Hi Wayne - the molten solder interacts with the organic "plug" material
resulting in outgassing that causes random, nonpredictable voiding which
can be problematic in some designs.
Dave
"Thayer, Wayne - IIW" <[log in to unmask]>
Sent by: TechNet <[log in to unmask]>
09/07/2011 03:48 PM
Please respond to
TechNet E-Mail Forum <[log in to unmask]>; Please respond to
"Thayer, Wayne - IIW" <[log in to unmask]>
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Subject
Re: [TN] QFN with solder slug
Has anyone tried just plugging without overplating? It is so little area
that I can't imagine the lost soldering surface area will mean anything
and it should be a lot cheaper and faster to plug than "vippo".
Wayne
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Robert Kondner
Sent: Friday, September 02, 2011 4:10 PM
To: [log in to unmask]
Subject: Re: [TN] QFN with solder slug
Julie,
What size vias are you filling and capping? You mention these are vias in
pads, I would guess these are fairly small vias.
Do you have a picture of one of your QFN locations where you have used
this process?
Thanks,
Bob K.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Julie Silk
Sent: Friday, September 02, 2011 1:48 PM
To: [log in to unmask]
Subject: Re: [TN] QFN with solder slug
We use a lot of QFNs, and manufacture some for our own use. We have
stopped using open vias -- no matter what the manufacturer says, the
soldering is inconsistent and adding solder to compensate for wicking away
can lead to the other issue of the ground pad lifting the part and causing
opens. Overprinting the groundpad (or pinpads) can also lead to
connection between ground pad and pin pad solder and siphoning of the pin
pad solder into the groundpad joint, causing opens. We use the fill with
epoxy and cap plate over method (which we call vippo, via in pad plated
over). This adds to the cost of the board because there are additional
drilling and plating steps, but we get consistent results, better quality
and the heat transfer is better. We don't plate-shut , which I would
guess is pretty specialized and expensive for thru vias. Dave's followup
sounded more like vippo. We never use the cap one side method. We have
reliability concerns with trapping acids in this blind hole. The issue
with voids is another good one. Definitely fewer voids with the vippo
method, but it can still be a problem. Optimization of the paste pattern
and process help. We have not tried the design to vent via diagonal
solder mask presenting in a paper at IPC2011. --Julie
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