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MARCH 2023 I PCB007 MAGAZINE 59 enhances the ability of the process to reduce lateral etch and undercut. Certainly, specific gravity of the solution is important as well. Maintaining the specific gravity within the upper range of the control limits reduces lateral etch. On the other hand, acidic etchants, such as cupric chloride, are used only on inner layers. is etchant is incompatible with metallic etch resists. However, acid etching provides a more favorable etch factor and less undercut than alkaline etching. It has been reported that con- trolling acid etchants at very low free acid nor- mality improves the etch factor 1 . ere were additional studies that compared etch factors with different etchants as well as photoresist thickness. e earlier work of T. Yamamoto, et al, 2 shows the beneficial effect of wider etch channels and thinner resist. e above referenced work also lends credence to the benefits of cupric etchant in terms of undercut vs. alkaline etchants. ere are circuit density limitations related to subtractive etching. is is a well-known fact of life. e longer it takes for the etchant to remove the unwanted copper, the greater the opportunity for undercut and reduced trace width. Moving to semi-additive processing and thinner copper foils or the use of dielec- tric films will improve the etch factor signifi- cantly. More on these processes in a future column. PCB007 Resources 1. "Fine Lines in High Yields, (Part CXXV): Fine Lines—Beyond the Limits of Semi-additive Process- ing?" by Karl H. Dietz, CircuiTree Magazine, Febru- ary 2006. 2. "Allowable Copper Thickness for Fine-Pitch Pat- terns Formed by a Subtractive Method," by Takuya Yamamoto, Takashi Kataoka, and John Andresakis, CircuiTree Magazine, June 2000, Volume 13, No. 6, pg. 112 (see also Proceedings of the Technical Con- ference, S-07-3, IPC Printed Circuit Expo, San Diego, CA, April 4-6, 2000). Michael Carano is an experi- enced executive in specialty chemicals, medical device, printed circuit boards, and semiconductor industries. To read past columns, click here. Figure 2: Over-developing due to early breakpoint. (Source: Tim Blair, Tim Blair LLC)

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