Advisory Board

By R. Wayne Johnson, Ph.D., Auburn University
While the summer’s $4/gal. gasoline prices have now thankfully dropped, it is inevitable they will rise again. So what does this have to do with advanced packaging? A lot! While we hear lengthy discussions of alternate energy, we will continue to use oil for the foreseeable future. According to the Bureau of Transportation Statistics there were 250,851,833 highway vehicles registered in the US in 2006. If all vehicles sold today were hybrid-electric (which still use petroleum) or full electric vehicles, it would take 10+ years to replace the current fleet. Presently, only a small fraction of vehicles sold are hybrid-electric, thus oil will be a primary source of energy for many years to come. However, oil discovery and production is becoming more difficult as we deplete a limited resource. Electronics (and advanced packaging) are important for measurements during well drilling and for production management over the life of the well.

During drilling, sensors used include temperature, pressure, radiation, acoustic, resistivity and inclination. Inclination sensors provide drill operators with knowledge of where the drill bit is located — some of these wells are 5 miles deep and like humans, the drill bit takes the path of least resistance. In addition, horizontal drilling is quite common. The other sensors provide information and geological data related to oil production potential. Sensors and the corresponding electronics have operating lifetimes of 1000 hours, but are exposed to high temperature (125-300°C depending on geography and depth), high pressure, vibration, and corrosive liquids/gases. Once the well is drilled, in-well sensors are used to assist production. It is common to have multiple horizontal wells feeding one main bore. Sensors and electronics to measure pressure, temperature, and resistivity are used to balance the flow from the different horizontal feeds. These sensors are expected to last for ~20 years at high temperature and pressure in a corrosive environment. Sensors and electronics are also used for natural gas wells in the same environmental conditions.

Similar sensors and electronics are of interest for well logging while drilling and production monitoring of geothermal wells. Water either naturally occurring (like Old Faithful Geyser in Yellowstone National Park) or pumped-in, flows through fractures in hot rocks below the earth’s surface extracting heat. Well temperatures can exceed 300°C. The resulting steam is used to turn turbines and generate electricity. The hot water can also be used to heat homes. Iceland has made major investments in geothermal energy and investments in the Western United States are growing.

Returning to the discussion of hybrid and full electric vehicles; guess what they need? Advanced packaging. The electronics content of these vehicles increases to include motor drives, battery chargers, and monitoring and control electronics to name a few. As with all things vehicle related, cost is a significant factor. Even with $4/gal. gas and government incentives (tax credits to purchasers), the payback period compared to a low-cost, fuel-efficient gasoline or diesel vehicle is significant. Thus driving down the cost of hybrid and electric vehicles is important to widespread acceptance. Reliability is the other key factor in the mind of consumers. Consumer expectations are for 10 years/100,000 miles.
Power electronics are not 100% efficient, and therefore dissipate power. Thermal management is a critical design element in power electronics packaging. Topics of research and development include thermal cycle reliability of substrates and die attach, performance of lead-free solders at higher temperatures, alternates to high-lead solders for die attach, and lower cost manufacturing processes. Reactive brazed Cu on Si3N4 and direct aluminum bond substrates are potentially better alternatives to traditional direct bond copper on alumina for thermal cycle reliability. Some papers have shown poor thermal cycle performance of lead-free SAC solders with high temperatures and long hold times. Further work is required on lead-free solder alloys and reliability. In the area of alternates to high-lead solders for die attach, low-temperature sintering of micro- and nano-Ag powders is being explored is in production at one company.

Thermal management systems to keep power electronics ‘cool’ add to overall vehicle weight and consume valuable space. One alternative is to allow electronics to operate at higher temperatures. This is synergistic with the well logging and monitoring electronics already discussed. The challenges for low-cost, high-volume solutions are significant. For example, as one approaches 200°C operating temperatures, the delta to the melting point of SAC lead-free solder alloys is too small. Many common polymers and molding compounds used in packaging and higher-level assemblies are reaching their limits at 200°C for a 10-year vehicle life. There will be trade-offs between higher operating temperatures and the complexity of the thermal management system as hybrid and full electric vehicles develop and mature.

The important issue for our industry is knowing that advanced packaging will play a role in solving the world’s energy challenges.

R. WAYNE JOHNSON may be contacted at Auburn University, 162 Broun Hall/ECE Dept., Auburn, AL 36849; (334) 844-1880; [email protected]

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