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February 2013

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From:
"Kuczynski, Michael" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Kuczynski, Michael
Date:
Fri, 22 Feb 2013 12:37:07 -0500
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As a librarian and fellow PCB guy her's my 2cents..

I have a feeling the vias are robbing the solder.

Are you using the vias because the slug tied to GND? Or doing it for thermal reasons? Or both?

If neither, take them out next spin of the PCB.
If you need them, reduce the finished drill size to .008 or smaller, as well as reducing the number of vias under there to 4 or 5.

-----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.

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