“Oh Magoo, you’ve done it again!” This often uttered phrase from a lovable fictional character, Mr. Magoo, was an indication of self-awareness that something was amiss. Typically, this meant that he had bumped into something or realized he was going in the wrong direction. Whatever his dilemma, it was usually due to his poor eyesight, which he refused to do anything about. This cartoon may seem trivial, but as is often the case, it has an important message for us that the ability to see things clearly can prevent a lot of headaches.
Although not required for your board to function, missing or unreadable silkscreen markings can create severe head scratching for your assembly service provider or may result in erroneous circuit behavior. The silkscreen provides identification information that helps to ensure your components are placed and oriented correctly. This is invaluable for diodes, transistors, quad IC packages, and other circuit elements that require specific directional signal or current flow for proper operation. Additionally, the silkscreen is where test point IDs, labels, and logos are printed. To be useful, however, the silkscreen markings and images must be clear and readable. Let’s take a look at how silkscreens are printed and then define design guidelines to help ensure that your silkscreens are indeed useful for your PCB development process.
PCB Silkscreen Printing Techniques
Applying identification markings and logos to circuit boards was once primarily done using stencils, similar to spray painting numbers on a car. However, just as other aspects of PCB development have advanced throughout the history of PCB design and manufacturing, so have the methods used to apply the silkscreen. Today, the three methods listed below are used to apply the distinguishing markings of the silkscreen to circuit boards.
🖶 Manual Screen Printing
This is the oldest method that is still used today, probably due to its speed. Here, epoxy ink is pushed through a stencil to create the text or imagery. The ink is then baked into the laminate. Although fast and easy, this technique yields the least accurate results.
🖶 Liquid Photo Imaging (LPI)
LPI is very similar to the method used to apply the solder mask to your board’s surface. Here, UV light is used to expose the silkscreen markings followed by the board being cured through baking. This method produces more precise results than manual screen printing but requires the most time of the three methods.
🖶 Direct Legend Printing (DLP)
DLP is the fastest method and produces the best resolution or most legible results. For this technique, acrylic ink is used with inkjet printers, similar to printing documents. Finally, UV light is used to cure the silkscreen markings. Now, let’s see how we can incorporate this information into silkscreen design.
Design Considerations for PCB Silkscreens
MANUFACTURING CONSIDERATIONS FOR PCB SILKSCREEN PROCESSES
|Font size||0.889 mm (0.035 in)||0.762 mm (0.03 in)||
0.508 mm (0.02 in)
|0.127 mm (0.005 in)||0.1016 mm (0.004 in)||
0.0762 mm (0.003 in)
Legibility and Accuracy
Good for larger font sizes
Setup and Application
Easy and quick
|Photoimaginable film (produces more waste than manual or DLP)||
Acrylic ink (cannot be used on silver surfaces or for applications such as aerospace)
In the table above, the three silkscreen techniques or processes are compared. As shown, they each have advantages and disadvantages. Understanding these attributes can assist you in designing your silkscreen.
DFM for HDI Printed Circuit Boards
How to Design PCB Silkscreens to Ensure Usability
Silkscreen designs may range from very simple to quite complicated. It is just as common to not make sufficient use of this opportunity to provide identification and markings to aid your board’s assembly as it is to include too much information. However, following the simple guidelines below will help you make the best use of your silkscreen.
Design Guidelines for PCB Silkscreens
1. Determine which process your CM uses to create silkscreens
Most CMs will not utilize all three of the silkscreen processes; therefore, you should find out what method your CM implements. This will impact your design in terms of size, clearances, and location of silkscreen markings and other elements, such as component pads, traces, and vias.
2. Account for silkscreen items during PCB layout
For readability, markings such as polarity, pin one, and reference indicators need to be as close to the associated components as possible. These additional real estate requirements should be considered as you are creating your layout to ensure there is no overlap or covering of the indicators.
3, Choose fonts based on CM’s method
Some PCB design packages allow you to create virtually any font style for your silkscreen. However, the number of fonts that your CM can produce may be much more restricted. You should consult with your CM to ensure that you are incorporating fonts that can actually be generated on your board.
4. Do not overuse silkscreen markings
It can be tempting to include indicators for all components and include other informational imagery that may be helpful for assembly, testing, or repair of your board. However, with the ongoing evolution towards smaller and densely packed PCBs, this is probably not possible. Therefore, you should prioritize the markings that you include as follows:
- Regulatory requirements; such as lead-free labeling required by J-STD-609A Marking and Labeling of Components, PCBs and PCBAs to Identify Lead (Pb), Lead-Free (Pb-Free) and Other Attributes from IPC.
- Manufacturer identification; including markings such as board part number, serial number, labels, and the logo.
- Assembly aids; such as polarity and pin one indicators. Reference indicators should be used based on available space.
- Testing aids; such as test point indicators, mode and/or jumper indicators.
Following the guidelines above on how to design PCB silkscreens will help you make the best use of identification and labeling on your board.
|Tempo‘s Custom PCB Manufacturing Service
Tempo Automation is the leading turnkey PCB manufacturer for prototyping and low-volume production in the industry. Our industry leading fastest board turnaround, software-driven process and commitment to quality enable us to provide you with the product and manufacturing experience. This includes silkscreen creation capabilities to satisfy any criteria your design requires.
And to help you get started on the best path, we furnish information for your DFM checks and enable you to easily view and download DRC files. If you’re an Altium user, you can simply add these files to your PCB design software.