I haven't tried to put down this part, but I did want to share my simple rule of thumb for stencil design for QFN pads:
Make the thermal pad look as much like the I/O pads as possible: Use a grid of deposits which are about the same size and overall density as the I/O pads. Probably for this part that will make the thermal significantly less than 50%, but often not much is needed. I usually end up potting almost everything we produce, so I can count on heat getting out through the potting material.
Wayne Thayer
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of stephengregory5849
Sent: Monday, August 10, 2009 9:39 AM
To: [log in to unmask]
Subject: Re: [TN] Actel QFN180 Assembly
I can tell you that the application note that Actel puts out for their
QFN180 is worthless. The reduction for the thermal pad that GA Tan is in the
ballpark, might even be a little too much. The problem with this part is
that there is such a razor thin balance between figuring out how much paste
you print for the thermal pad and getting decent I/O pad joints. The three
row I/O pad pattern make things all the more difficult. We were dealing with
single and double row QFN's without any problems until we ran across this
beast.
Oh, as far as trying to rework the part, good luck! It ain't happening. We
even paid a company who does nothing but rework for a living to try and
rework the part and it gave them fits. They were able to get the part back
down after much difficulty, but the board never worked. Don't know if the
part died during rework or if it was something else. Just know it wouldn't
program when we got the boards back.
Steve
----- Original Message -----
From: "Tan Geok Ang" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Monday, August 10, 2009 2:24 AM
Subject: Re: [TN] Actel QFN180 Assembly
Hi Grivon,
There is some guide from Actel :
http://www.actel.com/documents/QFN_AN.pdf which I don't think it will work
well. Go for a volume of 55% at the thermal pad (and a pad opening of 9mils
(5mils stencil thickness) with slight rounded radius at the corner of the
opening. What is your problem and stencil openings? With your result, you
will able to know where gone wrong. FYI, we used only two rows of QFN as of
now and lots of LGAs. Hope it help.
Regards
GA Tan
________________________________
From: TechNet on behalf of GRIVON Arnaud
Sent: Fri 8/7/2009 6:04 PM
To: [log in to unmask]
Subject: Re: [TN] Actel QFN180 Assembly
Coming back on an old post from Steve (at his former employer...).
Does anybody have good stencil rules for the assembly of multi-row QFNs and
especially triple-row such as ACTEL QFN180?
Like Steve and probably many others, we tried to find the right aperture
balance between I/Os and central thermal pad without success...
A. Grivon
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