In addition, if you have trace extended to the pad, you can print more solder paste (using large opening) to artificially provide thicker paste volume using thinner stencils. The surface tension of the solder will "pump" additional volume to the pad symmetrically (easy to level the device). A trick documented long ago in mid 90s (forget which SMT book got image of the layout... was used in extreme cases).
jk
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Phil Bavaro
Sent: Tuesday, January 11, 2011 5:27 PM
To: [log in to unmask]
Subject: Re: [TN] Trace Widths Entering SMT
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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