At 02:49 PM 11/17/99 -0800, Rodney Carrico wrote:

>To the best of our knowledge....
>The current carrying capacity of a blind micro-via is calculated just as you
>would for any other via or flat conductor - by finding the minimum square
>area of the conductor's cross-section, then using the IPC-2221 "Conductor
>Thickness and Width" charts.

This may be good as an approximation, but the heat dissipation will vary
for a trace depending on whether it is a surface trace or a buried one. I'd
think a via might be somewhere in between, and a blind via would be even
more like a buried trace. Fortunately, vias are typically fairly short.

>For a via, the exercise is simply to "unroll"
>the barrel of the hole, and figure out the minimum amount of copper area at
>the junction of the barrel and the pad layer.  This intersection can be the
>weakest point in the circuit in through or blind vias, unless there are
>thinner traces along the current path.  I recommend derating the final
>numbers by 50% in the case of blind laser-drilled vias, because of process
>variances from vendor to vendor in hole surface integrity and final plating.
>
>min. dia.  x  pi (3.1414)  x  Copper thickness  =  sq. area

The formula is an approximation which assumes that the copper thickness is
small compared to the diameter.

A more accurate formula would be

D - min. dia.
T = copper thickness.

A = pi * (D * T - T^2)

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Abdulrahman Lomax
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