How Important Are Solder Masks and Silkscreens

Solder Mask

Solder Mask (also known as Solder Resist, or Solder Stop Mask) is a thin polymer that is applied to the surface of a Printed Circuit Board (PCB) to seal the copper of a Printed Circuit Board from the environment. All colours of LPS solder masks are available at Bittele Electronics at no extra cost. Bittele Electronics also includes double-sided solder mask in its standard service.

There are two main purposes for this masking:

Prevent Oxidation
Prevent Solder bridges

When metal is exposed to moisture and oxygen a reaction occurs where the metal loses electrons and its properties are changed, this is also known as rust. Some metals like iron turn into a brittle reddish powder when oxidized. Copper, on the other hand, retains its structural integrity when oxidized, however, it loses its electrical properties instead. This can be quite a big issue for PCBs since they depend on the electrical properties of copper to function correctly. This is where solder mask becomes highly useful in that it seals the exposed copper away from the atmosphere thus preventing any oxidation. The exposed copper pads that are needed to solder components are treated with a surface finish such as Immersion Gold. Gold is highly resistant to oxidation and thus prevents oxidation on the copper pads.

Solder Bridges occur when an unintended short is created by solder bridging the gap between two electrical elements such as traces, pads, or leads. The solder mask acts as a barrier to prevent these shorts from forming. This is especially useful in the case of Integrated Circuits where the gap between pads can be as small as a few thousandths of an inch. At Bittele Electronics can apply solder mask dams to spaces as small as 8mils for a board with 1oz copper weight. An extra 2mils of space is needed for 2oz copper weights and an extra 1mil of space for any colour of solder mask that is not green.

A solder mask is also useful in the mass assembly of PCBs. With the solder mask protecting the copper traces a solder bath can be used to tin each exposed copper pad on a board in one application without allowing any solder to come into contact with sealed areas.

During reflow, the wicking effect of the solder and tinned pads combined with the solder mask will ensure that no solder bridges form while the components are properly soldered.

Silkscreen

At first glance, a silkscreen layer may mistakenly seem to be unimportant, after all, it has no direct impact on the electrical functionality of a PCB. However, due to the valuable information provided by the silkscreen it can be determined to be extremely important to the functionality of a PCB. A silkscreen can provide the following information:

Polarity of Parts
Location of parts through Reference Designators
Identify Test Points
Identification numbers unique to each board
Warning Symbols

This information is invaluable when assembling a PCB, it acts as a safety net that can provide simple visual feedback of deeper problems. For example, a resistor being soldered onto a location that is identified as C5, this means that the pick and place does not align with the reference designators on the silkscreen layer. At worst this could be a hint that many passives are incorrectly placed and the pick and place files needs to be regenerated. Bittele’s thorough DFM check will eliminate the risk of such issues. In the case where no silkscreen is provided, we ask that you provide an assembly drawing so that we may verify that the centroid date matched the PCB. You can also provide us with silkscreen layers to be used for reference and not be printed on the PCB.

At the end of the day the silkscreen layer is another means of providing information about the PCB. Most importantly it is information that cannot be separated from the board, therefore all warning symbols, unique identification numbers, certification logos, and manufacturer logos will be on display when viewing a board.

For information on designing a legible silkscreen please see this article: Making Readable Silkscreen Layouts for your Printed Circuit Board (PCB) Design