Hi Richard - we are on the same page except for the use of QFNs on high
performance designs. As you detailed, solder joint integrity is a function
of solder height. However, the center pad of a QFN puts another factor
into the integrity equation as the soldered surface area of the QFN makes
a big difference. The NASA DoD lead-free program is just releasing a data
set which includes a QFN with a center pad and it performed very well in
many of the test sequences. I recommend folks go look at the data and
assess how it would work on their products/use environments. Here is the
link to the NASA DoD:
http://www.teerm.nasa.gov/nasa_dodleadfreeelectronics_proj2.htm
Dave
"Stadem, Richard D." <[log in to unmask]>
12/13/2011 04:12 PM
To
"[log in to unmask]" <[log in to unmask]>, TechNet
E-Mail Forum <[log in to unmask]>
cc
"[log in to unmask]" <[log in to unmask]>
Subject
RE: [TN] TI Paste Recommendations
Yes. Dave brings up many good additional points, just as I knew he would!
J
Here at GD, and I know the same is true of many other high-rel OEMs, we
strongly discourage the use of QFN package types. Even if they can be
soldered in such a way to overcome the issues Dave and I discussed, there
often remains the issue of delta CTEs. With such small and short solder
joints (remember that H affects the modulus of elasticity directly) the
packages have been known to fail after far fewer thermal cycles than their
compliant-leaded counterparts.
From: [log in to unmask] [mailto:[log in to unmask]]
Sent: Tuesday, December 13, 2011 3:53 PM
To: TechNet E-Mail Forum; Stadem, Richard D.
Cc: [log in to unmask]
Subject: Re: [TN] TI Paste Recommendations
Hi folks -- one more comment to add to Richard's good information is that
the center pad on a QFN is the primary feature for controlling surface
tension influences for the component. If you use too much solder paste you
can cause solder joint wetting/alignment defects of the smaller outer
pads. Additionally, the amount of solder paste and the specific pattern
you use on the center pad has a large influence on how many voids you end
up with in the center pad solder joint. QFNs are a neat component
technology but have some specific assembly demands that we didn't have
before their introduction on designs.
Dave Hillman
Rockwell Collins
[log in to unmask]
"Stadem, Richard D." <[log in to unmask]>
Sent by: TechNet <[log in to unmask]>
12/13/2011 03:01 PM
Please respond to
TechNet E-Mail Forum <[log in to unmask]>; Please respond to
"Stadem, Richard D." <[log in to unmask]>
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cc
Subject
Re: [TN] TI Paste Recommendations
QFNs are very typically subjected to an aperture reduction. Usually the
peripheral pads are reduced by 10-15%, and if a large belly pad is
present, the stencil aperture reduction is typically between 40% to 60% of
the pad area.
To understand why, think of it like this:
A chip capacitor, resistor, or a leaded SOIC, QFP, etc. all have someplace
for the molten solder to flow other than the pad itself. Chips have endcap
terminations, molten solder can wet out to the edges of the pads and run
up the sides of these endcap terminations. Likewise with leaded
components. In all of these cases, the solder has somewhere to go; there
is no real constraint that would cause the surface energy of the molten
solder to go "out of bounds" in the form of solder spattering.
In the case of QFNs, if you print solder paste in a one-to-one ratio with
the pads, the molten solder has no place to wet and flow. With the
component body mass sitting on the resultant puddles or blobs of molten
solder on the belly pad and the peripheral pads as well, spattering
occurs, and that is why there are so many solder fines trapped under the
QFN part. To prevent this, the stencil aperture is reduced such that the
smaller amount of solder has someplace to flow.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Joyce Koo
Sent: Tuesday, December 13, 2011 2:40 PM
To: [log in to unmask]
Subject: Re: [TN] TI Paste Recommendations
For QFN, 7093 is the guideline you want. QFN is rather different compare
to the BGAs.
Joyce Koo
Materials Researcher - Materials Interconnect Lab
Research In Motion Limited
Office: (519) 888-7465 79945
Mobile: (226) 220-4760
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Gerald Bogert
Sent: Tuesday, December 13, 2011 3:28 PM
To: [log in to unmask]
Subject: Re: [TN] TI Paste Recommendations
December 13, 2011
Solder paste size can depend on the part manufacturer recommendations. For
example, Freescale for their BGA P013 processor recommends that the PC
board solder pad and stencil aperture diameters should generally be the
same; a 1 to 1 ratio.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of William Noel
Sent: Tuesday, December 13, 2011 2:52 PM
To: [log in to unmask]
Subject: [TN] TI Paste Recommendations
Hello all - I am working on creating a 24 pin QFN footprint for my CAD
library. Is there a valid reason for making the solder paste smaller than
the actual pad outline? I notice that TI consistently seems to recommend
a smaller paste outline than pad outline. Additionally, they recommend a
series of small square paste squares to appear on the thermal pad rather
than a single smaller square of paste. Essentially, IPC-7351
recommendations are different than the TI recommendations. Any help?
Thank you.
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