Agreed on the necessity of balanced trace area or at least having smaller traces
than reported on the one side... I believe that having 2x20mil traces going into an
0402 pad will make it heat up slower due to the larger area, delaying the initial
solder flow. This may keep this side of the comp from adhering and cause
tombstoning as well. Is there a via involved? Solder will wick down into a barrel
and cause standing up, too.
Tim
Date sent: Fri, 2 Feb 2007 14:47:49 -0000
Send reply to: "(Designers Council Forum)"
<[log in to unmask]>,
Matthew Lamkin <[log in to unmask]>
From: Matthew Lamkin <[log in to unmask]>
Subject: Re: [DC] 0402 tomb stoning
Originally to: [log in to unmask]
To: [log in to unmask]
> AFAIK you are not wrong Mario, that sounds spot on.
>
> Clare, do you really need more than 1 trace exiting it?
> Is it an RF or high speed circuit? otherwise if it's going into high volume manufacture
> it's better being evenly balanced.
>
> If you really must have 2 traces on one side, then put 2 (or a thicker one) on the other.
>
> Matthew Lamkin.
>
> 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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