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Comment by pjc50 | Hacker News Reader

This side of the business is actually pretty straightforward to learn and be guided by your PCB assembly house and design software. Ask the PCB fab for the stackup, punch 90 ohms into the calculator, and get a number in mils for the PCB software.

And the dirty secret is that for short runs, testbeds and hacking tools you can often cheat on the controlled impedance a bit to produce abominations like USB-over-FFC and three-ended ethernet cables.

Now, hot air rework, that's a serious manual skill that I respect.

(I suppose the difference versus learning software development work is that failures are expensive...)

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> Ask the PCB fab for the stackup, punch 90 ohms into the calculator, and get a number in mils for the PCB software.

This sounds like gibberish to me.

(I know it's not, just that the notion that it's "straightforward to get into" is maybe off the mark.)

The stackup is what controls the layers of a bare PCB; signal, power, ground, etc [1]. An engineer at your board supplier is likely involved in determining the stackup with the EE and PCB designer. The stackup and material selection of the bare board is one of the things that controls min/max trace width, proximity, etc. Once you have that info, you can plug it into your CAD tool of choice, for instance in Mentor Xpedition it's CES [2]. Makes things like differential pairs a lot easier.

1. http://blog.optimumdesign.com/hdi-layer-stackups-for-large-d...

2. https://www.mentor.com/pcb/xpedition/constraint-manager/

Good SMT rework is 60% flux, 20% isopropyl alcohol, 10% magnification and 10% practice. The trick is surface tension - if the surfaces are properly wetted, surface tension pulls the part into alignment as if by magic. Achieving good wetting is as simple as getting the solderable surfaces spotlessly clean and deoxidized, then using copious amounts of flux. If your preparation is good, the parts almost solder themselves.

Fine-pitch BGA rework can be genuinely challenging, but the core skills are remarkably straightforward. A complete novice can rework QFNs and 0603 passives with very little practice if they're taught the correct techniques. I actively prefer working with SMT over through-hole.

I teach a course (at various hacking events) titled "Surface mount electronics assembly for terrified beginners" and the thing that basically every participant reports after is that they expected it to be harder. We go from "never knowingly touched a PCB" down to 0.5mm pitch QFNs and 0402 passives in the space of an hour or so, so what you're saying about novices is absolutely correct. (if you want a course like that near you, contact me)

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