Tool and tips for good copper distribution
Evenly distributed copper across layers, and choosing symmetrical buildups (stackups), are two important ways for avoiding PCB manufacturing quality issues, and problems during component assembly. We explain below why that is.
Copper thickness and planarity (within tolerances) are two important PCB quality criteria. When copper is unevenly distributed or if the layers of a multilayer PCB are not symmetrical in number or thickness above and below the horizontal centre of the PCB, the imbalance may cause mechanical instability.
This imbalance can result in bowing (curvature) and/or twisting (distortion) of the PCB because the individual components, such as the copper and glass-fibre reinforced resin in the case of FR4 material, expand and contract at different rates when heated and cooled. Heating and cooling of the PCB occurs during the pressing, etching, electroplating processes, and the application of the soldermask, as well as during the soldering of components.
Measuring bow and twist of the PCB
PCB manufacturers have optimised their production processes to keep bow and twist to the minimum possible. Eurocircuits only delivers PCBs that are within the permissible tolerance. We explain how bow and twist is measured and what tolerances are permissible in our article Bow and twist on a PCB.
The PCB designer has the greatest influence on bow and twist and therefore, the following rule should be applied:
- The copper should always be distributed as evenly as possible on all layers.
- A symmetrical layer structure should always be chosen:
- Both in number and thickness.
- Unless an asymmetrical buildup is crucial for the application.
Tips for even copper distribution on PCB and panel
Good copper distribution does not only mean higher mechanical stability and thus less chance of distortion (bow and/or twist). Well-distributed copper has another important function. It is the prerequisite for uniform thicknesses of the copper layers and plated-through holes during the electroplating process.
If the copper density is low, there is a risk that too much copper will be deposited. In this case, the copper layer and the hole wall would be too thick; if the copper density is high, the copper layer could be too thin and the copper sleeve of the hole too weak. If a layer has low- and high-density areas then the above will also apply, resulting in uneven copper plating across the layer.
We have compiled the most important tips and examples for good copper distribution for PCBs and panels on a separate page Plating index solutions.
Plating Simulation tool shows copper distribution
In accordance with Eurocircuits guiding principle “Right First Time”, we have integrated a Plating simulation in our free to use online PCB Visualizer tools. Simply upload your PCB data to the Visualizer, activate the PCB Checker and click on DFM Information. The plating simulation tool shows the copper distribution on the PCB. It calculates the Plating index based on the copper density and distribution on the outer layers.
The software divides the PCB into cells. The copper density of each cell is compared to the average copper density of the entire PCB and a colour is assigned to that cell. Areas marked blue are at risk of underplating, areas marked red are at risk of over plating.
A low Plating index score causes too much variation in copper thickness
With a Plating Index of 1, no problems are expected during the electroplating process. A smaller value means non-uniformity (uneven copper plating) and is highlighted by red and blue areas on the visualised image of the PCB. A plating index of less than 0.40 leads to problems in manufacturing, increases scrap and can affect the quality and reliability of the finished PCB.
Basically, tolerances in the copper thickness are process-related and cannot be avoided. The reason: The metal lattices of the copper do not build up evenly in the galvanic process, there is also a very small amount of copper removed during each of the cleaning steps.
In addition, the start or base copper thickness has a manufacturing tolerance of up to -10%. For more details, please see our page Tolerances on Copper Thickness.