Round pads sometimes work well because
during reflow the surface tension of the molten
solder will pull more uniformly in every direction. Square pads tend to have
the end fillet larger than the side fillets, and because of this and the fact
that the paste on one end or the other may go liquidus
first, it is easier for the surface tension of the molten solder to lever up the
smaller 0402, 0201 and 00105 chips.
Another way to overcome this is to use
standard square pads, but purposely set up the pick and place machine program
to place the components at about a 30 degree angle. This way the molten solder
needs to straighten the part out first before it can lever one end or the other
up. By the time it straightens the part out both pads are in liquidus and the movement ensures contact with the molten
solder on each end of the chip, and prevents the tombstoning.
It may not completely end the issue, but it certainly reduces it.
And horseshoe-shaped paste deposits on
square pads have been known to work very well. I don’t know why. Ask your
mother.
R. Dean Stadem
Consulting Engineer
Analog Technologies
Corp./Lumagine, Inc.
11441 Rupp Drive
Burnsville, MN 55431
(952)894-9228
-----Original Message-----
From: DesignerCouncil
[mailto:[log in to unmask]] On Behalf Of
Heidel, Jeff
Sent: Wednesday, June 09, 2010
5:26 PM
To: [log in to unmask]
Subject: Re: [DC] Follow up ~ IPC
0201 land-pattern
Is
anyone using round pads for their 0201s? I have seen some artwork
recently with round pads and I was wondering what the advantages or
disadvantages would be.
From:
DesignerCouncil [mailto:[log in to unmask]] On Behalf Of Pete Waddell
Sent: Wednesday, June 09, 2010
3:30 PM
To: [log in to unmask]
Subject: Re: [DC] Follow up ~ IPC
0201 land-pattern
Fred,
I asked around a bit this morning, especially I asked Mike Buetow what he hears
from CMs. The verdict I hear is, it depends on the CM. I’m gonna do
some more research and let ya know what I find.
p.
From:
DesignerCouncil [mailto:[log in to unmask]] On Behalf Of Fred Dark
Sent: Wednesday, June 09, 2010
4:21 PM
To: [log in to unmask]
Subject: [DC] Follow up ~ IPC 0201
land-pattern
Hi All - to date what is the
recommended land pattern for an CC0201 to avoid issues at assembly...?
Regard's, Frederick Dark Jr.
Manager Senior PCB Designer
Crestron Electronics, Inc.
22 Link Drive Rockleigh N.J. 07647
P:201.750.7004 ext.11320
F:210.767.5772
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