Moisture Sensitive Devices (MSD's) Many
components and particularly IC's are susceptible to
absorbing moisture from the environment. MSD's are
almost always shipped from the manufacturer or
distributor in sealed bags along with desiccant and
humidity indicators. It's very important that the bags
remain sealed until the parts are ready to be placed
on the board. If there is any doubt about the moisture
content of a part it should be baked-out for 48 hours at
150C in order to drive out any moisture. The reason
for that is if the moisture were to remain in the part
when it is soldered to the board it could "popcorn" and
damage the part. In some cases the damage may be
visible but often times it creates a micro-crack and the
part just won't work.
Packaging: Most assembly houses prefer to
have their parts on tape and reel so they can load
them onto the pick and place machine. In the case of
BESTProto we don't need full reels or even leaders or
trailers on the tape. Our pick and place machine can
feed "snip-its", which are from a full reels of parts that
are cut down to shorter pieces.
Passive Components: As in the case of
resistors and capacitors (R's and C's) and other
smaller components, they tend to have the highest
scrap rates for a couple of reasons, primarily because
they're tiny and they also make up the majority of the
parts on the board assembly. Thankfully they're
normally inexpensive (fractions of a cent in some
cases) and therefore not worth spending much time
trying to salvage. The rule of thumb is at least 20 extra
pieces of each P/N and if there are a fair number of
the same P/N on the board then we would suggest
rounding your order up to the next 100 pieces
whenever practical. This is especially true when
ordering small quantities from the distributors
mentioned previously. One thing to keep in mind when
you order small parts that come on tape and reel, if
you order 60 pieces of a given P/N you will likely
receive six separate snip-its with ten individual parts
on each. Conversely, if you order 100 pieces you will
receive one continuous snip-it with all 100 parts on it,
which is much preferred. Having all the parts on one
continuous strip will help ensure a faster and more
accurate build and minimize waste.
Active Components and Larger Parts: Items
like IC's and other parts in the 12mm-24mm size
class are somewhat easier to work with and are there
are fewer of them compared to R's and C's however,
it's still doesn't hurt to have a few extra on hand during
the build. In most cases an extra 5-10 pieces is more
than sufficient. These parts are physically larger and
tend to be more expensive with longer lead-times. If
something does happen during the assembly
process it makes sense to try and salvage them as
compared to R's and C's.
Higher Value, Larger BGA Type Parts: In
cases like this exact quantities are fine. Again, they
are larger and less common then the other part types
mentioned so the potential for fallout is greatly
decreased. And unless you feel there is a potential for
fall-out extras are not normally required.
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