CircuiTree - January 2009 - (Page 18) Tech Talk By Karl Dietz Fine Lines in High Yield (Part CLX) Fine Line Etching Revisited – Part B P revious Tech Talk articles1,2 looked at the limitation of forming fine line circuits due to undesirable lateral etching (undercut), not just in the print and etch process, but also in pattern plating, panel plate/tent and etch, and even in semiadditive processing. It appears that neither photoresist resolution nor plating capabilities but rather etching presents the ultimate limit for these traditional circuitizing techniques. This column, the second of a twopart series, looks at ways to extend the subtractive technology to achieve finer features. Some technical advances described here have found commercial acceptance whereas others don’t appear to have gained traction. There are several problem areas associated with etching, some interrelated, some independent (see Figure 1). Benefits from process remedies may be seen in one or more of these problem areas. The first problem area shown in Figure 1 was addressed in Part A3, and the latter three are dealt with here in Part B. Most of the top-to-bottom etch variation (see Figure 2) stems from the so called puddling effect, which causes a slower etch rate near the center of the top side of the panel due to liquid buildup, which is not the case on the bottom side because of the faster liquid run-off facilitated by gravity. The practice of flipping the panel halfway through the etcher doesn’t actually eliminate the puddle effect but cuts it in half by spreading it over both surfaces. The use of suction bars, alternating with spray bars on the top side of the etcher4 as a means of removing the puddle by mimicking gravity on the panel top side, has improved etch uniformity significantly (see Figure 3). Another technique that is being practiced to reduce the puddle effect is the use of a spray pressure profile whereby the etchant stream enters the spray bar at the center, which is where the center of the board travels. The spray pressure is highest at the center, tapering off at both ends of the spray bar. Similar results are achieved by timing the sprays on/off, synchronized with the January 2009 • circuitree.com board movement, to deliver more spraying action to the center of the board. Figure 4 summarizes ways to minimize uneven shadowing of the board surface due to obstructions to the spray, which can lead to uneven etching. Modern conveyorized etchers apply these techniques. Because conveyor wheels obstruct sprays, so-called conveyorless transport systems have Etching Problems • Top to bottom variation due to puddling effect • Etch variation due to obstructions in the way of the spray pattern • Etch rate differences due to circuit pattern lay-out • Undesirable lateral etching (etch undercut) Figure 1 Problem Areas in Etching Reducing Top to Bottom Variation (Due to Puddle Effect) • Panel flip in etcher • Use of suction bars on top side (Pill, Germany) • Use of spray bars with pressure profile • Use of timed sprays become popular. They are not really conveyorless but most or all wheels have been either replaced by traveling ledges supporting rigid boards on both edges or edge grips for thinner panels that hold the panel under slight tension. If conveyor wheels are being used, they are spoked wheels as opposed to solid wheels to minimize the obstruction, and the wheels are staggered to randomize the obstruction. If the etcher features support guides for thin panel transport, these guides are also randomized (for example, by oscillating guy wires). The circuit design pattern also introduces etch rate variability as copper etches slower in narrow etch channels versus wide open copper areas (Figure 5). If several circuit lines run parallel and are separated by equal width, narrow spaces, the outside lines will etch faster than inside lines and will therefore become narrower, Reduce Etch Rate Differences Due to Circuit Patterns • Use of “dummy lines” to protect external lines of multiple line arrays from over-etching • Use of pulsed spays (Impulse Process, L.P. Chemie). Increased etch rate in narrow spaces from 0.3 micron/sec to 0.8 micron/sec. • Combined chemical & electrochemical etching (Obducat, Sweden) Figure 2 Measures to Reduce the Puddle Effect Figure 5 Minimizing Etch Rate Differences Due to Circuit Patterns Figure 3 Illustration of an Etchant Extraction System (Source: Pill) Reducing Etch Variation Due to Obstructions to Spray Patterns (Shadowing) By Randomizing or Avoiding Obstructions • Use of “conveyorless” transport (edge grip, no wheels) • Use of spoked wheel (vs solid wheels) • Use of staggered wheel array • Randomize thin panel support guides (e.g. oscillating guy wires) 3 Functional Conductor Lines. Outside Lines Etch Faster Than the Center Line. 3 Functional Conductor Lines, Flanked by Two “Dummy Lines.” Dummy Lines Etch Faster Than the Functional Lines. All Functional Lines Etch at the Same Rate. Figure 4 Reducing Etch Variation Due to Shadowing Figure 6 Addition of Nonfunctional Lines to Eliminate Pattern-Related Etch Rate Differences 18 http://www.circuitree.com
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