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84 The PCB Magazine • September 2015 this undesirable performance. The plasticizers should be inert with regard to the photochem- istry (i.e., they should not absorb in the region where sensitizers absorb or otherwise interfere with the photopolymerization). It is also desir- able that the plasticizers have a low tendency to absorb water for improved storage stabil- ity. Plasticizers often belong to the family of phthalates or polyglycols. Polyglycols can also act as foam control agents in the developer and stripping chemistries. Fillers Fillers can be highly crosslinked polymer beads, or carbonates and silicates. They serve as tack modifiers at the resist surface and they may improve thermal shock performance in solder- mask resists. Coating Agents To improve the quality of the coating or the coating speed, certain surfactants have been used successfully. These may be polyethylene- oxides, which can also act as plasticizers and foam control agents. Adhesion Modifiers Aromatic triazoles, thiazoles, and aliphatic imidazoles can be part of the resist formulation to tailor the chemical adhesion forces between the resist and the copper surface. These com- pounds tend to form copper complexes and are also often employed as antitarnishes to protect freshly cleaned copper surfaces from rapid re- oxidation. With regard to the adhesion of the resist to the polyester coversheet (Mylar ® tack), resist adjuvants have been in use to achieve the proper balance of tack: Too little adhesion to the polyester may cause premature coversheet release during lamination or during panel han- dling before development; too much polyester tack can cause resist to peel off with the cover- sheet. Antihalation Agents Radiation, which is not absorbed by the resist, can scatter or reflect off the substrate into non-exposure areas, blurring the image and limiting resolution. Such undesired po- lymerization in non-exposure areas was first observed with silver halide film in the form of halo ghost images, thus the name antihala- tion. Ideally, antihalation agents are applied as a layer between the resist and the backing sup- port structure where they can absorb reflected radiation. However, in PWB imaging the resist touches the copper directly, and antihalation agents have to be mixed into the resist matrix. There, they are less effective and also lower the photospeed since they also absorb direct inci- dent radiation. Dyes, Pigments, and Dye Precursors Color and color contrast are resist attributes that are only remotely related to its functional- ity. The blue dye that is typically added to the resist offers good contrast to the copper color to help in the visual inspection of developed boards. The conversion of a dye precursor to a dye during exposure creates a print-out image to visualize the latent image of the polymeriza- tion pattern. The print-out image is sometimes used to verify the registration of the image rela- tive to registration holes. PCB References 1. Fine Lines in High Yields, Part XXXVII: Contact Printing Problems: Radiation in Non- Exposure Areas and Imaging of Dirt and Defects (Part 2), Karl H. Dietz, CircuiTree Magazine, Sept. 1998. 2. Fine Lines in High Yields, Part XCIV: Dry Film Photoresist Components and Their Func- tions (Part A), Karl H. Dietz, CircuiTree Maga- zine, July 2003, pg. 48. 3. Fine Lines in High Yields, Part XCV: Dry Film Photoresist Components and Their Func- tions (Part B), Karl H. Dietz, CircuiTree Maga- zine, August 2003, pg. 40. Karl Dietz is president of Karl Dietz consulting llc. he offers consulting services and tutori- als in the field of circuit board & substrate fabrication technology. to view past columns or to reach Dietz, click here. Dietz may also be reached by phone at (001) 919-870-6230. kArl'S tECh tAlk DRy FILM PHOTORESIST STRuCTuRE AND COMPOSITION continues