One of the planet’s most challenging applications is operating in a harsh environment like downhole drilling. Precision equipment designed for this type of application must be able to survive extreme pressure, shock and vibration, as well as offer a small form factor and long battery life. However, perhaps the biggest challenge for design engineers is how protect these functions from the impact of extreme temperatures.
Written by Jeff Watson and Maithil Pachchigar, Applications Engineers at Analog Devices.
On average, the geothermal gradient is about 25°C/km, but in some regions it can be even higher. In addition, as the global demand for energy rises, oil field service companies are increasingly motivated to develop hotter wells. As it is not possible to cool electronics in this environment, there is now an increasingly urgent demand for precision instrumentation that can operate reliably above 200°C. Designers in this field are therefore constantly pushing the limits of technology to make it possible to explore areas never attempted before.
Reliability is always paramount in design engineering, however it is even more salient in this industry because of the high cost of failure. A case in point is that it can take more than a day to retrieve and replace an electronics assembly on a drill string operating miles underground; and to operate an offshore rig can cost more than $500,000 per day, meaning a costly setback for the firm.
The aviation sector is an example of another industry with an increasing need for high temperature-resilient electronics. Today there is a growing trend towards a ‘more electric aircraft’ where the traditional centralised engine controllers are replaced with distributed control systems. This places the engine controls closer to the engine, which greatly simplifies the interconnections and reduces the weight of the aircraft by hundreds of kilograms. Meanwhile, it is increasingly common for hydraulic systems to be replaced with power electronics and electronic controls to improve reliability and reduce maintenance costs. Ideally, these control electronics need to be very close to the actuators. Both these developments produce a high ambient temperature environment.
Before high temperature rated ICs were developed and became available off the shelf, electronics designers were forced to use components above their rated specification. Whilst some standard temperature ICs may have limited functionality above specification, it is an arduous and risky endeavour with no guarantee of reliability or performance. Engineers must identify potential candidates, thoroughly test and characterise performance over temperature, and qualify the reliability of the part over a long period of time. Performance and lifetime of the part are often substantially derated and could vary considerably between manufacturing lots. This is a challenging, expensive, and time consuming process that designers would rather avoid. Additionally, as design temperatures move towards 175°C and higher, advanced packaging is necessary to achieve reliability even for short durations of time.
While there a large number of different applications for high temperature electronics, there are several common requirements in their signal chains. The majority of these systems require precision data acquisition from multiple sensors, or high throughput rates. Furthermore, many of these applications have stringent power budgets because they are running from batteries and cannot tolerate additional heat coming from the electronics. Therefore, a low power data acquisition signal chain is required, consisting of sensors, precision analogue components and a high throughput ADC.
Although HT rated ICs are now commercially available, there is still a limited selection of circuit building blocks. In particular what doesn’t yet exist on market are, low-power precision ADCs that have a sample rate higher than 100kS/s and are rated for operation above 200°C. This is a major pain point for circuit designers who need to acquire and process wider bandwidth signals or want to multiplex channels.
To meet this need, Analog Devices last year released the high temperature qualified ADC AD7981 with operation specified from −55 to +175°C. The architecture is based on Analog Devices’ proprietary charge redistribution capacitive DAC technology. The CMOS fabrication process enables excellent performance at elevated temperature partly due to the matching and tracking of these capacitors over temperature. In addition, optimisations have been made to the acquisition circuit to improve precision at high temperature.
Another key function was incorporated in the design of the AD7981, to maximises battery life in harsh environments by scaling power linearly with throughput rate, dissipating typically around 4.65mW at full throughput of 600kS/s and 70µW at 10kS/s. The AD7981 powers down automatically between conversions in order to save power. This makes the part dually suited for low sampling rate applications, even of a few Hz, and enables very low power consumption for battery-powered portable systems.
However, only half the battle is creating a high performance silicon that operates at high temperatures. Robust packaging is absolutely critical for integrated circuits to survive high temperature environments. The package must provide adequate protection from the environment and reliable interconnect to the PCB while having a form factor appropriate for the mission profile of the system. The small footprint 10-lead MSOP package of AD7981 is designed for robustness at extreme temperatures, including monometallic wire bonding and is qualified for up to 1,000 hours of operation at the maximum temperature rating.One of the major failure points in creating reliable packaging at high temperature is the wire bond. This failure has been a particular issue in plastic packaging commonly found in the industry, where gold bond wires and aluminium bond pads are the standard. Elevated temperature accelerates the growth of AuAl intermetallic compounds. These intermetallics are associated with bond failures such as brittle bonds and voiding, which can happen as quickly as hundreds of hours.
Au ball bond on Al pad, after 500 hours at +195°C (left); and Au ball bond on OPM pad, after 6,000 hours at +195°C (right)
In order to avoid these failures, over pad metallisation process is necessary to create a gold bond pad surface for the gold bond wire to attach. This monometallic system does not form intermetallics and is proven to be reliable in our qualification testing with over 6,000 hours soak at 195°C.
It is clear that some of the most challenging applications in harsh environment systems are those that face extreme high temperatures. But armed with the right components and new high temperature rated ICs, design engineers now have the power to conquer this challenge with off the shelf parts that are high precision, low power and have well qualified reliability.
A complete reference design incorporating the ADC, completely qualified for +175°C, is available at www.analog.com/CN0365.