Mail Archives: geda-user/2013/07/21/12:02:02
Good points. I agree that environment -- shock & vibe -- also play a
role in deciding whether to stand the part up, or lay it down. I
would imagine that the amount of space you have over the board will
also play a role.
As for standing the part up, there are heatsinks which are mounted to
the PCB, and the tab is screwed to the heatsink. What do you think
about them w.r.t. the shock/vibe issue you raise? I think you have
expertise in design for extremely harsh environments, and know more
about this than I do.
The one problem with laying the part down is that in general I don't
know how to calculate the heat dissipation of a large PCB pad. With
standing parts and commercial heatsinks, the heatsink comes with a
rating allowing you to compute whether it will handle all the power
dissipated by the part. For computing the dissipation allowed by a
PCB pad I generally look around for an app note with a recommended
footprint. Maybe there is some commerical thermal analysis program
out there which can compute the thermal dissipation of an aribtrary
pad?
The good news is that this Infineon part comes in a TO-263 variant.
It is designed to lay down, and use the tab as the heat sink. I found
a TI app note which gives specs for the thermal conductivity of the
TO-263 package, and a clear drawing of the recommended footprint.
http://www.ti.com/lit/an/snva328a/snva328a.pdf
Maybe Rob should consider this package?
Stuart
On Sun, 21 Jul 2013, Bob Paddock wrote:
> On Sun, Jul 21, 2013 at 10:27 AM, Stuart Brorson <sdb AT cloud9 DOT net> wrote:
>
>> 1. Since the output is on both pin 4 and pin 8 (the tab), you can
>> stand the part upright, with the tab in the air,
>
>> 2. You can lay the part down on the board,
>
>> Which one you want to do will be largely determined by how much power
>> the part will dissipate.
>
> The other major determining factor for stand/lay, be sides space and
> power dissipation, is the system environment.
>
> If the unit is subject to high vibration levels (this is a motor boat control?)
> or high G levels (dropping the motor when mounting), standing parts
> tend to break (or be ripped in the case of G forces) their leads at
> the solder junction after a while.
> Alas no way to know how long "a while" might be.
>
> I've seen this failure most with standing electrolytic caps. Some
> have had the leads break *inside* the case so the part looked fine.
> :-(
>
> I've learned the hard way it is best to lay parts down when you can.
> Don't overlook the stress of bending the leads to get that done, as
> that can also lead to system failures.
>
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