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November 2002

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Date:
Wed, 27 Nov 2002 08:15:10 +0800
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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]



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