How Solder Mask is Used in PCB Circuit Board Assembly
Used in PCB Circuit Board Assembly
Solder mask is an essential layer of material on the surface of a printed circuit board (PCB) that protects copper traces and pads from solder. The mask also defines pads and areas for component placement, enabling automatic assembly machines to correctly place components on the board. In addition, the solder mask prevents oxidation of copper and enhances its solderability.
While it is possible to make a PCB without a mask layer, it is not recommended as it can lead to many problems. Without a mask, solder can wick into neighboring traces and pads, creating shorts. In addition, the lack of mask can make it difficult to inspect and rework the board. The mask layer also provides insulation for the circuit, preventing abrasions that could damage it.
The mask material is made of a photo-sensitive emulsion that is printed with the exact pad and trace pattern of the circuit. It is then exposed to UV light, causing the emulsion to harden in the open print areas. Once the mask has been hardened, it is peeled off and cleaned to remove any unhardened emulsion remaining on the board.
Depending on the application method, the dry film mask can either be applied as a sticker or through a lamination process. In the latter, a sheet of dry mask material is bonded to a liner that matches the pcb circuit board assembly design, then fed into a laminator. A combination of heat and pressure activates the adhesive, fusing it to the bare copper. The board is then cured in an oven or tunnel.
How Solder Mask is Used in PCB Circuit Board Assembly
Another type of mask is a liquid mask that is screen-printed on the PCB. In this case, a photo-sensitive emulsion is applied to a tightly woven mesh screen. The emulsion is then exposed to UV to harden the areas that will be printed with solder mask. A blade then forces the mask material through these openings to deposit the desired amount on the bare copper. The excess is wiped off with water and the board is placed into a curing oven or tunnel to complete the process.
As a finishing touch, the board is inspected using automated optical inspection (AOI) machines to look for defects like voids, bridges between traces, or stains that may compromise solder mask protection. If any of these are detected, they are corrected with a touch-up application.
Considering the importance of solder mask, it is vital to design it properly. This involves ensuring adequate clearance between pads and traces based on component sizes, manufacturing tolerances, and a variety of other factors. It is also important to maintain consistent clearance around pads – varying pullback allowance risks nearby shorts during thermal shock events. Finally, it is important to verify that the mask vendor’s reported property test results are accurate. These tests can help avoid costly mistakes during production and ensure that the final product meets your exact specifications.