Shouldn't there be more vias to take the heat to the bottom?
T > To: [email protected] > From: [email protected] > Date: Mon, 17 May 2010 09:57:50 +0100 > Subject: Re: [kicad-users] Preventing thermals on special pads of SM packages > > 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 > > > > ------------------------------------ > > 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 > > >
