It’s necessary to have a good strategy for component references in place when designing PCB’s. The fight for space on a PCB means more and more designers are stuck in a tug of war between including and not including component references on the silkscreen.
This is the conclusion I’ve come to recently, while designing a board where the component reference on the silk on some parts was six characters long. What made it worse was that the parts were only 0402 / 0603 size parts. The silk reference was MUCH bigger than the component. Naturally this caused me some placement problems over above the normal placement issues faced when using silk component references in a design. Because of company standardisation and the system in place it wasn’t possible to change the reference, meaning that the component reference that is longer than my first name has stayed with that design.
In order to help others avoid these mistakes when setting up a component library and system of component references I’ve written out a step by step guide to minimising the space needed for your component references while keeping things readable.
1) Font Size:
The font size needs to be readable use 1mm as a minimum, anything smaller will not easily be legible. For a more comprehensive approach:
1.5mm – Leaded Components (these are usually large and can support this size).
1.2mm – Standard size for SMT Components.
1.0mm – Used for small SMT Components when space is tight.
2) Maximum Number of Characters
Decide now, how many characters you will use as a maximum, except in unusual cases. 4 Characters should be sufficient to give you enough for one component type and 999 components of that type. It would be very rare (I never say never) to then need to use 5 characters and out of the 999 you can use, 9 will only be two characters long and 99 will be three characters long.
3) Component Type Letter
At the start of our reference there is usually a letter to denote the component type R for resistor and so on. There are times where this could be two characters, for example, SW for switch and IC for an Integrated Circuit. If the component reference is to be kept to a minimum, we could use one character as there are 26 letters in the English alphabet, we should have enough for most parts. This system could be used:
C – Capacitor
D – Diode (could include LED in this, some systems differentiate)
F – Fuse
J – Connector
L – Inductor
M – Mechanical (Includes fixing, heat sinks, screening cans and other such parts)
Q – Transistor or Mosfet
R – Resistor
S – Switch
T – Transformer
U – Integrated Circuit
And so on…
This link is a page from Tom Hausherr’s “PCB Design Optimization Starts in the CAD Library” which has a more comprehensive list of component types and how their represented in component references. What is used in design depends on what kinds of components are used in anyone’s systems. The key thing is, there are no more than TWO letters used.
4) Numbering Convention
Starting off an component with 1 and increasing this as components are placed into a design saves space. Systems where the schematic page number is denotes first is becoming more common, but this wastes a character. There are also systems that start of as 100, this wastes even more space. When setting up a design systems the pro’s and con’s need to be weighed up, but saving characters greatly reduces the ‘wasted’ space taken up with a component reference.
All aspects of an electronic design system have their own compromises to be made and they will come back and haunt you, whatever you do – there is no magic formula. But in this age of decreasing sizes, increasing densities, think about the component reference on this silk layers, it may save you tearing your hair out!