My Cheapo Chineese 3020t mini CNC Build (more like improvements) leanring all the way

[A_Camera]

Here is a video, the most relevant section about Flux starts at 7 minutes in, the astute of you will identify the difference in working with fresh unoxidized surfaces vs. oxidized surfaces such as those in the job Ravi is undertaking and the correlation in my recommendation of using flux.

I know very well what flux does and when to use it. In fact, the very same demonstration is done in your video, just like I am saying in post #56.

https://youtu.be/vIT4ra6Mo0s?t=8m1s

He is soldering large surface, so of course use of flux is a good idea. He is soldering to a copper plate with a blow torch from beneath, not a PCB, and what he does is the same as soldering pipes. So of course, use of liquid flux is a good idea, as it is demonstrated at the 7 and 8 minute marks. When soldering PCB manually flux is unnecessary because the solder wire already has enough. More important is HOW to solder and which tools are used. If the tip is too large or too small, or the temperature is too low, or the solder is applied to the tip and brought to the surface and so on...

Quite the opposite to you, if I make a PCB and see that it has a lot of oxide, or old flux or anything greasy on it I use PRF 6-68 to REMOVE flux, oxide and almost any other dirt, and that makes soldering really easy. I also use the same if I buy electronics from eBay and see that the PCB is badly cleaned or not cleaned at all, and is full of sticky flux. I routinely clean all my PCBs after manually soldering because there is always just too much flux rest after manually soldering PCB.