Hi, Sefika,
SM-782 offers formula at the beginning of the spec to use for calculating
land patterns if your component does not have an RLP (registered land
pattern) number, and the component's data sheet doesn't offer any
recommendations either.
Otherwise, there are a number of land pattern calculators on the Net,
including IPC's own one at the following URL:
http://search.ipc.org/sm782/default.asp
I have found several things, though, that I'll share with you.
I have found rare occasions where I have a data sheet that gives a land
pattern for an RLP device that also appears in SM-782. I quickly noticed
that the data sheet land pattern and the one given in SM-782 did not
match. When I tried to verify one of them using IPC's formula and their
on-line calculator, I got different answers again. So there is no
standardised way of calculating land patterns.
Even for RPL's, the "ideal" land pattern is dependant upon the soldering
process you use and the nature/layout of your board. For example, wave
soldering requires larger land patterns than Convection Reflow soldering
in many cases, and features such as (even partially) unmasked traces and
nearby via holes can act as "solder thieves" that rob your component of
solder.
Even with all the calculators and so on around, plenty of people are
still experimenting with pad sizes and shapes to get the best, reliable
solder joint. There is thus no exact science in calculating footprints.
I adopted a hierarchy approach where information conflicted. First, if the
data sheet gave a land pattern, I used that. If the data sheet didn't have
a footprint recommendation, but quoted an RLP number, I used the SM-782
footprint, and if that too failed, I used IPC's on-line calculator to work
it out from teh dimensional data that is usually given in the data sheet.
There have been several, quite lengthy threads on this subject in the past
year or so at least, so if you look up the TechNet archives on the IPC
website and search for "land pattern", I'm sure you'll find lots of good
advice. There is no "one size fits all" with footprints, though. You have
to take your best shot at it and see how it works. If it doesn't, then you
have to try adjusting it by experimentation and testing.
Peter
Sefika Ozkal <[log in to unmask]> 26/11/2002 10:32 PM
Sent by: TechNet <[log in to unmask]>
Please respond to "TechNet E-Mail Forum."; Please respond to Sefika Ozkal
To: [log in to unmask]
cc: (bcc: DUNCAN Peter/Asst Prin Engr/ST Aero/ST Group)
Subject: [TN] Footprint
Hello,
I have a problem about plastic small outline package type.
I could not find the recommended footprint for two devices which are having
SO8 and SO20 packages.
Is there any one who knows to calculate the footprint values?
Or, is there a standart I could use to solve this problem?
I could not find aanything from IPC-SM-782A. Am I in a wrong way?
I would be waiting for your answer.
Best Regards,
ªefika Özkal
Electronics Engineer
ASELSAN Inc.
Microelectronics, Guidance and Electro-Optics Division
Electronic Design
Address : P.O. Box:30, Etlik 06011 Ankara,TURKEY
Tel :+90 312 847 53 00 Ext:4698
Fax :+90 312 847 53 20
e-mail : [log in to unmask]
--------------------------------------------------- Technet Mail List
provided as a free service by IPC using LISTSERV 1.8e To unsubscribe, send
a message to [log in to unmask] with following text in the BODY (NOT the
subject field): SIGNOFF Technet To temporarily halt or (re-start) delivery
of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL) To
receive ONE mailing per day of all the posts: send e-mail to
[log in to unmask]: SET Technet Digest Search the archives of previous posts
at: http://listserv.ipc.org/archives Please visit IPC web site
http://www.ipc.org/html/forum.htm for additional information, or contact
Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315
-----------------------------------------------------
---------------------------------------------------
Technet Mail List provided as a free service by IPC using LISTSERV 1.8e
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Technet
To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL)
To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest
Search the archives of previous posts at: http://listserv.ipc.org/archives
Please visit IPC web site http://www.ipc.org/html/forum.htm for additional
information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315
-----------------------------------------------------
|