Also, if there is a large trace going to one side of a 2 terminal device, and a 
small trace from the other side, it can cause a heating imbalance, one side will 
reflow before the other (the side with the smaller trace), and sruface tension 
could pull the part toward that pad and possibly cause a tombstone. 

Blair

On Tue, 11 Jan 2011 14:27:02 -0800, Phil Bavaro <Phillip.Bavaro@L-
3COM.COM> wrote:

>I will hazard the guess or give at least one rational why they are
>asking for this, as I have asked for similar rules in the past.
>
>When you have a pad that is edge defined and not soldermask defined,
>having a 50 mil trace come in on one side will create a bigger pad in
>effect.  This will affect the skew that occurs when the part floats on
>the molten paste during reflow.  We ask for the smaller "spokes" so that
>the swimming effect is minimized and the part stays put in the center of
>the pad.
>
>The smaller the component and pad, the more that this affects the
>alignment of the component.
>
>Think of it as a parking lot and your Manufacturing Engineer is trying
>to keep all the cars parked parallel.
>
>Hope this helps....
>
>Phil
>
>-----Original Message-----
>From: TechNet [mailto:[log in to unmask]] On Behalf Of Cote, Clint (GE
>Intelligent Platforms, US)
>Sent: Tuesday, January 11, 2011 9:58 AM
>To: [log in to unmask]
>Subject: [TN] Trace Widths Entering SMT
>
>Our Manufacturing Engineer requires us to have a maximum 0.020" trace
>entering an SMT pad.  This is regardless of the size of the pad. He will
>allow multiple 0.020" traces to the same pad. For instance he will allow
>3 0.020" traces into a pad but reject one 0.050" trace. Does this make
>any sense?
>
>Thanks,
>Clint Cote
>

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