Yep! Been there, as have many others!
What you have to bear in mind is that the pictures in SM-782A deal only
with RLP's (Registered Land Patterns) and/or JEDEC Numbered packages. I
found, in practice, that there are not many RLP's, relative to the number
of components I found on our boards, so we had a hard time determining what
land patters we should adopt for most parts. Parts that look similar to RLP
parts are usually not so similar.
SM-782 gives generic patterns that may be suitable for most purposes, but
not all. What works in practice for your boards is, I'm sorry to say, up to
you, and depends on trace sizes and widths, what other components any given
part is in proximity to, board structure and composition, and a whole host
of other things. It's a good starting point, but you may find you need to
tweak the patterns a bit in due course.
Particularly if your parts don't have RLP or JEDEC numbers, you can't rely
on SM-782 to give you a ready answer, and if you take an answer for a
component that "looks similar", it will be different from any land pattern
that your data sheet might give. Personally, I made an executive decision
in the end with my boards, when none of the designers could make up their
minds - if the OEM provides a land pattern, use that first (and check which
soldering process it's intended for - usually it's convection reflow, but
sometimes it's Wave Solder where the patterns are different sizes). If the
data sheet doesn't give a pattern, but states an RLP/JEDEC Number, use the
pattern given in SM-782, and if both fail, use the formula in the front of
SM-782 to calculate the patterns for yourself.
Be warned, though, that you'll get different answers for every different
method and calculator you use. Frustrating? You bet, but it only goes to
prove yet again that this is still not an exact science. In spite of
package sizes, board materials, lead materials adn everything else changing
constantly, there's nothing like having history and hard experience to
"know" whether somethingis likely to be right or wrong. Without either, you
pays your money and takes your choice - then monitor the results closely to
see how well or how badly your guesstimates work out and learn for the next
time.
I did receive a note a while back from our Dungeon Master, Jack Crawford,
to say that IPC would be working on improving the consistency of Land
Patterns - any progress to report yet, Jack?
Good Luck!
Peter
"Black, Paul" <[log in to unmask]> 07/01/2003 01:25 AM
Sent by: TechNet <[log in to unmask]>
Please respond to "TechNet E-Mail Forum."; Please respond to "Black, Paul"
To: [log in to unmask]
cc: (bcc: DUNCAN Peter/Asst Prin Engr/ST Aero/ST Group)
Subject: [TN] SM-782A
My company is in the process of relayout of a board, and we are trying to
use SM-782A for component layout. However, we are seeing a number of
discrepancies between the recommended layout from the component
manufacturer and the SM-782A standard. The differences include pad length
and width, and distances between pads and pad rows. Does anyone have
experience with this problem, and what is your recommendation?
Thanks in advance,
Paul Black
Manufacturing Engineer
Kronos Incorporated
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