Copper Thickness Requirements for Flex Circuits
Most designers don’t pay attention to important parameters like copper thickness and flex PCB thickness until very late into their project - only to realize that they have to scrap the entire project and start over because they got it wrong.
You have to be mindful of copper thickness because it contributes to the overall thickness of the flex board, along with the solder mask itself. So it is not a trivial measurement that you could ignore.
So why is copper thickness an important metric to consider?
For starters, copper will determine the current carrying capacity of your board and also impact properties like impedance and thermal performance. These are important properties that will impact the long-term durability and functionality of the flex circuit.
It is useful for both the designer and circuit fabricator to understand the impact that copper thickness will have on the overall circuit
Let’s take a look at how copper thickness requirements are determined for flexible PCBs:
- Copper thickness can provide contact to connector areas
- The board’s current carrying capacity, that is calculated from the cross-sectional area of the trace
- The board’s overall conductivity which is calculated from the metal type of the trace and its cross-sectional area
- Thermal properties that are a function of trace profile and metal type
- Impedance in high-speed boards which is mostly determined by factors such as the distance from the signal to ground plane, surrounding dielectric constant, and cross-sectional area of the copper.
The weight of the copper is also used by designers as a thickness measurement. It is common in the industry for circuit fabricators to buy copper foil with amounts varying from half an ounce to 2 ounces and so forth. This metric (in ounces) depicts the weight of copper in a square foot of foil. The number is universally accepted by most fabricators, material suppliers, and designers and is also found in IPC 6013.
Although it may seem odd to most designers, the weight of copper as a unit of measure has existed in the industry for years. This is mostly because the weight of the copper is related to its use as a building material.
It is worth noting that when electronic designers determine thickness, weighing copper may be a bad idea since a fabricated flex circuit undergoes various processes with new additions and subtractions of new material.
You can determine copper thickness by using an instrument that can accurately measure copper thickness in inches or micrometers.
The table below shows the conversion of copper from ounces to inches.
The table only goes up to 3 ounces per square foot, although you may opt for much heavier. Some fabricators also offer extremely thick copper amounts with weights of up to 20 ounces.
Note that the industry allows for a +/- 10% difference in the specified copper foil thickness from the raw material supplier and the actual thickness.
Most specifications for flexible circuits will determine copper thickness using weight. However, this can lead to some ambiguity because copper plating on two-layer flex PCBs may add an ounce of copper to the surface of a trace.
Specifying the thickness using weight may be inaccurate, especially if it is indeed a finished thickness. Moreover, designs for controlled impedance only work if copper plating is restricted to the via sans copper plating on the surface of PCB trace conducts. This can reduce the variability of trace thickness and may call for a type of product in the industry known as ‘button plating’ or ‘pads only plating’.
These terms are useful in applications that require controlled impedance designs.
1.0 Ounce vs. 0.5 Ounce - A Dilemma
Designers new to circuit design wonder if they should go with 0.5 ounces or 1 ounce of copper thickness. In practice, the electrical distinction between the two thicknesses is negligible. Moreover, there is no industry standard that fabricators follow when making circuits. Some stackups use 1 ounce, others will use 1 ounce, and they may even mix them on different layers.
The primary difference between both thicknesses is the DC resistance. Thicker copper foil will make the DC resistance of traces and planes smaller.
You may opt for heavier copper if you are building high-current flexible boards. Heavy amounts are also beneficial for boards that need to be functional at higher temperatures. You can follow IPC standards to determine the thickness of your copper layer, namely IPC-2221 for plane thickness and IPC-2152 nomograph for traces.
Flex circuits Processes that Change Final Copper Thickness
Some fabrication processes can also add or subtract the copper thickness, thereby affecting the final copper thickness. One common process is micro-etching which is used to prepare a surface for coating or plating. The process may remove a small amount of copper from the circuit. On the other hand, a process like electrolytic plating can increase copper thickness.
Circuit fabricators may directly measure the changes in copper thickness in mils and microns.
A popular method for measuring copper thickness is to carry out a cross-section, also known as a micro section. This test is destructive, this is why it is common for fabricators to use coupons found in unused parts of the processing panels. The coupons represent the actual thickness of the copper.
Copper thickness also varies across panels depending on the current density for electroplating. Current density is calculated from the copper trace pattern, so it is common to find part numbers to have various differences.
Crosshatched copper patterns are very common in panels. This feature can be used to minimize the variability in thickness because of current densities. For the most part, copper plating thicknesses tend to be thin toward the center and thicken on the outer edges of the panel.
It is highly recommended to think about flex circuits in terms of their functional requirements. This will help designers get closer to an accurate number for the copper thickness. Talking to a fabricator will also help them get closer to manufacturability and functionality. Fabricators can provide recommendations for tolerances and cooper thicknesses as well as the most accurate method for measurements.
For more information, get in touch with the experts at Hemeixin PCB here.