I just went through the same pain, adn the answer for now is that you have
to use either the 3 formulae in SM-782 or IPC's on-line calculator for
packages with no defined land pattern. The trouble is that where you can
correlate results, SM-782 and the calculator usually give different
results, so which one is right. Also, if you use any other soldering method
than convection reflow, there is even less certain guidance.
Jack Crawford has stated that IPC "were working on these issues" (how's it
coming along, Jack?), but it's a Pandora's box. There area so many
variables like substrate material, component package varieties (even within
the same species - every manufacturer has its own version of the "standard"
package), assembly process, etc., that it will be quite difficult to turn
this trial and error art into a science. Sorry not to be more encouraging.
I determined the following heirarchy of rules for our guys:
1. If you have a land pattern in a data sheet, use it.
2. If the dtata sheet doesn't have a land pattern, but does have an RPL
(registered land pattern) number or JEDEC number, try SM-782 and use the
land pattern there, if there is one.
3. If 1 and 2 fail, use the IPC calculator and take the heel and toe solder
joint figures required from table 3.4 in SM-782. If in practice later, the
resultant pattern doesn't work, well, you're largely on your own to sort
out a better solution.
There's a chance here for someone to make a name for themselves here by
determining a reliable formula and save everyone re-inventing the wheel .
Maybe Daan can start a database on his site for successful land patterns
people have worked out for particular substrate materials/OEM component
packages/soldering process/operating conditions/.../... that other mortals
could use as a library. What d'ya reckon, Daan? People can add to it
themselves if the want to share their findings.
Peter Duncan
"Reid, Lorraine"
<Lorraine.Reid@TRI To: [log in to unmask]
VIRIX.COM> cc: (bcc: DUNCAN Peter/Asst Prin Engr/ST
Sent by: TechNet Aero/ST Group)
<[log in to unmask]> Subject: [TN] Land Patterns
10/26/01 06:21 PM
Please respond to
"TechNet E-Mail
Forum."
Can anyone point me to a good reliable resource for land patterns for
devices which are not in IPC-SM-762?
I need to verify patterns for devices such as TSSOPs, QSOPs, and SSOPs,
none of which seem to be covered in the standards!
thanks in advance,
Lorraine Reid
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