Hi team! Ok, let me provide an alternative response to Richard's position.
We have used QFNs on many avionics products -both commercial and military
- for over 5 years. Using the IPC-9701 specification test protocols, QFNs
with soldered center pads have been demonstrated to be as reliable as BGAs
under -55C to +125C thermal cycling conditions. QFNs do require processing
focus as the solder paste print and reflow profiles are critical to having
acceptable solder joint integrity and process yield success. One of the
critical parameters to having acceptable solder joint integrity is the
soldering of center pad on the QFN packages. The soldered center pad
creates a large solder joint that can offset the CTE mismatch - several
industry studies have shown that QFNs without soldered center pads have
lower solder joint integrity than their counterparts. I recommend you
look at two reference sources: (1) the IPC-7093 standard which covers the
Design and Assembly of Bottom Terminated Components which includes QFNs;
(2) The NASA DoD Lead-free Solder Project did extensive testing covering
thermal cycle, vibration and drop shock with a test vehicle that included
QFNs. The link to those reports is:
http://teerm.nasa.gov/nasa_dodleadfreeelectronics_proj2.htm
Read thru the information and then decide if QFN component technology is
applicable for your product. Good Luck.
Dave Hillman
Rockwell Collins
[log in to unmask]
From: "Stadem, Richard D." <[log in to unmask]>
To: <[log in to unmask]>
Date: 08/07/2013 07:13 AM
Subject: Re: [TN] QFN solder voiding
Sent by: TechNet <[log in to unmask]>
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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