Great points you bring up. As you say, it is truly an apples and oranges comparison since most commercial electronics are at least sufficient for their intended use and do not have to take 9Gs of stress at 40K feet at 0 degrees Celsius. My projects at work need to meet these standards because Federal Acquisition Regulations require it, and because operational failures carry unacceptable costs (loss of life and lots of $$$). That is why the solder and fabrication efforts I deal with at work must meet IPC J-STD... so outside of critical applications, J-STD is basically a soldering beauty contest more than anything(in my opinion). I cite it only because I am intimately familiar with it, and at least it is a standard that can be applied based on visual inspection of fabrication or repair work involving soldering.
In practical reality, the best solder joints need to be clean with a full flow/melt and no oxidation or contaminants in the joint... and the best way to get that is to do it right the first time in a wave solder machine or reflow oven and without the need for rework. A somewhat imperfect looking (or even ugly) solder job that meets these critieria is better than a beautiful-appearing solder joint that has been reworked 2+ times. A reworked solder job allows contaminants and oxides into the joint, and the excess heat loading from the soldering iron can cause damage (e.g. delamination of the PCB, lifted pads, thermal degradation of components mounted in proximity to the resolder site, Kirkendall voids and other damage that is not apparent to the unaided eye). The less heat you apply to the board, the better off it is. In untrained hands, soldering irons cause more problems than they fix. A gunsmith friend of mine loves Dremel tools and is thankful that they are sold to the public, because improper use of Dremels is responsible for most of his business (fixing the screw-ups of people trying to modify their own guns).
For a keyboard, it would be ideal for a truly skilled fabricator to do all soldering by hand without a single rework and pass J-STD. Since this would be ridiculously expensive in any quantity, the next best would be for soldering to be performed in an automated wave solder machine (even if the joints looked a little imperfect) and later functionally QA/QC'd by a skilled inspector. The scenario of fabrication being done by "underpaid Chinese peasants" should give us some pause... since any and every instance of rework is a cumulative compromise in reliability, even if the joints end up looking beautiful.
OK, I need some time to absorb this.
But you are basically saying that reworking is bad. I can see why – eg my soldering iron gets dirty and it is impossible to get every bit of that black stuff off. I was also worrying about impurities getting into solder joints and possibly causing problems in future.
What about total desoldering and resoldering?
IE total removal of original solder, impurities and all. I typically do that for one session, then thoroughly clean the soldering iron, and go back and solder. I know there will still be some impurities, but in general I get nicer looking, shiny convex joints better than when I reflow the solder/ add more solder to an existing joint. But I still have various black splotches, sticky places, etc. that tell me I didn’t do a perfect job and I need to get better. I am not familiar with solder paste, flux, etc.
I am really asking this out of my own interests. I am expecting to get more POS boards in future and fully expect to have to desolder all the black switches I hate so much. OK, after I do that, what’s the best way to resolder to the highest possible standard? What should I do to ensure maximum possible quality?
Assume that 1) I am using leaded solder; 2) I am using a very ordinary, cheapo soldering iron.
I only have cotton pads and copper solder wool (whatever you call that) to clean the soldering iron tip.
As I am not a professional and I only expect to have to solder a few more keyboards before I settle on my ideal layout (I’m already near there), I see no point in spending hundreds on a soldering iron, wave oven or more professional stuff. I just want to improve my work so that I can have decent quality without expending too much resources (on equipment, training/ practice, etc.) Naturally I do not expect my stuff to function in zero gravity. But I'd like to think that my work is good enough to last the next couple decades of normal desk use without temperature extremes. My work does have to withstand the occasional bump eg when I move my keyboard I might drop it a couple of inches or have similarly modest impacts.
