Peter was talking with me about using vias in thermal pads and how some still leave an open aperture the size of the thermal pad for their solder paste... 
This can cause the solder paste when it melts to be wicked down the via holes starving the part for solder... depending on the diameter of the vias and how many there are... 
One method to reduce this effect is to change the stencil aperture to a pattern that avoids the vias and helps distribute the paste over the thermal pad area evenly... 
See this illustration by Texas Instruments... 

http://www.ti.com/ods/images/SNLS302D/sample_layout_DAP_snls302.png

Another thing that helps is to reduce the diameter of the via to reduce its wicking volume... smaller holes wick less... 
I have used .008 in. diameter vias for that purpose before... 

Or... you can have the via holes filled and planarized... thus no extra solder is wicked down the holes... 
Partial filling tends to cause problems... 
For example: Trapped gases will expand in the high heat of the reflow oven and cause solder balls to occur... 
This is bad for your design when installed in the electronics application it typically must work in because solder balls are conductive and can cause shorts between pins and exposed pads or get loose in vibration and bounce around to other assemblies and cause the same sort of problems.

Also I have found that filling vias with solder does not improve their thermal transfer characteristics very much... Solder is not that great a heat conductor. Copper is the best thermal conductor typically for our PCB applications... so thicker copper plating in the holes or copper filled vias are bound to be far superior for thermal transfer reasons than filling the vias with solder. 



William Brooks, CID+




_____________________________________________________________________________
Scanned by IBM Email Security Management Services powered by MessageLabs. For more information please visit http://www.ers.ibm.com

This email is intended only for the use of the party to which it is addressed and may contain information that is privileged, confidential, or protected by law.  If you are not the intended recipient you are hereby notified that any dissemination, copying or distribution of the email or its contents is strictly prohibited.  If you have received this message in error, please notify us immediately, by replying to the message and deleting it from your computer.

WARNING: Internet communications are not assured to be secure or clear of inaccuracies as information could be intercepted, corrupted, lost, destroyed, arrive late or incomplete, or contain viruses.  Therefore, we do not accept responsibility for any errors or omissions that are present in this email, or any attachment, that have arisen as a result of e-mail transmission.
_____________________________________________________________________________

______________________________________________________________________
This email has been scanned by the Symantec Email Security.cloud service.
For more information please contact helpdesk at x2960 or [log in to unmask] 
______________________________________________________________________

---------------------------------------------------------------------------------
DesignerCouncil Mail List provided as a free service by IPC using LISTSERV 16.0.
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF DesignerCouncil.
To temporarily stop/(restart) delivery of DesignerCouncil send: SET DesignerCouncil NOMAIL/(MAIL)
For additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815
---------------------------------------------------------------------------------