Hi,

Please correct me if I'm wrong but 'tombstoning' occurs when the solder on
one side cools faster that on the other side and the force of the cooling
solder pulls the part in that direction. Your example, 2x20mils into one
side (side A) and 8 mils into the other (side B): Side A will cool
significantly faster than side B. The solder will be hardening on side A
when on side B the solder will still be molten. The force exerted by the
more rapidly cooling side can pull the part out of the molten side resulting
in a tombstone. 

Trace width isn't really the issue. It is one of balance. If you have 8 mils
on one side you should have 8 mils on the other, then cooling will be
balanced and no tombstones. I don't know of a circumstance where a 0402 part
would require traces any larger than 10 mils anyway.  

Best regards,
 
Mario Irigoyen
-----Original Message-----
From: DesignerCouncil [mailto:[log in to unmask]] On Behalf Of Hillman,
Clare
Sent: Friday, February 02, 2007 3:38 AM
To: [log in to unmask]
Subject: [DC] 0402 tomb stoning

We are currently in discussion with our production board assembler about
0402 components, in particular pad sizes and connecting traces.  They
have identified that our pad sizes and trace routings could potentially
cause an uneven thermal distribution between the pads resulting in the
component tombstoning.

 

We're in the process of modifying our 0402 footprint so that it meets
current IPC standards but the board assembler is also recommending that
we change the size of traces that connect to the pads.

 

e.g. one pad is connected, using an 8 thou trace, to an IC pin.  The
other pad is thermally relieved to a large ground shape using 2x20 thou
traces.  The recommendation is that we reduce the 20 thou traces to 8
thou.

 

Any advice would be gratefully appreciated.

 

Regards

Clare Hillman, C.I.D.

 


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