Solder mask, also known as solder stop mask, is a thin layer of polymer used on a PCB board to prevent the solder points from forming a bridge. The solder mask also prevents oxidation and is applied to the copper traces on the PCB board.
Here’s a quick answer, then we’ll dive into some details as not all solder masks are equal!
What are the PCB solder mask types? PCB solder masks work as a protective coating on the copper traces to prevent rust and stop the solders from forming a bridge causing a short circuit. There are 4 main PCB solder mask types - Epoxy Liquid, Liquid Photoimageable, Dry Film Photoimageable, Top and Bottom-side Masks.
But did you which of the different PCB solder mask types you should be using? That’s right, there are different types for different uses ...Let’s take a look.
The 4 Types of Solder Mask
Solder masks come in different makes and materials. How and which solder mask is going to be used depends on the application.
Top and Bottom Side Masks
Top-side and bottom-side solder masks are generally used by electronic engineers to recognise openings in the green solder mask layer. This layer is pre-added by epoxy or film techniques. The component pins are then soldered on the board using the openings registered using the masks.
The conductive trace pattern on the top-side of a board is called top traces. Similar to the top-side mask, the bottom-side mask is used for the flip side of the board.
Epoxy Liquid Solder Masks
Epoxy is the cheapest alternative among solder masks. Epoxy is a polymer that is silkscreened on the PCB. Silkscreening is a printing methodology which employs woven mesh to support the ink-blocking patterns. The mesh allows for the open areas to be identified for ink transfer. In the final step of the process, thermal curing is used.
Liquid Photoimageable Solder Masks
Liquid photoimageable solder masks are also known as LPI and is actually a mixture of two different liquids. The liquid components are mixed just before the application to ensure higher shelf life. It’s also one of the more economical products among the four different PCB solder mask types.
LPI can be used for screen printing, curtain coat or spray applications. The mask is a mixture of different solvents and polymers. As a result, a thin coating can be extracted that sticks to the surface of the target area. This kind of masks serve the original intention of solder masks but the PCB does not require any of the final plating finishes that are generally available today.
Contradictory to older epoxy inks, LPI is sensitive to UV light. The panel needs to be covered with the mask. After a brief ‘tack cure cycle’, the board is exposed to UV light using photolithography or a UV laser.
Before the mask is applied, the panels need to be cleaned and ensured that there’s no sign of oxidation. This is performed with the help of a special chemical solution. It can also be done using aluminium oxide solution or by scrubbing the panel with a suspended pumice.
One of the most common methods of exposing the panel surface to UV light is using contact printer and film tools. The film’s top and the bottom sheets are printed using emulsion to block the areas which are going to be soldered. The production panels and the film are fixed in their position using tooling on the printer. Subsequently, the panel is exposed to a UV light source at the same time.
Another technique uses direct imaging using a laser. But in this technique, there’s no need for a film or tooling since the laser is controlled using fiducial markers on the panel’s copper template.
LPI masks can be found in varying colours including green (matte or semi-gloss), white, blue, red, yellow, black and many more. LED industry and laser applications in the electronics industry has encouraged manufacturers and designers to develop more sturdy white and black materials.
Dry Film Photoimageable Solder Masks
When using dry film photoimageable solder masks, vacuum lamination is used. The dry film is then exposed and developed. After the film has been developed, openings are located to create a pattern. Following this, the components are soldered to the copper pads. Then electrochemical processing is used to layer the copper onto the board.
The copper is layered inside the holes as well as on the trace areas. Tin is finally used to help protect the copper circuits. In the last step, the film is removed and the etched marks are exposed. This method also uses thermal curing.
Dry film solder masks are generally used for high-density wiring boards. As a result, it will not flood into the through holes. These are some of the positive factors of using dry film solder masks.
Which Solder Mask for My Board
The decision to use which solder masks depends on various factors - including physical dimensions of the PCB, the final application it’s going to be used in, holes, components to be used, conductors, surface layouts, etc.
Most modern PCB designs you get a photoimageable solder mask. So, it’s either going to be a LPI or a dry film solder mask. The surface layout of the board will help you settle on the final choice. A LPI mask is preferable if the surface topography is not uniform. If you use dry film on an uneven topography, there might be gases trapped in the spaces formed between the film and the surface. As a result, LPI is better suited here.
However, there’s a downside to using LPI as well. Its thickness is not uniform across the board. You can also get different finishes on the mask layer and each has its own application. For example, a matte finish will reduce solder balls in case you’re using solder reflow.
Building Solder Mask Into Your Design
Building solder mask into your design is integral to ensure that the mask application is at an optimum level. When designing the board, the solder mask should have its own layer in the Gerber files. Generally, it’s advisable to use a 2mm border around the feature just in case the mask isn’t perfectly centred. You should also have a minimum distance of 8mm between the copper pads to ensure that solder bridges don’t form.
Solder Mask Thickness
The thickness of the solder mask will depend on how thick the copper traces are on the board. Usually, 0.5mm of solder masks is preferred to mask the traces. In case, you’re using liquid mask then you’re bound to have a varying thickness on the different features. The empty laminate areas will likely have 0.8-1.2mm of thickness while the areas with complex features (like the knee) will have thin spread (approximately 0.3mm).
Getting Advice on Which Solder Mask
Understanding how to design your PCB with solder mask and implementing the same requires extensive experience in different steps of the PCB manufacturing process. The final application of the PCB also needs to be taken into consideration when planning these details. That’s why you should always talk to an expert when deciding your mask options. Talk to us today for a free consultation.
In conclusion, a solder mask design has serious implications on the functioning of the application. It takes a crucial role in preventing rust and solder bridges (which can cause short circuits). As a result, your decision will need to consider the different factors mentioned in this article. Hopefully, this article helped you in gaining a better understanding of PCB solder mask types. In case of queries or if you need any help, then just simply get in touch with us, we’re always happy to help.