Aluminium Nitride Substrate Metallisation: DPC versus AMB

An aluminium nitride substrate is only useful once it has a conductor on it, and the way that conductor is attached decides almost everything about the finished part. The ceramic itself offers a thermal conductivity of one hundred and seventy watts per metre per kelvin or more, but the interface between the metal and the ceramic can add resistance, limit the copper thickness and set the number of thermal cycles the assembly survives. Two processes dominate the market, and they are suited to different products.

Why Metallisation Decides the Result

The purpose of the metallisation is to carry current into the die and to carry heat out of it at the same time, while holding a dielectric barrier between the two. That combination of duties is why a simple adhesive is unacceptable, since organic adhesives are poor thermal conductors and degrade long before the ceramic does.

A good metallisation therefore has three properties. It bonds to the ceramic without an organic interlayer, it is thick enough to spread heat laterally under the die, and it holds that bond through thousands of temperature cycles without lifting at the edges of the pattern.

Making the Substrate First

The ceramic is produced from high purity powder that is formed into a sheet, usually by tape casting, and then fired at high temperature in a nitrogen atmosphere. The sintering schedule controls grain size and oxygen content, and both of those influence the thermal conductivity of the finished part more than any other variable.

Oxygen is the particular enemy. It dissolves in the aluminium nitride lattice and scatters the phonons that carry heat, so a material that starts at two hundred and twenty watts per metre per kelvin can fall to below one hundred if the oxygen content is not controlled. Buyers should ask for the measured value on the delivered batch rather than the catalogue figure.

<img src="https://www.gopcba.com/wp-content/uploads/2026/06/Reliabillity-testing.jpg" alt="Aluminium nitride substrate with direct plated copper pattern” />

Direct Plated Copper in Detail

Direct plated copper, usually shortened to DPC, builds the conductor by deposition. A thin seed layer is sputtered onto the ceramic, a pattern is defined by photolithography, and copper is electroplated up to the required thickness. The process borrows heavily from semiconductor fabrication, which is why the achievable line width is fine.

The advantage is resolution. Lines and gaps of fifty micrometres are routine, and the copper can be built from ten to two hundred micrometres thick, so a dense pattern with many small pads can be produced on a single substrate. The limitation is that plating thickness variation across a large panel adds up, and very thick copper becomes slow and expensive to deposit.

Active Metal Brazing in Detail

Active metal brazing, or AMB, takes the opposite approach. A braze paste containing a reactive element is printed between the copper foil and the ceramic, and the assembly is heated in a vacuum or a controlled atmosphere so that the braze reacts with the ceramic and forms a metallurgical bond. The copper is then etched into the required pattern.

Because the copper arrives as foil rather than as a plated layer, AMB reaches thicknesses of three hundred to eight hundred micrometres, which is what high current modules need. The joint also survives thermal cycling better than most alternatives, so AMB has become the standard choice for silicon carbide and insulated gate bipolar transistor modules in traction and industrial drives.

Thick Film and Thin Film Alternatives

Thick film processing prints a metal paste and fires it, producing a conductor of perhaps ten to twenty five micrometres. It is inexpensive and adequate for sensor elements and control circuits, but the paste is a composite and its conductivity is well below that of solid copper, so it cannot carry several hundred amps.

Thin film is the opposite extreme. Sputtering and photolithography give very fine features with tight tolerances, which suits radio frequency matching networks and precision resistors, but the conductor is only a few micrometres thick and the current capacity is correspondingly small.

Copper Thickness and Current Capacity

Copper thickness is chosen from the current and the acceptable temperature rise, in the same way as on any other board, but the geometry is different. On a ceramic substrate the conductor is wide and short, and the ceramic underneath removes heat from the back of the copper, so the usual charts for traces in air do not apply directly.

In practice a direct plated copper substrate carries tens of amps per conductor, and an active metal brazing substrate carries several hundred. The limit is often set by the die attach rather than by the copper, because the solder or sinter layer between the die and the conductor contributes a thermal resistance that no amount of additional copper can remove.

Reliability Testing

Thermal cycling is the test that matters. The assembly is cycled between two temperature extremes, typically minus forty and one hundred and twenty five degrees Celsius or wider, and inspected at intervals for cracks in the ceramic, delamination at the copper edge and degradation of the die attach.

Power cycling is a separate test and a harsher one, because it heats the die electrically and produces a steep gradient across the joint rather than a uniform change. Insulation testing at the rated voltage and a copper peel test complete the usual set. A supplier should be able to show results from all four rather than a single figure quoted in a datasheet.

Design Rules and Handling

Keep the copper pattern away from the substrate edge, since the edge is where the thermal gradient is steepest and where cracks initiate. Round the corners of wide conductors, because a sharp internal corner concentrates stress under cycling. Where the copper is present on both faces, keep the patterns roughly balanced so that the part does not bow during cooling.

Vias through the ceramic are produced by laser drilling or by a co-fired process, and both give a much larger diameter than an organic board would allow, so a via array is planned rather than a single hole. The relevant selection criteria appear in via in pad or plated through, and the plating side of the process is covered in electroplating and via filling.

Cost and Supplier Assessment

A bare aluminium nitride substrate is expensive, and a metallised one far more so. Direct plated copper carries the cost of vacuum deposition and photolithography, so it is priced per panel area, while active metal brazing is priced on copper thickness and on the yield of the high temperature bond. Both rise sharply with size, because the probability of a defect scales with area.

When assessing a supplier, ask for the batch thermal conductivity, the copper thickness tolerance, the peel strength and the thermal cycling result rather than a capability list. Joining methods that avoid a brittle interface also deserve attention, and the alloy questions raised in lead-free versus leaded solder apply to the die attach as much as to a conventional board.

FAQ

Is direct plated copper or active metal brazing better? Neither wins outright. Direct plated copper gives finer lines and lower cost for moderate current, while active metal brazing gives thick copper and better cycling for high current modules.

Can the two processes be combined? They can be used on the same part in principle, but the extra steps rarely justify the cost. Most designs choose one route for the whole substrate.

What substrate thickness is typical? Between a quarter of a millimetre and one millimetre. Thin substrates lower the thermal resistance and are more fragile, so the choice follows from the handling and clamping arrangement in the final assembly.