Etching Fine line

The capability to etch fine-line circuits is also improved through the use of thinner copper foils. Although electrical considerations can limit the use of very thin foils on innerlayer circuits. 

There has been a continual increase in size and complexity of PCBs with a concurrent reduction in conductor and hole dimensions. Conductors can be less than 250 p.m wide some boards have conductors less than 75 pm wide. Multilayer boards greater than 2.5 mm thick having hole sizes less than 250 pm are being produced. This trend may, however, eventually cause the demise of the subtractive process. It is difficult to etch such fine lines using 35-pm copper foils, though foils as thin as 5 pm are now available. It is also difficult to electroplate holes having high aspect ratio. These factors may shift production to the semiadditive or fully additive processes. 

Optical Exposure. Multicomponent LB films were prepared from solutions of novolac/PAC varying in concentration from 5-50 wt% PAC, and transferred at 2.5 -10 dyn/cm. The films were composed of 15 - 20 monolayers, with an average film thickness of 30 nm, as measured by ellipsometry. Exposures were performed with a Canon FP-141 4 1 stepper (primarily g-line exposure) at an exposure setting of 5.2 and with a fine line test reticle that contains line/space patterns from 20 to 1 pm (40 to 2 pm pitch). They then were then developed in 0.1 - 0.2 M KOH, depending on the PAC content The wafers received a 20 min 120°C post development bake to improve adhesion to the Cr. Finally, the Cr was etched in Cyantek CR-14 chromium etchant, and the resist and Cr images were examined by SEM. 

Dry etching is a well established pattern-transfer technique in IC technology, characterized by a very good ability to pattern fine lines and by a high fidelity of pattern transfer [31]. It has become an important complementary technique for fabricating microstructures, after it has been adapted to the specific requirements of very deep etching. 

Bare die and other chip devices are attached with electrically conductive or nonconductive adhesives to ceramic substrates having defined circuit patterns produced by thin-film vapor deposition and photo etching of metals or by screen printing and firing of thick-film pastes. W ith recent advancements in fine-line printed-circuit boards, adhesives are also finding use in attaching bare die to PWBs, a technology known as chip-on-board (COB). 

Etching. It involves removal of metals and dielectrics and may include both wet and dry processes. Copper can be etched with cupric chloride or other isotropic etchants, which limit the practical feature sizes to more than two times the copper thickness. The uniformity of etching is also critical for fine line feature circuits. New anisotropic etching solutions are being developed to extend the fine line capability. 

As consumer products become smaller and offer more services to the user, one frequent consequence is that they contain increasing numbers of physically smaller components. A typical example of this trend is the hearing aid where the emphasis is on providing a more powerful and efficient device which will improve the wearer s quality of life. The Acuris P hearing aid from Siemens incorporates a moulded interconnect device (MID) made from Ticona s Vectra E820i LDS liquid crystal polymer (TCP). This novel plastic permits complex, fine-line circuit patterns to be laser-etched and then plated. The MID itself, whose function is to connect the microphone module to other electronic components in the unit, is approximately 3 mm wide and 25 mm long. 

Copper foil profile is important for etching of fine-line circuits. Figures 7.23 through 7.26 illustrate the difference between standard- and low-profile foils. As can be seen in those photos. 

Aramid-reinforced laminate and prepreg allow fast microvia hole formation and at the same time maintain the performance characteristic of a smooth surface for fine-line conductor imaging. The ablation speed of non-woven (aramid) laminates and prepregs is close to that achieved when using nonreinforced materials such as resin-coated foil, dry film, or liquid dielectrics. Since aramid laminates are very stable, they allow the fabrication of doublesided, very thin, etched innerlayers, which are then pressed to a multilayer package in a single... 

Half-etching yields a few benefits. Window formation is made easier, and formation of fine-line conductors is also made easier. H2O2/H2SO4 etchant is often used for this purpose for even thickness. As mentioned previously, this half-etching procedure is also becoming common for Yag laser users. 

Kickelhain, X, New Excimer Laser Technology—Ultra Fine Lines (15 pn) without Etching, poster presentation at Electronic Circuits World Convention 8, Tokyo, Japan, September 1999. 

Another foil type offered by laminate supphers, reverse-treated foil (RTF), offers an advantage for producing fine lines. The RTF copper has adhesion promoter applied to both sides and is classified by 4562 as code R (reverse-treated bond enhancement [cathode side] stain-proofing on both sides).This approach provides advantages to imaging fine lines. When the copper tooth is reversed, the fabricator can improve fine definition by allowing the etch chemistry process to stop at the surface of the laminate. 

Fine-line boards in high-volume production may require special fine-hne etchants, high-resolution photoresists, thin-clad laminates, controlled plating distribution, and thin base foil processing. Care must be taken to balance etching and thickness factors. In some cases, fine-line etchant additives have actually made cleanout more difficult in spaces of 0.003 in and narrower. 

There are two basic etchant needs to be met. The first is traditional foil etching for print and etch, plate/tent and etch, and pattern plate and etch. Virtually all processes in the United States and Europe use constant-rate systems for alkaline ammonia or cupric chloride etchants for this purpose. The second need is developing technology for specific precision very-fine-line etching—including foil thinning and thin metallization clearout for HDI constructions and fine features. (See Sec. 34.7 for additional discussion and mention of additional chemistries that may be useful for these applications.)... 

The basic process of chemical etching to form featnres has been studied and modeled extensively. However, the best sdentific knowledge can only proceed to an nnderstanding of factors influencing the process. The practical execntion of the process to mannfacture useful circuits contains elements of experience in the best practice of several processes and choices in the selection of materials and process variations to achieve technically sonnd, cost-effective, and manufacturable results. In order to make fine-line drcnit products, it is necessary to nnder-stand the process fundamentals so that the limitations may be understood and then overcome. The intent of this section is to briefly review the nnderlying sdence and discnss factors and practices for precision etching. 

Don Ball, The Surface Mechanics of Fine-Line Etching, Printed Circuit Fabrication, vol. 21, no. 11,1998. 

Possible fine-line capability is illustrated for each copper thickness in. Fig. 62.4 Currently 18- and 35- m copper foils are standard. Copper foils 12 /rm and 9 /rm thick are becoming the new standard to enable finer pattern etching. Thinner copper foils are available in sput-tered/plated adhesiveless laminate materials. The fine-trace etching capability depends very much on the manufacturer, especially the exposure process and etching process. 

Ferric chloride or cupric chloride solution is recommended for use as the etching chemicals for flexible circuits, especially for high-density circuits. An alkaline solution has a higher process speed, but is not good on fine circuits because of unstable etching rates. As an alkaline solution makes some chemical attacks on polyimide surfaces, it should not be used for fine line requirements. 

There are appropriate correction factors for the etching rates of each combination. Between conductors, patterns, thickness, and etching, solutions should be introduced for the photo masks, based on the actual trials, especially for the multiple parallel fine lines very common to high density flexible circuits. Usually, there are remarkable differences in etching factors between middle conductors and edge conductors, as shown in Fig. 63.13. 

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