are you using nitrogen?  low residue paste may have less tack force  
to hold it down if your oxygen level is high in the oven and or TAL  
time is high (for what ever the reason you need that to solder the  
other stuff).  check your paste ramp profile recommendation from your  
vendor, you need to get the ramp correct in order to prevent  
tombstone.  my 2 cents.
good luck.
          jk
On Jul 24, 2014, at 3:57 PM, Vargas, Stephen M wrote:

> 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
> [log in to unmask]<mailto:[log in to unmask]>
>
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