Hullabaloo!! Wally, I am with you all the way.
Theoretical reasonings for siding with oblong lands are not
same as practical results. There are no signifcant gains for
using oblong lands. Both oblong and square lands provide a
good soldering result. In fact, it is more of headache to
generate oblong shapes and apply them for photoplotting as per
your response.
Some of statements " Oblong stencil openings provides better
solder paste release" are theoretical and there is only less than
5% practical truth for that statement. Major deciding factors for
good solder paste release are based upon:
- rheology and condition of solder paste being printed
- screen printing parameters such as separation speed
- stencil opening wall
- aspect ratio
- underneath stencil wipe frequency
There is no etched-in-stone rule that stencil opening shapes
must be same as smt lands. Any shape and size are
acceptable provided that it produces good soldering joints and
fillets, and there are no process defects such as solder balls,
solder bridges, solder paste skip etc...
regards
mpark
NII, Norsat International Inc
>>> "Wally Doeling (wallyd)" <[log in to unmask]> April 1,
1998 9:19 am >>>
OK, but I disagree!
We at Sequent use rectangular pads and do not see a
problem with
concentrating the solder at the joint.
We do not require that the entire pad be covered with solder
on the
final product. This allows the assembly supplier
to modify his solder stencil and make it oblong for better
release of
the paste.
The solder deposit shape does the concentration of solder
fillet
along with the wettibility of the lead. What we see is that
all of the solder flows to the solder joint and leaves very little
on
the unused portions of the pad.
The advantage of routability with the oblong pad is not
something
that adds much to the design as routing on a grid and
using 45 DEG. corners is not compatable with a round
feature.... not
a significant gain in my view.
The biggest problem with a oblong pad is that it either has
to be
drawn with the CAD system (creates a large Gerber file)
or has to be constructed as a complex flash by the
photoplotting
vendor. All of this for no gain in my view!
We did a test of oblong pads with rectangular pads on the
same
design. When we received the board, we could almost
not identify the two designs, as the square corner is
rounded in the
PCB etching process to a extent that it approaches
the oblong.
The question of pad shape vs stencil opening shape should
be
separated, and are different with most assembly houses.
Many articles have been published about different opening
shapes in
stencils. All are a advantage in each assembly house
under their processes. To think that the pad shape dictates
the
stencil opening is a bad basic assumption!
Please give me some feedback on the above. I would like to
hear your
reasoning for your position..
[log in to unmask]
-----Original Message-----
From: Gary Ferrari [SMTP:[log in to unmask]]
Sent: Tuesday, March 31, 1998 8:27 PM
To: [log in to unmask]
Subject: Re: [TN] oblong lands
Marletta,
It is preferred to have oblong lands to assist in concentrating
the
solder fillet
around the component lead. The IPC-SM-782 indicates
oblong as
optional but should
read preferred.
Regards,
Gary Ferrari
Executive Director
IPC Designers Council
(860) 350-9300 Fax (860) 350-9309
[log in to unmask]
>>> Marletta Tucker <[log in to unmask]>
03/31/98 03:13PM >>>
What is the deciding factor on whether or not an SMD parts
land is
squared off
or oblong? Could/should all SMD parts have oblong lands?
@ @ @ Thanks,
@ @ @ ///
@ * Marletta Tucker
@ @ > 1921 Hurd Dr. M/S 2-21
@@|\ _ Irving, Tx. 75038
@ | 972-518-7619
# [log in to unmask]
We have enough youth, how about a fountain of SMART?
The above views are my own, and do not represent those
of Abbott Laboratories.
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