Solder Mask Applied in PCB Printed Circuit Board
Solder mask is a photoimageable resist applied to PCBs to prevent the copper traces from connecting to one another when soldering. It is usually green but it can also be blue, black, white and clear. It can be printed by a variety of methods such as silkscreen printing or electrostatic spraying. The printing method will determine how good the mask adheres to the surface. The choice of color will also depend on the surface topography and whether a liquid or dry application is used.
The most common way of applying the solder mask is by screen printing. This is the most cost-effective, although the quality of the mask will be dependent on the printer. The production panel is placed under the print head, and a pattern of ink (emulsion) is spread over the board by an automated machine with a squeegee. The tension, density and angle of the squeegee will all have an effect on the printing.
Once the printed circuit board pcba emulsion has been applied to the production panel, it is then exposed to UV light through a developer. This will strip off any unhardened resist and leave the hardened ones behind. The panels are then tack-dried to prepare them for mask application.
How is Solder Mask Applied in PCB Printed Circuit Board?
The tack-drying process also removes any grease and impurities from the panel. It will also increase the roughness of the surface to enhance bonding with the ink used for the mask. The production panel is then imaged with the correct solder mask pattern using a stencil. The stencil will be a positive image of the required copper surface, and any areas that should not be soldered are blocked. This will then be printed onto the production panel.
A key design consideration for the mask is the solder mask dam, which must be adequate to prevent the formation of solder bridges during assembly. The minimum dam distance is usually set at half the width of the conductor pitch, but it can be reduced for fine conductive patterns. Insufficient spacing between pads can cause solder bridges to form, which will result in poor connections and unreliable circuit boards.
It is also possible to incorporate openings in the solder mask for special requirements, such as signal leads and test points. These openings can allow manual soldering and provide easier access to the circuit components. They can also help to improve the accuracy of solder contact and ease measurement and debugging processes.
The best place to set these parameters is within the Allegro PCB editor, which allows for full control of the pad and solder mask shapes and sizes. These settings can be configured for manufacturability rules in the Constraint Manager, making it easy to ensure the correct pad stackup and avoid fabrication and assembly problems.
In addition to protecting the copper traces from accidental electrical connections, the solder mask layer can also reduce the chance of factory damage caused by slivers of mask that flake off during manufacturing. These slivers are known as “flake failure” and they must be minimized.
