Dry film photoresist

Fig. 8. Protection of a plated through-hole in a PWB during etching by using a tenting dry-film photoresist.

Typical of the temporary or manufacturing aid coating systems is the RISTON dry film photoresist for printed circuit (PC) board fabrication. This was the first of these systems developed. The RISTON product stmcture and the basic steps in its use are shown in Figure 2. It consists of a photopolymer sheet laminated between a Mylar cover sheet and a polyolefin separation sheet. It is manufactured as a continuous web (see Coating PROCESSES, survey), and is suppHed in roUs of varying width and photopolymer composition. 

Fig. 2. Schematic of the RISTON dry film photoresist process, (a) Removal of polyolefin separator sheet and laminate resist to clean surface, using special laminator (b) exposure to uv source using positive or negative phototool (positive to plate negative for print-and-etch) (c) removal of the protective Mylar, which is readily removed by hand and (d) development using a special processor (3).

The microstructure was realized by a dry-film photoresist technique and based on established techniques from printed circuit board technology [142], Dry resists are available as thin films, e.g. of thickness 50 or 100 pm. The resist films are encased in other polymer materials which are later removed. The resist films can be deposited on various base materials such as silicon or polymers giving mechanical stability. Lamination is carried out with a roller laminator. Then, exposure is made and spray development without any solvents follows. The process steps can be repeated at multi-laminated structures. Closed structures can be made in this way. 

Sunfort dry film photoresist and Pimel photosensitive polymide resins. Asahi Kasei Construction manufactures Hebei ALC panels and Neoma foam insulation panels for the residential, commercial building and civil engineering... 

Ripsom, J.R. and Wopschall, R.H., Optimizing Dry Film Photoresist Application for Fine Line Innerlayers. Proceedings of ther AES Merrimack Valley Printed Circuit Workshop, March (1985). 

Wallig, L.R., Dry Film Photoresists in The Multilayer Printed Circuit Board Handbook, edited by J.A. Scarlett, pp. 111-153, Electrochemical Publications, Ayr, Scotland, United Kingdom... 

Use Cold cleaning, vapor degreasing, resins application, dry-film photoresist processing, adhesive solvent, in aerosols as solvent and vapor-pressure depressant. 

Striptron Stripper. [Dow] Inhibited methylene chloride with additives solvent stri] for dry film photoresist and screen inks. 

A method for studying the photo-induced polymerisation of photoresists and solder masks at the substrate/ photopolymer interface, using ATR and IR spectroscopy is described. The technique is especially useful for dry film photoresists and gives information on cure levels affected by exposure dose and photoresist thickness. The photoprocessing of Vacrel 8100 series solder mask and Riston 3100 series photoresists from Dupont are used as examples. 23 refs. 

Yoshimura, K. Sato, M. Dry-film photoresist materials containing hydrophilic chain-transfer agents and their precision patterning. Jpn. Kokai Tokkyo Koho IP 2006227221, 2006 Chem. Abstr. 2006, 145, 281069. 

Tseitlin, G. M. Kuznetsov, I. V. Alkali developable dry film photoresist with high thermal resistance. Russ. RU 2190869, 2002 Chem. Abstr. 2003,138, 311548. 

Dry-films, as introduced in 1970 and based on a patent of Celeste s, have largely replaced wet resist systems for the fabrication of printed circuits. Most pronounced advantages of dry-film photoresists over wet photoresists are the simplified handling and the diminished chance on pinhole formation. Furthermore, the availability of a range of well-defined thicknesses (25, 38 and 50 pm are mostly used) is especially important during electrolytic metal deposition. 

In applying dry-film photoresists, the bottom foil of the system is peeled of just before the photoresist is laminated on the substrate. The top foil of the system (polyester) remains in place until developing starts, so that it protects the resist system before and during exposure. During exposure, it also acts as a barrier to oxygen. Dry-film photoresists can be organic-solvent developable (1,1,1-trichloro-ethane) or aqueous base developable (1 % Na2C03) The latter system is preferred because of the less hazardous nature of the developer. 

The imaging for additive metalization uses a permanent dry-film photoresist. For additional layers of dielectric, an adhesion promoter is applied to the snrface copper. Figure 23.3 outlines the IBSS process. 

FACT-EV. FACT-EV (Fujikiko Advanced Chemical Technology-Etched Via Post) is from Fujikiko of Japan. As with the SSP process, the via is a solid-plated copper post. In this case, the process uses standard dry-film photoresist to define the posts and a thin-liquid dielectric to coat the plated posts. Unlike SSP, however, the process is sequential and each two sets of layers are processed on the prior layers. The process is outhned in Fig. 23.16. 

The photosensitive polymeric systems used as photoresists in the interconnect industry are either liquids or dry films formed from a liquid solution. Dry-film photoresists are the industry standard, but liquid photoresists have become widely used, particularly for fine-feature circuits. Both types of photoresist can be used for a variety of processing requirements. 

FIGURE 26.4 Dry-film photoresist components. (Reprinted with permission of DuPont Electronics, DuPont Technical Literature.)... 

The chemical and mechanical properties of dry-film photoresists have been tailored to withstand various plating solutions and acid or ammoniacal-etching solutions. They can be grouped in terms of the processing chemistry used to develop the image ... 

TABLE 26.1 Summary for Dry-Film Photoresist Types and Chemistries... 

Electrolytic Cleaning. A unique inline conveyorized tool built by Atotech uses electrolytic cleaning. - The antitamish is removed electrolytically along with oils and fingerprints in a uniform process with very httle copper removal. The surface texture is then altered by microetching and passivated prior to dry-film photoresist lamination or other coatings. 

Dry-Film Hot Roll Lamination. In this process, both temperature and pressure are used to laminate the dry-film photoresist to the panels. After stripping off the polyethylene... 

FIGURE 26.13 Contrast curve for a negative-acting dry-film photoresist, percent film thickness remaining versus exposure dose (mJ/cm ). 

Photoresist patterning has different constraints for etching and plating. For etching, a thin photoresist is desirable to maximize the etchants attack in the channel developed in the photoresist. Resolution does not limit the process, since very small spaces can be resolved in either liquid or thin dry-film photoresists. The etching process itself is key to conductor formation. 

Screen-printed vinyl resists and dry-film photoresist are the most durable for deep etching or chem-milling. A dip in a 10 percent nitric or chromic add solution wUl remove residues from the surface or edges of conductor lines that may be left on some alloys. Dilute chromic add has also been used for this purpose. Spray rinse thoroughly with deionized water after etching. 

Horak, J., Dincer, C., Bakirci, H., Urban, G., 2014. A disposable dry film photoresist-based microcapillary immunosensor chip for rapid detection of Epstein-Barr virus infection. Sensors and Actuators B Chemical 191, 813-820. 

The main applications of methyl chloroform in the electronics industry are in circuit board fabrication, where it is used to develop dry film photoresist, and in the semiconductor industry where it is used for secondary cleaning. 

---