Genny,

Looks to me that the via holes for the thermal management have sucked all of
the solder from under the QFN leaving at best 50% of the thermal pad cross
section connected? - this could be an issue. However - back to the solder
balls.

I have noticed a couple of things about this assembly. The first and most
obvious is of course the solder ball issue. More telling is actually the
extent of the voiding in the joints, together thet might be pointing to a
profiling issue?

I also noticed that there is very little solder build up on what would be a
toe fillet in a QFP on the right and top side of the package (I also notice
also that the two outside joints at the top are showing heavy metal loading
at the toe , as well as the top and bottom joints on the right side -
although less so - is this a touch up with a soldering iron?) this leads me
to suspect that the QFN is probably not co-planar with the board but has the
top right corner raised somewhat from the board, so the package may have had
some rocking movement while being reflowed as the solder melted and moved
around?

Just my thoughts, fortunately you are able to actually look at it!!

John

 
 
John Burke
 
(408) 515 4992
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Genny Gibbard
Sent: Monday, July 21, 2008 11:56 AM
To: [log in to unmask]
Subject: Re: [TN] Solder balls

The solder balls are near the arrows.  Faint grey round objects.
This device does have a heat slug, and there are plated thru holes under
it for heat transfer.

And yes, our X-ray does allow oblique viewing.  What are you suggesting
we look for?  

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Mike Sewell
Sent: July 21, 2008 12:53 PM
To: [log in to unmask]
Subject: Re: [TN] Solder balls

Genny,

Does your new X-Ray allow for oblique viewing?  Your solderballs will
stay round - vias will appear as columns.  If not you can always prop up
the board on one side but magnification/focus will change and the image
won't be as "pretty".

Mike

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Igoshev, Vladimir
Sent: Monday, July 21, 2008 1:21 PM
To: [log in to unmask]
Subject: Re: [TN] Solder balls

Hi Genny,

Here is a dummy question: Why are you talking about QFN and solder
balls? I guess what you see are kind of PTH for heat transfer, but not
solder balls.

Vladimir

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Steve Gregory
Sent: Monday, July 21, 2008 1:54 PM
To: [log in to unmask]
Subject: Re: [TN] Solder balls

Hi Genny!

Sorry it's taken me a bit to get your picture posted, but I just got
back from 2-weeks vacation and a week in Colorado going to a
IPC-771/7721 instructor certification class, so I've been catching up on
things this morning. But here is your picture:

http://stevezeva.homestead.com/files/solderballs-QFN.jpg

Myself, I wouldn't worry about them tiny solderballs. They probably
aren't going anywhere, and besides if they did, they're so small what
problem could it cause?

Steve

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Genny Gibbard
Sent: Monday, July 21, 2008 10:52 AM
To: [log in to unmask]
Subject: [TN] Solder balls

Have you ever gotten a much better analytical tool available to you, and
have it cause major panic due to what you can see now that you never saw
before?
We got a major fancy dancy brand new X-Ray.
Now we see things like the attached picture.  Steve, could I trouble you
to post it for me?  
 <<solderballs-QFN.jpg>>
Our old X-Ray would have never seen this.
Would you fail these solder balls?

It is a QFN, 0.5mm pitch, 0.3mm pad width with a heat slug.
I think trying to replace this device by hand would cause more trouble
than we see right now.

Genny 

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