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November 1999

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Subject:
From:
"Hiteshew, Michael" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Tue, 23 Nov 1999 15:18:20 -0500
Content-Type:
text/plain
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text/plain (61 lines)
Hi Stella,

> ----------
> From:         Neyman, Stella[SMTP:[log in to unmask]]
>
> PCB designers please help!
> We are designing 2 oz copper PCB with the plain which suppose to carry 30
> amps.
> 1. How many vias would be necessary to transfer 30 amps from one side to
> another?
>
        With a via diameter of .031", with 1 oz copper in the via (ignoring
overplating with solder),
        the equivalent cross section in square mils is 2Pi(15)(1.4) = 132
sq. mils. This cross section
        can carry approximately 4 amps with a 10 degree (C) rise in
temperature above ambient.
        Therefore, you would need at least 8 vias this size. The more vias
you use beyond 8, the lower
        the impedance from side to side. Low impedance from the power supply
(and ground!) is
        important for the function of analog and high speed circuits.

> 2. Does the via need relief (wagon wheel) or may be solid?
>
        Thermal relief is only necessary when soldering a lead to a pad or a
plated through hole.
        Thermal reliefs are not required (or even desirable) on power or
ground vias. Some people
        use them to verify by sight, when viewing artwork plots,  what is
connected to the plane.

> 3. What should be the via size (0.020" or 0.031")
>
        With an .020" via, the equivalent cross section in square mils is
2Pi(10)(1.4) = 88 sq. mils.
        Each via will then carry about 3.5 amps with a 10 degree (C) rise in
temperature above ambient.
        Obviously, you'll then need to use 9 vias (minimum) to carry 30
amps.


 Michael Hiteshew
 Lockheed Martin Launching Systems
 [log in to unmask]
 (410) 682-1259

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