you either need holes in the shield or do it twice (put on shield  
after...).  (or get a grid type of open shield and put top cap  
(mechanical snap on) after... or just metallized tape of some sort...  
IMHO.  (one of the new designer goes wild thing?   been there... it  
is one of those you can  make it at lab bench using slow ramp, but  
hard for the production oven - vib plus fast ramp on panel format).   
best of luck.
jk
On May 10, 2018, at 3:35 PM, Graham Collins wrote:

> generic caps, and 3 different part numbers (and from 2  
> manufacturers) - so unique to the caps but I don't think it is a  
> part / date code issue.
>
> I can try rotating it next run, that is true. With the EMI shield  
> on it most of the heat will be coming up thru the PCB anyway so I  
> don't think that.
>
> Locations are scattered around the board - most but not all are  
> next to the EMI shield, but it is pretty tight under there - the  
> EMI shield is only 12x18mm.
>
> Graham Collins
> Senior Process Engineer
> Sunsel Systems
> (902) 444-7867
>
> On 5/10/2018 4:30 PM, SALA GABRIELE wrote:
>> Hi,
>>
>> - kind of terminal finishing ?
>> - terminal oxidized ? (data code)
>> - try to load PCB in reflow oven by rotating 180°  (front-rear :  
>> rear to front)
>>
>> - Electrostatic Charge on chip ?
>>
>> The metal EMI could cause thermal unbalance around chips
>>
>> Have a good luck
>>
>> Gabriele
>>
>> -----Messaggio originale-----
>> Da: TechNet [mailto:[log in to unmask]] Per conto di Graham Collins
>> Inviato: giovedì 10 maggio 2018 21:20
>> A: [log in to unmask]
>> Oggetto: [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
>>
>>
>> ---
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