Electrical and Thermal Requirements 12.8.1 Electrical Considerations

Electrical interfaces, based on the long history of the microelectronics industry, are the most standardized. For hermetic components feedthroughs are glass matched, glass compression, or ceramic. Glass matched seals rely on glass and metal com­binations with similar coefficients of thermal expansion to form an oxide bond that results in a hermetic seal. Compression seals rely on glass and metal combinations that have greater thermal expansion coefficients, thus allowing the creation of a com­pression seal providing hermeticity. Al2O3 ceramic seals are used as an alternative to glass. Integrated ceramic feedthroughs that are sometimes referred to as co-planar or strip-line connectors, can contain conductors designed for RF, DC, and/or ground signals.

Standard leadframes are used primarily for flatpacks although some machined housings utilize them as well. The standard leadframe consists of co-planar par­allel leads connected to a common tie bar that is used for electroplating contact. Typically, the lead portion of the leadframe is 0.010" thick and 0.015" wide and has an overall length of either 0.743" or 1.000" (including the 0.125" wide tie bar). The leads are spaced either 0.050" or 0.100" center-to-center. Special leadframes with varying dimensions such as thinner cross sections, longer overall lengths, non-standard lead spacings, etc., may be obtained. Round pins, sometimes called straight pins, are used for all package styles but are most commonly used in plug-in type packages. Round pins are typically 0.018" in diameter but other sizes are avail­able. Pin length can range from 0.100" to as long as 1.000". When round pins are used in flatpacks, they usually will have one end flattened to create a small area for wire bonding. High power leads are used to transmit high current signals through sealed conductors. These usually have copper-core under a sealing alloy like Kovar or Alloy 51. There are many materials that are able to conduct high current but many are not suitable for glass sealing.

Date: 2015-02-28; view: 1018