Handling Silkscreen On Pads

Handling Silkscreen On Pads
Parker Dillmann |   |  Engineering

One of the issues commonly seen in PCB design is silkscreen information overlapping exposed copper on pads. If the ink from the silkscreening process is on a pad that is to be soldered it will likely inhibit the solder from flowing and will result in a bad joint. In general, it is a good idea to never have silkscreen on any exposed copper that is to have solder applied.

When the gerber files are sent to a board manufacturer they are typically reviewed for issues including slikscreen on pad. In most cases the manufacturer will adjust the gerber information to prevent this by removing or moving the overlapping information. Although these issues are typically caught it is not good design practice to rely on it.

In some cases, the manufacturers will not make changes and create a board that has the issues (They make the board exactly as the gerber files tell them).

Luckily, fixing silkscreen on pads with EDA tools is simple. Getting into the habit of creating component footprints with no silkscreen on pads is worthwhile and prevents issues in manufacturing.

The following picture shows a SOIC-8 package where the pad width was selected to be too long. The red boxes around show areas where the silkscreen (green in this example) overlaps the pads.

The solution to this issue would be to reduce the width of the pads or move them further out from body of the component. In the following image the pads were moved from the body such that there is 0.01″ of space between the silkscreen and pad.

0.01″ (10mils) is a good general number for spacing between silk and copper to prevent issues.

There are cases where the designer wants information such as dimensions and assembly information to appear on the board in their EDA tool. The following picture shows an example of dimension lines crossing into pads.

When adding such information, it is important to not use the silkcreen layer. In this case, the dimension layer 48 in Eagle was used to draw the dimension lines. The information on this layer is not included in the exported gerber files that are sent to the PCB manufacturer and will not show up on the silkscreen.

Making sure that pads and exposed copper is free from silkscreen is good design practice and prevents manufacturing issues.