NO WAY!!!
Forced convection ovens are better than IR ovens, near-IR ovens are better than high-T IR ovens. The absorptivity of the materials in an electronic assembly can range from near 1.00 to near 0.00, thus you get large thermal imbalances; one of the reasons forced convection oven were developed was for this reason.
Werner
-----Original Message-----
From: Carl VanWormer <[log in to unmask]>
To: [log in to unmask]
Sent: Wed, Jan 12, 2011 10:49 am
Subject: Re: [TN] Trace Widths Entering SMT
Novice question . . .
I was told that the heating imbalance "goes away" in the IR reflow
process, since "the whole thing" has been brought up to temperature at
"the same time".
Question: can we "mostly ignore" potential thermal imbalances caused by
asymmetrical traces shape/width for IR reflow process boards? (or have
I been lied to, again . . .)
Thanks,
Carl
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Blair Hogg
Sent: Wednesday, January 12, 2011 6:46 AM
To: [log in to unmask]
Subject: Re: [TN] Trace Widths Entering SMT
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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