Getting back into the soldering saddle (Indium SAC305 vs. Kester K100LD with and without extra flux)

One of a few snapshots I took of the same soldered pins, in between twiddling the focus height adjustment on a microscope. In this first of three images, the uppermost bits of the pins are the most in-focus features. — One of a few snapshots I took of the same soldered pins, in between twiddling the focus height adjustment on a microscope.
In this first of three images, the uppermost bits of the pins are the most in-focus features.
This and the next two photos are taken of a small region on the prototype board on which I used Kester K100LD with no application of additional liquid flux.

It had been a little while since I’d last wielded a soldering iron, so I put myself through something like a set warm-up exercises: soldering multiple lengths of single-row pin headers into some small prototyping boards. I dug out two spools of solder wire: Indium-brand SAC305 and Kester’s K100LD and a small dropper bottle of Chip Quick no-clean liquid flux (CQ4LF-0.5). All of which are several years old now.

Another of the snapshots I took of the same soldered pins, in between twiddling the focus height adjustment on a microscope. In this second of three images, the tops of the humps of the solder joints are now most clearly visible. — In this second of three images, the tops of the humps of the same solder joints from the preceding image are now most clearly visible.
Ideally, the solder in a joint is supposed to by steeply sloped rather than camel-humped.
So, all else being equal, the joint on the left is probably better-executed.

Both spools of solder wire have adhesive-backed labels stuck to the round flat faces of their rims and that’s where one will find the D.O.M. for both: late 2017 for the Kester wire and early 2016 for the Indium. There may have been a use-by date on some long-discarded bit of outer packaging for the Kester product, but only the Indium wire has it on the rim-face label: 26 JAN 19. That’s five-plus years in the rearview as I write this. The Chip Quick flux has a manufacture date on the bottle in YYMMDD format: 200909 (September 9th, 2020). No expiration date for the flux is given, though, as with the Kester solder wire, one may have been specified on some packaging I absentmindedly tossed into the wastebasket when I bought it a few years back.

Searching online, I found Kester’s technical data sheet for K100LD solder alloy [PDF] and the Storage, Handling and Shelf Life section says the wire may turn dull gray but that this doesn’t indicate that it should not be used. The wire on my spool is still nice and shiny. A couple of lines down, it says that the warranty period for alloys that don’t contain ≥70% lead (applicable to K100LD, which is lead-free) is three years post-. So the K100LD wire I’m using here is well beyond its warranty period but seems to work well enough.

Chip Quick’s data sheet for CQ4LF-0.5 liquid flux, no-clean (in 15mL bottle) [PDF] has this to say regarding shelf life: Refrigerated >24 months, Unrefrigerated >24 months. Refrigerating flux and/or solder never occurred to me. In any case, it, too, seems to work just fine.

Picture #3. Finally, here we see the base of the joints and the surrounding bit of prototype board surface, featuring a caramelized blob of cooked flux ichor, said flux being from the solder wire itself (it's flux-cored). — Finally, here we see the base of the joints and the surrounding bit of prototype board surface, featuring a brittle, solid caramelized puddle of cooked flux ichor.
These pins were welded onto the protoboard that got Kester K100LD but no extra flux and so the charred goop came from the solder wire itself (it’s flux-cored).

Trying each solder alloy alone (they’re both flux-cored) and then again with extra flux seemed like an easy way to make things more interesting. Here are the combinations in the order that I muddled through them:

Indium SAC305
Indium SAC305 + extra flux
Kester K100LD
Kester K100LD + extra flux

Each protoboard got slotted onto a solderless breadboard into which I’d already pressed six separate pieces of pin header, each 5 pins long. I turned the temperature adjustment for my soldering station up to 400 degrees Celsius, at or near the top of the recommended range for lead-free alloys like these, and got started.

First up is a clip of me working on the first of the four aforementioned protoboards (the SAC305-with-no-liquid-flux one). My technique could always be improved. For instance, I can see the slag/crud on the tip of the iron when I hit the first couple of pins in the video beloww and a wipe over the cleaning sponge and re-tinning would have been prudent. In my defense, I did that before starting the next row of pins.

I picked this bit of footage to share because of the visually interesting sputtering effects to be seen. The cooling solder blob girding the last pin in the first row shown spits pseudopods like a T1000 dropped into a vat of molten steel. It looked neat enough that I excerpted that sequence of frames and converted it into an animated GIF file to use for this post’s thumbnail. The next pin, the first in another series of five, does the same but, just before congealing, pops in a way that leaves a large crater on the upwards-facing side of the joint.

Soldering some pins into a prototyping board using Indium-brand SAC305 without any additional flux.

The second clip, embedded below, comes from the fourth protoboard, the pins of which got K100LD as well as a generous helping of extra liquid flux. The aesthetically pleasing detail here is the spinning-rainbow-colored-ring effects created by the reflection of that microscope’s LED ring light in the surfaces of the shrinking air bubbles in the liquid flux. When the air bubbles between and around the leftmost two pins disintegrate, the same lone white ring reflection is visible in the pool of flux around each of those pins as was present next to the remaining pins.

Soldering some pins into a prototyping board using Kester K100LD solder wire with additional flux.

Did you notice that I accidentally plugged an empty through-hole pad in the row above the target row? Such snafus aside, the joints in the fourth protoboard turned out the best in terms of exhibiting the desired pinched-cone cross-section and staying shinier after cooling than the joints on the other boards. If ranked from least-good to best in terms of outcome, the ordering of alloy+flux/no-flux combos would end up being the same as in the list above. SAC305 (years past its expiry date, mind you) with no added flux yielded, in my not-too-terribly-skilled hands, the least wonderful outcomes and K100LD with additional flux gave me the best joints.

This jibes with a simpler trial I did back in 2018, without any extra flux and when I was even worse at soldering than I am today: Lead-free solder recommendation: Kester K100LD for shiny lead-free solder joins. The SAC305 wire used that time was a Kester-branded product well within its expiration date (assuming it had one) and it’s possible that I could obtain better results today. Those joints/joins are terrible and shaped very unacceptably like dollops of whipped dessert topping. Live and learn.