Alumina Ceramic PCB Manufacturing

The Workhorse of Ceramic Boards

When a laminate can no longer take the temperature, the voltage or the heat flux, the design moves to a ceramic substrate. Among the ceramics, alumina is the one used most widely, because it combines a useful thermal conductivity with excellent insulation, good mechanical stability and a cost that is manageable. Aluminum nitride conducts heat far better, but it also costs considerably more, and for many power and high voltage applications alumina is simply the sensible choice.

Where the dielectric losses and the highest heat fluxes dominate, the calculation changes, but the majority of ceramic board work is still done on alumina.

What Alumina Is

An alumina board uses a high purity aluminum oxide ceramic as the insulating base, with the circuit formed by one of several metallisation processes. The purity grade is the first specification decision: ninety six percent alumina is the industrial standard and offers the best balance of performance and cost, while ninety nine percent alumina improves the dielectric behaviour and the stability for high reliability work, at a higher price.

Substrates are typically supplied between a quarter of a millimetre and one and a half millimetres thick, with panel sizes tailored to the application, and the metal layer is silver or gold for thick film circuits or copper for direct bonded constructions.

Thermal Performance in Context

Alumina conducts heat in the range of twenty to thirty watts per metre kelvin. That is modest compared with aluminum nitride, which sits at one hundred and forty to one hundred and seventy, but it is dramatically better than the alternatives that a designer would otherwise use: FR-4 conducts around a third of a watt per metre kelvin, and an insulated metal substrate around one to three.

The practical effect is that alumina lowers the junction temperature of a power device substantially compared with a laminate, extends the life of light emitting diodes and insulated gate bipolar transistors, and improves the long term stability of the system, without paying the premium that the higher conductivity ceramics demand.

Electrical Performance

The dielectric strength of alumina is more than fifteen kilovolts per millimetre, which is what makes it suitable for high voltage circuits where the insulation distance matters as much as the thermal path. Its dielectric constant is between nine and nine and a half, and its loss tangent is below one thousandth, so it behaves well at high frequency as well as under high voltage.

Crucially, those properties remain stable at temperature and over long periods, which is the advantage that an organic substrate cannot match in a demanding environment.

Mechanical and Thermal Reliability

The coefficient of thermal expansion of alumina is six to seven parts per million per degree Celsius, which is close to that of silicon. A close expansion match reduces the mechanical stress on the die attach and the solder joints as the assembly heats and cools, and it is one of the reasons ceramic substrates are used in packaging where a large silicon device has to be bonded reliably.

Alumina is also hard and wear resistant, and it tolerates thermal shock well, so it survives the rapid temperature changes of a switching power circuit. Our notes on PCB capabilities describe the ceramic processes available.

Metallisation Options

The choice of metallisation follows from the current and the precision required. Thick film, using silver or gold paste fired onto the ceramic, suits lower current circuits and is well established and economical. Thin film gives finer conductor geometry, which is useful where the circuit is dense.

Direct bonded copper is the option for high current power modules: a copper layer is bonded directly to the ceramic, and the copper thickness is chosen from the standard options of three, five or eight tenths of a millimetre according to the current and the thermal requirement. Direct bonded alumina is used widely in insulated gate bipolar transistor modules, electric vehicle controllers and high power inverters.

Design Rules

Ceramic circuits are not laminates, and the layout rules differ. The minimum conductor width and spacing are wider than on an FR-4 board, because the metallisation and the etching processes set their own limits. Via formation is restricted to mechanical drilling or laser processing, which constrains the routing options. Sharp corners and abrupt changes in conductor width concentrate stress and should be avoided, both in the circuit and at the edge of the metal pattern.

Because the processes are specialised, discussing the design with a manufacturer that runs ceramic production before the layout is finalised avoids the situation where a design is complete but cannot be built at an acceptable yield. Our PCB manufacturing group supports this review.

Quality Control and Test

Ceramic boards are inspected for dimensional accuracy and surface condition, and the metallisation is checked for adhesion, which is the property that determines whether the joint survives thermal cycling. Electrical testing verifies insulation and continuity, including the high voltage insulation that the application requires.

Reliability testing then applies thermal cycling and, where relevant, damp heat exposure, with electrical measurements taken before and after so that any degradation is visible. Our notes on PCBA testing and quality management describe how the results are recorded.

Where Alumina Is Used

Power electronics and power modules are the largest application, followed by light emitting diode and laser packaging, electric vehicle electronics, industrial automation and control, and high voltage sensing and protection circuits. The common requirement is a combination of heat removal and electrical insulation that a laminate cannot provide.

The choice between alumina and aluminum nitride then comes down to the heat flux. Alumina handles the majority of medium and high power designs; aluminum nitride is reserved for the cases where the thermal resistance of the substrate would otherwise limit the design.

Cost

The cost of an alumina board is driven by the purity of the ceramic, the thickness, the metallisation process and the copper thickness where direct bonded copper is used, and the order quantity. Prototype quantities are expensive per piece because the tooling and setup are spread over few boards, while volume production brings the price down considerably.

Compared with aluminum nitride, an alumina solution typically costs substantially less, so the question is whether the thermal performance of alumina is sufficient. If it is, using the higher conductivity material adds cost without a benefit. Our notes on quality management cover how the specification is fixed with the customer.

FAQ

What is the thermal conductivity of alumina? Between twenty and thirty watts per metre kelvin, which is far better than FR-4 or an insulated metal substrate but lower than aluminum nitride.

Which purity grade should be used? Ninety six percent covers most industrial applications; ninety nine percent is used where the dielectric performance and the stability requirements are higher.

When is direct bonded copper needed? When the circuit carries high current, as in insulated gate bipolar transistor modules and electric vehicle controllers, because the copper layer carries the current and spreads the heat.

Is alumina suitable for high voltage? Yes. Its dielectric strength exceeds fifteen kilovolts per millimetre and remains stable at temperature and over time.

Why choose alumina over aluminum nitride? Cost. Alumina costs considerably less, and it is sufficient for most medium and high power designs.

Conclusion

Alumina is the practical ceramic for power and high voltage circuits. Its combination of useful thermal conductivity, high dielectric strength, a close expansion match to silicon and a manageable cost makes it the default choice, with aluminum nitride reserved for the designs where the heat flux genuinely demands it. Good design practice for ceramic, a metallisation process matched to the current and a supplier that controls adhesion are what make the board reliable.