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74 The PCB Magazine • January 2016 HIGH-PERFORMANCE LAMINATES of copper. These multilayer lay-ups are separated by separation sheets and inserted into a multi- layer press where the prepreg softens, conforms to the adjacent copper, bonds, and fully cures during the press cycle. After multilayer lamina- tion, the multilayers are taken apart (broken down), their edges are cleaned, and multilayer processing continues (drilling, through-hole metallization, outerlayer circuitization, etc.). Non-thermoset resins, or resins that have insufficient "pot life" after mixing the resin component with the curing agent, present a problem for multilayer manufacture. Such res- ins include PTFE, LCPs, polyimides and others. Bondplies for PTFE have been introduced that combine PTFE with thermoset resins. LCPs have been processed into multilayers using several different LCPs with different softening/melting points (i.e., a hierarchy of melting points) to avoid the softening (swimming) of completed, individual layers during the bonding process. Polyimides have used bondplies consisting of polyimide coated with acrylates. The high- performance, non-epoxy resins have caused concern because of special considerations in drilling and desmear operations. The desmear chemistry removes the resin drill smear from the copper of innerlayers. This chemistry has been pretty much standardized, consisting of alkaline potassium permanganate, and any oth- er special chemical requirements are a barrier to entry. Special desmear chemistry for PTFE has been in use, based on complexed, metallic so- dium (tetra-etch). Flex laminates, for all practical purposes polyimides, are produced in a different process- ing sequence. The aromatic diamines and dian- hydrides are dissolved on a solvent such as pico- line, and cast onto belts or drums where the sol- vent is evaporated, and amidization, then imidi- zation occur to form the polyimide film which is then peeled off and stabilized (stretched) for dimensional stability. The polyimide film can be treated by proprietary surface treatments for improved copper adhesion, and the copper clad laminate can be formed in sheets in a lamina- tion press. Alternatively, polyimide film and copper foils can be continuously laminated to form rolls of flex base material. Downstream processing (resist application, exposure devel- opment etching, and stripping) can also be per- formed roll-to-roll. PTFE-based high-performance laminates are also formed in a different process. The starting material is typically a PTFE aqueous dispersion. It may be mixed with fillers. The woven glass fabric is guided through a trough containing the dispersion where the glass cloth is coated with the dispersion. The coated glass cloth then trav - els up a treatment tower where it reaches a tem- perature at which the water evaporates and the PTFE dispersion sinters into a contiguous film. This process can be repeated until the proper resin/glass ratio is reached. The CCL roll stock is then cut up into sheets, interleafed with copper foil and laminated in a specialized lamination press which is capable of reaching the desired high lamination temperature. A continuous fabrication process for CCL was developed by GIL Technologies and is worth mentioning here because of its technical merits, although the technology is not practiced anymore, according to the information avail- able to the author. The process involves the use of a thermoset resin (thermosetting polymer al- loy—TPA), the components of which are mixed in-line without the use of solvent. The result- ing composition is coated onto a moving band of copper foil. A glass fabric is then rolled onto the resin coated copper, another coating of resin is applied, the second copper foil is then rolled on to cap off the laminate. The uncured laminate then travels through a curing zone and is wound up in a roll from where it can be processed into sheets. Some of the advan - tages of the process include low manufacturing investment cost (no prepreg manufacture, no treatment tower), and the ability to mix the di- electric layer components in-line (i.e., no resin shelf-life issues). PCB 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