[time-nuts] Vaperware Parts and pulse stretching circuits

[Gerhard Hoffmann]

Am 25.04.20 um 13:41 schrieb John Ackermann:
> I do have a microscope (cheap Chinese unit, maybe $400 with articulated arm and the works) and it does make things much easier.  But as long as you can see the work, you can do the job.
>
> It's not that hard to do small pitch parts.  I usually do the best I can soldering individual pins, knowing their will be bridges, then clean up with solder wick and *lots* of no-clean flux.  You can never have too much flux.  I've found a 1.6 mm chisel tip is a good all around size for SMD work, though I have a 0.8 mm chisel available for when things get tight.
>
> The hardest part is getting the first couple of pins tacked down so the part is square on the pads.  After that it's fast.
>
> John

I have about the same here, plus a cheap Chinese Ayoue852 hot air 
station. Exchanging the Weller for a Metcal was the biggest improvement 
after the LED ringlight for the microscope.

In my quest to scrutinize the 1/f region, I have built some chopper 
amplifiers and the newest one will have GaN transistors that are nekkid 
chips with jut 4 tin bumps below. No case, just the passivated chip, 1 * 
1 mm, EPC2038. Low channel resistance, even lower capacitance -> low 
charge injection. Resistors are 0603.

Fearing I could not handle them, I made a minimum version of the switch 
itself as a test structure in an unoccupied corner of a different 
project. But soldering did take just 3 minutes, it was surprisingly 
easy. Just keep the air flow low enough, or you will have trouble to 
find the chips again. The thick-liquid flux helps to fix the chips in 
place.

Legible part numbers on the board are hopeless at this scale. The board 
was made by PCBway, there were some discussions about having solder mask 
ON part of the pads, and some discussions with our German customs that 
you cannot buy 10 boards for $10 or so.

The chips are the gray squares between the 2 vias on the left and the 4 
huge coupling capacitors.

Cheers, Gerhard


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