Hi Alan,

I see you had many responses now that I have had the opportunity to check my Technet folder. You are probably wishing you never asked the questions at this point but, I wanted to alert you to a thermal study done by one of our engineers concerning heat slugs several years back.

In a nutshell, the results of the thermal modeling concluded that covering the thermal pad on the board completely with soldermask has a negligible impact on heat transfer. Unfortunately I do not have the study to show you to back this up. 

Covering the pad entirely with solder mask works fine when the device pin is not electrically connected. However, that tends to be rare. The issue then becomes how do you make the connection from device to PCB if the pin needs to be electrically connected (not connected internally to other pins). The solution that works for us was to leave the thermal pad corners exposed. 

Please note that I am only speaking for my local design team and not on behalf of my entire company.

Hoped this helped.

Best Regards,
Rick

BTW, I believe North American sparrows circle left while Australian sparrows circle right. Don't know what other sparrows do.



-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Alan Young
Sent: Friday, February 22, 2013 12:23 PM
To: [log in to unmask]
Subject: Re: [TN] 48-pin QFN via-in-pad solder slug problem.

Hi folks,

Please forgive me if this topic does not belong to this group.

Let me explain my predicament.
I'm a PCB layout guy who has been asked to look at a soldering issue we are having with a 48-pin QFN 7mmx7mm [EMBER CORP - EM357-RTR], the solder slug under the part is not soldering/bonding to the PCB. The pcb footprint is made per the manufactures specification with a 5.3mm x 5.3mm solder slug pad with 9 vias finished drill size of 0.254mm. There are no soldermask features on the component or solder side over the vias.

When the board has been reflowed, all of the 48 pins solder well but the slug has not bonded to the pcb. There are signs of solder in the slug vias on the opposite side of the board. If the part is ripped off the board, the slug is clean and solder free.

I do not have any information about the paste screen setup or the oven profile.

The questions I have are;

1 - Any ideas on why this is happening?

2 - Are the vias in the slug causing this problem?

3 - Would soldermaks capping the vias on the component side help resolve this?

4 - Would plugging (no capping) of the vias with epoxy help?

5 - Do north African sparrows fly faster then European sparrows (just seeing if anybody is reading this :-)

I do have some photos and x-rays of the device if this will help.

Again any help or comments would be greatly appreciated.

Regards,

Alan Young.



This e-mail message and all attachments transmitted with it may contain legally privileged and confidential information intended solely for the use of the addressee. If you are not the intended recipient, you are hereby notified that any reading, dissemination, distribution, copying, or other use of this message or its attachments is strictly prohibited.

______________________________________________________________________
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] ______________________________________________________________________

______________________________________________________________________
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] 
______________________________________________________________________