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84 PCB007 MAGAZINE I JANUARY 2023 meet specifications—especially considering the current state of expertise and technology in the United States. For U.S. manu- facturers, UHDI is very much a high-cost low-yield process. e industry has been exploring dif- ferent ways to make fine features more obtainable, but so far, if you want to have a high-volume production facility, there is no way around SAP. Although the situation puts the U.S. at a disadvantage, what if it we could skip the middle plat- ing processes entirely while still achieving UHDI level resolu- tion? is may be the PCB fabri- cator's dream, but a subtractive- driven process might make this dream a reality. Here is how a subtractive-driven process could theoretically benefit manufacturers: • More floor space • Reduced fabrication time • Less need for specialized workers • Reduction of plating chemicals in storage • Reduction of waste from plating processes • Less process maintenance If you can eliminate the need for the middle plating processes, your PCB fabrication pro- cess will become more efficient. To realize this goal, the industry would need to overcome several barriers. Revitalizing Copper Etch Chemistry To become independent from SAP, we need to obtain better etch factors. Recall that etch factor is the ratio of downward etching to side- ways etching. In the field of copper etching, the current standard etch factor is 3:1, but can go as high as 4:1 depending on the etchant being used. If we want to side-step SAP and have a subtractive approach to UHDI resolution, we need to obtain a bare minimum etch factor. Feature size requirements will depend on the final copper thickness needed (see Figure 1). As shown from the diagram, the finer the features, the bigger the jump in etch factor requirements. For example, if we want to etch finer features on panels with thicker copper layers, our etch factors must compensate for that extra etch depth. Unfortunately, regarding copper, there is currently no sufficient way to dramatically improve etch factor. While there is some room for improvement on the fluid dynamics end, to reach etch factors of 20:1 or 40:1, we will need to see an improvement in the base chemistry. An improvement could look like an entirely new etchant, or an additive to the etchants that are currently used through the industry. As I mentioned in my August 2022 column "e Etch Factor," we did get fairly close to having straight sidewalls with a PCB etchant via an additive to ferric chloride etchant that helped restrict the sideways etch. is etching pro- cess—called PERI etch—never took off in PCB manufacturing because it requires an etchant that can't be regenerated, thus significantly increasing the cost of etching. However, since we have had modest success in approaching Figure 1: Estimated required etch factor to obtain various feature dimensions on 1-ounce and 2-ounce copper.

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