Electronic – What type of solder should be used to assemble Adafruit cobbler

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.

Don't waste your time trying to solder them, it won't work. Most distributors will make up cables for you, it shouldn't be expensive. Or. choose a connector system that is easier and cheaper to assemble.