When I first learned how to make footprints several years ago this was given
to me a rule as well, however, ours was anything under 4 mils we had to cut
the pads back to the edge of the body. This applied mainly to soic type
packages. The reasoning was that if the heel fillet touched the package body
during assembly there was a potential for the contact to melt the case and
thus cause a short. Other companies I have worked for since and others I
have talked to about the rule did(do) not have that policy( or in some cases
had never even heard of it) and as yet I have not heard of any issues.
However As I am the "lowely librarian" :-) that does not mean that I just
wasn't in the loop.......
Thanks,
Dharma
KAW/USA Ltd.
39 Simon Street, #4
Nashua, NH 03060
603-886-8711 x212
603-881-8763 fax
Confidentiality Notice: This email and any accompanying documents contain
information that is confidential, privileged or exempt from disclosure under
applicable law and is intended for the exclusive use of the addressee. This
information is private and protected by law. If you are not the intended
recipient, or have received this email in error, you are hereby notified
that any disclosure, copying, distribution or use of the contents of this
document in any manner is strictly prohibited.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Wayne Thayer
Sent: Monday, December 11, 2006 7:57 AM
To: [log in to unmask]
Subject: Re: [TN] Question on assembly
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
---------------------------------------------------
Technet Mail List provided as a service by IPC using LISTSERV 1.8e To
unsubscribe, send a message to [log in to unmask] with following text in the
BODY (NOT the subject field): SIGNOFF Technet To temporarily halt or
(re-start) delivery of Technet send e-mail to
[log in to unmask]: SET Technet NOMAIL or (MAIL) To receive ONE mailing per
day of all the posts: send e-mail to
[log in to unmask]: SET Technet Digest
Search the archives of previous posts at:
http://listserv.ipc.org/archives
Please visit IPC web site
http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information,
or contact Keach Sasamori at [log in to unmask] or 847-615-7100
ext.2815
-----------------------------------------------------
---------------------------------------------------
Technet Mail List provided as a service by IPC using LISTSERV 1.8e To
unsubscribe, send a message to [log in to unmask] with following text in the
BODY (NOT the subject field): SIGNOFF Technet To temporarily halt or
(re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet
NOMAIL or (MAIL) To receive ONE mailing per day of all the posts: send
e-mail to [log in to unmask]: SET Technet Digest Search the archives of
previous posts at: http://listserv.ipc.org/archives Please visit IPC web
site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional
information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100
ext.2815
-----------------------------------------------------
---------------------------------------------------
Technet Mail List provided as a service by IPC using LISTSERV 1.8e
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Technet
To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL)
To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest
Search the archives of previous posts at: http://listserv.ipc.org/archives
Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815
-----------------------------------------------------
|
