The continued shrinking size of portable electronics, as well as the trend to pack more features into the same size packages, are cramping the space with which designers have to work. When circuit board separation in these devices is as small as 5 mm (0.2 inch) or less, or when component contact pad pitches are under 1.27 mm (0.05 inch), mechanical electrical connectors can be problematic—co-planarity between circuit boards is harder to maintain for connector alignment, position tolerances are more demanding, and they are more susceptible to shock and vibration. In addition, small mechanical connectors may incur higher tooling costs for tight tolerances.
To counter these difficulties, engineers at Tyco Electronics have developed an elastomeric-based connector, called STAX™, that uses a simple mechanism that not only ensures connectivity but only requires low manufacturing tolerances. Small cross-section strips (roughly 0.5-5.0 mm high and 0.9-2.5 mm wide for a dual contact-row connector) of low-durometer silicon rubber are fabricated with alternating conductive and non-conductive layers (see figure). Impregnated into the alternating conductive layers are metallic particles, so when the strips are clamped between stacked circuit boards or between a component and a board, multiple contact paths are created for reliable connections. The force of the mating components is all that is needed to maintain connectivity; no separate connector mating action is required.
John Seibert, product manager, notes the conductive particles are either proprietary gold-coated or silver-filled. The silver-filled STAX connectors are used with higher current applications with pads having flat mating surfaces. The smaller-sized population gold-coated particles are geared for chip-on-glass connections, such as connecting to an LCD display. He notes the resistance between the indium tin oxide (InSnO) contact on the glass is critical to maintain rise and fall times. InSnO is a rough semiconductor so the small particles get into the surface valleys, providing many points of contact.
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At the Design News webinar on June 27, learn all about aluminum extrusion: designing the right shape so it costs the least, is simplest to manufacture, and best fits the application's structural requirements.
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