agree.  normally speaking, if you use nitrogen, the TAL time might  
need to adjust to lower and peak temp might need a bit lower...  
however, it depends on the overall parts and layout of the design.
        jk
On Jul 25, 2014, at 7:18 AM, Graham Collins wrote:

> Hi Stephen
> Two quick tests
>  - run it with Nitrogen off (assuming you are reflowing in Nitrogen)
>  - rotate the board 90 degrees on the conveyor
>
> After that I'd look at the stencil, cut the amount of paste down.  
> Given your comment about qualification I'm guessing that changing  
> the paste would involve jumping through a lot of hoops...
>
> I had a board once, same deal - 0603 parts tombstoning like crazy,  
> pads were too big and no way was that going to get fixed.  We  
> started running it in air for reflow, problem went away.
>
> regards,
>
> Graham Collins
> Senior Process Engineer
> Sunsel Systems
> (902) 444-7867 ext 211
>
> On 7/24/2014 4:57 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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