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PCB Technical

PCB Technical - How to choose the correct solder mask for PCB circuit boards

PCB Technical

PCB Technical - How to choose the correct solder mask for PCB circuit boards

How to choose the correct solder mask for PCB circuit boards

2021-08-25
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Author:Aure

Solder mask is a critical process in PCB (Printed Circuit Board) manufacturing, designed to protect the metal parts of the board from oxidisation and to prevent the formation of conductive connections between the pads. This step is particularly important in the PCB manufacturing process, especially when soldering processes such as reflow or wave soldering are used, as these processes make it difficult to precisely control where the molten solder lands on the board, and the soldermask provides the necessary control. Sometimes referred to as ‘soldermask’, soldermask is a more appropriate term as it is not a layer of solder that covers the entire board, as is commonly misunderstood.


Types of PCB Solder Resists

All soldermasks consist of a polymer layer applied to the metal leads of the printed circuit board. There are several types of PCB soldermask available, depending on cost and application requirements. One of the most basic soldermask options is a liquid epoxy resin printed on the conductors using screen printing technology, a process similar to painting through a stencil. The soldermask can be rendered in a variety of colours to suit different needs.


Liquid Epoxy Soldermask

Liquid epoxy solder mask is the most basic option, where liquid epoxy is applied to the PCB using screen printing techniques. This is the lowest cost and most widely used soldermask process. In this process, a woven mesh is used to support the ink resist pattern. Liquid epoxy is a thermosetting polymer that becomes hard when cured by heat. The colour of the soldermask is formed by mixing a dye into the liquid epoxy and during the curing process.


Liquid Photoimageable Soldermask (LPSM)

More advanced soldermasks are applied using a dry film or liquid resist lithography process, which is similar to that used for photoresist exposure in semiconductor manufacturing.LPSM can be applied by screen printing or spraying, where spraying is usually a more economical option. A more advanced and precise method is to use a photolithography process to define soldermask openings for pads, vias and mounting holes.


In the LPSM process, a photolithographic foil is first fabricated to match the desired soldermask based on a Gerber file.Then,the board is thoroughly cleaned to ensure that there are no dust particles under the cured soldermask. Next, both sides of the board are completely covered with liquid LPSM. the black portion of the photolithography foil defines the areas where the conductors are desired to be exposed, while the areas of the board desired to be covered with the soldermask are left clear.


The soldermask is typically coated with an epoxy or photosensitive polymer.After the LPSM is applied,the circuit board is dried in an oven and placed in a UV developing apparatus. The photolithographic film is carefully aligned on the dried board and the board is illuminated with UV light. exposed areas of the LPSM material are cured by the UV light and the unexposed areas are cleaned off with a solvent, leaving a hard soldermask.


Dry Film Soldermask (DFSM)

DFSM soldermask uses a similar photolithography process as LPSM. Both PCB solder mask types are exposed during the lithography process. Unlike liquid coatings, dry film soldermasks are applied in the form of a soldermask diaphragm using a vacuum lamination process. This vacuum lamination step ensures that the unexposed soldermask adheres tightly to the board and removes air bubbles from the film. After exposure, the unexposed areas of the soldermask are removed with a solvent and the remaining film is cured in a heat treatment process.


Top and Bottom Soldermask

The two types of soldermask often referred to in other guides on PCB solder mask types are top and bottom layers.These terms refer only to a specific soldermask layer placed on the top or bottom of the board and do not relate to a specific manufacturing process or a specific type of soldermask material.


Final Steps: Curing and Surface Preparation

After applying the above media, the boards need to be cleaned to remove all dust. Next, they undergo a final hardening and curing process.Liquid epoxy soldermasks are thermally cured as they are not exposed to UV light. The LPSM and DFSM films, on the other hand, will be cured by UV exposure during the lithography process. After exposure, these films will be further cured and hardened by heat treatment.


Regardless of the type of PCB soldermask used,the resulting soldermask will leave exposed copper areas on the board.These exposed areas need to be protected against oxidation using a surface treatment plating. The most common surface finish is Hot Air Solder Levelling (HASL), although other popular finishes include Electroless Nickel Immersion Gold (ENIG) and Electroless Nickel Electroless Chemical Palladium Immersion Gold (ENEPIG). If applicable, additional holes are left in the diaphragm layer for the flux layer. The flux layer is used to attach pads or other components to the printed circuit board and is treated differently depending on the manufacturing process.

soldermask

How to choose the correct solder mask for PCB circuit boards
The solder mask contains a polymer layer that can be coated on the metal traces on the PCB circuit board.There are different types of mask materials, and the best choice for your PCB board depends on the cost and your application. The most basic solder mask option is to use screen printing to print liquid epoxy on the PCB. It's like spraying a topcoat with a stencil.

Fancier solder mask uses dry film or liquid solder mask for light imaging. Liquid photoimageable solder mask (LPSM) can be screen printed like epoxy or sprayed on the surface, which is usually a cheaper method of application. Dry film solder mask (DFSM) must be vacuum laminated to the circuit board to avoid bubble defects. Both optical imaging methods have been developed to remove the part of the mask that solders the pad to the component and cure it through a baking process or UV light exposure.

Solder mask is used as epoxy resin or photoimageable polymer.

Which solder mask should I use?

The determination of the appropriate solder mask depends on the physical dimensions of the circuit board, holes, components and conductors, surface layout and the final application of the product.

First of all, if you have a PCB solder mask, it will be used in aerospace, telecommunications, medical or other "high reliability" industries, check the industry standards of solder masks, and your general application. Some specific requirements supersede any other content you learn on the Internet.

For most modern printed circuit board designs, you need a photo-imageable solder resist. The surface topography will determine whether to use liquid or dry applications. Dry application spreads a uniform thickness on the entire surface. However, if the surface of your circuit board is very flat, the dry mask will adhere best. If you have complex surface features, then you may best use the liquid (LPISM) option for better contact with trace copper and laminate. The disadvantage of liquid applications is that the thickness of the entire plate is not completely uniform.

You can also obtain different finishes on the mask layer. Discuss with your PCB manufacturer what products are available to them and how it will affect production. For example, if a solder reflow process is used, a matte finish will reduce solder balls.

PCB manufactured using solder reflow process requires solder mask. The smoothness of the mask will affect the quality of reflow soldering.

How thick is my solder mask?
The thickness of your solder mask mainly depends on the thickness of the copper wire on the circuit board. Generally, you need about 0.5 mil solder mask on your trace. If a liquid mask is used, it must have a different thickness from other functions. In the empty laminate area, you can expect a thickness of 0.8-1.2 mils, while on complex features (such as the inflection point of a circuit), it may be as thin as 0.3 mils.

As with any other manufacturing parameters or process, you should consider the sensitivity of the final application and plan your design accordingly. It is always important to discuss manufacturing options with the manufacturer. They can even suggest better options based on their abilities.

How to add solder mask to the design?
When designing the printed circuit board, the solder mask should be its own layer in the Gerber file.Check the design rules of the solder mask. Generally, if the solder mask is not completely centered, you need to use a 2mil border around the function. The minimum distance between pads is usually 8 mils to ensure that the mask is sufficient to prevent solder bridges from forming.