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32 DESIGN007 MAGAZINE I OCTOBER 2022 Feature Q&A With Tara Dunn For this issue on ultra HDI, we reached out to Tara Dunn at Averatek with some specific questions about how she defines UDHI, more about the company's patented semi-additive process, and what really sets ultra HDI apart from everything else. Do designers want to learn a new technology? What about fabrica- tors? We hope this interview answers some of those questions that you may be having about these capabilities and what it could mean for your designs. Q: How do you define ultra HDI? What is the cutoff in mils or microns? A: at is an excellent question. At this point I think it means different things to different people depending on where their current "HDI" capabilities are. IPC has created an ultra-HDI working group and I believe the The Learning Curve for Ultra HDI definition they are working with is that to be considered ultra-HDI, a design needs to include one or more of these parameters: Line width below 50 mm, spacing below 50 mm, dielectric thickness below 50 mm, and micro- via diameter below 75 mm. Q: Averatek has developed the A-SAP™ semi- additive process, which can produce traces down into the UHDI space. Can you clear up the differ- ences between mSAP and A-SAP, and what this means to designers and design engineers? A: In general, SAP, or a semi-additive process, is a process that starts with a very thin layer of copper and then builds the trace patterns from there. One common differentiating factor in these two approaches to SAP is the starting copper thickness. Typically, copper thickness that is 1.5 microns or above would be consid- ered mSAP, or a modified semi-additive pro- cess. Because the copper is a little thicker than other SAP processes, it requires more etching, which can have impacts on trace width and space and also the sidewalls of the trace. is