Why use a Ceramic Base?
Many applications need removal of heat from hot spots rather quickly and efficiently, while effectively dissipating it over the entire surface of the PCB. With epoxy-based PCB material such as FR-4, this is impossible, as epoxy has poor thermal conductivity. Therefore, using epoxy-based materials in such applications may form local hot spots, leading to a reduction in the life of semiconductor junctions.
Ceramic substrates such as beryllium oxide, aluminum nitride, and alumina each have very high thermal conductivity. Compared to the thermal conductivity of 0.8-1.1 W/m-K for FR-4, the thermal conductivity of beryllium oxide ranges from 170 to 200 W/m-K, while for aluminum nitride the range is from 140 to 180 W/m-K and for alumina the figures range from 28 to 35 W/m-K. Therefore, PCBs made of ceramic substrates are capable of distributing heat more evenly, thereby allowing components mounted thereon to function more efficiently.
Hybrids with Ceramic Base
Typical uses for PCB’s with a ceramic base are hybrids or microelectronic circuits. Hybrids may comprise multiple devices, very often in die form, which are hermetically sealed inside a metal enclosure or within an impervious conformal coating. This effectively insulates and isolates all components to a high level, protecting the constituent parts from ingress of moisture. Manufacturers fire the insulating layers and conducting tracks of hybrid substrates at 850°C for nearly 10 minutes, while curing the conductive adhesive device termination at 150°C for about two hours. In comparison, FR-4 PCBs can withstand maximum temperatures of about 80 to 200°C for short periods, with damage to PCB structure starting beyond this.
Hybrids also utilize components such as on-board resistors capable of long-term operation up to 250°C. Additionally, it is possible to trim these resistor values by adjusting them with laser trimming, while the circuitry is functionally operating. This allows dynamic tuning as a process in automated production.
Direct Copper Bonded Ceramic PCB’s
The technology involving Direct Copper Bonded (DCB) Ceramic PCB’s uses a special process that bonds the copper foil to one or both sides of the ceramic substrate at appropriately high temperatures.
Although the DCB substrate is super-thin, it has high thermal conductivity, excellent electrical isolation, high bonding strength, and fine solderability. It is possible to etch it like the regular FR-4 PCB, but it can carry much higher currents. This feature has made the DCB ceramic PCB popular as the base material for construction of interconnection technology of electronic circuits using high power semiconductors. Its capability of handling higher currents also makes it the basis of the Chip On Board (COB) technology, representing the future in packaging trend.
DCB Properties
The DCB ceramic PCB possesses some very desirable and excellent properties such as high corrosion resistance, good adhesion, excellent electrical isolation, high thermal conductivity, and high mechanical strength. Additionally, it has excellent thermal cycling capabilities along with high reliability.
As the coefficient of thermal expansion of the DCB substrate is nearly the same as that of a silicon chip, it is possible to solder the chips onto a DCB board directly, thus saving on the interface layer and labor while reducing the cost. This process reduces the solder layer, lowers the thermal resistance, and increases the yield of the finished products by reducing cavities.
Conclusion
For more information on Ceramic based PCB’s, please don’t hesitate to contact the team at PCB Global. Many of our online capabilities are displayed on our website, however, for specialised PCB requirements, please email you design files to sales@pcbglobal.comfor a rapid and competitive quote as well as advice for improving your board’s capabilities and efficiencies.
