A welding mask, also known as a solder blocking mask, is a thin layer of polymer used on PCB board to prevent solder joints from forming Bridges. The welding mask also prevents oxidation and applies to the copper traces on the PCB board.
What is PCB solder resistance type? The PCB welding mask acts as a protective coating on the copper trace line to prevent rust and prevent solder from forming Bridges that lead to short circuits. There are 4 main types of PCB welding masks – epoxy liquid, liquid photogramable, dry film photogramable, and top and bottom masks.
Four types of welding masks
Welding masks vary in manufacture and material. How and which welding mask to use depends on the application.
Top and bottom side cover
Top and Bottom welding mask Electronic engineers often use it to identify openings in the green solder barrier layer. The layer is pre-added by epoxy resin or film technology. The component pins are then welded to the board using an opening registered with the mask.
The conductive trace pattern on the top of the circuit board is called the top trace. Similar to the top side mask, the bottom side mask is used on the reverse side of the circuit board.
Epoxy liquid solder mask
Epoxy resins are the cheapest alternative to welding masks. Epoxy is a polymer that is screen printed on a PCB. Screen printing is a printing process that uses a fabric net to support an ink blocking pattern. The grid allows identification of open areas for ink transfer. In the final step of the process, heat curing is used.
Liquid optical imaging solder mask
Liquid photoconductive masks, also known as LPI, are actually a mixture of two different liquids. Liquid components are mixed prior to application to ensure a longer shelf life. It is also one of the more economical of the four different PCB solder resistance types.
LPI can be used for screen printing, screen painting or spray applications. The mask is a mixture of different solvents and polymers. As a result, thin coatings can be extracted that adhere to the surface of the target region. This mask is intended for soldering masks, but the PCB does not require any of the final plating coatings commonly available today.
In contrast to older epoxy inks, LPI is sensitive to ultraviolet light. The panel needs to be covered with a mask. After a short “cure cycle”, the board is exposed to ultraviolet light using photolithography or ultraviolet laser.
Before applying the mask, the panel should be cleaned and free of oxidization. This is done with the help of special chemical solutions. This can also be done using an alumina solution or by scrubbing the panels with a suspended pumice stone.
One of the most common ways to expose panel surfaces to UV is by using contact printers and film tools. The top and bottom sheets of the film are printed with an emulsion to block the area to be welded. Use the tools on the printer to fix the production panel and film in place. The panels were then simultaneously exposed to an ULTRAVIOLET light source.
Another technique uses lasers to create direct images. But in this technique, no film or tools are needed because the laser is controlled using a reference mark on the panel’s copper template.
LPI masks can be found in a variety of colors, including green (matte or semi-gloss), white, blue, red, yellow, black, and more. The LED industry and laser applications in the electronics industry are encouraging manufacturers and designers to develop stronger white and black materials.
Dry film photoimaging solder mask
A dry film photoimagable welding mask is used, and vacuum lamination is used. The dry film is then exposed and developed. After the film is developed, openings are positioned to produce patterns. After this, the element is welded to the brazing pad. The copper is then laminated onto the circuit board using an electrochemical process.
The copper is layered in the hole and in the trace area. Tin was eventually used to help protect copper circuits. In the final step, the membrane is removed and the etching mark is exposed. The method also uses heat curing.
Dry film welding masks are commonly used for high-density patch boards. As a result, it doesn’t pour into the through-hole. These are some of the positives of using a dry film welding mask.
Deciding which welding mask to use depends on various factors – including the physical size of the PCB, the final application to be used, the holes, the components to be used, the conductors, the surface layout, etc.
Most modern PCB designs can obtain photoimageable solder resist films. Therefore, it is either LPI or dry film resistance film. The surface layout of the board will help you determine your final choice. If the surface topography is not uniform, the LPI mask is preferred. If a dry film is used on uneven terrain, gas may be trapped in the space formed between the film and the surface. Therefore, LPI is more suitable here.
However, there are downsides to using LPI. Its comprehensiveness is not uniform. You can also get different finishes on the mask layer, each with its own application. For example, in cases where solder reflow is used, the matte finish will reduce solder balls.
Build solder masks into your design
Building a solder resist film into your design is indispensable to ensure the mask application is at the optimal level. When designing a circuit board, the welding mask should have its own layer in the Gerber file. In general, it is recommended to use a 2mm border around the function in case the mask is not fully centered. You should also leave a minimum of 8mm between the pads to ensure that Bridges do not form.
Thickness of welding mask
Thickness Welding mask will depend on the thickness of the copper trace on the board. In general, a 0.5mm welding mask is preferred to mask the trace lines. If you are using liquid masks, you must have different thicknesses for different features. Empty laminate areas may have a thickness of 0.8-1.2mm, while areas with complex features such as knees will have thin extensions (about 0.3mm).
In summary, welding mask design has a serious impact on application functionality. It plays a vital role in preventing rust and welding Bridges, which can lead to short circuits. Therefore, your decision needs to take into account the different factors mentioned in this article. Hope this article can help you better understand the TYPE of PCB resistance film. If you have any questions, or just need to contact us, we are always happy to help you.