A quick comment:
Adding teardrops (aka snowman pads, doubleflash) is very easy for a
fabricator to add with a CAM station (global edit). From a design
perspective it may be more difficult. Some software (CAD) requires the
teardrop placement to be specified with respect to the pad location.
If added to an edge or corner of a pad it may not end up on the
location where the trace connects to the pads. (It's getting better
though. Some design software packages are getting smarter. :-))
Now on the electrical performance side.....Be very careful if the design
is for RF! (Such as telecommunications) If the designer has tweaked
and peaked his design for HF and has modeled his design from
an EMC/EMI perspective, adding teardrops to a short trace
can greatly change the net capacitance to the point to where
it may not perform as designed. : /
It's always best to ask the customer up front or get a blanket approval.
IPC-2221 has great (suggested) designer guidelines showing teardrops
and fillets.
Dave Hoover (aka, Groovy)
-----Original Message-----
From: Glynn Shaw [mailto:[log in to unmask]]
Sent: Monday, July 10, 2000 8:29 AM
To: [log in to unmask]
Subject: Re: [TN] Tear drop question for inner layer and outer layer
Good morning Ken.
IPC authorizes tear dropping on innerlayers, so we do it automatically on
all inners, with or without a specific design note, providing there is a
note mentioning compliance with IPC...
Tear dropping inners is always better, it hurts nothing from a design
perspective and improves manufacturability. Inners move and shift and
shrink during lamination, so teardrops prevent substandard connections if
the movement is in the direction of the trace feeding the innerlayer pad.
Outers is another story. Since the outer is visible, changes we make would
alter cosmetics. We do not tear drop outers unless specifically authorized
by the customer. Outer layer registration stays where you put it so if you
do a good job of registering the image, for most holes and pad sizes, it
shouldn't be necessary to increase clearances to meet minimum annular ring
requirements. However, drill sizes keep getting smaller and smaller. When
small drills hit a glass bundle, they may be diverted from their vertical
path through the board and exit the bottom of the board in a slightly
different and random direction from where they entered the board. The
result is nice registration on one side of the board and random
misregistration on the other side of the board. This phenomenon is called
"drill wander". Tear drops will improve reliability on the small holes in
this case.
At 05:14 PM 7/7/00 -0700, you wrote:
>Is tear drop always better? Should I add as a fab note to add tear drop for
>inner and outer layer as and when needed by fab house? or have my design
guy
>add it for them?
>
>Any comments will be highly appreciated.
>
>re,
>ken patel
>______________________________________________________
>Ken Patel Phone: (408) 490-6804
>1708 McCarthy Blvd. Fax: (408) 490-6859
>Milpitas, CA 95035
>
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