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August 2013

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From:
Bob Landman <[log in to unmask]>
Reply To:
Bob Landman <[log in to unmask]>
Date:
Wed, 7 Aug 2013 12:53:38 -0400
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I'm 100% with Dave on this one. We mfg industrial fiberoptic communications equipment.  We can't avoid TQFNs as the laser transmitters and photodiode receiver ICs are only available in TQFN packages or die (we do not use die). 

I'm checking with the IC manufacturer as the package datasheet isn't clear about this; that we purchase gold plated padded QFN and TQFN devices.

We specify that 63/37 Sn/Pb water soluable solderpaste be used on our ENIG boards.  An older design is HASL tin/lead. 

We've been using these parts since 2002 and have never had a reliability problem with either the center pad nor peripheral pads. We specify that the peripheral board pads be slightly extended beyond the package for probing and assurance that solder is attaching to all the pads.  The center pad is the tricky part. Either a cross of paste or if Mydata paste jet is used, a thinner layer of paste is applied to the center pad or the solder on this pad will elevate the package so it won't evenly solder the peripheral pads.

We do not do RoHS nor do we trust no-clean solder flux.  Our boards are all put through a standard water wash process.  We don't want any dentrites (or tin whiskers).

Bob Landman

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem, Richard D.
Sent: Wednesday, August 07, 2013 11:50 AM
To: [log in to unmask]
Subject: Re: [TN] QFN solder voiding

I respect the information in the alternative response. If all of the steps Dave lists below are followed, the components should be reliable.
But they definitely are more problematic, so if a leaded package is available, I would choose that over the QFN. While the X-ray image may show the resultant belly pad SJ quite clearly, it cannot always tell you that you have full solder wetting on the PWB pad AND the QFN belly pad.  A laminar X-ray may be required. I have seen too many examples of a dome of solder under the component wetted to the board pad but not to the bottom of the component. This can happen if the aperture reduction that is required is too much, or if for whatever reason the solder on the peripheral pads hold the component up such that the wetting does not occur on the belly pad, or the amount of paste on the belly pad is too thin due to a printer set-up issue, or the part reflows with a slight tilt, or...........
So if there is any variation in the process, the reliability immediately goes south.

From: [log in to unmask] [mailto:[log in to unmask]]
Sent: Wednesday, August 07, 2013 9:45 AM
To: TechNet E-Mail Forum; Stadem, Richard D.
Subject: Re: [TN] QFN solder voiding

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]<mailto:[log in to unmask]>



From:        "Stadem, Richard D." <[log in to unmask]<mailto:[log in to unmask]>>
To:        <[log in to unmask]<mailto:[log in to unmask]>>
Date:        08/07/2013 07:13 AM
Subject:        Re: [TN] QFN solder voiding
Sent by:        TechNet <[log in to unmask]<mailto:[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]<mailto:[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]<mailto:[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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