A new method for fabricating thick-film hybrids lets designers place both logic and power circuitry on the same ceramic substrate. Developed by Welwyn Components Ltd. of Bedlington, the U.K., and AB Mikroelektronik of Salzburg, Austria, the Patchwork technology is designed to withstand the rigors of operation in automotive applications between −50°C and 150°C.
Prior to Patchwork, designers of automotive hybrid circuits needed two substrates to implement power and control circuitry. In the resulting two-hybrid approach, each substrate was mounted to a metal base plate along with any additional discrete power components. With Patchwork's single-substrate solution, packaging for a complete power-electronics control can be reduced to a single hybrid module. This saves space and improves reliability, compared to other designs (see the figure).
The new hybrid technology consists of conventional thick-film elements—silver-palladium and silver-palladium-platinum conductors—for creating small-signal circuits and thick-film copper conductors for fabricating high-current elements. The copper ink is a proprietary material developed by Welwyn and its sister company, IRC, of Corpus Christi, Texas.
Wide Range Of Thicknesses
The ink can be screen-printed in layers as thin as 25 µm for control circuits or, through a series of multiple screen printings, built up to thicknesses as high as 170 to 200 µm for power conductors and current-sensing elements. With that much copper on the substrate, traces can handle a maximum current density of 4.5 A/mm at 70°C. This value corresponds to free air conditions where the track area is greater than 10% of the substrate area. Higher current density is possible at lower percentages of substrate area.
The Patchwork technology provides an interface between the conventional thick-film elements and the 25-µm copper film. It relies on specially developed dielectrics that permit crossover and multilayer structures. Another one of Patchwork's innovative features is its use of a lead-free solder capable of operation at 150°C. The solder, a proprietary mix of tin, silver, and copper, permits the reflow soldering of components directly onto the copper circuits.
Additionally, the technology uses heat-dissipating adhesives that meet the automotive industry's high-temperature requirement. Despite its use of different materials and construction, Patchwork is fabricated using conventional thick-film production methods. The companies say that because of this, the technology is cost-competitive with existing hybrid solutions.
For additional details on Patchwork, point your browser to www.welwyn-tt.co.uk/prass1m_fr.htm. Or contact the IRC Advanced Film Division Sales and Marketing Department at (361) 992-7900 or at www. irctt.com. In the U.K., contact Welwyn at +44 1670-822 181 or at [email protected]. AB Mikroelektronik can be reached at +43 662 44991-0 or at [email protected].