One of the issues I hear about most often in
the PCB
assembly business relates to what should be
taken into consideration when providing parts kits for
board assembly. Many times kits are sent out with the
exact amount of parts being provided based on the
quantity per board and the number of boards being
built. The assumption being that everything will go
perfectly and there will be no loss of parts. That
position is completely understandable when you
consider that a high percentage of prototype
customers are designers or companies that don't
have boards built outside all that often. Plus the idea
of providing kits to most assembly houses is
somewhat foreign as many CM's (contract
manufacturers) aren't set up to accept kits and prefer
to provide the components themselves.
When building prototypes and small
production runs,
the economies of scale aren't there as you would
have when you buy parts in larger quantities. Typically
time is of the essence so buying just the parts you
need from stocking distributors such as Digi-Key,
Mouser or Newark makes more sense. They aren't
the cheapest guys around but they usually have good
stock on hand and their delivery is fast and accurate.
Several factors should be considered when
ordering
electronic components. Since there is no such thing
as a "perfect build", there will be some scrap. There
are number of factors that contribute to generating
scrap and most CM's do everything they can to
minimize it. As to what causes scrap, there are a
number of factors such as when the automated
assembly machine is first set-up to run a particular
board, some parts may be lost due to the way the
parts are presented to the machine on the tape. Parts
may also be lost while the machine is running if it mis-
picks the part or in the case of some placement
machines, the part is tested for its electrical value and
if it doesn't meet a certain spec then it is rejected.
There could also be problem with the feeder, broken
cover-tape, etc.
General Kit Guidelines: Parts should all be individually packaged by P/N (part
number) and clearly marked with the BOM line item
number referenced on the outside of the package. It's
imperative that the parts somehow be tied back to the
BOM. In addition, it's helpful to have the manufacturer
and P/N listed which will normally be the case when
the part is shipped from the distributor or
manufacturer.
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