> I have the basic question:
>  Why do we need annular ring on inner layers? If the answer is no then why
>  IPC is asking for 1 mil annular ring on Class 3 products. The problem
arisen
>  due to microBGA application.
>
>  re,
>  ken patel

Ken,

Speaking from recent experience (with polyimide laminates anyway), to have
the support of the copper from pads (even though they were un-used) on the
inner layers, would have most likely prevented the fractures that we
experienced on the inner layers of a fab that eventually shorted under power
that the assembly experienced in actual operation. Our ICT did not catch this
problem as it did not subject the assembly to the voltages that were
experienced during the operation of the assembly.

Inner layer pads will change the basic physics that occur when drilling a
laminate as far as what I've begun to learn...drill bits don't cut the same
way through different materials, and it stands to reason. The way materials
react and the forces being applied when being cut will change when going from
fiberglass (and all the different resin systems) to copper, to fiberglass, to
copper, to fiberglass, etc. as it passes through the material...

My measly 2-cents...

-Steve Gregory

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