SDG #086 What's the best flux for soldering? A comparison with PCBs from JLCPCB

Flux is the cheapest part of a soldering setup and the one that makes the biggest difference to results, yet most of what is sold online is unbranded gel in a syringe with no datasheet behind it. This comparison puts sixteen fluxes through the same set of tasks on identical test boards so the differences come down to the chemistry rather than the technique.

The line-up spans the full price range: budget syringe gels including Mechanic MCN-UV80, Best BST-223-UV80, Relife RL-421-OR and RL-422-IM, Topnik Zel Flux and KSS S800, European branded products such as Weller F-SW21, AG Termopasty TK83 and LP-1, CIF F42202 and Pro’sKit 8S005, plus the reference-grade options MG Chemicals no-clean gel, Kester no-clean and Amtech NC-559-V2-TF. A counterfeit Amtech NC-559-ASM is included as well, which is worth watching because relabelled fake Amtech is extremely common and does not behave like the genuine product.

What matters when judging flux is not just whether solder flows. Tack and viscosity determine whether it will hold a fine-pitch part in place, activity determines how well it wets oxidised pads and old joints, spatter and smoke affect how pleasant it is to use under a microscope, and residue behaviour decides whether you can leave it on the board or have to clean it off. Several of the cheap gels perform surprisingly well for drag soldering and rework, while others leave sticky, hard to remove residue that is a poor choice for anything you intend to keep. The practical takeaway for buyers is that you do not need to spend a lot, but you do need to know which specific products are worth the money.