[time-nuts] OT stuffing boards: was GPS interface/prototyping board

Steve Wiseman sjwiseman at gmail.com
Fri Jun 24 15:01:24 EDT 2016


On 24 June 2016 at 14:23, Attila Kinali <attila at kinali.ch> wrote:

> Unlike what most people seem to think, small batches of PCBs have always
> been a business for some assembly companies.

For my sins, I am one of those... (Cambridge, UK).
Yes - semi-manual assembly is the way it goes, especially for the
active parts. It's just not worth teaching the machines and loading
the parts for small runs. Typically, passives with more than 20
instances I'll load onto the machines, then do the rest by hand on a
manual placer.

Stencils - not any more. I use a dispensing robot, which is fine down
to 0.4mm pin pitch as long as the ambient temperature is right (35oC,
quite deeply unpleasant to share a room with). No more cleaning
stencils, throwing away paste, or wishing that the customer-supplied
stencil wasn't unhelpful in one of a thousand ways. It also means that
I can go from CAD data to built boards in less than a day, if I ply my
local PCB house with enough cash...

I definitely concur with the 'make it SMT as much as possible' plan -
pin-mount stuff is a pain. Also, QFN is far preferable to QFP, as
catalogue suppliers don't always manage to ship fine-pitch stuff
without bending legs in one direction or another. Reworking a duff QFP
(or fine-pitch SOP) can take as long as assembling a whole board. With
small volumes, there's no real statistical process control, you just
do what you think will work, fix any defects, and update the big
logbook of results.
Hacked reflow ovens have a place, but there are some parts that simply
won't solder with IR - the heat load to get the balls to melt is more
than it takes to kill the part. LTC's modules are especially bad, but
any BGA runs a risk. I'm a recent and thorough convert to vapour phase
(which can also be done in a homebrew manner).

Also - Pleeeeease overbuy components!
those extra few 0402 resistors cost you a penny. Finding the one that
pinged off or the machine threw on the spitback pile - impossible.

Sorry about the offtopic. (I'm also a moderate frequency nut and EMC
chamber owner, so am starting to get nutty about RF amplitudes, which
is a whole new game...)

Steve


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