航空航天级薄膜基板：电子可靠性的基石

In military and aerospace systems, thin film patterned substrates literally serve as the foundation of some of the most critical electronic assemblies, including RF, microwave, millimeter wave, and high-speed digital circuits.

These substrates are typically made by depositing extremely thin layers of conductive and dielectric materials on bases built from materials like alumina, aluminum nitride, quartz, or certain high-performance titanates with various dielectric constants up to a k of 4000.

The goal: to create precise, stable electrical pathways that improve electrical performance and increase reliability even under extreme conditions such as rapid temperature fluctuations, high vibration, exposure to radiation, and long mission lifetimes where maintenance is impossible. An additional benefit is that thin film circuits are often smaller and weigh less than other technologies which can be important for airborne and spaceborne systems.

This level of precision is essential for electronics embedded in low earth orbit (LEO) satellites, drones, spaceborne communications, radar payloads, navigation systems, and signal-processing electronics, where even slight impedance or phase shifts can degrade link quality, reduce antenna beam accuracy, or impair payload measurements

Thin Film Substrates

Compared to thick film or regular printed circuit boards, thin film technology allows for extremely fine detail, often with lines and spaces just a few microns wide, which helps ensure electrical performance is consistent from one part to the next.

With accurate conductor line widths and spacing, designers can control the characteristic impedance of transmission lines, coupling between RF traces, and even manage thermal loads within RF circuits.

At high frequencies, even a small change in the width of a circuit trace can affect impedance or detune a filter, so this level of control is critical. It also makes it possible to fit more circuitry into a smaller space while keeping unwanted electrical effects to a minimum.

“Functionally, thin film substrates provide the stable, high-precision electrical platform on which critical aerospace and military electronics are built. They allow complex circuits to be integrated in ways that minimize signal loss, maintain phase coherence, and reduce noise—attributes that directly impact range, clarity, and security of communications, as well as the resolution and accuracy of sensing systems,” says Scott Horton of Johanson Technology. For more than 60 Years, Johanson Technology of Camarillo, Calif. has designed and manufactured passive components and substrates including thin-film circuits, multi-layer ceramic capacitors, EMI filters, and other critical components for high reliability applications.

Military and Aerospace-Grade Versions

Compared to commercial RF substrates, those for military and aerospace are manufactured to tighter dimensional tolerances and undergo more extensive screening. This includes controlling the dielectric constant, measuring conductor thickness with micron precision, and stress testing for vibration, shock, and thermal cycling.

High-purity gold, platinum, or other noble metals are often used for conductors to ensure low resistive loss, corrosion resistance, and consistent performance over decades. The dielectric layers are also applied with precise thickness control to achieve exact impedance targets and reduce parasitics, which is critical for multi-gigahertz wireless operation.

Space-qualified thin film substrates are manufactured to far tighter tolerances and can be subjected to extensive screening/testing before integration. This includes thermal cycling, random vibration and shock testing to ensure performance.

Across all these military and aerospace applications, thin-film patterned substrates are designed and can be screened to the highest-level reliability standards, such as MIL-PRF-38534 for hybrid microcircuits and MIL-STD-883 for environmental and mechanical stress testing.

“This combination of precision, thermal performance, and rugged construction makes them one of the few substrate technologies capable of meeting the performance and reliability expectations of modern defense and space electronics,” says Horton.

Substrate Selection

Horton adds that the selection of the correct substrate is key because of the influences on the mechanical and electrical function of a design.

Johanson Technology offers several dielectric k values and different metallization schemes to meet customer needs. The substrates can be metallized or nonmetallized. Metallized substrates can be patterned to customer specifications by chemical etching, abrasive etching, and patterned plating.

Single-sided or double-sided metallization is available as well, and each side can have unique design requirements and metallization for specific uses.

Adhesion layers like titanium-tungsten or chromium help the metal stick firmly to the ceramic, while gold, copper, or nickel provide good conductivity, solderability, and compatibility with wire bonding. The thickness of the gold layer is controlled so wire bonds stay strong over time.

Attention should also be given to the coefficient of thermal expansion (CTE) and thermal conductivity parameters of circuits involved. Spaceborne applications can see large temperature swings from external sources (i.e. solar radiation) while higher power systems can create higher operating temperatures during operation. These higher temperatures must be mitigated during the design process.

Aluminum nitride, specifically, conducts heat extremely well, spreading it away from hot components and helping it last longer, while alumina offers strong mechanical stability and high voltage tolerance. Special “filled vias” inside the substrate lower thermal resistance and electrical inductance, helping both with heat transfer and with maintaining clean signal paths.

“In applications where heat conduction is critical, such as in the vacuum of space, designers must consider thermal conductivity of available substrates or use filled-vias as an alternative thermal path within a design,” says Horton.

Johanson Technology also has advanced experience incorporating vias into substrate designs with demanding requirements. Vias can be offered as either filled, through-hole or plated-through. The process involves lasering through a substrate from the top to bottom layer and then sputtering or plating over the substrate via opening. Both options are effective in routing electrical signals from one side to another.

“Plated-through vias are often recommended for low-power applications while filled vias are ideal for high-power because they help transfer heat away from the substrate. Filled-vias also offer the lowest RF inductance,” says Horton.