Solder flux, if you purchase it on its own, typically has aN expiration date of 2 years. But what about the flux cores inside solder wire? That os gemerally considered not to expire, what gives? Okay, so it turns out that some manufacturers do list an expiration date. And actually, especially if solder wire is left exposed to open air, the flux core will go bad near the tip. But, kept in a sealed bag, many folks have reported the solder working just fine several decades later. As for solder flux, it can be restored by putting a few drops of alcohol in it if it thickens. But solder paste, due to the solder powder and flux being mixed together, it doesn’t fare so well when it expires.
A flux wetting test is useful to determine if flux is good or bad. What is that? Basically, if you drip it on to a metal surface to be cleaned and fluxed, does the flux spread evenly across the surface? Or does it tend to stick together? Proper wetting means it will spread as much as possible evenly across the surface, for a considerable surface distance.
20200723/DuckDuckGo how long does flux in solder last
20200723/https://www.evilmadscientist.com/2013/solder-expire/
20200723/http://www.circuitinsight.com/programs/54448.html
20200723/DuckDuckGo flux wetting test
20200723/https://superiorflux.com/role-of-flux/
Now, all that discussion about using flux. My rationale, I need it so that I can properly clean connections already applied to the circuit board that have developed an oxide layer, but I want to rework them. Is using flux really the answer? Well, yes, looks like so, according to this source on proper soldering. Alas, looking in here, I also learned that I was not doing a bunch of my solder joints correctly. A proper solder joint should have a concave fillet, if you see a convex fillet, you used too much solder. Why? This is mainly in regard to inspection to check if both the lead and the pad were both properly wetted. A ball-like convex solder fillet may be hiding defects beneath the surface, a lesser amount of solder for a concave fillet cannot so easily hide defects under the surface. Indeed, this article is in reference to NASA’s electronics workmanship guide.
20200723/DuckDuckGo oxide layer on board solder
20200723/https://www.seeedstudio.com/blog/2019/08/07/13-common-pcb-soldering-problems-to-avoid/
Also, another important note, Seeed Fusion is another turnkey PCB assembler, they do the complete process of not just printing your printed circuit board, but they can also pick-and-place the discrete components onto it. Another Chinese one, that is. So, now that brings our total number of PCB Assembler choices to three.
- JLC PCB (Hong Kong)
- NextPCB (China)
- Seeed Fusion (China)
One thing I didn’t really figure out, how many of these allow you to order assembled PCBs in quantity of one? Seeed Fusion does PCB printing in quantity of one, not sure about PCB assembly.
Important! The Seeed Fusion article mentioned how to tell leaded solder joints apart from lead-free solder joints, and now thinking it through carefully, I can attest that I did see the difference and am personally able to tell them apart visually. Leaded solder joins, when done ideally, will look very smooth and shiny, whereas lead-free solder joints, even when done ideally, will have a duller and more matte look to them.
Yes, now I think back. Have you bought a Raspberry Pi with the pin header already soldered on? Take a good look at the solder around the pin header, since there is ample amount over there. You will notice that the solder looks rather dull and matte. Why is that? Shouldn’t it look clean, smooth, and shiny? No, it should not. That is precisely because Raspberry Pi boards are RoHS and therefore manufactured with lead-free solder. On the other hand, if you look at any vintage-style electronics with through-hole components, the underside of the board with the solder joints will look very clean, smooth, and shiny.
That being said, it is much harder to tell what kind of solder has been used on a board by looking at surface-mount components than it is by looking at through-hole components, simply because less solder is used and most of it is visibly occluded by its nature of being between the pads and the leads, or maybe even totally hidden underneath a no-leads component such as Quad-Flat No Leads (QFNL) or Ball Grid Array (BGA).