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54 PCB007 MAGAZINE I FEBRUARY 2022 f i n e - l i n e i n te r p o s e r s with sub 25-40 mm lines and spaces is severely challenged and perhaps near impossible to cre- ate copper features with these dimensions. A fully subtractive process is where foil copper, plated copper, and even panel- plated copper requires a significant amount of c o p p e r to b e e tc h e d . A l o ng w i t h u n d e r c u t concerns, as well as the etch factor, one can see the limitations of such a process. Let's review some of the options for fine-line and high-den- sity applications. Microvia technolog y appeared in the mid- 1990s to allow fine-pitch area array semiconduc- tor packages to be sur- face mounted. Now, microvia technology is used not only on the surface of the board, but also to interconnect to embedded devices— both formed and inserted—and allow "any layer via" board construction for multilayer applications. e IPC Roadmap identifies where the proficiency within different global locations impacts processing operations. ere are not many technical issues between geogra- phies, but they can be significant. Sometimes the technical variation deals with what is more prevalent in portable devices produced around the world. Since a fair number of products are produced for semiconductor carrier applica- tions, the metrics on rigid interconnections reflect Japan/Asia/Europe capabilities. Build-up Film (Ajinomoto) e ABF process from Ajinomoto has taken hold in the high density and ultra-high density circuit designs. e process utilizes a dielectric film that then is metalized with a thin deposit of electroless copper. e ABF film is prepared prior to electroless copper by chemically roughening the surface (Figure 1). A brief overview of the process is detailed in Table 1. Resin Coated Copper (RCC) Of course, there are other options as well. e following process uses a material known as RCC or resin-coated copper (Table 2). RCC has been on the market for over two decades. e dielectric resin is coated onto a copper foil and does not contain any glass. While the un-reinforced resin works well with laser via formation, the RCC does tend to "move" somewhat more than rein- forced resins. Figure 1: Via formation and film roughening prior to metalization. (Source: Ajinomoto)