Graham,
Here is something else to consider.  While at BlackBerry we had an issue with tombstoning that could be traced to unequal sizes of the solderable terminations of the components that lead to unbalanced wetting forces on the two ends of the components.

Regards,
Bev

-----Original Message-----
From: TechNet <[log in to unmask]> On Behalf Of Stadem, Richard D
Sent: Thursday, May 10, 2018 4:06 PM
To: [log in to unmask]
Subject: Re: [TN] 0201 tombstoning

If the caps are very rectangular, by that I mean they do not have rounded shoulders on the endcaps, you might try purposely placing them at a slight angle, perhaps 30 degrees from the long axis. The shield may be causing a thermal imbalance through the board between the pads of caps in certain locations. Purposely placing the caps at a slight angle requires the molten solder to first straighten a rectangular part out in order to pull either end up. This slight delay is sometimes enough to allow enough solder to melt on both pads and thus prevent the tombstoning.
Look at it this way; it is much easier to pull up the end or side of a large rectangular bed than it is to pull up any corner.
If the corners of the caps have rounded corners, then this may not help at all, but it is worth trying. In fact, the rounded corners on the caps are often precisely why the caps tombstone and not the flatter chip resistors of the same size which tend to have sharper corners.
 I have had this little trick work for me many times. It does not require a huge pick/place program change, most techs can make this simple change in just a few minutes if you just want to try it on a couple of boards.


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Graham Collins
Sent: Thursday, May 10, 2018 2:20 PM
To: [log in to unmask]
Subject: [TN] 0201 tombstoning

Hey Technet

I've got a problem with 0201s capacitors tombstoning.  No resistors, just a few of the caps.  This is a lead-free build, no nitrogen, and there is an EMI shield installed on top prior to reflow (solid shield, no openings).  Thermal layout looks OK, no glaring copper imbalances. The boards were inspected prior to placing the EMI shield, placement accuracy was good, paste accuracy was good.  Some of the parts are adjacent the EMI shield side wall, but not all.

The only idea I've got to try is to reduce the size of my paste aperture.  It is 13.5x11.2 mils, on a 4 mil coated stencil, I'm pondering going down to 10x10 mils.

We don't see this with other 0201 placements, but this is the only one we are building that has an EMI shield on top.

Any other ideas?

--
Graham Collins
Senior Process Engineer
Sunsel Systems
(902) 444-7867