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48 The PCB Magazine • January 2015 1. Minimization of undercut and increase of fine line yields (Figure 1) 2. Ease of handling as opposed to working with ultra-thin copper foils 3. Adhesion of the half-etched foil is usually greater than the adhesion of the ultra-thin foil to the resin materials as well as the plated copper in the SAP (semi-additive process). In subtractive etching, as the copper is etched down, there is lateral removal of the copper as well. If one can minimize the amount of copper foil that must be etched (either through thinner copper foils, semi-additive processing, copper foil half-etch) undercut can be minimized and finer lines and spaces achieved. It is well understood that the three most fun- damental metrics of organic interconnections and substrates are imaging feature sizes, hole formation technology and size, and the plat- ing types and thicknesses used for interconnec- tions. These same parameters have been used for nearly four decades to quickly quantify the capability of a fabricator to profitably produce traditional boards. The ability to image conduc- tor lines and, perhaps even more important, the insulating airspace between them, is considered a key characteristic. With surface mount com- ponents, a dramatic decrease in plated via hole diameter requirements occurred and, as a result, via holes have become simple vertical intercon- nections. Now, under competition from laser drilling, both drill bit and machine technology have driven mechanical holes' capability much smaller. When microvia technology was intro- duced in the mid-1990s, high-density intercon- nect really took hold into the mainstream of printed circuit fabrication. But What About Fine Lines? True, the via, whether blind or through-hole, is critical for the board design. And with HDI technology leading the way to increased rout- ing density, finer lines and spaces are required. While there are several means to achieve finer lines and spaces, the author will present the half-etch process for this month's Trouble in Your Tank. FINE LINES AND SPACES WITH HALF-ETCH PROCESSES continues Figure 1: example of etch factor and undercut. (source: Dr. Karl Dietz) trouBlE in your tank

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