For thru-hole work (which it sounds like you are doing) I would go with an active
flux that is water soluble. You need to wash the residue off since it is corrosive
and can cause shorts. I have had a number of boards mysteriously come back to life after
a good cleaning.
For thru-hole parts that may get damaged by cleaning (LED lenses) or SMD parts
with heatsink pads I would use the no clean flux.
I have been using the flux pens from Kester. 2331-ZX for water soluble flux and
the 951 for no-clean. To clean the water soluble flux I use an "acid" brush with
the bristles clipped to around 0.5". I run the board under the hottest tap water
I get (probably around 140-160degF) and scrub with the brush. If I do a small
repair job I will clean with the isopropyl alcohol I have on my bench.
Here some of the whys --
Why you need flux -- to get a quality joint at a lower soldering temperature.
Using a liquid flux lets you spread the flux better over the two surfaces
to be soldered.
Why choose an aggressive flux -- an aggresive flux is more tolerant of soldering
temperatures. As Leon mentioned this would be a good candidate for a hobbyist.
I like the 2331-ZX for this. Since these leave ionic compounds you need to clean.
Why choose no-clean -- eliminates a process step and the issues associated with
the extra process step. For example for soldering XLamp LEDs I use a no-clean
flux rather than risk scratching the lens.
For more details take a look at http://www.finishing.com/Library/flux.html
and the Kester website.
Yes, if it's an acid flux, you can't use it with electronics like they say. Definitely sounds like plumber's flux. I would return it to Frys.
Best Answer
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