- Product size is compact as several copper weights can exist on the same layer of circuitry.
- Possibility of integrating high-current circuitry with their control circuit in a dense structure.
- Vias, heavily plated with copper, allow higher currents to flow from one layer to the other, while assisting in transferring heat to an outer layer or heat sink.
- Possibility of making high power density planar transformers on the PCB.
Manufacturers purchase material for PCB or printed circuit board from laminate suppliers. The substrate comes pre-clad with copper sheet on one or both sides. The pre-clad is available in various substrate materials and copper thicknesses. This allows construction of single-, double-, or multilayered PCBs with different thicknesses and finished copper weights, as the measure of copper thickness on a PCB is usually in ounces (oz).
A PCB copper thickness of 1 oz means one ounce by weight of copper spread evenly and pressed flat over an area of one square foot. The resulting copper thickness measures 1.37 thousandth part of an inch, or more commonly 1.37 mils. Manufacturers generally fabricate PCBs with 1 oz copper thickness. Unless the customer specifically orders a different thickness, the manufacturer will assume and quote for 1 oz copper thickness.
For most purposes, 1 oz copper thickness will suffice. Increasing the thickness of copper increases the cost and complexity of fabricating the PCB, as processing thicker copper takes more time and is a more difficult task. Rather, designers prefer to increase the width of the copper trace on the PCB to allow it to carry more current:
| Copper Weight (oz/ft2) | Copper Thickness (mils) | Track Width (mils) | |||||||
|---|---|---|---|---|---|---|---|---|---|
| 1 | 1.37 | 62.5 | 125 | 250 | 250 | 1000 | 2000 | 4000 | 8000 |
| Current (A) at 20 °C | 4.6 | 7.6 | 12.5 | 20.7 | 34.2 | 56.6 | 93.6 | 154.7 | |
Table 1: Track Width and Current Carrying Capacity for 1 oz Copper Weight
Importance of Copper Thickness
Apart from the dimensions of the PCB itself, the single most common property that designers change is the copper thickness. Changing the copper weight can lead to unexpected changes in other properties of the board, calling for a DFM/DFA review process. These changes include:
Copper Thickness: This is obvious, as copper weight is directly related to copper thickness— higher weight means thicker copper and vice versa.
Via Annular Ring: Changing the copper weight results in a change in the minimum annular ring required for a via. This is an important design specification as it affects the reliability of an electrical connection to a via, as the drilled hole may not always centered perfectly.
Track Width and Spacing: The copper weight significantly influences the track width and spacing on a specific layer. This in turn affects the stackup of the board and the calculations for impedance control. Therefore, changing the copper weight can lead to a cascading effect for tight track width and spacing, and impedance control.
Minimum Conductor Clearance: Copper weight on a layer affects the minimum conductor clearance on that layer. Heavier copper weight results in a thicker copper layer, requiring deeper etching, and results in deeper undercuts. To compensate for the deeper undercuts, designers need to provide wider conductor clearance.
Cost of PCB: Heavier copper weight means the PCB has more copper in that layer. If the PCB is multi-layered, the amount of copper increases proportionately and this could add up to a significant increase in the cost of the project.
| Copper Type |
Copper Weight (oz/ft2) |
Copper Thickness (Mils) |
Minimum Annular Ring (Mils) |
Track Width/Clearance (Mils) |
Minimum Clearance (Mils) |
||
|---|---|---|---|---|---|---|---|
| Inner Layer |
Outer Layer |
Inner Layer |
Outer Layer |
||||
| Standard | 1 | 1.37 | 3.5 – 4.0 | 3/4 - 4/5 | 4/4 - 4/6 | 4 - 5 | 5 - 6 |
| High or Heavy | 2 | 2.74 | 5.0 – 6.0 | 4/5.5 - 5/7 | 5/6 - 5/8 | 6 - 7 | 7 - 8 |
| 4 | 5.48 | 6.0 – 8.0 | 5/8 - 6/10 | 6/8 - 6/10 | 9 - 10 | 10 - 12 | |
| 3 | 4.11 | 6.0 – 8.0 | 5/8 - 6/10 | 6/8 - 6/10 | 9 - 10 | 10 - 12 | |
| 5 - 20 | 5.6 - 28 | - | - | - | - | - | |
| Extreme | 24 - 200 | 33.6 - 280 | - | - | - | - | - |
Table 2: Design Requirements for Different Copper Weights
Copper Thickness & PCB Fabrication
The PCB industry uses 1 oz as the most common copper weight, resulting in 1.37 mils as finished copper thickness. In practice, the manufacturer plates up the initial copper weight to a higher value while fabricating the PCB. Therefore, the designer must indicate the finished or plated value of copper weight for their PCB when submitting Gerber files.
PCB manufacturing involves a combination of process for etching and plating copper. Manufacturers begin by using circuit layers of thin sheets of copper foil, mostly 1 oz, which they etch to remove the unwanted copper, and then plate to add copper thickness to the various traces, pads, planes, and plated through holes. Laminating the circuit layers into a complete package on an epoxy-based substrate completes the PCB.
Manufacturers will not add any additional charges for PCBs with copper weight of 1 oz. However, above 1 oz they will usually add some extra cost and lead time. Manufacturers will usually assist customers get an idea of the extra cost and lead time if they choose copper weights higher than 1 oz. Apart from additional cost and lead times, copper thickness above 1 oz also adds to shipping charges as the weight of PCBs increase.
The extra cost for copper thickness above 1 oz comes from the necessity for additional labor, process engineering, and quality assurance. For instance, increasing the thickness of the copper cladding on the laminate requires more time for etching, while observing specific DFM guidelines. More care during the assembly process is necessary as the thicker copper affects the thermal profile of the board, causing it to absorb significant amounts of heat during the reflow stage.
Traditionally, etching features with heavy copper caused unacceptable undercutting and uneven trace sidewalls. This led to the development of techniques involving specialized etching and plating, such as differential etching along with high-speed and step plating. Fabricators have enhanced these techniques further to offer negligible undercut and straight sidewalls.
Plating a board with high copper thickness also increases the thickness of copper in the plated through holes and on the sidewalls. Fabricators now offer boards with a mix of heavy copper and features with standard copper thickness on a single board. This allows a reduction in layer count, smaller footprints, and low impedance power distribution along with potential cost savings.
High Copper Thickness
Although there is no standard definition for high or heavy copper, the general acceptance is high or heavy copper ranges from 2 oz to 20 oz, and extreme copper ranges from 24-200 oz. Additional copper thickness allows the PCB to cater to frequent exposure to excessive current flow, operation at elevated temperatures, and safe operation even under recurring thermal cycling. With a high tolerance capacity, the heavy copper board can withstand applications in rough situations such as those required by defense and aerospace industries.
Advantages of high copper thickness boards are:
Applications that require power conversion, high power distribution, heat dissipation, planar transformers use PCBs with high copper thickness. Industries such as solar, automotive, computers, military, industrial controls, and those making welding equipment have great demand for heavy copper-clad boards.
Conclusion
It is possible to connect features with high copper thickness seamlessly to circuits with standard copper thickness on the same PCB. However, the fabricator and the designer must discuss Fig 1: Etching High Thickness Copper manufacturing abilities and tolerances prior to settling with the final design for placing heavy copper and standard features together with minimal restrictions.
