Notes from the Board Room — On Silkcreen
Imagine a map with all of the lines for roads, rivers, political boundaries, and so on presented in perfect detail. If the labels for those terrains and borders were missing, the map wouldn’t be of much use. The marking on a PCB is kind of like that.
To the end user, the details that are inked onto the board have no function or meaning. The “pick and place” robot doesn’t care either. The in-circuit test (ICT) fixture can’t read. Today’s components are so small and so densely packed that room for adding marking next each one with the corresponding reference designator (1) is a rarity. For these reasons, some companies will omit the step altogether once the product goes into mass production.
Still, marking the board is an important final touch that usually takes a back seat to the metal and mask layers. It’s also the last process step on the board fabrication. Stocking, kitting, hand assembly, rework, bring-up, inspection, import/export, and disposal all rely to some extent on what is printed or screened onto the board.
Safety
As a minimum, setting aside a space for a bar or QR code will assist in tracking a device through the factory and beyond. There may also be contractual or legal requirements to label something as UL (Underwriters Laboratories) listed. These are the folks who make sure stuff doesn’t arbitrarily catch on fire or readily burn in the presence of such. They evaluate and certify that all of the various components meet specific safety requirements as well. Their mark of approval is an valuable marketing tool (at the very least). It can be added to the board in ink if the product passes the battery of tests.
Next on the scale of importance is the polarity mark of any device that would misbehave if installed backward. I recall my days on the factory floor when a tech put a board onto the GenRad (now Teradyne) test fixture. She started the test, and there was a loud explosion as the primary power diode blew up in her face. It was installed backward where it fit just fine. She was okay, but we were all pretty startled. We visually inspected all of the work in progress (WIP) before any further testing.
The polarity mark can be created without ink. We use a small opening in the solder-mask next to pin-one of an IC, the cathode of a diode, the positive pin of a cap, and so on. You just have to make sure you’re not creating an opportunity for solder to bridge the gap between the pin and whatever metal is being exposed by the mask opening. Plan ahead during placement for this important piece of the puzzle!
Environmental
IPC-1066 outlines the various ways that a PCB or product should be marked to identify it as lead-free or indicate other dangerous ingredients. The scope goes beyond the bare board out to the entire object.
Component footprints
Component outlines are helpful to have. We want the outline to be visible with the component in place. Revision C of the sprawling IPC-7351 document considers three possible methods for component outline marking. Our hardworking friends at PCB Libraries published an excellent transitional document that provides guidance on the three methods. They break it all down to density requirements. One thing in common is that the pin-one dot is now incorporated into the package outline as an extension of the outline. It remains within the so-called courtyard for each type (as illustrated below). The red rectangles represent the courtyards, and the thick black lines show the part marking.
If I had to choose one, I’d go with the nominal packages. The smaller pads of the Least Environment worry me when it comes to applications where a lot of shock, vibration, temperature extremes, and long life expectancy are part of the deal. Automotive is one such application.
Reference designators
The last in line for priority is the individual component identification. Within those, there is a hierarchy that begins with the obvious priorities of connectors, lights, test points, potentiometers, or anything that would be touched in the course of bringing up the system. Active components are next followed by the hoard of passive resistors, inductors, and capacitors. The prevailing idea is to mark as many as you can. When it comes to test fixtures, the game changes a little. You can see here where J30 has every pin’s function marked. The LEDs show what it means when the light comes on while the passive parts are entirely ignored regarding PCB marking.
Pro tip: When you have a swarm of puny parts that need marking, you can use the same methods used by the cartographers of yesteryear when they were faced with a mess of short streets with long names. Arrange all of the reference designators over the components and move all of them to an open area. Put a perimeter around both the parts and the reference designators and give them a common label such as A.
General marking
Aside from the UL and tracking data mentioned above, we also like to see some vendor specific information. Their logo, a date code, country of origin and a graphic of a garbage can be overlaid with the international “No” indication of the slashed circle are typical requirements. We also want our own part number and revision for both fabrication (etched on the bottom) and assembly printed on the top. Serial numbers, ESD warnings and other esoteric information as required will be printed, stamped or labeled onto the board.
Military grade brings on its own set of rules including a minimum text size that would have us forgo about half of what you see here. Traceability is their primary concern.
On notes
The way we provide the fabricator with our wishes is pretty straightforward. Here’s a sample:
Notes: Unless otherwise specified.
A. Mark primary side of PCB with white epoxy non-conductive, non-nutrient ink.
B. Remove any ink from exposed copper.
What is left unsaid is whether they are to use silkscreen or another printing process. Fabrication and assembly drawings are all about what-is and not about how-to.
That’s all I’ve got for now. If you think I missed anything or have any questions, I would be delighted to help you out or receive your help as you see fit. Comments appreciated.
