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January 2015 • The PCB Magazine 49 FINE LINES AND SPACES WITH HALF-ETCH PROCESSES continues Half-etch Process This really is the drive for sub-35 micron lines. And the half-etch process is one way to achieve this goal. The basic concept of half- etch is to remove a set amount of copper from the surface in as uniform a fashion as possible. By etching the copper foil down to a lower thickness, the etch factor and resulting under - cut are greatly reduced. Consequently, mini- mal copper is lost from the circuit trace. To ensure this operation is successful, both chem- istry and equipment configuration designs are critical. First and foremost, etching chemistry should etch down in the Z-axis as fast as pos- sible. Secondly, the etching equipment must be designed to move the solution across the copper foils uniformly. With respect to the half-etch process, the etching chemistry should be designed to re- move the copper in a controlled fashion, im- parting minimal roughness to the remaining copper. I have found that the desired etch rate on the copper foil (whether HTE or ED foil) for the half-etch process is 0.10–0.25 microns per second (4–10 microinches per second, or 240–600 microinches per min- ute). One can achieve higher etch rates if desired. However, there needs to be a bal- ance between chemical consumption, etch equipment compatibility and overall process control. Removing copper too fast could cause the heat to rise in the etching chamber. In addition, it is beneficial to process the in- nerlayer foils through an acidic cleaner that will remove chromate and mild soils prior to actually etching the copper. This helps to en- sure a clean virgin copper surface that permits the etching chemistry to remove copper uni- formly. Results In Table 1 the results of production runs at a large volume fabricator are listed. The objec- tive is to reduce the thickness of 12 micron cop- per foil to three micron. As shown, the results are consistent and uniform. The data points are gathered over several production shifts each week. Equipment Equipment capability is a key part of process of half-etching. And the etching equipment must continue to evolve in order to improve quality and reduce labor costs. Horizontal etch- ing equipment used for half-etch must prevent "puddling" on the top side of the layers. Failure to prevent puddling will lead to more copper being etched from the top side than the bot- tom. Summary Half-etch technology is production proven in meeting high density circuitry requirements. Thinner copper is effective in achieving sub-35 micron lines and spaces due to the total copper thickness that must be etched. PCB michael Carano is with omg electronic Chemicals, a devel- oper and provider of processes and materials for the electron- ics industry supply chain. To read past columns, or to con- tact the author, click here. Table 1. trouBlE in your tank

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