another hack

A pantographic cutter might be cool. I could trace a sketch.

As noted, we have a Tormach nc mill, a cute little Sherline nc mill, and we used to have an official nc PCB routing machine, but we got rid of it.

I did another iteration on my pulse driver circuit, which is beginning to look like it might work, in about 10 minutes of Sharpie marking and Dremeling.

I'm not adverse to nc machining when it makes sense. I have written three compilers for nc machines, mostly big to gigantic stuff. The Whitney punch press would shake the building every hit; that was fun.

Right. For $100, I get two square feet of gold plating. That must be

On 12/11/2015 11:36 AM, John Larkin wrote: [...]

That happens with well humidified FR4 when you put it on a hot air reworker. Never occurred to me to check the spin-down time.

We're engineers. We timed it.

Ok, let's see if the diode can handle some RF power. I would guesstimate the contact surface area of the common 7/16 DIN connector at about 800 sq-mm: That's probably off by an order of magnitude in either direction. However, with variable contact surface area, alignment problems, air gap clearances, and my lousy math, the errors might cancel each other and magically produce a usable guess(tm).

The junction is 0.01 sq-microns or 1*10^-8 sq-mm. Assuming uniform current density, uniform contact surface, and only one diode, the ratio of diode area to contact area is: 1*10^-8 / 800 = 1.3*10^-13

A typical cellular xmitter belches about 40 watts per antenna. Into

The portion of that which goes through the diode is: 0.9A * 1.3*10^-13 = 1.2*10^-13 A = 0.12 picoamps

Power dissipated in the junction is about: P = I^2 * R = 1.2*10^-13 * 100 = 1.4*10^-24 watts P = 1.4 yoctowatts which is a very small amount of power that I don't believe is sufficient to blow up the diode.

But to be sure, will the self forming nickel-oxide diodes survive 1.4 yoctowatts and make my life miserable by mixing RF signals and creating intermod, or will the power be sufficient to vaporize the diode(s) and save me from a weekend of excessive worry?

Mine survived bias up to about 0.2V pretty consistently, so that's 400 uW DC. However, the resistance goes up exponentially with thickness past about 20 angstroms. Mine were right around 25 angstroms, based on curve fitting the junction parameters (which actually fit amazingly well when you do it right). So just a few atomic layers difference results in the thin places hogging all the current. You might well be off by 5 or 6 orders of magnitude in your calculated junction area.

Cheers

Phil Hobbs

On Fri, 11 Dec 2015 08:36:25 -0800, John Larkin Gave us:

Skin effect is insignificant. Do I need to send you a link?

Yes, post it here.

Shiny!

Did you use a magnifier to do that? I'm extremely nearsighted so I just take off my specs and put on shop glasses. And a mask for glass, though I haven't done a board in a while.

Didn't go out the door like that, then? ;>)

Mark L. Fergerson

Nothing yet? Try this:

Interesting!

Is OSP finish ever considered as an alternative to ENIG for such RF stuff?

--sp

RF stuff on such materials "never" get solder mask, OSP, or any other coating (won't stick to teflon, interferes with ribbon bonding). Most of the time the boards are stored in sealed bags or in dry boxes - standard practice for the general class of applications. Worst case, if boards have been exposed to open air for a long time or have been inappropriately handled, they get a wash in a lab grade detergent and some time in a plasma cleaner (Ar, sometimes with a small amount of forming gas (NH4 aka ammonia)) to restore solderability, minimize epoxy bleed, improve ribbon bond adhesion...

I've run TDR, 30 ps rise time, on microstrip traces that had sections of solder mask removed. I couldn't see any difference.

Maybe I could find a good microstrip board and try removing solder mask some places, and removing the ENIG down to bare copper in other places. I'm guessing that won't show up either.

I can see what is probably the glass fiber weave in the FR4.

I have a Mantis magnifier, which I value close to my wife and kids.

I'm nearsighted too, but not enough to work on the smaller stuff. The Mantis is fabulous.

I shoot pretty good pics right through the hood, for illustrating ECOs and such. There is an expensive version that has a built-in camera.

Up to about 500 MHz, we would solder mask everything. The danger from external contamination was far greater than anything the solder mask could bring to the table. This is for marine radios, where salt water contamination is common and where a solder mask really helps. Also, no conformal coatings except where demanded by the customer or where some errant designer decided to use high impedances in a marine environment. Bad idea.

The choice of cleaner varied by the type of contamination, but we just washed the PCBs with a mild detergent in a washing machine and rinsed them with deionized water. At the time, we used wave soldering, with water soluable flux. Vapor or IR reflow soldering will probably require different techniques.

I've done PIN switches, filters, couplers, preamps and such on Duroid

FR4 and solder mask are fine for lumped circuitry up 500 MHz at a few watts but when the power gets above 2-300 WCW FR4 is simply too lossy - it melts and become even lossier, burns and you end up with an ash tray. At higher power levels a teflon/ceramic or glass fiber material with tightly controlled dielectric properties is required. The 5880 class of materials is very low loss and is preferred until you get to something like 1 kW cw at 500 MHz - it gets too hot and solder melts. The alternative is the 6002 material which has much higher thermal conductivity - 1200 WCW at 1 GHz for example.

Losses due to solder mask are probably insignificant (it just isn't where the E fields are). Losses due to Ni are - a PIN diode switch at

- a useful improvement in S/N and power handling when loss budgets are tight (4% here, 4% there - it adds up).

We've used a Parylene coating on finished circuitry where water accidents could occur. Not something you do in you own shop but the coating is very thin, durable and gets into small spaces. Not sure how well it works at really high impedance (giga ohm) but for normal electronics that might get splashed, it covers everything important. I think if I were doing stuff for chemical labs, it would be my first choice (I hated scrubbing green stuff off expensive electronics back when I did that stuff). I've used the krylon trick but it doesn't get under things. I also tried dipping or spraying with LPS-1 - good for routine humidity and mild lab fumes but not for a direct splash.

Most metallic magnetic things quit working at high frequencies. Ferrites are better than, say, Metglas in the 100s of MHz. I wonder when the nickel quits being megnatic.

Right. We don't gold plate the bottoms of the traces!

Losses due to Ni are - a PIN diode switch at

I work in time domain, mostly low power stuff, usually around 1 ns edges but sometimes down to 40 psec. Some fast serial stuff, like 1 GHz SERDES and PCIexpress. Ordinary boards, FR4/ENIG, seem to work fine. But I wouldn't notice the equivalent of 0.5 dB loss.

On Fri, 11 Dec 2015 14:56:05 -0800, John Larkin Gave us:

"megnatic"?

Try firretes and neckil and matgles on your matellic devices.