Depending on the assembly method this may end up costing more to
assemble.
Lets assume you are using mixed TH/SMT and the back side SMT are attached
via glue dot.  If you have 50 10K resistors on the board and only 2 need
to
be placed on the back then you would be shooting glue dots for 48 extra
parts.
This is more expensive than screening paste.  As a general rule it's
better to
place as few parts as possible on the bottom side.  You'll have higher
first
pass yields and that saves time and money.

There are any number of possible reasons for the engineer wanting this,
your
job should be to research the capabilities of your assembler.  That's
what
PCB Designers do, take the requirements of electrical, mechanical,
thermal,
assembly, fab, etc. and roll all requirements into a functional
producible
product.

===========================================
Clint Cote
===========================================


Mark Koitmaa wrote:
>>I have been asked by an engineer that I am working with to restrict
>the top
>>side/bottom side placement of parts according to part values (all
>parts are
>>0805's).  For example if I place a 10K resistor on the bottom side,
>all
>>10K's should also go to the bottom side.  He does not want me to mix
>parts
>>of the same value between two sides.
>>
>>He tells me this will significantly reduce assembly costs.  I have
>never
>>heard of such a thing.  Is he correct?  I believe the extra layout
>time
>>will negate any potential savings in assembly setup.  Any comments?
>
>I'm sure he is right that it will reduce assembly costs. How much is
>another question.
>
>If it were only a matter of set-up costs, I doubt that these would
>counterbalance the nuisance of making sure that parts of a value are
>only
>on one side, and nuisance = design cost. Further, with some boards,
>this
>requirement could impact noise performance.
>
>But his requirement might also affect per-unit costs. I would think
>that it
>might depend a bit on exactly what equipment is being used to do the
>pick-and-place. This cost saving, though, would probably be small.
>
>This could be one of those clever ideas which is penny wise and pound
>foolish. Every day of delay getting a product to market might cost
>more
>than is saved by shaving the last penny off of assembly costs.
>
>But if this product is already on the market and this is a production
>redesign, and the production quantities are really large, well, a
>million
>pennies will buy a fair amount of design time.
>
>And it is not terribly difficult to do what he is asking, unless the
>design
>is really dense.
>
>Now, if the design is not dense, we'll have to ask why it is not so,
>since
>one could probably save *more* by making the PCB board smaller. That's
>not
>just pennies.
>
>[log in to unmask]
>Abdulrahman Lomax
>P.O. Box 690
>El Verano, CA 95433
>

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