Photosensitive Polyimides in RF Microsystem Design

In radio frequency (RF) microsystem design—where speeds are measured in gigahertz and space in microns—even the smallest material choice can make or break system performance.

That’s where photosensitive polyimides (PSPIs) quietly shine.

These materials are helping engineers build smaller, faster, and more reliable RF systems—powering everything from communication satellites and 5G infrastructure to radar modules and wearable electronics.

What Makes PSPIs Special?

Photosensitive polyimides combine two powerful traits:

• They’re tough. PSPIs withstand high temperatures, mechanical stress, and harsh environments.

• They’re patternable. Like photoresists, they respond to UV light and can be developed into fine patterns—no dry etching or plasma tools required.

This makes them more than just insulation or protection—they become an active part of the circuit structure.

Why PSPIs Matter in High-Frequency Design

At frequencies above 30 GHz, material performance becomes critical. Poor choices can lead to:

• Signal loss

• Heat buildup

• Parasitic interference

• Device failure

Here’s how PSPIs help mitigate those risks:

Key Property	Why It Matters for RF
Low Dielectric Constant (2.8–3.5)	Reduces signal delay and unwanted capacitive effects
Low Loss Tangent (< 0.005)	Minimizes energy loss in filters and transmission paths
High Heat Resistance (>300°C)	Tolerates soldering, high power, and harsh operating conditions
Excellent Adhesion	Bonds well with materials like silicon, glass, and AlN
UV Photopatternable	Simplifies manufacturing, shortens cycles, lowers cost

Where PSPIs Make a Big Impact

1. High-Density Interconnects (RDLs)

Used in fan-out wafer-level packaging (FOWLP) and glass interposers, PSPIs act as low-loss insulating layers between signal traces and vias.

Why it works: They fill gaps cleanly and allow precise via patterning—without lasers or etching.

2. RF Filters and Couplers

In thin-film circuits operating at 26–40 GHz (Ka-band), PSPIs provide flat, uniform dielectric layers with low loss.

Result: Lower insertion loss, better signal control, and smaller device size.

3. Passivation and Solder Dams

In compact RF modules, PSPIs prevent solder bridging and protect sensitive circuits from moisture and mechanical wear.🛡️ Essential for: Medical, defense, aerospace, and high-reliability electronics.

Manufacturing Made Simple

A standard PSPI process flow:

Spin coat → Soft bake → UV expose → Develop → Final bake

No dry etching, plasma, or complex tooling needed.

• High-resolution features (down to 10 µm)

• Excellent planarization for multi-layer stacking

• Saves time, reduces rework, and cuts capital equipment costs

We have used PSPIs like HD-8820 to fabricate multilayer circuits on glass and AlN substrates—delivering performance with agility.

Challenges and How to Handle Them

Challenge	Solution
Moisture Sensitivity	Use low-absorption PSPIs or apply barrier coatings
Tight Process Window	Fine-tune bake and exposure steps; regular tool calibration
Cracking in Thick Layers	Apply multiple thin coats; optimize cure schedules

What’s Next for PSPIs in RF?

As RF technologies push toward 6G, terahertz, and flexible electronics, PSPIs are evolving too:

• Lower-Dk formulations for ultra-high-frequency antennas

• Flexible grades for wearables and aerospace modules

• New chemistries like PBO-based polyimides for improved performance and environmental resilience

They’re also crucial in heterogeneous integration—bringing together MEMS, photonics, sensors, and logic into one compact module.

Invisible, But Indispensable

If you're building next-generation RF systems, photosensitive polyimides are one of your strongest allies. They help you:

• Shrink size

• Improve performance

• Simplify fabrication

• Increase reliability

  
  

  “Invisible when perfect. Essential when removed.”

For engineers working at the bleeding edge of frequency, form factor, and function—PSPIs aren’t just a material.