I often have to work with MLF/QFN devices, which have a thermal pad on the bottom. There are two considerations here. Firstly there is the heatsinking requirement, and secondly if you get the copper design wrong the chip will float on a central blob of solder, resulting in unreliable soldering of the pins.
For the thermal pad footprint for a 32 pin device I arrange 8 square pads around a central via, and I place solder resist over the via. I number all the (thermal) pads as "33", so I only end up with one extra pin in eeschema. I connect together the pads and the via with a grid of thick tracks. The use of a tented via in this way means that the via will be solidly connected to the heatsinking copper zone on the reverse side, whilst the tenting prevents solder wicking through the via. This arrangement has worked well for me. An alternative arrangement might be to use nine untented vias with very small drill holes in the same pattern. This would give better thermal contact between board and component, but I don't know how small the holes would have to be to prevent solder wicking, or whether they would end up so small that their heat transfer capability would be compromised. If that were the case I guess you could use more vias with a smaller annulus. However, whilst I would be interested to know if this is a better method, I've no idea what size the holes would have to be, I don't have the means to do the necessary experimentation, and the arrangement I use currently works well enough for me. Regards, Robert. On 15/05/2010 22:30, Karl Schmidt wrote: > Today, there are many surface mount parts (MOSFETS, driver-chips etc.) that > depend on a solid copper > connection to aid in dissipating heat. Those pins should not have a thermal > created to a ground > plane. What is the best way to prevent the generation of this thermal? > > ( I think this should be an attribute of a pin type in eeschema - but it > isn't there .. there might > have been a 'T' attribute in PADS - might have been in the pad-stack > definition? - if memory serves > me right. I think it could default to T unless told not to do so). > > > I think I can create a zone with thermals turned off - and kludge it up to > work. > > This wasn't much of an issue in the past, but is rather common with the SM > boards of today - > probably should have some way to do this.. > > I want to write this up.. > > -------------------------------------------------------------------------------- > Karl Schmidt EMail [email protected] > Transtronics, Inc. WEB http://xtronics.com > 3209 West 9th Street Ph (785) 841-3089 > Lawrence, KS 66049 FAX (785) 841-0434 > > Action speaks louder than words but not nearly as often. -- Mark Twain > > -------------------------------------------------------------------------------- > > > ------------------------------------ > > Please read the Kicad FAQ in the group files section before posting your > question. > Please post your bug reports here. They will be picked up by the creator of > Kicad. > Please visit http://www.kicadlib.org for details of how to contribute your > symbols/modules to the kicad library. > For building Kicad from source and other development questions visit the > kicad-devel group at http://groups.yahoo.com/group/kicad-develYahoo! Groups > Links > > > > > > > No virus found in this incoming message. > Checked by AVG - www.avg.com > Version: 9.0.819 / Virus Database: 271.1.1/2878 - Release Date: 05/16/10 > 19:26:00 >
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