Hi Jeff,
Printing paste in a grid is the common approach for surface mounting of
large QFNs, but a large QFN is typically only about 10-11 mm sq. With
surface mounting of packages similar to a QFN, called a thin array plastic
package (TAPP), the body sizes we have run have been as large as 15x15mm and
the paste grid has been the approach used. The key issue is not to have too
much solder on the PCB pad ("thermal pad") that is opposed to the
package/substrate pad, or it is possible that the small contact pads will
not solder to the boards (solder pool height on the grid vs. on the contact
pads; final solder thickness). The surface tension of the molten solder
grid should allow the tops of the first pools to melt to allow the solder to
spread across the pad on the bottom of the substrate while the grid pools
spread into the areas not printed on the PCB, pulling the substrate down to
towards the PCB. Since the outer contact pads are likely to melt first, as
you note, the solder pool heights and resulting thicknesses need to be
balanced somewhat so that the molten solder on the PCB does not float the
substrate and break the solder connections on the few contact pads. If too
little solder is put on the thermal pad, spreading across the substrate pad
and the thermal pad can cause the solder on the few contact pads to squeeze
out and short to the thermal pad/substrate pad, although a 0.5mm gap is
reasonably "large".
In the design guides of some companies, and in some published work, it
is shown that approx 50% void area in the solder connection between the
substrate pad and the PCB thermal pad is allowed with little or no negative
impact to thermal sinking into the PCB or electrical resistivity between the
PCB thermal pad and the substrate pad. So, as long as the joint achieve
this and there are not shorts or opens, you may be good to go. It may take
a few stencil design iterations to achieve the optimized results. Some
folks have evaluated a fairly elegant starburst patterns of grids with
smaller openings towards the center and aperture increasing in size as you
move radially from the center out. Some grid designs print no past in the
center of the pad to allow paste spreading inward and outward. A slower
ramp in reflow may help as well.
Best regards,
Leo
Leo M. Higgins III, Ph.D.
Vice President Technical Support Operations
ASAT, Inc.
3755 Capital of Texas Highway, Suite 100
Austin, Texas 78704
ph 512-383-4593
fx 512-383-1590
[log in to unmask]
www.asat.com
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-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Jeff Brown
Sent: Wednesday, November 23, 2005 11:39 PM
To: [log in to unmask]
Subject: Re: [TN] Unusual soldering challenge
Hi Zil,
Unfortunately it's too late to revise the board this time, but I will
pass your suggestion on for next time.
Many thanks,
Jeff
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Zilber Gil
Sent: Wednesday, 23 November 2005 11:01 PM
To: [log in to unmask]
Subject: Re: [TN] Unusual soldering challenge
Hello Jeff,
We had similar components short time ago, and we create a grid in the
aperture of the stencil, similar to what Ivan suggested. The reflow pass
well, but after examine the component with X-ray we saw air trapped in
the solder (about 20% of the area). Thus, I suggest designing the pad on
the PCB the same way as the grid in the stencil.
Regards,
Gil Zilber
Technologist, Elta system Ltd.
Tel. +972-8-8577240
Mobile +972-54-4983222
http://www.iai.co.il/site/en/iai.asp?pi=17887
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Jeff Brown
Sent: Tuesday, November 22, 2005 7:46 AM
To: [log in to unmask]
Subject: [TN] Unusual soldering challenge
Hi Everyone,
We have a problem with a fairly unique component that requires soldering
to a PCB. I'm really hoping that someone has seen this before as it
looking really awkward.
The component is mounted on a piece of fiberglass about 25mm x 16mm. The
footprint consists of a ground pad which takes up 95% of the area, and
four signal pads with 0.5mm clearance from the ground pad.
The manufacturer recommends a grid of stenciled paste for the ground pad
due to the large area, but we are concerned that if the paste melts from
the outside edges in, then it will at a minimum result in flux
entrapment, and at worst, the trapped flux will boil and lift the
component, or move the molten solder causing shorts to the signal pads.
I imagine that even if we used solid paste pattern that this would
probably only make the problem worse.
Or are we just being overly pessimistic?
Any thoughts on options would be gratefully received... Currently
wondering about the possibility of applying BGA balls to the package so
that there is some clearance between the board and the component.
If anyone is interested, I can send the component datasheet to them
directly.
Thank you all in advance
Jeff Brown and Josh Vear
EOS Space Systems
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