Many years ago (too many to count), my classmates and I used to divide our days equally between learning and playing. Without a doubt, our Kindergarten curriculum must have been designed by a child or an adult who was a child at heart. Although we had lots of fun, we also managed to conquer the alphabet, acquire some basic writing skills and start the journey to understanding mathematics. Yet, one of the most important things I learned occurred every day after playtime was over: our teacher reminding us that there is a place for everything and everything belongs in its place. That simple admonition to put things in their proper place can be applied to many more complex situations, such as designing PCB traces to optimize signal current flow.
A major concern when designing your board layout is signal integrity, which is a measure of the variance of a transmitted signal along its path. For PCBs, the ability of a signal to minimize current loss depends on the degree of impedance encountered along its trace. The trace may be a planar path on an external (surface) or internal layer or through a plated via such that the direction of current flow is perpendicular to current flow through the layers1. Traces are conductors usually made of copper and defined by their width, length, weight and thickness. Current flow is controlled based on the choice of these parameters. For cases where trace length and width are fixed due to board density and spacing restrictions, current flow can be optimized by adjusting the PCB copper thickness or weight.
To use PCB copper thickness for current flow optimization, we need to understand the relationship between them. But first, let’s take a look at copper trace fabrication and its implications for signal integrity.
How is PCB Copper Thickness Fabricated?
The fabrication of the copper traces of your board, whether on the surface or for internal layers, is done by etching, one of the major PCB manufacturing steps. Layer fabrication begins with a laminate sheet that is completely covered by copper. Etching is the process of removing excess copper from traces and other areas, such as component pads and annular rings for vias, using an ammonia-based solution.
Typically, these sheets begin with preset copper thicknesses. For example, for 1oz (0.0014in or 0.035mm) copper thickness, the sheet would have a 0.5oz (0.0007in or 0.018mm) layer of copper on both sides. This thickness can be etched down, decreased, or plated up, increased, to the desired specification. Due to the manufacturing process for the sheets (rolled or electro-deposited), the surface has a degree of roughness that helps with bonding to the dielectric material. However, this can have an increasingly adverse effect on signal integrity at higher frequencies. At frequencies above 100MHz, the skin effect, where current flow is typically near the internal surface of the trace as opposed to through the conductor center, occurs, causing the effective resistance of the trace to increase and oppose current flow.
Traces for plated vias must be added after the holes are drilled. A process for accomplishing this is electrodeposition where the board is placed in an electrolytic solution containing copper ions. A direct current is applied to set up a potential across the via and ions are distributed throughout the via. The distribution of the copper ions throughout the via or the filling determine the quality of signal flow along via traces.
How Does PCB Copper Thickness Affect Signal Flow?
From the PCB manufacturing perspective, PCB copper thickness and weight are fixed at circuit board material selection. From the design vantage point, these parameters may be varied after fixing your trace lengths and widths to optimize signal flows or vice versa to improve signal integrity. As stipulated in IPC-2152 the Standard for Determining Current-carrying Capacity in Printed Board Design, trace parameters: PCB copper thickness, copper weight, trace width and length are interrelated and dependent on electrical and thermal material properties. Although interweaving all of these mathematical relationships may seem too complex to utilize, there are closed-form equations to help you determine the trace parameters that will yield the best signal flows.
How to Use PCB Copper Thickness to Optimize Current Flow
Following the procedure below and using constants:
🖩 Determine optimal signal power, P (W) and voltage, V (Volts).
This should be known quantities from circuit operation design.
🖩 Determine trace impedance, R (Ω).
This should be set and can be controlled during board manufacture.
🖩 Determine the maximum current flow for the trace, I (A).
Current can be determined from either of the following equations:
I = V2/R or I = P/V or I = √(P/R)
🖩 Determine the expected temperature rise, T (°C).
This should be known or estimable from circuit operation design.
🖩 Determine the trace cross-sectional area, A (mils2).
Use curve fitting constants, k, b and c, to approximate the IPC-2152 curve that relates area to current carrying capacity.
- For internal layers: k = 0.024, b = 0.44, c = 0.725
- For external layers: k = 0.048, b = 0.44, c = 0.725
A = (I/(k×Tb))1/c
🖩 Determine the trace width, w (mils).
This should be set.
🖩 Determine the PCB copper thickness, 𝜟 (mils).
𝜟 = A/w
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Your PCB design software may allow you to set copper thickness or instead it may require that you use copper weight, which is also common among contract manufacturers (CMs). These parameters are directly proportional and you can convert to copper weight using the following equation:
Copper weight(oz) = (copper thickness(mils)) / 1.37
Essentially, one square foot of copper has a weight of 1 ounce.
Tempo Automation, the industry leader in fast, precise PCB prototyping and low volume production provides a range of PCB copper weights to enable you to optimize the signal flows for your design. We also provide expert support to ensure that your boards are built in accordance with your design intent.
|Tempo‘s Custom PCB Manufacturing Service
And to help you get started on the best path, we furnish information for your DFM and enable you to easily view and download DRC files. If you’re an Altium user, you can simply add these files to your PCB design software.
If you are ready to have your design manufactured, try our quote tool to upload your CAD and BOM files. If you want more information on PCB copper thickness or how to use trace parameters to improve signal flow, contact us.