Hi Steve,

If you have more than enough solder for your class level, then I suggest to
print less paste on the toes of the lands/pads.  Perhaps 10% less removed
from the toes of the stencil apertures will reduce the tipping forces
exerted on the long terminations.

Bill Kunkle
MET Assocs. 

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Vargas, Stephen M
Sent: Thursday, July 24, 2014 3:58 PM
To: [log in to unmask]
Subject: [TN] Tombstoning

Hello All:

We are experiencing a high rate of tombstoning on two particular package
styles (0508 and 0612 capacitors) on an assembly here. The rest of the board
solders at a normal defect rate. Here are some of the things we have looked
at and some aspects of our process:

I've tried using two different profiles (straight ramp to peak and a ramp,
soak, spike).
I've moved the parts from our high speed chip shooter to our flexible
placement machine to optimize placement accuracy.
The pad layout (which is not an option for change due to the product having
already been qualified by our customer) is very close to the manufacturer's
recommended layout and the board finish is immersion silver.
We are printing 1:1, no aperture micro-modifications.
Stencil thickness is 6 mils. I'm concerned about moving to a thicker stencil
due to having 20 mil pitch parts on the board.
Our paste is a low residue / pin probe-able no-clean 63/37 (again not an
option for change).
We also looked at which side of the device is connected to ground, assuming
that this side of the device would heat up more slowly causing a tombstoning
condition pivoting at the non-ground side. But there was no trend indicating
that this was the case.
Solder mask height measured in between the two pads and it was determined to
be at the same height as the pads

I've attached a link to the datasheet for one of the devices for reference.
I'm wondering if the forces of physics make this part more susceptible to
tombstoning due to the terminations being on the long side of the device.
Any ideas on how we can improve our yields? Thanks.

http://www.avx.com/docs/Catalogs/licc.pdf

Regards,
Steve Vargas

If you don't have time to do it right, when will you have time to do it
over?
John Wooden

Polaris Contract Mfg Inc
15 Barnabas Rd
Marion, MA 02738
774-553-6192
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