Can we build capacitors on a PCB board?

Question

For the magnitude of nF or µF capacitors, I hope I can build them on a PCB board. The capacitor is like a two metal layer and something between them.

Is this possible?

Not buying the capacitor, just design the capacitor on the PCB board. Double metal layers on the PCB board.

Answer 1

You will have a hard time achieving 1 nF by just laying out copper on a standard two-layer FR-4 board. The capacitance is given roughly by the parallel plate equation:

\$ C = \epsilon{}A / d \$

In this case

\$ C = (4.7)(8.854\times{}10^{-12}) A / (1.6\times{}10^{-3}) \$

or

\$ C = A (2.6\times{}10^{-8}\mathrm{F/m^2}) \$

Meaning you'd need .038 m² or 380 cm² of copper area to achieve 1 nF. I used 4.7 as a typical dielectric constant (relative permittivity) for FR-4 and 1.6 mm as a typical board thickness.

It is not uncommon to make pF scale capacitors by parallel copper regions, but it's normally done in multilayer boards where the d term can be much smaller. This kind of constructed capacitor can achieve lower ESR and ESL than a discrete capacitor, so it is valuable for bypassing power supplies in very high frequency circuits.

There are also companies that make special materials that can be laminated up in a multilayer PCB to provide a high-dielectric-constant layer, enabling construction of even larger capacitor value by metal patterning. 3M is one. These are often called embedded capacitors or buried capacitors. Contact your PCB fabrication shop to see if they support this type of material.

Answer 2

It is possible to build capacitors that way, but you can forget µF. It would most likely be in the pF range.

I think the formula for calculating the capacitance of a plate capacitor would be appropriate here. \$C = \dfrac{\varepsilon A}{d}\$

It will be hard to build a large area on a PCB and you can not make the plate separation arbitrarily small as it will we hard for you to build it that way and you also probably will want it to be able to have some voltage across it.

And yes,this means that you get capacitance on the board from the traces, it's usually not a large value but it matters, especially if you have long traces close to each other and you are running a high frequency.

Answer 3

For a capacitor on a PCB, we need to look at the common formula for a parallel-plate capacitor with an area A, a distance of d between the plates, and a relative permittivity \$\varepsilon_r\$.

\$C=\varepsilon_0\varepsilon_r \frac{A}{d}\$

Let's use some common numbers: Our PCB has an area of 100 mm x 100 mm = 0.01 m², the thickness of the core is 1.5 mm, and FR4 (a.k.a. "PCB-type epoxy") as an \$\varepsilon_r\$ of approx. 4.2. Thus,

\$C=8.85 \cdot 10^{-12} \frac{\mathrm{F}}{\mathrm{m}} \cdot 4.2 \cdot\frac{0.01\space\mathrm{m}^2}{0.0015\space\mathrm{m}}\$

\$C=248\space\mathrm{pF}\$

Even if we used a thinner dielectric (FR4 core), and maybe even a multilayer board for more than two plates, getting towards nF will be big, and we are far from getting into the µF range.

However, you can use some capacitors on the edges of your board, and distribute their voltage across the board using two copper planes acting as a capacitor. The discrete capacitors paralleled with your PCB capacitor can act as one nearly perfect lumped capacitor, giving your fast logic or power design the warm fuzzies.

You won't use a PCB capacitor if you need exact or big values, but you can use it to create a really good power distribution system on your complete design.

Answer 4

A more esoteric form of capacitor uses fringing fields and lays both electrodes out on both layers in a intertwined fractal pattern. There is no closed form solution and it is very manufacturing tolerance sensitive, so practically useless in this case. Boost in capacitance would be in the range of 4X to 5X. Just mentioned for completeness. NOT at all advised.

Answer 5

As an experiment last year I attempted to build a capacitor by wrapping sheets of aluminum foil separated by a sheet of paper around a roll a few times. I think I only got something around 20 nF or so. Very little. Would be hard to get anywhere near that on a pcb since I was using relatively huge sheets of Al.

Answer 6

Is this possible? YES!

If I take your question verbatim and literally, you can build caps of that magnitude on PCB of a very large size. I don't know the equation of calculating the PCB size but I assume it would be fairly larger than the cost of the capacitor that you want to build on the PCB.