We do not use QFNs if at all possible. Their reliability issues are very well known and well-documented. Their use is discouraged in the military electronics/high rel industry.
And before all of you folks out there tell me "oh, we use them all the time, but you have to know how", well, we know how, but why bother?
As an industry consultant outside of GD, I have been involved in countless numbers of calls for QFN issues. I know how to set up a reliable assembly process for them, but again, why bother with all of that extra cost?
Bottom line is that there is a reduced reliability inherent with using a package that has radically different CTE than the substrate it is soldered to, they have no compliant leads to absorb some of the CTE mismatch, the wetting characteristics are difficult to balance across all of the peripheral pads, and voiding is almost always an issue.
With stand-off mitigations, the issues are much improved, but only at extra cost and it requires two stage processing.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Kerr, Bryan (UK)
Sent: Wednesday, August 07, 2013 6:53 AM
To: [log in to unmask]
Subject: Re: [TN] QFN solder voiding
Hi Wayne
Interesting. We are using the same kind of materials to process and unfortunately have many more than one to fit so hand tin is not an option. Previous work by myself has shown that extending time above liquidus makes no real difference to voiding levels. For high rel processes using mildly active fluxes it may be that commercial QFN's with tin finishes are simply not compatible if voids are to be minimised ? As you say, stronger water based fluxes would give you a different result I'm sure - opens up the process window but not an option for some.
Anyone any thoughts on how much voiding on a peripheral pad would be an issue ? Any thoughts on QFN reliability in general (with or without stand off mitigations !)
Regards
Bryan
-----Original Message-----
From: Wayne Thayer [mailto:[log in to unmask]]
Sent: 07 August 2013 12:45
To: TechNet E-Mail Forum; Kerr, Bryan (UK)
Subject: RE: QFN solder voiding
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Hi Bryan-
I've had problems with those. I think the main problem is stripping the tin oxide. I speculated that perhaps because I was using Rosin flux and Pb based soldering temperatures, it didn't strip that oxide as well or quickly as higher temperatures with more highly active aqueous fluxes. Try extending the time above liquidus.
On the other hand, if you only have to make one, you can hand tin, or even bump, the pads on the QFN, and then reflow. For me, that has always led to the most void-free joints on these parts.
Wayne Thayer
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Kerr, Bryan (UK)
Sent: Wednesday, August 07, 2013 7:38 AM
To: [log in to unmask]
Subject: [TN] QFN solder voiding
Hi Folks
I am familiar with most of the industry thoughts and standards on voiding in the centre paddle of QFN's but has anyone any experience with voiding in the peripheral pads ? From my experience this can vary fairly widely and there are no standards that I know of which would cover this - unless anyone can enlighten me ?
Bryan Kerr
Principal Quality Engineer
CMA Lab and Process Engineering
BAE Systems Maritime Services
Manufacturing Hillend
Hillend Industrial Estate
Hillend
Nr Dunfermline
Fife
Scotland
KY11 9HQ
01383-836097
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