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Solder flux and resistance

When testing a high impedance board (up to 1 G ohm) I found that the SMD 0805 10 Meg ohm resistors measured only 6 meg ohm. The problem was traced back to the liquid flux used with Rohs solder. (100 Meg and 1 G resistors are through hole and measured correctly.)

I then wanted to measure the resistance of different =91flux blobs=92. I made big solder blobs on 0805 SMD pads (spacing between pads was about

0.030 inches) I used a Keithely 610B electrometer to measure the resistances. Resistance was first measured with the flux left in place and then the board was cleaned with isopropyl alcohol (or hot water and detergent for the one water soluble flux) and measured again. The solders tested were all from Kester.

#44 rosin core (the old standard) #245 low residue rosin core #48 Sn/Ag/Cu (Rohs) rosin core #331 water soluble flux

All the rosin based solders had resistances in the 3 - 7 x10^11 ohms range before cleaning. The 245 was a bit lower than the other two, (about 1/2 the resistance), but this was a =91one of=92 test and I may have just had a bit more or less flux left in place.

The 331 had 3 Meg Ohm of resistance before washing. After cleaning the 331 resistance went up to 5x10^11 ohms the same as the others. I left the rosin flux on over the weekend and found no real change. (The average resistance changed with the day (Humidity?) but they all tracked together.

On Tuesday I cleaned the rosin flux off the boards, all the boards then had about the same resistance. (#44 and #48 showed no change with cleaning, the #245 went up in resistance a bit.)

I also tried a humidity test on the boards by breathing on them. The rosin fluxed boards went down by about a factor of 2 or so. (A bit hard to determine because I found that I could also =91spray=92 charge onto the boards.) The water based flux went down by more the 90%.

Today, (at the suggestion of Grant on SED), I =91burned=92 some 44 flux with a soldering gun. I made lots of black icky stuff. The resistance was 3X10^11 ohms the same as all the other boards. (It=92s a humid day here in Buffalo.) Interestingly the 331 today has a resistance of only 0.4 X 10^11. Apparently there is some film left on the board by the water soluble fluxes and this makes it a humidity sensor. (As I think John L. has said.)

I have one question before finishing. The technician who will be soldering the first production boards complained that the Rohs solder and rosin flux (#48) is ugly looking and one gets big globs of soldered with out his beloved liquid flux. Are there any rosin based products to help? I do have an old jar of rosin at home. Is there anything that is easier to dispense onto a SMD PCB? Or some other =91tricks of the trade=92 when working with Rohs solder/ rosin flux and surface mount parts?

George H.

Reply to
George Herold
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We've had intermittent problems with boards that we send out to contract assemblers who use water process. If they do everything right (correct solder temp profile, really agressive spray wash, super clean water) leakages will be in the picoamps. Any residual ionic contamination (loves to hide under surface mount parts) makes for humidity-sensitive time bombs. And a little ionic conduction can allow electroplating type effects.

We had one batch of boards that were terrible. Turns out the assembler's water wasn't clean enough. We bought a water pic toothbrush thing and hand-blasted around and under all the critical parts with distilled water, then baked, then conformal coated. Lots of work. New assembler.

"No clean" flux is awful too. Rosin flux + 63/37 solder is wonderful, hand soldered or paste.

ROHS is the pits, so we don't do that on our aerospace stuff. I know people who buy expensive ICs and send them out to shops that strip the plating (or remove all the BGA balls!) and replate/reball with lead-based solder.

To be conservative, you might consider sticking with leaded resistors for the critical ones.

John

Reply to
John Larkin

Thanks for that, I'm hand soldering prototypes on matrix card with

60/40 rosin core (Ersin multicore) and any rework usually burns the rosin, knowing it's not making unexpected resistors is good :)

Grant.

Reply to
Grant

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" "No clean" flux is awful too. Rosin flux + 63/37 solder is wonderful, hand soldered or paste."

I think the Kester 245 is what is called a no clean flux. It seemed only a bit worse than old #44. Resistance of 4 X10^11 ohms vs 7X10^11 ohms... hardly matters for my values. (And on humid days both number go down)

The flux (in Rohs #48) seemed to be the same as was in #44. The solder may be worse.

Yeah we talked about that. Make the 10 Meg a through hole and all my problems may go away.... Still can't I get the flux stuck under some other surface mount part in the circuit, a cap or opamp? Seems safer to just use rosin for all the high impedance circuits.

George H.

Reply to
George Herold

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This was a 'one of' experiment, your results at home may vary.

It sounds like you need a temperature regulated soldering iron. I've got a Weller with temperature 'set point' tips. The 700F with a sharp tip works fine and never any burning.

George H.

Reply to
George Herold

...

Oh sure, it's the indication of trouble I was after. I don't have a high range resistance meter here. You found more trouble with water wash flux than with cooked rosin :)

Yes, I know the Weller, used them for years, at work, sometimes at home, and I chose 700'F tips too. I don't like that slight tip jump when the Weller's tip 'clicks' in sometimes at a critical part of work. As well, they attract the iron based leads from some components, very annoying.

Got a new Hakko FX-888 65W temperature controlled iron recently (couple weeks ago), still getting used to it. Quite pleased with new iron so far.

With the old soldering iron I had to turn temp up a bit to get heat flow, thus it was often too hot :( Not enough metal in the tips, so temp regulation was poor.

Here's a photo of latest project, some rosin is slightly cooked, none is black, nor even very dark:

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second image.

Using the Hakko at ~350'C, ~650'F with 60/40 for this medium-fine work.

Trouble with these prototypes is changing components and rework can cook the old rosin, but if it gets too dark I clean it off.

Not happening with new iron, but I had the old one for few years, and that sticks in my memory.

Grant.

Reply to
Grant

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I'll was thinking I should post a picture of black icky. (Monday?)

George H.

Reply to
George Herold

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