Role of Solder & Paste Masks in PCBs
For improving the quality of soldering on printed circuit boards (PCBs), the electronic industry uses two types of masks—solder and paste masks. As the names are very similar, it is easy for newcomers to the industry to be confused about the usefulness and functioning of the two. In reality, the industry uses the two essential masks for entirely different purposes. In this article, Rush PCB explains their individual functioning, and the advantages of using them.
Solder Mask for PCBs
As the name implies, the solder mask creates the soldermask on the PCB, allowing soldering on selected areas of a PCB, while masking the others. Most commonly, the soldermask appears on the PCB as a green layer on its two outer surfaces. The green layer covers most of the PCB surface, except for copper pads that will accept component leads for soldering. Therefore, if a copper trace or pad is under the green soldermask, it will not be possible to make solder adhere to it.
Copper traces and pads on a PCB, if left uncovered, readily tarnish when they encounter air. The tarnish is due to the formation of copper oxide, a compound that does not conduct electricity, and does not allow soldering. Moreover, the presence of Sulphur in air causes the formation of copper sulphate, a corrosive substance. The net result, for a PCB with exposed copper on it, is a degradation of its quality when stored for some time.
To get over this problem, Rush PCB takes recourse to two processes. The first is to cover those parts of the PCB that will not undergo soldering with a green soldermask, and the other, to apply surface treatment to the exposed copper. The two processes ensure no copper surface on the PCB encounters air, while making it is easier to solder components on the PCB during assembly. With a soldermask present and surface treatment on the PCB, Rush PCB ensures the possibility of storing bare boards for a longer time, without any damage, until it is time to assemble them.
The common implement to apply the green soldermask on a PCB is the solder mask. The actual mask is a negative or complement of the green soldermask, and therefore, covers all the copper areas that will undergo soldering. When applying, the green color penetrates the open areas of the mask, and deposits on the PCB, covering the areas the mask exposes.
Paste Mask for PCBs
This mask or stencil is applicable to the use of solder paste, controlling the amount of paste the assembler dispenses to each solder pad on a PCB. Applying the proper amount of solder paste using a paste mask is an important process during assembly, since the presence of excess solder paste may cause solder to overflow, thereby creating shorts during reflow soldering, while inadequate solder paste may create dry soldering on some components.
Rush PCB prefers making paste masks or stencils of stainless steel, although it is possible to make them with other materials as well. For accuracy, the openings in the stencil must be laser cut and the surface finished with chemical etching. This creates a smooth inner surface in the openings, allowing for a smooth deposition and no sticking of paste on the stencil walls.
The thickness of the paste mask determines the volume of solder paste the process deposits on the copper pads. For dense PCBs, it is essential to have a thin stencil to adequately control the amount of solder paste deposition. For good quality of soldering, it is essential to accurately register the stencil on the PCB.
Advantages of Solder Mask and Paste Mask
Rush PCB uses both solder and paste masks on PCBs to ensure the quality of soldering. We need the solder mask while fabricating the board, and use the paste mask while assembling it. Both are indispensable for ensuring high quality. For instance, in HDI boards, we use the solder mask to create soldermask dams in between closely spaced pads, thereby preventing molten solder from overflowing and bridging neighboring pads during reflow soldering.
For PCBs using fine-pitch components, such as BGAs and gull-wing ICs, the pitch between neighboring pads may not be enough to allow placing a soldermask dam. For such PCBs, Rush PCB recommends using soldermask defined pads. While the pads are of a larger dimension, a smaller soldermask opening defines the opening for the solder paste.
Designing a paste mask or a stencil is an involved subject, and eminent PCB manufacturers such as Rush PCB have evolved their own special techniques for fabricating them.
Please consult Rush PCB with your design today, and get expert advice on the type of solder and paste masks necessary. Visit our website, or give us a call for a free quote.