23 Sep
2023
23 Sep
'23
6:28 p.m.
On Sep 22, 2023, at 9:01 PM, Martin Bishop via cctalk
<cctalk(a)classiccmp.org> wrote:
...
Down to 0.5 mm pitch gull wing legs can be readily hand soldered by a wireman. Surface
mount double sided PCBs can be hand assembled (by me), in small quantities, by restricting
the components to 0603 and 0.65 mm pitch using an under board heater and hot air. The
fine pitch stuff goes on first. OTOH a good wireman can hand solder 0402 components. So,
don't back off completely from SMD. You "only" need: a quartz heater, a hot
air wand, and a solder paste dispenser (you spit it on by hand). Note that surface
tension squares everything up and moves the solder onto the pads; which need to be (hand
assembly) oversize. NB this approach is for very low volume / zero budget / assemble
tonight work; for serious boards / volumes you send the kit to the assembler.
I used to not mind sub-mm lead pitch; but my most recent bout with SMD (with a TSSOP
multi-RS232 transceiver) was enough trouble with my older hands that I'm more hesitant
now. Then again, the fact that I didn't make the footprint quite wide enough
undoubtedly didn't help. Leadless packages may actually be easier -- no micro-legs to
break off. Next time, maybe.
BGAs are another story - dodge them; e.g. use
https://digilent.com/shop/cmod-a7-35t-breadboardable-artix-7-fpga-module/ or similar to
apply FPGAs in a 0.1" DIL form factor, at LVTTL levels. You could build an OmniBus
interface with two of these, some bus interface chips 74LVC for Rx, something OD (and not
too fast) for Tx, and a "host" processor.
There have been some good articles about assembling SMD boards (I think including at least
modest size BGA) using a toaster oven with a suitable heat controller. John Wilson of
Ersatz-11 fame made such a controller (an Arduino daughterboard). For solder paste, some
PCB shops make stencils quite cheaply, or partner with a maker of those; OSH is one I
remember. They might be the classic stainless steel ones or a less expensive (and less
but sufficiently durable) material. I should try this one of these days.
paul