FR4 for coil flatwork?

[Ronsonic]

I did a little searching and it looks like there's at least one person using FR4 for flatwork, but not a lot of mention of it here. Or at least that I found with the Search feature.

Just wondering if there's a consensus on it - sound okay? Any surprises? Interesting anecdotes?

It's probably far more rugged and and a bit more expensive than needs be, but it's technology I already work with. I can do a layout on the computer using PCB software and have parts in the mail long before I could tool up to cut anything nearly that accurate. It would also make wiring easy and onboard electronics simple if I ever went that way.

[salvarsan]

Just wondering if there's a consensus on it - sound okay? Any surprises? Interesting anecdotes?.

Bill Lawrence used it a lot, particularly on the Strat form factor flats. Sometimes it was FR4, sometimes it was black XXXP.

My opinion is that FR4 is more microphonic than vulcanized fibre, less so than formica and carbon fibre+resin.

[FunkyKikuchiyo]

It works wonderfully. Like vulcanized fibre it is a fantastic dielectric and is used to that end often. If it didn't perpetually have a "cheap circuit board" look to it I suspect more people would be using it. It is a lot easier to machine and keep tighter tolerances... working with it by hand can be a pain, though.

It also doesn't take on moisture like fibreboard does - so that may mean greater life and less risk of oxidation creeping down around polepieces... I'm not sure on that, though.

[Ronsonic]

What I was planning to do is just lay it out and have them done with my batches of PCBs. There is a choice of board color and conformal coat, the traces can be bright tin or gold plated. Those "traces" could also be used for patterns, words, logos, etc. and ground fill patterns can be used for shielding and decoration. So even if I do something without a cover, I'm not afraid of that "cheap circuit board look." Hell, it's the 21st Century, maybe circuit board is a good look for some people. I'm thinking all black with gold highlights.

Not inexpensive, but I'm thinking custom cut traditional fiberboard isn't either, and this is technology I'm already handy with. So even if the things cost $10-12 per pup in prototype quantities, it's still just a couple bucks instead of launching a new learning curve (yeah, as if pickup design wasn't its own dark art. At least some of the components will be familiar). The thing I was concerned about was if guys here had tried it and found it to irremediably sound bad or otherwise suck in some way that can't be anticipated without trying it.

Thanks for the input. Sounds like there isn't a big reason to not try it.

[salvarsan]

Belatedly, I realized that, since the flats are PCB's, you can also get them silk-screened any way you like.

The idea of shielding using a ground fill speckle (unconnected dots) instead of a flood or a crosshatch seems plausible since it would have less of a path for eddy currents.

[Joe Gwinn]

The idea of shielding using a ground fill speckle (unconnected dots) instead of a flood or a crosshatch seems plausible since it would have less of a path for eddy currents.

The dots need to be grounded to act as a shield. But a sheet of copper foil (on the FR4) with cuts more or less perpendicular to the wires in the coil will work. The cuts block the eddy currents while allowing electrostatic fields to be shorted to ground.

[David King]

A simple, repeating, tree-like fractal would be nice. Don't forget though-plating of the magnet holes will assure grounded magnets. No eyelet staking needed anymore. I can't imagine how you could spend more than a buck or two per set unless you were paying for gold plating and processing by blue moonlight. Aero and Bartolini have been doing it this way for decades.

[FunkyKikuchiyo]

I'm able to do FR4 cheaper than forbon... but I haven't sourced forbon bulk like most people here.

Why would you need to worry about eddy currents if it is only on the flatwork? Doesn't it need to go around the coil to have an impact on the sound? Wouldn't it being just on the bottom negate the effects?

[peskywinnets]

The dots need to be grounded to act as a shield. But a sheet of copper foil (on the FR4) with cuts more or less perpendicular to the wires in the coil will work. The cuts block the eddy currents while allowing electrostatic fields to be shorted to ground.

Ok...you're talking about copper clad board here? Why would you want to put "a sheet of copper foil" onto the FR4? ..ie it's already coated with copper in the first place ! (as in double sided FR4)

[salvarsan]

Ok...you're talking about copper clad board here? Why would you want to put "a sheet of copper foil" onto the FR4? ..ie it's already coated with copper in the first place ! (as in double sided FR4)

Not everyone who reads this knows that FR4 is that green stuff used for printed circuit boards (aka PCBs).

Strictly speaking, FR4 is the dielectric irrespective of the metal cladding, but that's (almost) insufferably pedantic.

David King's point about plate-through holes and not having to stake brass eyelets is excellent.

[peskywinnets]

Aaahh....ok, such is my hobby-esque dabbling in pcbs, that I now see FR4 written just to mean PCB board, but you are quite right the FR4 technically only refers to the laminate that PCB makers then put the thin sheet of copper onto.

But I come back to my point....if wanting shielding....don't be about adding 'copper foil' to FR4...it's already been done for you if you buy copper clad PCB board! (& if going this route don't use FR4, it's nasty stuff to inhale when cutting - use the economy paper fibre SRBP based board ....it's *very* strong & cuts easier, it's cheaper too http://cgi.ebay.co.uk/160x100mm-S-S-...item5d2d93c390 )

[Ronsonic]

David King's point about plate-through holes and not having to stake brass eyelets is excellent.

That is very much what I'm looking at this for.

For anyone not familiar, FR4 is the base, substrate material for printed circuit boards, it's a glass fiber reinforced resin. Very strong, dimenionally stable, excellent dielectric AND in the 21st Century it is very simple for me to draw up what I want using circuit board design software and send the code out to have the boards / flatwork produced. I can have almost any shape cut with near any holes, traces and solder pads that make sense. It is pretty well standard that the boards can have traces / patterns / shielding on both sides and that those would be covered with a solder mask/conformal coating where I specify. Unless there's someone I don't know laser cutting flatwork to spec real cheap, this is the easiest way I know to get the things done.

A simple, repeating, tree-like fractal would be nice. Don't forget though-plating of the magnet holes will assure grounded magnets. No eyelet staking needed anymore. I can't imagine how you could spend more than a buck or two per set unless you were paying for gold plating and processing by blue moonlight. Aero and Bartolini have been doing it this way for decades.

Yep, was planning to mount the magnets that way. Not only no eyelets, but I can put the terminals for the coil where I want and run trace to connection points. I can even stash a little extra length at the coil start inside the coil.

In prototype quantity I'm paying less than $2 a square inch for DS, through plates, coated, screened boards. In quantity it gets cheaper fast.

[Berman]

And if you look a bit you can find the pertinax in a dark brown color, very close to black.

Cheers Hermann

[Joe Gwinn]

Why would you need to worry about eddy currents if it is only on the flatwork? Doesn't it need to go around the coil to have an impact on the sound? Wouldn't it being just on the bottom negate the effects?

Any metallic object near the coil will have eddy-current effects, which effects may or may not be desired.

[FunkyKikuchiyo]

Any metallic object near the coil will have eddy-current effects, which effects may or may not be desired.

For whatever reason I thought I heard the contrary, but the more I think about it, the less it makes sense. Perhaps it was a skin-depth thing that got me confused. In any case, thanks.