(background chorus) I want my...I want my...parts void free!
Steve
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Whittaker, Dewey
(EHCOE)
Sent: Wednesday, August 7, 2013 1:47 PM
To: [log in to unmask]
Subject: Re: [TN] QFN solder voiding
Striking a familiar chord; Excellent!
Dewey
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Bob Landman
Sent: Wednesday, August 07, 2013 12:28 PM
To: [log in to unmask]
Subject: Re: [TN] QFN solder voiding
Gotta move those semiconductors, gotta make those smart phones!
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Whittaker, Dewey
(EHCOE)
Sent: Wednesday, August 07, 2013 3:11 PM
To: [log in to unmask]
Subject: Re: [TN] QFN solder voiding
Thinking you can have Quality For Nothing and your chicks for free,
leads to a career in dire straits.
Dewey
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of David D. Hillman
Sent: Wednesday, August 07, 2013 11:51 AM
To: [log in to unmask]
Subject: Re: [TN] QFN solder voiding
Hi Bob - I think we can sum up Richard's, your and my experience with
QFNs as they require good due diligence and attention to achieve good
reliability and manufacturability. The use of QFN components should be
properly investigated and characterized prior to implementation on
production product. The IPC-7093 standard is a great resource for that
task.
Dave
From: Bob Landman <[log in to unmask]>
To: <[log in to unmask]>
Date: 08/07/2013 11:54 AM
Subject: Re: [TN] QFN solder voiding
Sent by: TechNet <[log in to unmask]>
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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