Choosing flux core solder wire for beginners (beyond just 63/37 vs 60/40)

[vestrue]

Following recommendations, I bought 0.5lb of a popular 63/37 .025" RA flux 2.2% a while back. It's been fine for the most part, but I've always wondered what else is out there. I want to share what I learned while it's fresh in my head & see how accurate it is. Hopefully it'll help some people in a similar position who are curious about choosing beyond just 60/40 vs 63/37, who don't know what type of solder they have, or are frustrated by not knowing why they have difficulties with their solder. This is meant to be a jumping-off point, for very basic needs, such as for a beginner hobbyist buying quality solder for the first time.


It helped when I realized that I'd been thinking about the situation wrong. I was focused on the metals in the solder, and how they might be different between brands/products. Everything began to click when I changed the focus to, "how are the fluxes different?"


The main questions to answer would be:
-Leaded or non leaded?
-Eutectic or not?
-What diameter wire?
-What flux should I choose?
-What percentage of flux should I choose



I've found most recommendations to be:
-Leaded for hobbyist use. Work in a ventilated area or with a fume hood. Don't eat/drink/smoke while soldering. Pay attention to what you touch, don't touch your mouth, etc.
-Eutectic (63/37) is easier to work with than 60/40. A novice is less likely to create cold solder joints with it
-Diameter depends on the work. You can always twist a thin diameter to double it up (as many times as needed), but can't cut a thicker diameter in half. Generally 0.015-0.031" is a range that allows for PCB work. Consider your needs- surface mount, through-hole, wire diameter
-Flux is available in a wide variety of strengths, R/RMA/RA/WS/NC, some needs to be cleaned with a specific flux cleaner, others with water, others can be left untouched. Some is electrically conductive, some is inert. Some dries clear, others not. Some can lead to future corrosion in certain uses. I don't know much beyond that, and all I know about halogens/halides is that they can effect activity, wettability, compatibility/corrosion, and other areas. Fortunately I don't think the basic stuff I do is delicate enough to worry about, but it's good to be aware of the precautions for future scenarios.
-Flux is typically available in 1.1%, 2.2.%, or 3.3% (volume of flux, not strength). I am not sure about how to choose between the 3, or how to balance choosing % in consideration of wire diameter. I get the feeling % is kind of job-specific



There are many good brands recommended such as AIM, Alpha Metals, Kester, MG Chemicals, Multicore, and Stannol, to name a few
I saw a lot of recommendations for Kester #44 and #245. It helped to understand that the numbers "44" or "245" refer to the type of flux that is used, not anything else.
Kester makes dozens of varieties of wire with "44" flux. You can choose a "44" that is: 60/40 or 63/37 or part silver (or many other options), a diameter ranging from 0.010-0.125, and a flux content from 1.1-3.3%.
I found their website to be pretty helpful and easy to understand. They point out the differences between fluxes (such as strength, how they need to be cleaned, what applications they may be designed specifically for). If you select 'view all' you can familiarize yourself with some of the differences between fluxes
You can't just walk into a store and say, '1 roll of Kester #44 please', you would need to specify, '1 0.5lb roll of Kester #44 0.031" diameter, 63/37, 66 core (where 66 is how Kester denotes a 3.3% flux content)'



If I were to make some recommendations to my earlier self, they would be:
-buy small sample quantities first
1) Decide on composition- 63/37 or 60/40. 2) Decide what diameter you want. 3) Decide what flux you think you want. 4) Decide what % flux you think you want.
-go on ebay, search for "solder diameter variety" or "assortment". Buy a pack for <$15, wait for it to arrive & play with it. Wires, PCB work, whatever you've got to experiment with.
-try a few flux varieties, or buy a sample pack. Some come with different % as well.
-it may come in handy to have those spare sample assortments in the drawer for the future
-I'm not sure whether it's best to get 1 do-all, or 2 that might be better suited for more specific scenarios. Depends on what you're typically soldering
-the 'right' solder changes based on the job, and is based a lot upon personal preferences (not to mention price & availability). Hopefully you can find 1-2 that satisfy most of your needs.

It might help to pick up some solder wick & dedicated flux. One of my best purchases was a tip tinner. The dedicated flux & brush pens like in the linked thread at the bottom of this post look nice.
This doesn't get into silver solder, lead/lead-free incompatibilities, tip incompatibilities, flux incompatibilities, work preparation, etc.

And of course don't overlook practicing and building your abilities! Investing in good tools is just part of setting yourself up for success & part of an enjoyable return while you're working



I wish I knew more. Hopefully this shines a little more light on how one solder differs from another, which flux core wire might work best for your needs, or why your current selection might not be working the way you'd like it to.

