed through a stencil. Woven mesh is widely incorporated to create a particular stencil that ensures that only the needed traces are exposed to the solder mask. In other words – woven mesh supports ink-blocking patterns.

As a result of the process, the open areas are completely coated with liquid epoxy with a certain degree of accuracy. This method is not considered a technologically advanced and precise one. But still, it mostly ensures the appropriate functioning of PCBs.

Liquid Photoimageable (LPI) Ink Solder Masks

LPI solder masks are mixes of ink compounds with multiple additives. Commonly, the substances are combined before the application process.

LPI solder masks may be either silkscreen-printed or spray-coated onto a PCB’s surfaces. The first method is very much similar to the above-described one.

In its turn, spray coating is just covering PCBs’ surfaces with ink-based aerosol applied by a spray machine. It is typically performed in a sealed chamber. Areas that must be left unexposed are typically protected / ink-blocked.

The interesting thing about the LPI method is that the ink-based liquid used in the process is UV sensitive. Therefore, all the PCBs are exposed to UV light to “cure” surfaces and make soldering solidify. One more time, areas onto which not soldering must be applied are protected from the mask and the UV light with a black film.

Dry Film Photoimageable Solder Masks

This method is of particular importance for the permanent protection of the outer layer of a PCB. Dry firm soldering is basically laminating a PCB using a polymer photoimageable solder mask.

For this purpose, the film is applied to components using a vacuum generating machine. Then, it is exposed to UV light to fully solder. The dry film already has openings in the places that must be left unexposed.

The process is rather simple and effective but suits only the perfectly flat, rigid PCBs.

SURFACE FINISH

Surface finish is a crucial procedure done with any PCB. It enhances components with multiple properties.

Why Surface Finish is So Essential

In the context of soldering a component, a proper surface finish also plays a vital role. Here are two main reasons to prove this claim.

  • Surface protection

Basically, as PCBs’ main layer is made of copper, it can be oxidized easily. In case of such deterioration happens, the soldering layer will not perform as expected.

The surface finish enables it to protect the copper from rusting and PCBs’ components from losing soldering bonds. In other words – PCBs just cannot be utilized in case no surface finish was applied.

Additionally, a proper surface finish enhances the shelf life of a PCB. It also increases the number of thermal cycles that an electronic component can sustain.

  • Functionality and appearance enhancing

Besides protection, surface finishes positively impact the functioning of a PCB overall. Depending on the operating environment, one or another type of finish may extend the expected period of PCB exploitation.

Also, some finishes can enhance the appearance of a PCB. They can make components look silvery, shiny, and visually attractive.

Methods of Surface Finish and Their Impact on Solder Mask

We also detail some methods of PCB surface finish that are available in the table below.

Types of Surface Finish Description
Metallic surface finish – process PCBs with the use of metal elements. HASL (Hot-Air Solder Leveling) A PCB is dipped into Tin-Lead molten solder, so the surfaces are fully coated with it. The excess solder is removed using hot airait knives.
It is a low-cost, well-known process. It ensures great solderability but is not suitable for fine pitch PCBs – ones with a high number of components per square inch.
Lead-free HASL It is mostly the same as HASL but utilizes Tin-Copper or Tin-Nickel molten solder.
It is still great for soldering. This method enhances PCBs’ shelf life and can withstand multiple thermal cycles. However, coating thickness is inconsistent. It is not suitable for pitch PCBs.
ENIG – Electroless nickel immersion gold It implies adding a nickel plating and the following immersion in a solution containing gold.
It ensures long shelf life, consistent thickness, and multiple thermal cycles. But it is an expensive and complex method.
Immersion Silver Basically, it is the depositing a thin layer of silver onto a PCB’s surfaces. It is done through a chemical reaction.
This finishing method is good for fine pitch PCBs, and it is rather inexpensive. However, it leads to planar microvoids and occasional creep corrosion.
Organic surface finish – does not apply metal elements onto PCBs OSP – Organic Solderability The thin layer of water-based organic aromatic compound. It is applied to a PCB as a spray.
Preservative The drawbacks – processed PCBs are difficult to test, not that reliable, and can sustain two thermal cycles only.

APPLICATIONS OF SOLDER MASK

Solder masks have a particular intended use. It is an obligatory component of a PCB and has the following purposes:

  • Protection from corrosion

The basic goal of soldering is to ensure that oxidation-vulnerable copper does not deteriorate over time. In case a solder mask is not applied, a PCB will likely stop working in a week or two after being crafted.

  • Protection from environmental influences and contaminants

Besides oxidation, PCBs are subject to environmental conditions during operation. It may include temperature fluctuations, moisture, dust, physical stresses, biological and metallic contaminants, etc.

Solder mask effectively minimizes such influences and ensures the long-lasting electronics component as PCB.

  • Functionality enhancing

In cases when a solder mask is applied inappropriately, it may result in multiple solder bridges occurring. They interfere with the normal functioning of a PCB.

In contrast, good soldering makes PCBs not deteriorate in their functioning over the years. Solder masks protect copper and substrate-grounded elements of a PCB.

  • Enable finishing/adding of other layers

Silkscreen can be printed onto copper, of course. It is even known that applying a PCB legend to a solder mask may increase the cost of such a procedure. However, depending on the type of PCB, it may not be possible to apply silkscreen on bare copper at all.

The other important nuance is that the types of PCB surface finish discussed above are intended to be performed after soldering. Without soldering, there is no appropriate finish. Without finish, there are no multiple advantages of PCBs.

