On  9 Jul 96 at 16:25, Jack Olson <[log in to unmask]> wrote:

 >As I was browsing through some previous discussions, I came across
 >the following question (yeah, I'm quoting myself). It was regarding
 >the use of Figure 3-4 in IPC-D-275 to determine an appropriate trace
 >width based on current.
 >
 >> As an example, say I need 5amps and 20 degrees over ambient is a
 >> safe assumption. The chart gives ABOUT 100 square mils, which
 >> corresponds to ABOUT a 75mil trace width on the 1oz copper curve.
 >> OK, how many of you use 75mils? How safe is this number? 
 >
 >Many of you responded (THANK YOU) but one issue was never brought up
 >and I am still curious about it. 
 >
 >According to IPC-RB-275 Table 7, a conductor width can be reduced
 >20% by a nick or scratch for a length of 0.5 inches and still be
 >acceptable, plating thickness can vary (as well as the 1oz copper
 >stock), traces can be over-etched, etc.
 >
 >So does the chart indicate that I should use 75mil aperture in my
 >CAD system, or is my goal to take all factors into account and try
 >to ensure the END RESULT is 75 mils minimum?  
 >
 >Am I splitting hairs?						Jack
 >

 Yes.
 If you're still concerned, use the line on the chart for a 10
degree rise. I've never been burned doing that. OK, bad pun, sorry.



---
Ross LaGue          < [log in to unmask] >          Dayton, Ohio

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