Our experience with this kind of tomb stoning has shown the root cuase to
be one of "symmetry".
1. Surface tension of one pad being higher than the other due to volume
difference ( paste or unequal pad sizes).
2. oven heating issues due to the ground plane/copper stack up - slower
ramp rate might help
thanks
-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Dehoyos, Ramon
Sent: Wednesday, June 08, 2005 7:51 AM
To: [log in to unmask]
Subject: Re: [TN] mystery tombstone cause
Want to add that faster preheat temp rise is a cause for
tombstoning. So slow the oven chains down.
Ramon
-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Dehoyos, Ramon
Sent: Wednesday, June 08, 2005 9:43 AM
To: [log in to unmask]
Subject: Re: [TN] mystery tombstone cause
Hi Ben:
Two of suggestions that have been made in the forum have been less
solder paste and match pad size. Solder pulls the part over to one side when
it melts one side faster than the other side due to surface tension. By
placing less paste, the pull is less. Another recommendation was rounded far
corner pads. Finally, solder mask between pads should be of less height than
pads. If solder mask is taller than the pads, one end of the part is lifted
from that pad, which will make it easier to tombstone.
Regards,
Ramon
-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Graham Collins
Sent: Wednesday, June 08, 2005 8:52 AM
To: [log in to unmask]
Subject: Re: [TN] mystery tombstone cause
Hi Ben
A couple of suggestions.
- is the solder paste well aligned with the board? If not, it would
be consistent in direction.
- is the pick and place machine placing the parts evenly in the paste
deposits?
If the part is more in one paste deposit compared to the other, it may
lift from the other.
- have you looked at orientation going through the oven? E.G. if you
reflow a board and always get tombstones lifted on the west side, and
then run the next board rotated 180 degrees, do the tombstones match?
If so, can you run it rotated 90 degrees? Ideally you want both pads to
reflow at the same time, so if the part is oriented perpendicular to the
direction of travel you should get best results.
Pad geometry can also contribute. We have a board with 0603s where the
pads are about 50% bigger than per IPC SM782, and the tombstoning rate
is relatively high, whereas we have 0402s on properly proportioned pads
and they never tombstone.
We also find that, with our particular paste (SnPb, RMA flux), running
without nitrogen pretty much eliminates tombstones because it slows down
the wetting.
Good luck.
regards,
Graham Collins
Process Engineer,
L-3 Communications Electronic Systems Inc.
Halifax
(902) 873-2000 ext 6215
>>> [log in to unmask] 06/08/05 09:09AM >>>
Technetters,
I have several SMT board styles which give me some trouble with 0306
capacitor tombstones, and through many attempts to reduce the number
of
these defects, I have found a condition that I can not seem to
attribute
a cause to. I have found that across a board, all the tombstones tend
to
lift in the same direction (for components in the same orientation).
I have looked at the build process, but different board styles result
in
tombstones in different directions, so I believe the cause to be board
related. I have looked at solder mask registration (misregistration
occurs in various directions), power/ground plane connections by pad
(changes from location to location), and solder bumps due to HASL
(wicked pads have similar results), and nothing has led me to a root
cause.
Any ideas???
Thanks,
Ben Gumpert
---------------------------------------------------
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
-----------------------------------------------------
---------------------------------------------------
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
-----------------------------------------------------
|
