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SMT Components Toughen Up for Rugged Applications
December 31, 1969 |Estimated reading time: 3 minutes
By Craig Hunter, Vishay Intertechnology
With the electronic content of so many systems on the rise, it's not surprising that surface mount technology is continually branching out into new applications. Even a few years ago, however, not everyone could have anticipated the extent to which SMT components have penetrated some of the more rugged applications for electronics in general. Manufacturers are using novel approaches to upgrade device capability and provide higher performance and higher reliability for these more demanding environments.
Take automotive, for example. A baseline requirement for automotive electronic components has long been a maximum junction temperature of at least 175°C. Until very recently this requirement excluded true surface mount parts from a number of these applications. How has this changed? Some discrete semiconductors that were limited to industrial applications have used internal and external package developments to make them suitable for higher-temperature sockets. A leadless and very thermally efficient surface-mount power MOSFET package modified from the original industrial-grade version with a heavier-duty epoxy die attach and molding compound can meet the 175°C maximum junction temperature requirement, for example. Another such adaptation to the automotive environment is a leadless industrial SO-8L package to which bent leads have been added; their effect is to make the package more resistant to physical stresses and therefore more flexible and resilient to the effects of temperature on PCBs and specific areas of the assembled PCB.
Passive surface-mount components are now perfectly at home in the automotive environment as well. Suppliers have recognized the increasing demand for electronic components in these applications and developed products to take advantage of this demand. You can now specify a thin film chip resistor in the 0603 case size with the required +175°C rating and an advanced rated power level of 150 mW at +85°C. In addition to its rugged temperature ratings, these modern passive devices are built to withstand humid environments with moisture resistivity specified to <0.5 % (+85 °C; 85% RH; 1000 h).
While temperature stability and high-temperature reliability are critical factors, they are not the only ones that producers need to meet. Even though there is a lot of overall space in a vehicle, size in specific applications within the car are important. You can now design-in a low-profile inductor with a less than 18 mm2 footprint and height below 10 mm. Electrical performance in these smaller inductors would still be considered very high power. Inductance values can be up to 22 µH, and the heat rating current ranges from 9 up to 44 A. Because these high-power inductors are SMT, they offer an inherently high resistance to thermal shock, moisture, mechanical shock, and vibration.
Another recent surface-mount inductor innovation uses a winged construction that allows a higher-current device to be placed on a cut-out PCB without exceeding a profile of 1.2 mm on either side of the board. Again, this enables designers to avoid compromising performance when specifying components.
Military applications can be even more rugged than automotive, and certainly the standards for reliability are as stringent as they come. Even so, SMT has a firm foothold in this demanding market with new advances like wet tantalum capacitors in true surface-mountable molded packages that allow military designers to eliminate the last through-hole component on their PCB. Ultra-high-precision foil resistors are also now available in 0603 chips offering a military-grade absolute TCR of ±0.2 ppm/°C from -55° to +125°C and any resistance value from 100 Ω to 5 kΩ. A new-generation flip chip voltage divider features dimensions of 3.81 × 1.27 mm but still achieves a TCR tracking of 0.1 ppm/°C between two resistors simultaneously etched on one piece of foil on a common substrate. This expands the options for designers of high-precision instrumentation amplifiers, bridge networks, differential amplifiers, and ratio arms in bridge circuits, medical, test, and military equipment.
Despite its ability to reduce size and weight (critical in today's medical, military, and automotive applications), surface mount technology might never displace all through-hole components in these applications, if only because a good number of components will continue to be used in circuits where there is no PCB for mounting. Considering recent component manufacturing developments, it's interesting to see how far manufacturers have come in SMT, and it will be even more interesting to see how they meet future challenges for rugged applications.
Craig Hunter is an SMT Editorial Advisory Board member and director, global Internet marketing at Vishay Intertechnology Inc. Contact him at craig.hunter@vishay.com.