Kerem Durdag
Director of Business Development, Sensors & Advanced Packaging
Vectron International
Hudson, N.H.

Edited by Robert Repas

A growing industrial demand concerns sensors able to withstand harsher environments than that required by earlier designs. Applications like deep-well data logging or automotive oil quality sensing need devices that can withstand temperature, shock, and vibration levels that far exceed conventional MIL-STD ranges.

For example, the oil and gas exploration industries need realtime data about ever-deeper drilling environments. Deeper holes need sensors and electronics that can withstand temperatures up to 200°C and pressures up to 10,000 psi. The goal to minimize maintenance costs and device failures has led to the design of sensors and instrumentation electronics using novel packaging and sealing methods that operate at temperatures up to 250°C.

A growing assortment of sensors under development for the automotive industry produces real-time data about emissions, intake air temperature and humidity levels, tire traction, and oil pressure. Mounted under the hood and body of vehicles, these sensors typically see temperatures that vary from –40 to 180°C.

The high temperatures of these extreme environments can induce myriad failures in electronic systems that rely on organic printed-circuit-board (PCB) material. Standard silicon (Si) semiconductor devices break down as temperature climbs because of rising intrinsic carrier density and leakage current.

Interconnection reliability is always of paramount concern in designing electronics for these harsh environments. Intermetallic formations at junctions or metal migration across conductor traces at high temperatures act as catalysts for connection failures.

Mismatches in the coefficient of thermal expansion of packaging materials can result in strain and fatigue-related failure modes. Temperature-induced change in the dielectric properties of a material can produce a significant change in capacitor value.

The introduction of novel semiconductor- wafer material and new processing technologies now make harsh-environment design possible. For example, in areas of 250°C and up, design engineers can prevent leakage and latch-up problems by employing siliconon- insulator (SOI) technology. SOI devices isolate parts on the IC dielectrically rather than isolating with reverse-biased junctions as in the standard Si process.

Likewise, the use of a wideband- gap semiconductor, such as silicon carbide (SiC), ensures reliability and better per-formance in applications above 300°C. Typically found in the Class III to V group of materials, wide-bandgap devices require that current carriers have more energy to breach operating junctions. This property reduces the effect of heat upon their operation.

Some harsh-environment electronics research focuses on the materials and methods used to build electronic circuits such as the use of the organic PCB material known as FR-4 with components attached by solder. Circuits built us-ing this method limit semiconductor and SMT-type passive components to operating temperatures below 175°C. Temperatures higher than 175°C start a delaminating process that breaks down the PCB material.

Designs for harsh environments replace the organic material with an inorganic substrate such as that found in hybrid microelectronics technology. Any soldering uses high-temperature alloys. Special metal-to-metal sealing features for the sensors and the adoption of no-lead processes and design rules let designers create components that have maximum operating temperatures of 250°C.

An additional benefit derived from higher operating temperatures is the elimination of any auxiliary cooling system. The absence of a cooling system usually means a significant reduction in equipment size and weight.

Moreover, availability of hightemperature components makes it possible to put electronics close to sensors. This makes practical nextgeneration applications in harsh environments. For example, the combination of a sensing device and smart-sensor electronics creates a real-time solid-state sensor that determines oil quality in mobile and fixed equipment. The strides in standardizing these product innovations could soon bring similar developments to other harsh-environment applications.