I hate to admit it, but I have a certain admiration for jewel thieves as they are portrayed in the movies. Now don’t worry, I am not endorsing a life of crime, nor am I contemplating a career change. Rest assured that I fully understand that in reality there isn’t any such thing as a “gentleman” thief, and instead it is all just robbery and common crime. However, I still have a fascination with these stories about smooth operators opening a wall safe and scurrying away with the diamonds before anyone realizes it.
Interestingly enough, the word “thief” is used in printed circuit board design as well, and it also relates to deftly stealing something covertly. In this case, however, we are talking about the theft of copper during circuit board fabrication, and the skill of this burglary is actually very important to the manufacturing of your circuit board. Does that sound like a wanted poster that you would be interested to find out more about? If so, then keep reading, and we’ll discuss copper thieving in PCB design and its importance to you.
What is Copper Thieving in PCB Design?
There are two similar terms used in PCB design; copper balancing and copper thieving. Although they are often used interchangeably, there are some important distinctions between the two:
This is the process of adding non-conductive patterns of copper to a circuit board in order to alleviate the possible bending of the board during manufacturing. PCB layer stackups that are not configured symmetrically, or individual layers with dense copper on one side versus the other, can all cause the board to warp during fabrication. Uneven copper in the board structure can also twist during the assembly process due to the excessive amount of heat used for soldering. To counter this, the PCB CAD design system will add area fills, or pours, to sparse areas of the board to copper balance the design.
This is also the addition of non-conductive patterns of copper to the circuit board, but in this case, it is to even out or “thieve” some of the electrical current used for plating the board. On areas of the board where there are isolated or “lonely” traces, the current applied to the board for plating can become more concentrated on those features. This scenario may result in an under-etching of the trace giving it a “mushroom” profile instead of the clean squared edges that were expected. You can see an example of this in the image below, where the cross-section of the trace on the left depicts serious undercutting, while the trace on the right is more squared off as desired.
The mushroom shape created by the under-etching can affect both the electrical performance of the trace as well as its structural strength. By reducing the metal in the trace, the characteristic impedance of the circuit could change and thereby introducing signal integrity problems into the board. At the same time, the reduction of metal will make the trace weaker than it should be, and the mushroom shape can potentially trap acid, leading to eventual alteration and/or destruction of the trace over time.
These problems can be averted however by adding copper thieving in the areas of the circuit board where the metal is sparse. With the same amount of copper distributed throughout the board, the plating current will be applied equally and the traces will all etch the same amount.
PCB traces cross-section, with the left side showing the mushroom shape of undercut etching
How Can You Apply Copper Thieving to Your PCB Design?
Most PCB design tools have the capability to add copper thieving automatically. These features will allow you to specify the shape, size, and spacing of the pattern that you want to use. As copper thieving is usually not electrically connected to the board, the designer has a lot of freedom in the patterns that they chose. The actual shape of the thieving pattern is not as important as long as care is taken to create the copper pours consistently and evenly throughout the design in order to balance the copper. You do however want to avoid acute angles in your thieving patterns as those can become acid traps. For this reason, a pattern of dots is often used.
You also have the opportunity to use these copper pours as more than just thieving patterns. By creating the copper thieving as part of the ground net and configuring it as a solid plane, you can help your board with additional EMI shielding. The caveat however is that this must be done in conjunction with the overall signal performance requirements of the design. Any additional planes like this must be positioned and spaced correctly so as to not have a negative effect on the signal integrity of the board.
Yes, there is a lot to consider when adding copper thieving to a printed circuit board, so don’t be afraid to ask for help with this step of your PCB layout.
Who To Turn To for More Information on Copper Thieving
There are some who would advocate leaving the copper thieving decisions up to the PCB fabrication shop, but this may not necessarily be the best design decision. While the fab shop will create thieving that will best fit the plating needs of the board, they won’t necessarily have all of the tools available to assess the electrical impacts of adding metal to the design. Adding copper to the design like this could have an effect on the electrical performance of the board that the fab house may not be aware of. It is critical that any metal changes to the board be done by the design engineers so that all of the potential effects can be taken into account.
The best resource that you have for creating copper thieving that will satisfy both the plating and signal performance needs of the board is your PCB contract manufacturer. By working together with these people you will have the expertise that you need to make the correct choices for adding copper thieving to your design. Your PCB CM understands the requirements of both manufacturing and electrical performance, and can either advise you on the creation of the thieving pattern, or do it for you.