Question about mixing flux types - Page 1
Forum section: electronics / general

author: Miti
author_rank: Super Contributor
timestamp: February 10, 2021, 04:15:44 pm
content: I've ordered recently some PCB assemblies from JLCPCB and my plan was to ask them to populate the Altera Cyclone IV in EQFP144 package, which they had plenty in stock when I designed the board. By the time I was ready to order, the stock got depleted to about 6 pcs and, even though I only needed 2, the system would not show as confirmed.Unfortunately, I did not add a hole for the power pad so I can solder from the bottom with the soldering iron, so I need to reflow the FPGA and the rest of the components that LCSC didn't have. I like using water soluble paste in all my assemblies that are washable. The boards come very clean and, if I dry them fast, washing them in the sink with tap water does not show any sign of corrosion after many years, so DI water is not needed. I'm talking hobby projects, not commercial, but even at work, the boards are washed with tap water and rinsed with DI water.My questions are:1. Can I use water-soluble paste under the power pad TQFP (EQFP?)? Would tap water, at tap pressure, go in a 0.05 - 0.1 mm space, under a flat component (per datasheet) to wash the flux?2. Can I mix no-clean (power pad) and water-soluble (everywhere else), then wash it? The JLC process seems to be NC, so there's already (very little) NC flux on the board.Cheers,Miti

author: Matt-Brown
author_rank: Regular Contributor
timestamp: February 10, 2021, 04:34:34 pm
content: There are some no cleans that cannot be washed. You need to find out what they used.

author: SMTech
author_rank: Frequent Contributor
timestamp: February 10, 2021, 04:38:26 pm
content: It's no clean so generally not necessary to remove, so why does it matter unless you have a very sensitive op-amp or something? NC flux residues are unlikely to budge at all with just water, you'd want a saponifier or other cleaning agent in there too. The fitted NC fluxed devices shouldn't give 2 hoots that you used WS flux/paste on one of the other devices, they don't have feelings.However following on from your other reply, attempt to clean off no-clean flux badly and you can make things worse so you might be better off using no clean for your other parts too.

author: Miti
author_rank: Super Contributor
timestamp: February 10, 2021, 04:57:43 pm
content: I would totally use NC everywhere just to be consistent... but... because I don't have a stencil, and even if I had I couldn't use it, I cannot deposit the right amount of paste on the TQFP pin pads, so I would have to hand solder or touch up the pins. That would need some flux from the bottle, and that must be cleaned. If that makes its way under the TQFP, it would be harder to remove.

author: SMTech
author_rank: Frequent Contributor
timestamp: February 10, 2021, 05:04:36 pm
content: You can get no clean flux in a pen, although I would probably be tempted to try and clean it off locally with an appropriate solvent and a cotton bud or something.

author: Miti
author_rank: Super Contributor
timestamp: February 10, 2021, 05:14:48 pm
content: Yes, regardless if it is pen or bottle, it would need cleaning afterwards, and that's a PITA.

author: David Hess
author_rank: Super Contributor
timestamp: February 11, 2021, 06:56:34 pm
content: Quote from: Miti on February 10, 2021, 04:15:44 pmI sure would not unless I was very confident in my washing. Even then I would avoid it if possible; water soluble fluxes are nasty if not completely removed.QuoteWhy not use no-clean flux on the entire board, unless the boards or components are old of course so require a stronger flux?

author: Miti
author_rank: Super Contributor
timestamp: February 11, 2021, 08:27:10 pm
content: I can say with confidence that NC flux is almost as nasty as water soluble if not activated. To activate it, it must go through high temperature, either through the oven, or through the tip of the soldering iron. I explained my dilemma in reply #3.

author: tooki
author_rank: Super Contributor
timestamp: February 16, 2021, 12:23:33 pm
content: Quote from: Miti on February 11, 2021, 08:27:10 pmAll of the following refers to separate fluxes, not the flux core of wire solder or the flux in solder paste.I’ve been doing corrosion testing with all the liquid and paste/gel/tacky fluxes I have at work and home: I strip about 12-15mm of insulation off stranded, bare copper wire with clear insulation. I apply the flux to both ends, and tin one end only.Most of the no-clean fluxes are not corrosive if they can dry quickly. But if they remain wet for longer (as when liquid flux wicks under the insulation), it actually can cause corrosion. (A test on copper clad board had a similar result with one RMA flux: when applied as a thin coat with a pen or brush, absolutely no corrosion, as it dried mostly within a few minutes, and fully within a few hours . But a small droplet, spread around with a syringe needle, took long enough to dry to cause green corrosion. When I came back the next day, it was fully dry, not tacky any more.)But the tinned side of the wires gives additional insight: the no-clean fluxes must reach a certain temperature to activate, and then a higher temperature to neutralize. This is very visible because the flux close to the tinned area doesn’t corrode, but the part father away does, because it didn’t get hot enough to neutralize it. With paste fluxes, which obviously cannot wick at all at room temperature, there’s a noticeable gradient: little corrosion where the flux was warmed just enough to melt and wick, but not activate, then an area where it was heated enough to activate, but not neutralize, giving the highest corrosion, and then the area where it got hot enough to neutralize.The upshot is that while reflowing in an oven certainly will guarantee that all of the flux has been heated enough to neutralize, hand soldering does not, since its heat is applied very selectively.

author: Miti
author_rank: Super Contributor
timestamp: February 23, 2021, 07:45:07 pm
content: Well, it happened exactly as I expected. I used NC to solder the remaining parts and, sure enough, the FPGA pins were bridging all over the place. I tried to clean them up without using additional flux... nope. So I cleaned the board with IPA, then with Vigon EFM, and I poured water soluble flux to clean up the bridges.In the end I washed the board under hot water and cold water brushing with a soft brush. The board looks squeaky clean but I don't know what's left under the TQFP.Time will tell...