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December 2005

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Subject:
From:
Chris Ball <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, [log in to unmask]
Date:
Mon, 5 Dec 2005 09:34:22 -0500
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Hi Bill-

I saw that in some of the earlier replies, you had been referred to the
footprint available in IPC-7351. Is there a problem with using that config
that we should know about?

I'd like to hear about how you decide to handle the vias in the thermal
pad. Having issues with another pak type at present. In our case, the parts
solder just fine but the wicking through the holes causes a flatness
problem when mounting to a heatsink on the opposite side. Potential for
poking through the sealpad and we need to be electrically isolated. Tenting
the bottom side (LPI) was not the answer....

All the Best,
-Chris




                       "Brooks,Bill"
                       <[log in to unmask]>                To:   [log in to unmask]
                       Sent by: TechNet                  cc:
                       <[log in to unmask]>                 Subject:    Re: [TN] 56 pin LFCSP footprint

                       12/02/2005 11:58 AM
                       Please respond to TechNet
                       E-Mail Forum; Please
                       respond to "Brooks,Bill"







Thanks...

I contacted AD and talked with a technical support representative... He's
looking into a footprint recommendation from their development team... He
also sent me an application note on the part. It seems it has a large
thermal 'belly pad', for lack of a better name, that gets soldered to the
board in the center of the part. They recommend .3 mm vias on 1mm centers
as
thermal vias under the part, but as to any hard data on choice of solder,
heat profiles, or the like, I did not see any reliable data. They mentioned
the idea of plugging the vias or tenting them, and the use of no-clean flux
containing solders because there is no way you would be able to clean under
the part... That has me a little concerned too. I have heard of troubles
with plugging vias on this forum before... but if I leave them open won't
they will most likely wick the solder away from the pad to the far side of
the board starving the thermal pad.

I have used a chip with a 'belly thermal pad' before but in small
quantities... this is a medium volume product and troubles with assembly
could be costly so I value the comments put forth greatly.

Thanks again for your experienced insight.

Best regards,


Bill Brooks - KG6VVP
PCB Design Engineer, C.I.D.+, C.I.I.
Tel: (760)597-1500 Ext 3772 Fax: (760)597-1510
Datron World Communications, Inc.
_______________________________________
San Diego Chapter of the IPC Designers Council
Communications Officer, Web Manager
http://dcchapters.ipc.org/SanDiego/
http://pcbwizards.com

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