Does soldering flux truly affect a circuit board operation?

[dann11]

I have read in one application note about troubleshooting of crystal oscillation troubleshooting that one possible cause of troubles in a circuit, like for example having a crystal is the PCB contaminants like flux, does it?


this is the article link:

**broken link removed**

"PCB contaminants, like flux, humidity, and finger prints, can reduce the impedance between nodes, which in turn can create a number of issues".

this the part of the application note that I am saying. It is at #12part.

 

[KlausST]

Hi,

does it?

Believe it.

I´d say it is not very likely, but possible.
Some flux residuals are hygroscopic, some have high or low pH. Some are slightely conductive (megaohms), some act like a dielectric.
Therefore they change ohmic and capacitive coupling on a PCB. Therefore with sensitive circuits there still is the need for cleaning the PCB.

Klaus

 

[dann11]

Hi,



Believe it.

I´d say it is not very likely, but possible.
Some flux residuals are hygroscopic, some have high or low pH. Some are slightely conductive (megaohms), some act like a dielectric.
Therefore they change ohmic and capacitive coupling on a PCB. Therefore with sensitive circuits there still is the need for cleaning the PCB.

Klaus

I see, but would you consider that it is a sensitive circuit and needs for cleaning the PCB?

 

[KlausST]

Hi,

I already said:

I´d say it is not very likely, but possible.

And how can anybody give a more detailed answer without seeing your circuit, your PCB layout and your part datasheets?

Klaus

 

[BradtheRad]

Flux residue can also catch and retain things such as:

* solder blobs
* metal filings
* carbon dust
* conductive deposits

These might provide paths for current and/or static charge.

 

[pjmelect]

I would agree with the previous posts "I´d say it is not very likely, but possible." I have never had any problems caused by flux residue even on sensitive circuits, I think that flux removal is more cosmetic than anything else. Saying that I don't know what flux you are using maybe some different fluxes to the ones I use can cause problems.

 

[KlausST]

Hi,

I think that flux removal is more cosmetic than anything else.

Definitely not.
Modern "no clean" flux residuals are almost invisible. So there is no need for "cosmetics".

In opposite to this: Often when "no clean" flux PCBs are cleand the still remaining residuals are awful. Now the last couple of % are visible. (With heat you can make it invisible again. )

***
I agree that in many cases the flux is not problematic. Controlling a LED for example. Who cares about a couple of nanaoamperes fluctuating LED current.

But with an Xray dosemeter, we trigger at about 3..10 picoamperes. It´s less than 1/100 of a nanoampere...You see what I mean?

***
Back to XTALs.
Sometimes there is a high ohmic resistor - some megaohms - across the XTAL to ensure startup.
Depending on the oscillator circuit there is a little possibilty that leakage currents caused by flux residuals prevent the oscillator from starting.
As said before: not very likely, but possible.

Klaus

 

[dick_freebird]

When I was a young technician we built large complex
logic boards out of LSTTL (Woot! Woot!) and TI9900
16-bit uPs, all soldered up by hand. At one point we
used a different solder vendor and the flux residue
grew conductive mold, lost us a whole customer's
worth of systems (and the only paying customer at
the time). Shortly after that, the company folded that
division and I went on unemployment.

So, yeah. Probabilities don't mean Jack when it's you
that got the lucky ticket. Luck perhaps being only
somebody else's occasional carelessness, randomly.