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72 The PCB Magazine • January 2016 acceptable limit, depending on the laminate specifications). The reasons that chemical stabil- ity is important have to do with the expected, detrimental change of electrical properties and the notion that weight loss in the form of vola- tilized breakdown products may cause blisters, delamination, and conductor breaks. Studies have shown that the curing agent dicyanamide ("dicy") in epoxy resins appears to contribute to poor chemical stability performance, so that laminate fabricators switched to non-dicy cur- ing agents such as phenol-based curing agents. However, other studies suggest that dicy can perform adequately with minor changes to the resin system. Conductive anodic filament (CAF) is a de- fect that can lead to short circuits due to electro- migration of copper ions, originating from me- tallic copper in metalized through-holes. Cop- per ions migrate along glass fibers, potentially forming a conducting path to another circuit. Such migration is facilitated by factors contrib- uting to copper corrosion and migration such as moisture, and by small gaps between the glass fiber and the resin which may be caused by poor adhesion of the resin to the treated glass surface. Current Material Technology The following is a brief summary of the ma - terial components of laminates and their func- tions: Metal Layers and Metal Coatings Copper is the metal of choice for the circuit, power and ground layers, and through-hole metallization. In the case of CCL, the copper comes in a foil that is laminated to the dielec - tric layer(s). The copper foil is either formed in an electrodeposition process (ED-foil), or is so- called rolled/annealed copper—a copper form that is preferred in flex circuits because of its flexibility. Copper foils are typically treated for enhanced topography and chemical composi- tion for adhesion to the dielectric resin. Such chemical modifications may include the deposi- tion of zinc or brass and the coating with silane coupling agents. The copper surface facing the outside may be treated with anti-tarnish such as chromium for protection against corrosion, and it may be topographically and/or chemically modified for improved dry film resist adhesion as is the case with double-treat foil. The final, circuitized circuit board outerlayer may have a metal coating such as gold, nickel/gold, tin, nickel/palladium/gold, or tin/lead to protect the copper surface from corrosion and enhance sol - derability or wire bonding. Reinforcements/Fillers The most common reinforcement is woven glass, the surface of which is typically treated for better processability and for better adhesion to the resin (e.g., with silane coupling agents). Non-woven glass reinforcements as well as or - ganic fibers such as aramids have also been used as reinforcements. They are used to improve di- mensional stability and lower CTE. Fillers serve a number of functions (e.g., to modify Dk and Df, to lower CTE and to enhance thermal conduc- tivity). In recent years there has been a demand for finer, more uniform glass weaves to improve impedance control and to enhance drillability, both mechanical as well as laser (UV, CO 2 ). The use of fillers requires dispersion know-how and surface treatment technology. The shape of filler particles is important too. To achieve low CTE in X, Y, and Z directions, one needs to employ fillers of near spherical shape. Dielectric Resin Historically, there has been a trend from low- performance resins such as phenolic/formalde - hyde resins to epoxies and to non-epoxy high- performance resins to improve electric prop- erties (lower Dk, Df), lower moisture take-up, improved chemical stability, and dimensional stability (modulus). Equally important to low Dk, Df values of the dielectric are the uniformity of these values across the entire surface of the board, and little change of Df, Dk as a function of temperature, frequency, and moisture con - tents. Low moisture take-up is desirable to avoid vapor formation (out-gassing) at high tempera- tures. Also, the presence of water typically dete- riorates the electrical properties and changes Dk and Df values as a function of moisture content. High-performance resin examples are given in several tabulations of this study: PTFE, other flu- oropolymers, BT, polyesters, polyimides, LCPs, hydrocarbons, rubbers, etc. karl's tech talk HIGH-PERFORMANCE LAMINATES