Other links
https://www.neurochrome.com/choosing-solder/
https://www.eevblog.com/forum/beginners/ra-vs-no-clean-flux/
https://www.eevblog.com/forum/projects/about-different-kinds-of-solder-flux/

[GreyWoolfe]

The main questions to answer would be:
-Leaded or non leaded?
-Eutectic or not?
-What diameter wire?
-What flux should I choose?
-What percentage of flux should I choose

-Leaded, non leaded requires higher temps to melt and is hard on tips.  That said, I do have some lead free that I bought for a side gig that never materialized.  I have used it to replace some bad caps in my Hantek scope and some experimental board level repair on some company equipment that has since gone obsolete and scrapped.
-Eutectic, there is no plastic stage to wait on, it melts and solidifies at almost the same temp-184C or thereabouts I think.  I use Kester 44 63/37, .031" and I have soldered 0603 and SOT-23 components with it with no issues on size.  If I need larger diameters, I twist some pieces together to get what I need.
-I always add a little extra flux.  MG Chemicals 835 RA flux with either a needle applicator or nail polish type bottle/brush.  Louis Rossman and Mike's Radio Repair say you can never have too much flux.  I do clean my work with IPA just because it looks better.

I still have a little of a 1 pound roll of 60/40 solder from my ITT Technical Institute days in the late 90's.  It is thicker and I just use it for soldering PL-259 connectors onto coax or Western Union splices.  It's almost finished and when it is gone, I will just triple up some of the 63/37 as needed.  Hope this helps.

[Fsck]

-Leaded or non leaded?
 I like leaded.
-Eutectic or not?
 I prefer eutectic, since the price is essentially the same.
-What diameter wire?
 I've been using 0.5mm, which is pretty thin
-What flux should I choose?
 I like RA, it's strong enough to deal with most cases and doesn't have many downsides. Avoid water-clean at all cost, that stuff is almost always corrosive if not cleaned properly.
-What percentage of flux should I choose
 You may not have a choice in the matter depending on the diameter of wire you buy. Kester for example doesn't offer many options (or only 1) for its thin solders. I always add extra flux though (also RA) to everything, then IPA clean afterwards.

[Prizmatic]

-Leaded or non leaded?
Always leaded for me.
-Eutectic or not?
Non-eutectic.
-What diameter wire?
0.75 for through-hole, 0.5 SMD.
-What flux should I choose?
RA, never needed anything else.
-What percentage of flux should I choose
2% for general purpose.

Add Warton Metals to your recommended list.

[nanofrog]

• Leaded
• Eutectic
• Diameter
• Through-hole, .031".
• SMD that doesn't require a microscope, .020".
• For mixed, .025" strikes a nice balance IMHO.
• RA.
• Works well for both new and oxidized parts & boards (allows you to keep it to a single roll).
• I clean mine if I want it to look nice.
• 3.3% as it's better to get a good joint and have residue than little residue and a shoddy joint. I also keep additional flux around to be sure.

[KL27x]

There is some concern that hand-soldering MLCC (multilayer ceramic caps), you can break the caps due to localized heating of the board causing warping. When the board cools, the cap leads are put under stress. It's just a theory, but maybe non-eutectic solder is better at reducing this effect?

As for diameter? For me, the thinnest of the wire doesn't matter to me. The thinner wire is better for even distribution of flux and for metering the right amount of solder without overdoing it. But in SMD soldering, you are almost always better off adding liquid flux to the board, before even picking up a soldering iron. And the components and pads and tip type are more than enough to leave the right amount of solder without trying to feed solder wire with your third hand. For this reason, I do 99.9% of my soldering with relatively giant 0.050" to 0.064" diameter solder. And 99% of my soldering and reworking is tight pitch SMD stuff. My tip and the pads are all I need to control the amount of solder to the joint.

The bigger diameter solder can extend over longer distances without drooping. And you don't have an insane feed rate akin to reeling in an angry 600 lb marlin. To put it in perspective, 0.064" solder has about 6.5 times as much solder per inch as the 0.025" stuff. It is very rare I find any advantage to use thinner stuff, and the only thinner stuff I have on hand is 0.032", collecting dust.

I used to employ "rosin boards" (see bottom left of the pic) for picking up solder while using the microscope with TFO bevel tips. But it's been almost a year since I used that method. You can see I put a solder feed right on my microscope and I have not looked back. I have it loaded with 0.064". The thicker wire gives enough solder to do a lot of work between advances.


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