Comparative Account: PCB Solder Mask vs. Paste Mask

Comparative Account: PCB Solder Mask vs. Paste Mask

PCBs or printed circuit boards being crucial and core parts of electrical and electronic devices and integral to their functioning need to be not only designed accurately but also protected from spillages, external damages, and more. So, core copper layer of a board is masked with a layer of protective material mostly of some polymer or resin to protect the soldered bridges and joints from oxidation, dust, contamination, and so on. This is applied on both sides of the board. In case of double sided boards, the copper layer stays between the layers of dielectric material, and the same sandwich process is followed for all the layers. So, masking covers the entire board including the dielectric and copper layers. This masking process is of broadly two types- solder mask and paste mask. So, what is the difference between PCB solder mask vs. paste mask, and how is the right method decided. This post offers insights on the differences between PCB solder mask vs. paste mask.

What is a Solder Mask?

As mentioned, solder mask is a lacquer-like polymer coat applied on the copper layer of the board. Mostly, it is green in color. However, depending on application specifications or business requirement, it may be blue, red, black, white, and so on. These masks when applied are in liquid form mostly comprising photoimageable fluid, epoxy, or some other polymer. This is done by spraying or silkscreen printed.

What is a Paste Mask?

Also known as stencil, the paste mask is a paste made of tin, which is applied on to the SMD pads. This process is automated and performed on an assembly. While the process can be done manually, it is tedious. The application and removal of the excess paste depends on the component mounting method used, such as for surface mount devices (SMD), thru-hole mounting, and so on.

Differences Between PCB Solder Mask vs. Paste Mask

The above definitions may give a basic idea about both these PCB masking methods. This segment offers more clarity and core differentiating factors between these solder mask and paste mask. Here are some pointers.

• In case of solder mask, it coats the entire board except for the joints and pads to be soldered and drilled holes, where components need to be mounted. This coat protects the board from metal oxidation, human fingerprints, layers of oil, cream, and other contaminants. As mentioned, this is a layer of liquid applied through screen printing or spraying. It may also be in the form of a dry film sheet which is a photoimageable material. Here, the board is subjected to UV rays, while protecting the thru-holes and solder pads. This way, the mask layer solidifies and the excess material is wiped off carefully.
• In case of paste mask, a layer of this paste is coated on surface mount pads. Made of tin or other metal solder particles along with a sticky flux, this paste layer cleans the board surface and the components yet to be soldered. The flux being sticky remains adhered to the holes until the soldering process is completed. This is a part of the solder reflow process, wherein the solder paste is melted in the oven and then solidified to form a solder joint. This solder paste can be applied manually, but as mentioned it is automated quite often as the manual process is cumbersome and time consuming.
• Jet printing is one of the methods used for solder paste application. By using the CAD system for PCB design and masking data, the printer deposits the paste on all the SMDs on board one by one. This method is far better than performing it manually; however, this is also time consuming to some extent.
• The use of fabricated stencils to apply solder paste has been the most effective and speedy method so far, and hence is widely used. Here, the solder paste can be squeezed onto the board and applied to several boards in just a few minutes. This is useful for high volume production runs, wherein it can be applied precisely, accurately, and at a high speed.
• Solder and paste mask for both top and bottom layers are represented distinctly in the PCB CAD system. The solder paste portions in a CAD system can be identified as paste layers coated on SMDs. On the other hand, solder mask portions can be distinctly identified as you can make out the drilled holes, vias, and SMDs to be soldered.
• The overall dimensions of the solder mask as well as paste elements would be most of the times same as that of the PCB pads or holes.

Masking your circuits boards using the right materials is just as crucial as the board design and fabrication processes. If you are an OEM who manufactures electromechanical devices and need to partner with a PCB manufacturer, ensure they have enough experience. Also, make sure they understand your requirements and customize accordingly. If you are new in this segment and not sure of the design, processes, and materials required for your PCBs, look for a full services manufacturer who offers end-to-end services. Twisted Traces is a custom circuit board manufacturer offering end-end services from ideation, design, fabrication, soldering, masking, testing, installation and aftersales. The company uses advanced PCB manufacturing processes, materials, and fabrication techniques.