In essence, soldering is nearly the obligatory stage of fabricating a high-quality, long-lasting PCB.

STEPS FOR APPLYING SOLDER MASK

In this section, we’ll review the steps to DIY apply a solder mask to a pre-prepared or old PCB. In this case, we’ll stick to the LPI method and be using the common household materials. If you follow the stages precisely, your newly reconstructed/DIY fabricated PCB will perform quite well.

Materials Needed to Solder a PCB

Before you proceed to the soldering, make sure you have the following tools and materials:

  • Rusty/Newly crafted PCB.
  • UV curable (photoimageable) solder mask resin.
  • Transparency film.
  • Isopropyl alcohol.
  • Paper towels.
  • Gloves.
  • X-Acto knife.
  • Silicone mat.
  • Cutting mat.
  • Superglue.
  • Ruler.
  • Tupperware tray.
  • Sharpie marker.

So, let’s get down to business. Here are the detailed stages of PCB soldering:

Solder Mask Design Preparation

1. Making a DIY template

Place a transparent film onto your PCB and copy pads and vias of the design using a sharpie. Ensure that pads are exceptionally dark to benefit the quality of further UV light curing.

Employ this manual method only if your PCB’s design is simple. If it is not – proceed to the next step.

2. Making a CAD template

If you are stuck with an intricate PCB design, you may make a custom template using CAD software. Autodesk Eagle – for instance.

Model your PCB and isolate pads and via layers. You may also want to locate mounting holes in the digital design, but it is not obligatory. Then, save your design as a PDF file

3. Printing a design on transparency film

In case you resulted with a PDF file from the previous step – just print it. You may use any printed material for this purpose. Just mount your film on the paper and press “Print”. And you are good to go.

If it did not work – change the paper type to glossy paper, you select the highest printing quality available. It will ensure that ink sticks to the film well.

4. Cutting an artwork out

Use an X-Acto knife and a ruler to cut out the artwork. Make sure to leave spaces around it so you can mount a film over a PCB properly.

5. Doubling the mask

In case you printed your design on the film, make sure you doubled it using a sharpie marker. The ink may not be enough to protect your soldering from UV light fully, so make the design on the film as dark as possible with the marker.

6. Clean your PCB

Make sure your PCB does not have any rust and dust. Any contaminants may deteriorate the quality of soldering and result in poor performance of a PCB further. You may want to use a scotch-brite pad to polish your PCB entirely.

7. Apply Resin

Place your PCB onto a piece of transparency film, and apply resin to it.

Remove the protective cap from the tube with the solder mask, and press against the plunger to disperse some resin.

8. Spread and squeegee resin

It is one of the essential steps of the process. Use another piece of transparency resin and lay it onto the PCB with the resin applied. Press onto the film. In this way, you will evenly spread the solder resin across the PCB’s surface.

Then, use a squeegee to spread the resin even better. Make sure all the corners are covered. Do not press too hard not to leave uncovered areas. Scrap the excess resing off the edges of the PCB.

9. Placing a film

At this stage, add drops of superglue on the corners of your prepared film with the design. Then, accurately place it onto your PCB (that is still located onto another piece of film). Make sure the PCB design is well located.

You may want to water parts of the film with alcohol, so they stick to each other better. But still, use glue drops on the corners to firmly stick top and back pieces of film to each other.

As a result, your PCB may be between two pieces of transparency film, the top of which is with a design accurately covering the PCB.

10. Expose the mask to UV light

This stage is simple. Just use a UV flashlight that is powerful and large enough to cover the entire PCB. Keep the flashlight about 2-3 inches away from the PCB and expose your solder mask to the UV light for 10 seconds.

Use instructions from the supplier of the photoimageable resin for learning more accurate conditions of UV exposure, as they may significantly vary.

11. Peel the film

Gently peel the top film away from the PCB. In case you see the resin is still liquid – you underexposed the solder mask.

To get the best outcomes, you may need several attempts to find out how much resin and how long the exposure time should be.

12. Get rid of the uncured resin

You may want to scrap uncured resin out of the PCB manually. Also, you may just submerge it into a container with isopropyl alcohol. Shake PCB in the container gently to help the resin separate from the cured solder mask.

13. Rub your PCB

Scrub away the remaining resin with a paper towel. You must see that copper pads are unexposed due to the marked design you prepared in the early steps.

14. Clean the holes

You may want to use a tool to poke out resin that is clogging holes in the solder mask applied to your PCB.

15. Final UV light cure

Expose the PCB one more time to the UV light. This way, you will make sure that your solder mask is fully hardened. It will help it to last for a longer time.

That’s it. As a result, you should have a finished PCB of a custom design that is ready to have components mounted onto it.

SOLDER MASK PROCESSING WITH INKJET

In recent years, multiple methods of ink-jetting started to replace conventional masking techniques. Inkjet machines are able to print solder masks or any other markings onto surfaces, including PCBs.

Basically, solder mask printing for rigid PCBs is called direct jetting or selective solder mask depositing. Inkjet equipment units can simply print a molten solder mask onto a PCB’s surface according to a pre-modeled design.

Ink jetting machines employ piezoelectric heads – microscopic actuators built behind the print nozzles. When an electrical charge is applied to them, they extrude precise volumes of ink/solder.

This method ensures the absence of solder bridges that leads to PCBs’ malfunctions. It is also a material-effective method as it minimizes the wastage of solder masks. It also does not involve labor effort so that any fabrics can handle this process error-free.