A solder mask is a protection layer of polymer that is used on printed circuit board surfaces to control solder spreading and avoid solder bridges in the assembly process. It also saves traces from corrosion and electrical leakage. Here details of the solder mask and its common uses for PCB will be explained. Let’s discuss the Solder Mask and its functionality in PCB Design.
What is Solder Mask / Solder Resist?
Solder mask also called solder resist, solder stop mask is then a lacquer layer made with a polymer that is applied to copper traces of board to minimize the effect of oxidation and avoid solder bridges to make on closely spaced solder pads. The solder bridges undesired electrical connections between conductors through small parts of solder. PCB board uses a solder mask to avoid connection. With conventional soldering techniques solder masks are also used for the automated soldering process like reflow or wave soldering.
When it is applied opening is made in the solder mask to connect components that are made with the use of photolithography. Solder mask is normally green colored, it also comes in different colors. There are different media used for solder masks based on the project. The low-price solder mask is epoxy liquid that is silkscreened through different designs on board. Other are photo-imageable solder masks and dry film photo-imageable solder masks.
How is the solder mask applied to the PCB?
Soldier masks are made with a polymer layer applied over the metallic trace on board. There are different types of mask material used. They are normally used based on board uses and the cost of the project. The printing of epoxy on the board with silkscreen is a very commonly used solder mask type.
Solder mask comes in different types of media based on application. The low-cost solder is epoxy that is silkscreened through different patterns on the board. LPSM can applied on board over the exposed pattern and provide open holes in the pattern for parts soldered to copper pads. DFSM is vacuum laminated on board that is exposed and created. All these solder mask passes through a thermal process when the pattern is defined.
Why Use Solder Mask on PCBs?
- it is used to prevent solder from spreading between neighboring pads in the assembly.
- It isolates the traces electrically to avoid shorts
- it minimizes corrosion and dendrite on traces
- Open pads are selectively metalized while the mask layer remains protected.
- It has a finished appearance with defined board colors.
- It increases the strength of pads so it is best to use on a flex board.
Solder Mask Properties
- It breaks the voltage rating and provides insulation
- It makes a strong connection between the board substrate and copper
- Line widths and spaces are easily made
- it has good CTE and strain tolerance
- it handles reflows, cleaners, and fluxes.
- it has good flame retardance
- It has resistance to corrosion and moisture
Types of solder mask on PCB
There are two main types of solder masks over bare copper onboard
- Dry film
- liquid photo imageable
Dry film is laminated and applied on the board during liquid photo imageable solder is used as liquid. Dry film is highly resistant to cracking and peeling and offers a smooth finish. But it is difficult to use and needs special types of instruments. Liquid photo imageable solder mask is easy to use but has a chance of cracking and peeling
Dry film Solder Maks
This type of solder mask is applied on a bare copper layer of the board whereas a dry film solder mask is used on the copper surface of the board and heated to make a film. it is helpful to save the board from wetting and corrosion.
Advantages of dry film solder mask:
- it makes consistent solder joints with less shorts and bridging. It also comes with a smooth surface, that increases the quality of solder joints.
- it also provides a fine-pitch component to use and offers good electrical insulation for traces
- Dry film solder masks are highly resistant to thermal shock and cycling makes a good option for use in very high and very low-temperature conditions.
- Dry film solder masks are highly resistant to heat and chemicals and bare copper offers good resistance to moisture and contaminants.
Liquid photo imageable solder mask
liquid photo imageable solder mask is material used on the surface of the board for protection of the circuit made on board. This type of solder mask is made with the use of photopolymer resin, a light-sensitive material that is cured through applying UV light.
LPSM is used on the board surface in liquid material and after that exposed to UV light for curing it. The UV light is applied to make the solder paste harden and make a protective layer on the circuits. The LPSM works as a barrier to avoid soldering flux to make the connection of the circuit and also saves circuits from mechanical damage. The liquid photo-imageable solder mask is the main component of the PCB manufacturing process and increases board durability.
Advantages
its main advantage is that it offers good protection for exposed copper circuits. This makes is dielectric materials that applied over copper and save it from making connections with conductive materials. it is used to save short circuits.
it also increases the physical look of the board since it comes in different colors used to make designs and patterns on the board. Due to this board, different components can be easily identified.
it also enhances the board’s durability on board. it is hard materials that resist wear and tear and increases board operating life. it helps to board easily work in harsh conditions.
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