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no-clean flux vs. impedance

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no-clean flux vs. impedance | 18 February, 2008

A PCBA contains a couple of parts that are not sealed and can't be water washed. We hand solder these with no-clean after everything else is water washed. The designer says the no-clean residue must be completely removed so that it will not affect the circuit impedance. Does this sound accurate, or is he being too carefull? Is there any way to quantitatively argue a case for not cleaning?

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no-clean flux vs. impedance | 18 February, 2008

In theory, no clean flux residues are electrically benign...period.

Here's a good test. Subject the board to some moisture/humidity, and have your circuit designer ohm-out any areas that may be affected by conductive material.

Option B: Have a lab, like Trace Labs, verify ionic cleanliness.

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no-clean flux vs. impedance | 18 February, 2008

If the hand soldering is done right and you use little to no extra flux (other than whats in the solder wire) you should be ok. You should still verify with some testing however... You need to make sure any flux used is activated, thus rendered benign.

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no-clean flux vs. impedance | 18 February, 2008

I should have mentioned... the only flux we use for the hand soldering is wire-core, no bottle flux. I think it should be a non-issue to leave the activated wire core flux residue. I'm just looking for some way to prove it to the designer.

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no-clean flux vs. impedance | 19 February, 2008

Get the ionic specs from your flux manufacturer, and show it to the designer. No-clean flux/solder is supposed to be low to minimal in ionic contamination, which allow the whole no-clean thing.

If he needs further proof, you can have the board ionically tested. Test it before and after soldering, to show the difference in the ionic contamination. It should be low enough to not effect his impedences.

Although, I will say that I've heard of cases of high frequency RF boards having super-sensitivity to ionic contamination.

Cheers ..rob

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no-clean flux vs. impedance | 19 February, 2008

The low solids flux that is commonly called no-clean can affect some circuits.

We are building a PCB that measures pico amps. The customer required that it be cleaned. We would rather used the low solids and eliminate a step.

The customer had us build 1 PCB with low solids flux paste. The board gets reflowed and the profile was to the paste manufacturers spec so we were sure that the flux was completely activated.

The customer then showed us the measurements this board was capable of and what a cleaned board was capable of. The clean board was more than twice as sensitive.

So a test would have to be conducted to prove whether it will actually cause problems.


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no-clean flux vs. impedance | 20 February, 2008

KRIKIES, a designer insisting on cleaning no-clean residues!

the BLAME-THE-PROCESS bandwagon travels everywhere, me lad!

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