Alan,

Here is my two cents worth.  You do not want to cap the vias with soldermask.  If you must filling the via, copper is the best or epoxy is another option.  You would have to talk to your PWB supplier to see what they have to offer.  BUT, first I would suggest changing you stencil design.  The intent being to keep the solder from draining to the hole. So you will design the stencil per the IPC 7525 guidelines with a 20-50% reduction.  I would look at doing a pattern that cover the holes but retains at least 50% of the paste coverage.  I am sure your stencil provider can help.  If they cannot I can suggest contacting Bill Coleman at Photo Stencil.  I know they can help you.  He is on the Technet but he probably just returning from APEX.

 
RUSSELL NOWLAND
ALCATEL-LUCENT        
REPAIR STRATEGY & SUPPLIER MANAGEMENT
14000 Quail Spring Parkway, Suite 100
Oklahoma City, OK 73134
T: +1 405 302 1660
F: +1 405 302 1622
M: +1 405 203 0034
[log in to unmask]

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Alan Young
Sent: Friday, February 22, 2013 11:23 AM
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] 
______________________________________________________________________