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December 2006

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From:
Wayne Thayer <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Wayne Thayer <[log in to unmask]>
Date:
Mon, 11 Dec 2006 07:56:37 -0500
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John-

Unfortunately, there are lots of exceptions nowadays to any "rule" like
this you try to set up.  There are just too many different packages and
different kinds of packages to make blanket statements.

The first package family which comes to mind for this application is
the "FN" styles:  "DFN" and "QFN".  The "N" is for no- lead (that's not
"lead" as in Pb, but as in legs).  These packages often have a large
contact under the body of the part for either thermal dissipation or
good RF ground.  I think they would be more appropriately called
"ball-less BGA's" because that is essentially what they are--the
manufacturers (and consumers) spend a considerable amount of money doing
ball attach on BGA parts, so industry has discovered that you can get
away without the ball for smaller parts under certain conditions.  These
"FN" styles all have body heights less than 6mils and you MUST put
solder beneath them to make proper electrical contact.  Other components
which violate the "rule" you are asking about are the "leadless"
devices.  These use "castellations" (vias sawn in half--see ceramic
crystal packages) or end-wrap terminations (such as most SMT resistors
and capacitors) for contact.  Again, you want solder to go beneath these
parts.

So I'm actually having trouble imagining where the "rule" might make
sense!  Perhaps it was made because of people complaining about shorts
under "FN" packages which are actually due to the manufacturers not
assigning a pin number to the big pad used for thermal/RF reasons--when
these get entered into the CAD library, many CAD operators aren't sure
how to handle them without a pin number, so maybe they left the pad off
the library model and thus the opportunity for a short occurs.

Wayne Thayer

>>> [log in to unmask] 12/9/2006 1:20 pm >>>
Hello

I was wondering if some manufacurers could comment on the following.

I have been using a footprint calculator and it recently added a new
feature.

The calculator asks for the height of the component body from the
board.

If the height is less then 6 mils then it will not allow the pads to
go
under the
body of the IC. This makes sense to me as then when the paste is
deposited
the body of the IC is not setting in the solder paste. In the old
calculator
and quite frankly in almost every decal tool or baseline I have seen
the
pads
run up under the body of the IC. I think most board deigners tend to
make
the paste layer identical to the surface mount pad.

I have not personally had problems with this. But I don't assembly
circuit
cards for a living.
I am building a lot of new decals and am running with this new thought
because
it makes sense from a common sense standpoint. However I worry about
heel
fillets.

DOES IT REALLY MATTER.

Any input will be appreciated
Thanks

John Foster

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