Hi Steve‎,

Have you tried a 2% silver paste yet? I've done this more than a few times and it really works. In fact that's the only paste I use now for our tin/lead boards.

Steve

Sent from my BlackBerry 10 smartphone.
  Original Message  
From: Vargas, Stephen M
Sent: Thursday, July 24, 2014 2:00 PM
To: [log in to unmask]
Reply To: TechNet E-Mail Forum
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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