At PCB Trace Technologies Inc, we use two techniques during PCB manufacturing—copper balancing and copper thieving—to achieve the high quality and reliability of our boards. Both are crucial processes but with different purposes. Copper distribution is an important factor related to manufacturing, and copper balancing, along with copper thieving, helps achieve an even distribution of copper on circuit boards.
Why is Copper Distribution Important?
When some parts of a PCB layer have more copper than other regions, the resulting variation in copper distribution leads to many defects in the board’s overall shape. This is especially visible during thermal cycling, such as during baking and soldering.
The heat causes physical expansion and contraction of the printed circuit board, but the copper foil and the substrate expand and contract at different rates due to the difference in their expansion coefficients. This generates internal stresses in the board, subsequently leading to its physical deformation, where the entire board is no longer in the same plane or is no longer perfectly flat. Depending on the nature and position of the unbalanced copper, a printed circuit board may suffer three types of deformation such as:
Warpage
The board may bend differently at different areas, leading to a wavy structure as against a perfectly flat surface.
Bow
The bending may be of a cylindrical or spherical nature. This can result in the center of the board shifting to a different plane from that of its corners, giving the board a bow-shaped formation.
Twist
Depending on the board thickness, one of the corners of the PCB may no longer remain aligned with the others. A corner may rise diagonally, while others may move downwards. This formation typically gives the board a twisted look like a potato chip.
Unbalanced copper distribution causing any deformation as above is not acceptable for a board under Class 3 standard. We use copper balancing and copper thieving to avoid such deformation of boards during thermal cycling.
Copper Balancing
We achieve copper balancing in our PCBs in different stages of design, such as:
Stack-Up
Typically, we set the central layer with the maximum copper thickness, balancing the remaining layers to match their mirror opposites. This is an important recommendation to avoid the deformation discussed earlier.
Symmetrical Power Planes
We typically place power planes symmetrically in pairs in the stack-up, taking care to use the same copper weight in a pair.
Core and Prepreg Layers
It is necessary to achieve copper balancing in the core and prepreg layers of the PCB. By ensuring a uniform proportion of copper in these layers, we maintain an overall copper balance in the PCB.
Grids
We design wide copper areas on the PCB as grids rather than solid planes. This helps to avoid the mismatch in copper density in that layer, reducing the tendency to bow and twist effects to a large extent.
Symmetrical Layers
We balance any excess copper area in a particular layer by placing copper thieving in a symmetrically opposite layer.
We use copper balancing to prevent deformation of the board during thermal cycling in the manufacturing and assembly phases due to uneven copper distribution. Copper balancing helps ensure the board remains flat and maintains its structural integrity.
What is Copper Thieving?
Copper thieving is primarily tiny grids of copper that we place in sparse areas of the board to balance out the presence of excess copper area in a symmetrically opposite layer in the board stack up.
These tiny grids of copper may be of any shape, circular or square, and as they are not connected to any net, they do not disturb the functionality of the board. However, apart from copper balancing, copper thieving has additional functions as well:
| Aspect | Copper Thieving |
| Primary Purpose | Balancing the excess copper in symmetrically opposite layers. |
| Plating | Aids in consistent plating in areas with low copper density. |
| Precision | Improves plating precision in PCBs with varying copper density. |
| Trace Performance | Prevents trace under-etching, thereby improving trace strength and performance. |
Considerations for Copper Thieving
At PCB Trace Technologies Inc., we follow the best design practices for PCBs. We consider the following aspects of copper thieving:
Clearance
To prevent interference, we maintain adequate clearance between copper thieving and signal traces. Typically, this is about three times the trace width. We enforce these clearance requirements by implementing them in the DRC or design rule check. In certain cases, we employ keep-out areas or guard traces for maintaining proper separation between the copper thieving and signal traces.
Net Assignment
Although copper thieving is typically not connected to any net, some applications do not allow floating copper. In such cases, we connect copper thieving to a designated net, mostly ground, using traces or vias.
Signal Integrity
In some cases, particularly in high-speed applications, thieving patterns can influence the signal integrity. In these cases, we adjust the thieving patterns to tapered shapes or meandering to mitigate impedance changes, adjusting the shapes to minimize impedance variations.
Solderability
We do not allow thieving patterns to hinder the soldering process or affect any solder joint quality. For this, we ensure that the thieving features do not encroach on solder pads or in areas dominated by component mounting. We take care to prevent solder bridging among thieving patterns by using suitable solder masking.
Placement
We strategically place thieving patterns to optimize copper distribution, while addressing areas with uneven copper coverage. For this, we identify regions on the board with limited copper presence for targeted implementation of copper thieving. Next, we concentrate the thieving features in areas that require additional copper balancing, taking care to avoid interference with critical signal paths and components. We also take care to check the signal integrity of traces that have thieving on layers directly underneath.
Benefits of Copper Thieving
Although placing copper thieving in a circuit board helps to achieve a uniform distribution of copper, it has additional benefits as well:
More Uniform Copper Distribution
By adding copper thieving or small copper patterns or features in areas on a board, we can balance out the copper density more uniformly. By doing this, we can attain a more homogeneous distribution of copper throughout the board, thereby preventing physical deformation during thermal cycling.
Etching and Plating Control
In subtractive PCB manufacturing, we typically etch away unwanted copper to realize patterns and traces. Placing copper thieving in sparse areas helps to maintain the copper density consistently, ensuring the etching process does not remove excess copper. The same applies to the plating process as well, as the plating process adds copper to the board.
Uniform Final Board Thickness
By adding copper thieving, we ensure the board has a consistent final thickness. With uneven distribution, the final thickness of the board can have variations, affecting its physical properties during the assembly process. By balancing the copper density with copper thieving, we can eliminate these issues.
Minimizing Warping and Twisting
Uneven copper distribution can lead to warping and twisting of boards during thermal cycling in the manufacturing and assembly processes. Thieving leads to more stable and reliable PCBs by ensuring an even distribution of copper across the board.
Dielectric Thickness Regulation
By adding copper thieving, we can additionally manage the thickness of the dielectric substrate between different copper layers. Managing the copper distribution ensures the usage of proper amounts of dielectric material, thereby preventing adverse problems like crosstalk between traces in adjacent layers.
Cost Reduction
Copper thieving, by ensuring optimal copper distribution, helps reduce material wastage during the etching process. By preventing over-etching, thieving not only reduces manufacturing expenses but also improves the board’s quality and reliability.
Conclusion
At PCB Trace Technologies Inc, we use a crucial technique like copper thieving in our printed circuit board manufacturing processes to ensure a more uniform distribution of copper across the board. This helps us prevent several issues related to uneven plating, over-etching, warping, and twisting during the manufacturing process, thereby improving the board’s quality and reliability.
