Hi Ioan!

The reasons to offset parts from the pcb are TCE mismatch and heat
dissipation.

From your email, it looks like you have through-hole parts, so you are
concerned about getting rid of heat, presumably.  You can get rid of
heat by putting the part way up on nice long leads so they get into some
kind of airstream.  Or you can get rid of heat by trying to get it into
the ground plane of the board the device is mounted to--in which case
you want it on the board, with some kind of thermal coupling material.
There are TO-225 sized resistor packages designed for SMT, and they do
great as far as getting the heat into the board.  For ancient through
hole parts such as you are using, you may find that the data sheets
specify a minimum lead length, as most of the heat is carried away via
convection off of the leads.  Thus you could calculate the offset from
the hole spacing and minimum lead length.

It depends!

Wayne Thayer 

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Ioan Tempea
Sent: Wednesday, May 14, 2008 9:06 AM
To: [log in to unmask]
Subject: [TN] Power components standoff

Hi Technos,

 

Is there any standard that states standoff height with respect to the
PCB for power components, like 1W and more resistors? The only number
that IPC-610 and J-STD-001 advance is minimum 1.5mm. Is there anything
available that relates power to height? I mean 5W comps should be higher
than 1W, but how high?

 

I am trying to standardize component prepping...

 

Thanks,

 

Ioan


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