de-flux pcb or not?

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What are peoples ideas on cleaning flux from boards after hand soldering? In
particular 511 crystal & 362 rosin lead/tin solders from multicore. Places
I've worked have had different attitudes. Any arguments for/against? I've
heard a lot of "it looks like sh@t" but in most cases it doesn't matter does
it?



Re: de-flux pcb or not?

"K Ludger"
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** Generally it does not matter  -  but sometimes it does.

Ordinary ( non acid) solder flux is non corrosive and non conductive WHILE
the surface remains hard and shiny.

However, if the surface becomes damaged, scratched or broken  -  then it
will absorb moisture from the air and become very conductive, very corrosive
and so very harmful to circuit operation.

So the " Clint Eastwood Criterion " has to be applied.

The one about punks and luck ......

BTW

I have seen valuable equipment written off simply because the makers had NOT
de-fluxed the PCB before installing all the op-amp ICs in sockets.



......  Phil




Re: de-flux pcb or not?

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Agreed




Because? - corrosive flux? humid usage environment?

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Re: de-flux pcb or not?

"K Ludger"
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** Because the DIL sockets had flux coated over them prior to IC
nsertion  - that is what wave or flow soldering does.

Then the IC pins punctured the normally impervious coating on the flux.

Then, the flux absorbed lotsa water out of the air.

Then the whole device ( a dual 1/3 oct. graphic equaliser made by Phonic of
Taiwan ) became ridded with DC leakage current flowing from DC supply pins
to adjacent input pins.

Result  =   massive " popcorn noise" in the output .

Useless and uneconomic to repair.

Fuckwits.



.....   Phil







Re: de-flux pcb or not?

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 We had a batch of boards manufactured with "fluxless solder".  Or, it doesn't
need flux cleaning after the wave solder stage.

 At least 90% of our boards would not function till we cleaned them, properly.

 Ended up sending what's left of the batch back so they can fix their stuffup.

 That was the first time we've seen this type of fluxless solder being used,
(and last time we wanted to know about it) and now demand the boards are
cleaned regardless of their process.
--
Linux Registered User # 302622
<http://counter.li.org

Re: de-flux pcb or not?

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Was there anything sensitive about the board John - high gain integrators,
very low currents???



Re: de-flux pcb or not?

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 60dB gain for one thing, some filtering, analogue switching and the rest was
digital (past the A/D stages of course).

 Oddly enough, we only really noticed the problem with the digital boards, we
never got to testing the analogue section because of that.

 And this isn't seriously high speed either, 8051 based CPUs that were clocked
lower than what the CPUs were rated to save power (can't remember exactly how
fast though, this was a while back).  I do remember they didn't go past about
6-7Mhz.

 The CPU PLCC through-hole sockets were especially troublesome, where cleaning
of the socket pins was required, and in some cases, removal of the socket,
clean underneath and re-solder back in again.

 Not happy Jan.
--
Linux Registered User # 302622
<http://counter.li.org

Re: de-flux pcb or not?

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I've had similar experience with PLCC sockets, they can be a bitch to track
down initially.



Re: de-flux pcb or not?

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Clean the boards, especially if you have high impedance areas. Ensure that your
cleaning
materials are clean as well. Do no allow fluxes to remain on component bodies
like smd
caps (0603, 0805, 1206), etc. If you have flux and dc across a cap, and a humid
atmosphere,
over time you will get deposition across the surface of the cap, leading to
lower impedance
across the component and possibly affecting circuit operation.


Re: de-flux pcb or not?

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What do you/others use for cleaning?

I generally clean with IPA, sometimes metho and sometimes spot rinsing with
distilled water. The IPA wash I usually repeat a few times to get rid of the
flux salts from between SOIC pads etc. I tried some "citrus" based wash
years ago but found it quicke and easier to just use IPA.



Re: de-flux pcb or not?

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 IPA alone.  As you do, sometimes twice over, first to loosen/dissolve, second
 to flush that mixture off.
 And it evaporates faster too, where I can shake it off, let evaporate for a
few minutes and I'm ready for testing again.

 Works well.

 I never liked metho, it didn't really do a good enough job because it was
watered down, and the additives leaves streaks when you're using it for
general cleaning.  And I didn't want to keep too many different chemicals
where I didn't have to.
--
Linux Registered User # 302622
<http://counter.li.org

Re: de-flux pcb or not?
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Funny thing about metho: Most brands used, Recochem comes to mind are
96% ethanol with water and methylating chemicals as the balance 4% work
fine for dissolving flux, but when you use 96% pure non methylated lab
ehtanol it just doesn't seem to work at all. It seems the additives make
all the difference in my experience.

    I do have to use a detergent wash and an IR bake afterwards to get a
  stain free flux removal.

Re: de-flux pcb or not?

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Know of anywhere cheaper for the IPA than Altronics at $20 for 2L?
cheers



Re: de-flux pcb or not?
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   I'd suggest ringing around various cleaning supplies and chemical
shops: I think it's used in aluminium cleaner formulations. I used
to buy it from a small fibreglass supplies shop for $12 for 2L if
you brought your own container.


Re: de-flux pcb or not?

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I use metho and a 1" soft bristled paint brush. That's because I'm too
tight to waste IPA if I don't need to.

John

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