Some useful comments from a friend:


Thru-hole boards in quantities less than a few thousand can be
cost-effectively assembled by hand (but the advantage over machine 
assembly
obviously diminish as quantities increase).  If production is likely 
to be
on-going in the long term then you should use machine assembly as soon 
as
practicable.  If you are only ever going to do say 100 units then 
machine
assembly (with the possible exception of wave soldering) will almost 
never
make sense.

The same general argument applies for SMT but the thresholds are 
probably 10
to 50 times lower, and the issue of lead pitch comes in.  We routinely
manually assemble low-medium complexity SMT boards with lead pitches 
down to
0.5mm and lead counts up to several hundred (QFP packages).  We don't 
do
manual BGA but could if we wanted to  - we'd just have to get the
appropriate equipment and practice a bit.  The thresholds will 
generally be
lower as the component lead pitch increases  - for simpler boards with 
0805
passives, SOT23/SOT223 etc. discretes, and smaller IC's in 1.27mm 
pitch
(SOIC packages) you might be happy to do those by hand in runs of a 
few
hundred.  You probably don't want to be doing more complex boards with
0.65mm (QFP/TSSOP) or 0.5mm (QFP) in quantities greater than about 10.

How many you can/should sensibly do is a complex balancing act with a 
large
number of input factors.  Your techniques can have quite a major 
influence
too.  I believe we have some pretty efficient/effective ways of 
assembling
SMT componnents  - especially at finer pitches.

So there is no single answer  - every board is different and must be
assessed on its own merits.  I certainly wouldn't be wanting to quote 
on a
simple per component or per lead basis  - that would be suicide on 
some
boards.

Regards,

Ken Mardle

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