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

Attila Kinali attila at kinali.ch
Fri Jun 24 09:23:26 EDT 2016

On Thu, 23 Jun 2016 19:28:15 +0000 (UTC)
Bob Stewart <bob at evoria.net> wrote:

> One more related question before this topic dies, if you don't mind.
> What about the other side of building: stuffing the boards.  My GPSDOs
> have about 120 parts per board, plus some custom work on the SMA connectors.
> Is there a service out there that will populate boards with SMT components
> for small orders at a reasonable price?  Small is 10 boards.

Unlike what most people seem to think, small batches of PCBs have always
been a business for some assembly companies. I know a couple of those
in Europe that are specialized in small volumes (<100pcs per batch) and
reject anything larger. If you google for "PCB prototype assembly" you should
find some in your area. The more electronic industry you have in your area,
the more you will find those assembly fabs. Especially if your electronic
industry consists of mostly small, specialized companies. There are also
some inter-regional companies specialized on hobbyist market, like macrofab.

The prices vary a bit, depending on where you live, but usually
using 2*(BOM cost) and/or (BOM Cost)+300 are good estimates for
how much it will cost to build a PCB. "Going east" might also be
a good strategy. I know one fab in Estonia (Alktech www.alktech.com)
that does offer pretty competitive prices, while giving better quality
than what you usually get from china. The advantage of "professional"
companies like Alktech over "hobbyist" companies like macrofab is,
that you get full professional support while the price does not differ much.
E.g. while macrofab only does 4 layer boards if you specially ask for it,
for Alktech getting any number of layers is standard procedure.

If you want to keep prices low, then the first thing you should do is
to minimize your BOM: The fewer different parts you need, the better.
Ie if you have 200R, 100R and 50R resistors in your design, replace the
200R and the 50R by series and parallel connected 100R resistor. The cost
of a single resistor (transistor, chip, ...) is usually much lower than
having to handle another reel/tray. Especially if the fab has to go onto
the bigger pick&place machine because the number of feeders needed didn't
fit on the small machine.

The second cost saving thing is to minimize through hole components. Use
SMD as much as you can. It doesn't cost much to place an SMD component.
After the pick and place (which can be manual work on small volumes) the
soldering is done in an oven. But soldering a THD part means that someone
has to solder it by hand, as it would be too expensive to perpare a machine
for this. You can take this even further to let the fab only populate the
SMD parts and populate the THD parts yourself, which then only costs a bit
of time. (In my experience, THD solder jobs vary quite a lot more in quality
than one would expect. So doing it yourself might even improve quality).

The last way to minimize cost is to use common components and give the
fab the freedom to choose replacement parts. This gives the fab the
opportunity to choose parts that they already have on stock or buy
in large volumes, which makes them a lot cheaper. But means that you
specify the parts such that they are only as restrictive as you need.
I.e. if most of your reistors can be 5%, then specify them as 5% and
not as 1%. Even odd percent numbers are ok, though usually frowned upon,
as the company will choose the next better rating they have on stock.

Oh: and one additional hint: do not get the PCBs yourself. Let the fab
buy them. Then they will be panelized in the way they like it the best.
There is of course a little price hit if they buy it, but it usually pays
off when taking NRE costs into account.

As for doing assembly yourself: Yes it's possible and a good way to save
money. But be prepared to experiment a lot until you get consistent
and good results. Unless you have some serious experience in machine
soldering you will need a couple of runs in the beginning to figure
out how to do it right. And some things only show after a couple of
months/years after soldering (like borderline cold solders, cracks,
whiskers, popcorn packages etc) that depend on the quality of solder
and settings of the solder process. Also, IR solder ovens do not work
well with anything that casts a shadow on the solder joint, like QFN
or BGA parts.. or even high parts (alu capacitors or connectors)
next to low parts (IC's, resistors). That's the main reason why industry
pretty much stopped using (pure) IR ovens for production and switched
to convection type and vapor phase ovens. 

An alterantive to the ubiquitous IR ovens are rework-heater plates. 
These are basically just electrical stoves with temperature control
and a PCB holder ontop. They are ment as helpers for reworking, when
large ground planes confound any attempt of using a soldering iron.
By heating up the PCB to 50-100°C it is easy to get the last few
degrees with an iron. These heating plates can go up quite high in
temperature and thus allow the whole board to be soldered/unsoldered
at the same time. For obvious reasons this works only with single sided
PCB designs, but usually gives a better and more even result than
with an IR oven, especially for QFN and BGA. 

			Attila Kinali

It is upon moral qualities that a society is ultimately founded. All 
the prosperity and technological sophistication in the world is of no 
use without that foundation.
                 -- Miss Matheson, The Diamond Age, Neil Stephenson

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