PCB Silkscreen

The silkscreen layer is the top layer of the PCB and serves as a reference indicator for placing components on the PCB. The graphics and text on PCB circuit board is usually printed with a permanent non-conductive epoxy ink. The color is typically white, although other colors are also available. Screen printing is used to specify useful information on the PCB board, which can help users during the assembly. It is used to mark component values, part numbers, test points, polarity, etc. The problems related to the material of silkscreen are rarely occurred, except maybe some discoloration after high PCB reflow temperatures for RoHS compliant soldering. Silkscreen problems are always caused by the design of the PCB.

Step 1: What Is Silk Screening?

Silkscreen is a layer of ink traces used to identify components, test points, parts of the PCB, warning symbols, logos and marks, etc. This silkscreen is usually applied on the component side; however, using silkscreen on the solder side is also not uncommon. But this may increase the cost. Essentially a detailed PCB silkscreen can help both the manufacturer and the engineer to locate and identify all the components. The standard colors we normally see are black, white and yellow. PCB software also uses standard fonts in silkscreen layers, but you can choose other fonts from the system too. For traditional silk-screening, you require a polyester screen stretched on aluminum frames, a laser photoplotter, spray developer and curing ovens.

Step 2: What Is the Purpose of Using a Silkscreen Layer?

The silkscreen is not applied to PCB boards by default. A customer has to ask for this and there is usually an additional charge for applying a silkscreen on the board. Here are some important benefits to opt for the silkscreen layer:

- It helps the engineers and technicians identify test points on the PCB board. This way they know where to take measurements from and helps when troubleshooting.

- It can be used to outline where components need to be placed on the board, this way users will not users forget to place components on the board. The silkscreen coating also helps with the warning symbols which are generally used to indicate the higher parameter values such as Voltage, Current, and Power.

- It can be used to specify values of inductors, resistors and other components reducing the probability of placing a component with an incorrect value on the board.

- Acts as a reference indicator to identify the component type and polarity orientation.

Step 3: Methods of Silk Screening

Manual screen-printing

Manual screen-printing is done when the line widths are greater than 7 mil (0.007”) and the registration tolerance is 5 mil. For this method, you require a stencil of the text and traces made of nylon. The ink is pushed through the stencil onto the laminate. Next, the board is baked in a curing oven for the ink to cure. The application and set up is easy but the result is the least precise.

Liquid Photo Imaging (LPI)

This method is used when the line widths are greater than 4 mil. Liquid Photo Imaging is quite similar to the process used for the application of a solder mask. In this, a liquid photo-imageable epoxy is coated on to the laminate and then exposed with UV light. After this, the board is developed and cured. It is much more accurate than manual screening.

Direct Legend Printing (DLP)

DLP is the most accurate of all these processes but is more expensive. In this process, an inkjet projector is used with acrylic ink that is applied onto the raw PCB directly from the CAD data. The ink is cured with UV light as it is printed. It should be noted though that the acrylic ink does not cure on boards with silver finishes.

Step 4: Importance of Silk Screen Technology

A properly designed silk-screen can prove to be highly useful as it can reduce the chance of error and can reduce time taken to spot the error. The silkscreen can easily label the passive components of the PCB no matter how packed the board maybe.

There are a few things you should keep in mind regarding silk-screening. For example, the silkscreen epoxy should not be printed over pads or land PCBs that will be soldered as it will melt into the solder joint. This is why it should be applied over the solder mask. While designing you should keep the component outlines away from the pins (almost at a distance of 0.25 mm). Also, you should check to see if the width of the silkscreen graphics is suitable for the design. Try to keep the line widths no less than 6 mil. Knowing more about PCB and silk-screening can help you to reduce your PCB costs. For example, applying silkscreen on only one side and choosing the standard colors will give you better overall pricing and give you the same benefits.

Step 5: ​References

Nasir, S. (2018, November 21). Use of Silk Screen Technology in Printed Circuit Board (PCB). Retrieved September 02, 2020, from https://www.theengineeringprojects.com/2018/11/us...

PCB Silkscreen. (n.d.). Retrieved September 02, 2020, from https://www.pcbway.com/pcb_prototype/PCB_Silkscre...

Team, O. (n.d.). Understanding Printed Circuit Board Manufacturing: Silk-Screening. Retrieved September 02, 2020, from http://www.omnicircuitboards.com/blog/bid/312861/...

What is the Silkscreen in a PCB? (n.d.). Retrieved September 02, 2020, from https://www.pcbdirectory.com/community/what-is-th...