Selecting The Material With the geometry established, material selection is the next step. There are a number of plateable resins with varying usage temperatures (Fig. 4). Acrylonitrile Butadiene Styrene (ABS) and polycarbonate (PC) are commodity thermoplastics for good, low-cost performance in nonsoldering applications. These applications are those in which the MID employs raised contacts to interconnect with another assembly. This is instead of the traditional connectors or wires that would be soldered to the pc board. Polysulfone (PSF) and its blends are engineering-grade thermoplastics with better heat resistance, dimensional stability, and performance.
Polyethersulfone (PES), polyetherimide (PEI), liquid crystal polymer (LCP), and polyphthalamide (PPA) are all high-temperature thermoplastics for products that encounter high heat or soldering. These materials may also be used when precision molding is necessary. Another resin, syndiotactic polystyrene (SPS), suits antenna applications. Resins with wide ranges of chemical properties are available as well for destructive industrial environments. Material suppliers who have their own trade names for their thermoplastics can furnish additional information about the properties of their resins.
Resin characteristics are particularly important in high-frequency applications such as antennas. In this case, there are two considerations. The ability to mold just about any shape allows antenna designers the freedom to create three-dimensional patterns for optimal radiation performance. Yet the electrical properties of resin also influence performance. Different thermoplastic resins are available to achieve the desired dielectric and loss properties.
Questra is an SPS resin comparable to LCP-, PC-, and PPA-type resins. Thermx is a copolymer that's comparable to LCP and PC as well. Zytel is a type of PPA, and Vectra is a kind of LCP. In short, low dielectric constants produce less crosstalk in connectors. High dielectric constants, on the other hand, are desirable for smaller antennas. Low loss indexes are desired for efficient antennas, while high loss indexes are required for insulation.
For designers wishing to implement MIDs, knowledge of material options, electromechanical guidelines, and process capabilities is just the beginning. Designing molded interconnect devices requires a new state of mind. It demands that the circuit-board designer think three-dimensionally. But doing so yields new levels of design freedom for the engineer and, ultimately, leads to more-efficient, lower-cost designs.
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