Flux fux, and 10 Meg ohm

meta question: Who does testing, and who decides what passes?

GH

That's kind of a business question, although if the testing is done out of house it should be backed up with spot-testing in-house.

Set some testing goals that take into account the fact that a little bit of leakage now probably means a lot later on.

--

Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com 

I'm looking for work -- see my website!

Too bad you can't make slots under the resistors, so that the underside is clean-able.

Does anyone make SM resistors that hold the body off of the board a bit?

--

Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com 

I'm looking for work -- see my website!

That's what I generally do. Makes the guard traces a bit of a puzzle to route, but it's a win anyway.

4-oz Cu on the top layer is one approach, but it isn't as good as slots. If hand work is okay, one can put a couple of pieces of fine magnet wire underneath the resistor while soldering, then pull them out and clean.

For big resistors in critical spots (e.g. TIA feedback) I often use through-hole parts specifically for their cleanability.

Cheers

Phil Hobbs

What sort of solder/flux did you use?

--

John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com

Sigh, I have no idea. The board was farmed out to a local house. I'm guessing some water soluble stuff... Should I clean with H20 and detergent. Dawn? Alconox?

George H.

We had one batch of boards, assembled by a contractor, that had conductive crud under parts; they had a dirty water-wash process. We bought a Water Pic and a few gallons of distilled water, and The Brat (working for us during the summer) blasted around and under all the parts, and then we baked them dry. She got soaked and was about as happy as a wet cat. She still whines about it.

We use rosin flux and solvent wash on anything that is high impedance/leakage sensitive. Water wash is fine for digital stuff.

New solvent cleaner gadget:

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics

I use ordinary rosin flux solder for PCBs, and then I scrub with alcohol, followed by detergent like Simple Green, then scrub and flush with hot water, and finally dry with a heat gun. I use mostly 0805 resistors. It looks like they have a thin layer of metallization on the ends of the ceramic substrate, and then there should be some thickness to the solder from the surface of the PCB pads to the terminals. So there should be some space under the body of the resistor where cleaning solvents and water should reach, and allow water to be removed by hot air.

The use of leaded resistors for high resistance seems a good idea. Another method for SMT resistors might be to place a thin strip of Mylar or Teflon tape between the pads to lift the component a few mils above the board, and then remove it after soldering to open up some space underneath.

Paul

MELFs would probably be better - not off the board, but low contact area and easier to clean.

Cheers

--
Syd

Not snickering : I have experienced your pain.

My mega-ohm detox recipe:

Ultrasonic aqueous wash and rinse, then blow with compressed air.

Repeat with organic solvent and rinse and after blown dry then oven bake, after test conformal coat, then vow to design out high value Rs.

I got the impression the compressed air drying was the ace step.

JL's water pik sounds like a good idea too.

If routing isn't part of your pcb process then a mere hole under the smd at least helps clear residues and lengthens creepage path.

piglet

What kind of flux are you using?

Regards,

Boris Mohar

Got Knock? - see: Viatrack Printed Circuit Designs (among other things)

void _-void-_ in the obvious place

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Grin. If you can tell me how to measure tiny little currents w/o high value R's... I'll sing your praises from the highest mountains.

(OK I know you can do tricks charging up a cap.. I guess I mean DC currents.)

There are only two "troublesome" R's per board. I'm going to rip 'em out clean it up, and install new R's with rosin flux.

Turns out these boards were made here.. (water based flux.) so it's a self inflicted wound. I don't know if that makes me feel better or worse. George H.

A charge dispensing loop works at DC too. Still has the leakage problem, of course, and is also vulnerable to microphonics.

Water-based flux is a nightmare. It's almost as bad as cleaning with drugstore IPA.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net

Hmm so do you pulse the input/balance current?.. also from a little cap.

Well the good news is the "mischief is managed". (I couldn't stand the thought of worrying about it all weekend.) I'm off to Ted's for a hot dog, onion rings, and chocolate shake.

Then home for some plumbing. Which I was supposed to do this morning.

George H.

Just be careful about the air. Shop air is often full of rusty water.

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics

Hot dogs are one of the Great American Foods and don't get enough respect.

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics

I usually use a CMOS SPDT mux with low charge injection and about a 470 pF cap. You connect the cap and a small series resistor between the pole and ground. One throw goes to the input and the other to ground. Pulse the switch when the input voltage gets above some value TBD. You need another cap on the input so that charge doesn't get lost during reset. You can also do it with an integrator that gets reset when its output goes above some voltage, still with an SPDT so that charge doesn't get lost.

Good!

Well, you gentleman-farmer types get enough exercise to walk off a lunch like that. I'd be comatose.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net

That could be bad.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net

You can buy aerosol cans of duster stuff. As far as I can tell, you can convert duster to freeze spray, and vice versa, by inverting the can.

I'd prefer to wash boards with very clean water and then bake them dry. Better yet, go rosin+solvent and avoid water.

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics