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Hot Vias

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New for December 2020. Trade studies on how to package MMIC chips for millimeter-wave will continue into the coming decades. One thing for sure is that RF wirebonds need to be replaced with something with less inductance and radiation. Hot vias are a possibility.  Bumped flip chip is another.

Hot vias are implemented in MMIC processes to route signals to the chip backside, as opposed to ground vias (which are "cold"). This topic seems to be trending as designers try to cope with wirebond inductance in millimeter-wave designs.

The inductance of a via hole is way less than that of a wirebond, see this page on wirebond inductance and attenuation.  That is the principal attraction to hot vias,

Here's a page on via hole inductance

What are the downsides to hot vias?

  1. You will need to pattern the attachment media (epoxy, or solder)., A solder bubble in the worng place will cost you an entire assembly.
  2. the substrate you attach it to will set the maximum dissipation.  Chances are that it won't have anywhere near as good thermal conductivity compared to a copper-moly tab.

This is not a new idea. It was patented back in 2001 by Lampen, Setzko, Kazior (a Darpa progam manager since July 2020), Kizner and Wendler at Raytheon: US Patent # 6,175,287 B1. "Direct backside interconnect for multiple chip assemblies"

 

Author : Unknown Editor

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