I hate to admit it, but I admire jewel thieves as shown in movies. Now, don’t worry, I am not endorsing a life of crime or 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 a common crime. However, I am still fascinated by these stories about smooth operators opening a wall safely and making a getaway with the diamonds before anyone realizes it.
Interestingly, the word thief is also used in printed circuit board design and 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 essential to manufacturing your circuit board. Does that sound like a wanted poster you want to learn more about? If so, keep reading as we discuss the importance of copper thieving in PCB design.
| Feature | Copper Balancing | Copper Thieving |
|---|---|---|
| Definition | Technique used to ensure an even distribution of copper across the board to prevent warping. Ensures a uniform thermal expansion and contraction | A technique used to add copper features to areas with low copper density to ensure consistent plating thickness and uniform copper distribution. |
| Components | – Copper layers
– Prepreg and core materials |
– Dummy pads
– Additional copper features not part of functional circuitry |
| Used in | – Multilayer PCBs
– High-density interconnect (HDI) PCBs |
– PCBs with large variations in copper density
– PCBs requiring high precision in plating |
| Manufacturing Considerations | – Layer stackup design must consider copper weight across all layers.
– Balancing requires adjustments to distribute copper evenly. |
– Areas of low copper density require thieving patterns.
– Thieving patterns can be dots, hatches, or solid patches, depending on the requirements. |
What is Copper Thieving in PCB Design?
Two similar terms occur in PCB design: copper balancing and copper thieving. Although often used interchangeably, there are some important distinctions between the two.
Copper Balancing
Adding non-conductive patterns of copper to a circuit board alleviates the possibility of the board bending during manufacturing. PCB layer stack-ups without symmetrical configuration 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 heat used for soldering. The PCB CAD design system will add area fills, or pours, to sparse board areas to copper balance the design.
Copper Thieving
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 with 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 expected clean squared edges. You can see an example in the image below, where the cross-section of the trace on the left depicts severe undercutting, while the trace on the right is more squared off as desired.
PCB traces cross-section, with the left side showing the mushroom shape of undercut etching
The mushroom shape created by the under-etching can affect both the electrical performance of the trace and its structural strength. By reducing the metal in the trace, the characteristic impedance of the circuit could change and thereby introduce 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 destruction of the trace over time.
These problems are avertable, 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 apply equally, and the traces will all etch the same amount.
How Can You Apply Copper Thieving to Your PCB Design?
Most PCB design tools can add copper thieving automatically. These features will allow you to specify the shape, size, and spacing of the pattern you want. As copper thieving is usually not electrically connected to the board, the designer has a lot of freedom in the chosen patterns. The actual shape of the thieving pattern is less important as long as copper pours consistently and evenly throughout the design. However, avoid acute angles in your thieving patterns, which can become acid traps. A pattern of dots achieves good results.
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 it requires conjunction with the overall signal performance requirements of the design. Any additional planes like this need correct placement and spacing so as not to negatively affect the board’s signal integrity.
Yes, there is much 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
Some 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 the tools available to assess the electrical impacts of adding metal to the design. Adding copper to the design like this could affect the board’s electrical performance, which the fab house may not be aware of. The design engineers must make any metal changes to the board to consider all potential effects.
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 with these people, you will have the expertise to make the correct choices for adding copper thieving to your design. Your PCB CM understands manufacturing and electrical performance requirements and can either advise you on creating the thieving pattern or do it for you.
