Smd flux

Hey,
I’m looking for a flux for smd components. I looked on you tube for some ideas and wanted to ask here. I can’t believe how expensive some are. I hate to squeak too loud but the most important thing is ease of use and clean up. I won’t be doing alot of smd but I like to leave the board really clean. Anyway, what are you using?
Thanks
 

lauda

Member
2015-03-28 5:03 pm
Great write-up, suzyj. I'm definitely going to take your formula for a spin.

I've been using Chip Quik SMD291 for prototyping, and as a tack flux it works well, with one caveat. When hand soldering, the residue that remains is very sticky and a little difficult to completely remove with 99.9% IPA. When using hot air, the cleanup has been much easier. I'm not a flux expert by a long shot, but it seems like when this flux isn't brought to high temperature for a while, it doesn't fully evaporate/bake/transform into something that's easy to clean. This isn't an issue when the heat is widely distributed (hot air). However, when you use an iron, the flux softens and flows under ICs, onto adjacent pins, etc. and, I suspect, thereafter ceases baking. It usually takes me several courses of IPA to flush the flux out of all its hiding places.

I've tried other tack fluxes, and so far they've all had quirks. The right one for you might be down to which quirk you're least bothered by.
 
Learned how to solder to ultrasonic crystals many moons ago (1979). It involved liquid flux. The technique translated very easily to SMD. tin the pad, place a small drop of flux, place the part, touch clean iron tip to solder and touch the fluxed spot with it. Done. I tired paste flux but could never get it to work as easily. And that's my $0.02 for the day!
 
Just make your own. Then you know what’s in it.

http://www.suzyj.net/2021/01/suzys-...1/suzys-super-rosin-paste-flux.html [/QUOTE]

flux.html[/URL]
Great write-up, suzyj. I'm definitely going to take your formula for a spin.

I've been using Chip Quik SMD291 for prototyping, and as a tack flux it works well, with one caveat. When hand soldering, the residue that remains is very sticky and a little difficult to completely remove with 99.9% IPA. When using hot air, the cleanup has been much easier. I'm not a flux expert by a long shot, but it seems like when this flux isn't brought to high temperature for a while, it doesn't fully evaporate/bake/transform into something that's easy to clean. This isn't an issue when the heat is widely distributed (hot air). However, when you use an iron, the flux softens and flows under ICs, onto adjacent pins, etc. and, I suspect, thereafter ceases baking. It usually takes me several courses of IPA to flush the flux out of all its hiding places.

I've tried other tack fluxes, and so far they've all had quirks. The right one for you might be down to which quirk you're least bothered by.
Good information. I like the idea of making my own like suzyj but also am kinda lazy. Just want something ready made. What can I say. But clean up is key for me. I haven’t done any smd yet but my experience with through hole components and rosin core Kester 63/37 left a real mess on the boards. I was able to clean it with isopropyl but it took alot and still didn’t come as clean as I’d like. Partly because I didn’t pay much attention to how messy it was getting. I’m trying to improve this time around. Thanks
 
SMT work is meant to be done submerged in flux, otherwise the air gets in, and things may oxidize, and even if 1 pin out of 50 oxidizes enough to prevent solder wettting it the whole job's ruined. So you bathe everything in flux. Solder paste is made of flux with particles of solder dispersed in it, so the solder never sees oxygen during the standard manufacturing process.

SMT parts (and solder paste) also have a fairly short shelf life due to oxide build-up over time, although as a hobbiest you can ignore this and hope it'll be fine, you wouldn't want to do this on a production line and risk the failure rate rising -- that quickly eats up your profitability (many PCBs have 1000's of SMT solder joints and all have to work for a board to pass testing!)
 
The shelf life for solder paste is due to the flux and solder balls separating, as the solder drops out of suspension. You can buy machines that vigorously shake the paste to get them uniform again.

Shelf life for SMD parts is due to moisture absorption in the plastic packages rather than oxidation of the pins. The packages absorb moisture from the atmosphere over time, and when they are suddenly heated during a reflow process they can crack. The methods to slow this are packaging with silica gel, and expired parts can be baked prior to use.

oxidation of pins takes a much longer time, and mainly affects through-hoke parts, that aren’t stored to the same standards that SMD ones are.
 
To me, this is really just amazing knowledge. From possibly someone who has worked in the industry for awhile? I used to work in the heart of Silicon Valley, with companies like IBM, Intel and AMD. With folks whose experience was vast and who shared it with humility. In some ways this diy community reminds me of those excellent days and surprisingly how much I miss them. I realize we’re just talking about flux, a seemingly innocuous detail, but those little things are bringing back good memories. So thanks everyone and I hope your day brings new knowledge and intriguing experience
 

kn0ppers

Member
2019-03-23 8:02 pm
SMT work is meant to be done submerged in flux, otherwise the air gets in, and things may oxidize, and even if 1 pin out of 50 oxidizes enough to prevent solder wettting it the whole job's ruined. So you bathe everything in flux. Solder paste is made of flux with particles of solder dispersed in it, so the solder never sees oxygen during the standard manufacturing process.

I surely don't use too little flux, I can assure you that :D

However with the right setup the flux application can definitely be optimized. It surely doesn't help squeezing flux in spots where it will neither be properly activated nor properly cleaned. Even "no-clean" solder is only "no-clean" if properly activated. And even then I want it off my board if possible. Just get a whole bunch of cheap syringe dispenser needle kits and you will always have the right one for the flux you are using and the packages you are soldering.
 

lauda

Member
2015-03-28 5:03 pm
In my experience, using solder paste is a joy, but you really need a stencil for it. They can be had pretty inexpensively through JLCPCB and possibly others. I've not been able to get good results using stencil-less application of solder paste, but I've only tried a handful of times and never with an electronically controlled pump+syringe.

If a project has only a few SMD parts and they are reasonably large, I opt to hand solder. With some experience, it takes no more time than soldering through-hole parts. If a design uses tight pitch ICs or 0603 or 0402 parts, I opt to use paste with a stencil, and either hot air or a toaster oven. If you go this route, good ventilation is recommended.
 

lauda

Member
2015-03-28 5:03 pm
Using a skillet, with or without sand as ballast, is another popular approach. Sparkfun has a decent writeup on a bunch of different methods. For my next stenciled project, I'm thinking of using a kitchen-grade infrared hotplate to preheat the board and then use hot air to push things over the edge. I think it might make for an easier process and may reduce thermal issues for the boards and parts.
 
Using a skillet, with or without sand as ballast, is another popular approach. Sparkfun has a decent writeup on a bunch of different methods. For my next stenciled project, I'm thinking of using a kitchen-grade infrared hotplate to preheat the board and then use hot air to push things over the edge. I think it might make for an easier process and may reduce thermal issues for the boards and parts.
I’m going to get a skillet and use sand. This is really all I’m willing to spend just now. The Controleo looks great but I’m not sure how much smd work I’ll be doing. Now I just need to settle on what flux paste I’m going to use.
Also, I really like Sparkfun. I’m really only doing a small number of smd parts and I’m honestly not very familiar with the different sizes. Some are 1206 and some are smaller. Stencils seem a bit beyond my current needs but are something I may use as my interest expands. I really need to just get some cheap parts and experiment with a few different techniques. Any suggestions?
Thanks guys
 
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