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All About Flex: Copper Thickness Requirements for Flex Circuits

An end user will specify the copper thickness of a printed circuit for different reasons. The most obvious reason would be for current-carrying capacity, but copper thickness also directly impacts thermal performance and impedance. All these are vital properties impacting the functionality and reliability of a flexible circuit. 

It is important for both the circuit fabricator and the customer to understand the functional need driving a copper thickness requirement. Some of the common functional requirements could be:

• Minimum thickness in a connector area to assure robust contact
• Current carrying capacity (calculated based on the cross-sectional area of the trace)
• Conductivity, which is a function of cross-sectional area and metal type of the trace
• Impedance in high-speed circuits which is driven by the cross-sectional area of the copper, the surrounding dielectric constant, and distance from signal trace to ground plane
• Thermal properties which are directly related to metal type and trace profile

Copper weight is used in the industry as a “thickness” measurement. Circuit fabricators commonly purchase copper foil with descriptions of ½ ounce, 1 ounce, 2 ounce, and so on. The number is the weight of copper in a square foot of foil. This metric is commonly accepted by the material supplier, fabricator, and end users and is the typical measurement defined by the industry in IPC 6013. The weight as a unit of measure has been around for decades, even though it seems a bit odd that in this age of microelectronics and high-speed circuitry we still specify copper by its weight, which is probably related to its use as a building material. But when an end user is determining thickness, weighing the part is impractical since a fabricated flexible circuit has undergone numerous processes with material additions and subtractions. 

The best way to determine thickness is to use an instrument that directly measures thickness in inches (mils) or micrometers (µm). Equivalent thickness that relates to a given weight is a linear relationship.  Below are some common conversions:

0.5 oz. = .0007" (0.018 mm) thick copper
1 oz. = 0.0014" (0.036 mm) thick copper
2 oz. = 0.0028" (0.071 mm) thick copper
3 oz. = 0.0042" (0.107 mm) thick copper

It is important to note that +/- 10% is the industry accepted purchased specification tolerance for copper foil thickness provided from a raw material supplier.

Drawing specifications will frequently define a flexible PCB copper thickness using weight. For example, “circuit to be 1-ounce copper.” This can lead to some ambiguity as copper plating on double-sided circuits can easily add an ounce of copper to the surface of a trace. By specifying thickness in this fashion, it is not clear if this is intended as a finished thickness or an original thickness. Additionally, controlled impedance designs work best when copper plating is restricted to the vias sans copper plating on the surface of the traces. This minimizes trace thickness variability and suggests a product category known in the industry as “pads only plating” or “button plating.” For controlled impedance designs, one of these terms should be called out in the drawing notes.

Affecting final copper thickness are the variety of fabrication processes that add or subtract copper thickness. Micro-etching is a common cleaning process used to prepare a surface for plating or coating. This process removes a small amount of copper. On the flip side, copper thickness is increased during electrolytic plating processes. The circuit fabricator will directly measure added (or subtracted) thickness in mils (1 mil = 0.001”) or microns (25 µm= 0.001”). 

The most accurate method for determining thickness is to do a microsection, also known as a cross-section. This is a destructive test, so it is common to use coupons located in unused areas of the processing panel. These coupons are located and sized to be representative of the circuitry copper thickness.

Copper thicknesses will vary slightly across a panel depending on current density from electroplating. Current density can be a function of the copper trace pattern so differences among various part numbers will occur. Cross-hatched copper patterns are sometimes seen on panels. This artwork feature is added to help minimize thickness variability due to current densities. As a general rule, copper plating thicknesses will tend to be thicker on the outer edges of the panel and thinner toward the center.

In summary, when defining the specific copper thickness for an application it is best to start with a discussion of the myriad functional requirements. Input from the fabricator will assure a match driven by tradeoffs to meet functionality and manufacturability. The fabricator can help recommend copper thicknesses and tolerances as well as the best methods for measurement.

Dave Becker is vice president of sales and marketing at All Flex Flexible Circuits LLC.