Arlon 85N PCB | Pure Polyimide Multilayer Boards

What Is Arlon 85N?

Arlon 85N is a pure polyimide, woven-glass-reinforced laminate and prepreg system rated for continuous operation at 250 °C. It delivers a glass transition temperature (Tg) above 250 °C by DSC, extremely low Z-axis expansion, and the long-term thermal endurance that standard FR-4 or even high-Tg epoxy systems cannot match. Arlon 85N PCBs are the baseline choice when a multilayer board must survive sustained high temperatures, aggressive thermal cycling, or lead-free reflow profiles with wide safety margins.

The material is manufactured by Arlon Electronic Materials (now part of the AGC group). It is qualified to IPC-4101 slash sheet /40 and /42 requirements for polyimide laminates and carries UL 94 V-0 flammability certification. Engineers specify 85N when the operating environment rules out every lower-cost resin system—think jet-engine-mounted avionics, downhole drilling telemetry, burn-in boards and high-power military radar modules.

Key Material Properties of Arlon 85N

Two numbers stand out. First, the Tg above 250 °C means the resin stays in its glassy state throughout every standard reflow profile—even the most aggressive SAC305 lead-free profile peaks around 260 °C, leaving only a thin margin with high-Tg FR-4 (Tg ~170–180 °C) but a comfortable one with 85N. Second, the Z-axis expansion of only 2.0 % through the full 50–260 °C range protects plated through-holes and microvias from barrel cracking during repeated thermal excursions, a failure mode that destroys reliability in thick multilayer boards.

Where Arlon 85N PCBs Are Used

Aerospace and Defense

Engine-mounted avionics, radar T/R module substrates and missile fin electronics routinely see ambient temperatures above 150 °C. A pure polyimide board like 85N survives thousands of –55 °C to +200 °C thermal cycles without delamination—a requirement called out in MIL-PRF-31032 and often verified by IPC-TM-650 2.6.7.2 thermal stress testing. For radar front-end boards where both low loss and extreme temperature are needed, designers sometimes pair a low-loss RF layer (such as a Rogers RO3003G2 substrate) with 85N structural layers in a hybrid stackup.

Downhole Oil and Gas

Measurement-while-drilling (MWD) and logging-while-drilling (LWD) tools operate at 175–200 °C for weeks continuously. Standard FR-4 and even BT-epoxy boards degrade rapidly above 150 °C. Arlon 85N’s Td above 390 °C provides the long-term thermal endurance these applications demand.

Burn-In and Test Boards

Semiconductor burn-in boards cycle between room temperature and 150–200 °C thousands of times per year. The low Z-axis CTE of 85N preserves via integrity across these cycles far longer than high-Tg FR-4, reducing costly board replacements.

High-Power RF Amplifiers

GaN power amplifier pallets can raise local board temperature well above 125 °C. When the amplifier’s matching network does not require ultra-low-loss PTFE but does need thermal resilience, 85N is a practical alternative to more expensive ceramic substrates like alumina ceramic RF boards.

Arlon 85N vs. Other High-Tg and Polyimide Laminates

Choose Arlon 85N when continuous operating temperature exceeds 150 °C, when the board must survive thousands of wide-range thermal cycles, or when military/aerospace qualification demands a pure polyimide resin system. Choose a low-loss thermoset like Isola I-Tera MT40 when signal integrity at multi-GHz frequencies matters more than extreme temperature survival. Choose a polyimide blend like Nelco N4000-13 when you need high Tg with a slightly lower Dk and Df for mixed signal-plus-RF stackups.

Selecting the Right Arlon Material for Your Application

Arlon offers several laminate families beyond 85N, spanning pure polyimide, PTFE-ceramic, and thermoset blends. The selector below helps you narrow the field based on your operating temperature, frequency band and dielectric requirements. Enter your constraints and it returns the best-fit Arlon grade along with key parameters.

If the selector returns 85N, you are in the right place—your application’s thermal demands are the dominant constraint. If it suggests an Arlon CLTE or AD-series PTFE-ceramic grade instead, your frequency is high enough that loss tangent matters more than temperature, and you should evaluate those families separately. For non-Arlon PTFE options at microwave frequencies, materials like Taconic TLX-8 laminates offer Dk around 2.55 with very low loss.

Fabrication Guidelines for Arlon 85N PCBs

Drilling

Polyimide resin is tougher and more abrasive than epoxy. Use carbide drills with a fresh edge; expect roughly 30–40 % shorter drill life compared to FR-4. Entry and backer materials should be standard aluminum entry foil and phenolic backer. Drill speeds of 150–200 surface feet per minute (SFM) with chip loads of 0.001–0.002 in/rev work well for standard through-holes. Deburr mechanically—polyimide smear is difficult to remove chemically.

Desmear and Plating

This is the single most critical process step. Standard permanganate desmear cycles that work on FR-4 are inadequate for polyimide. A plasma desmear (CF₄/O₂ gas mixture) is strongly recommended, followed by a permanganate cycle for etchback control. Incomplete desmear leads to plating voids in via barrels—the most common Arlon 85N fabrication defect. After desmear, electroless copper deposition and electrolytic plating proceed normally, but the plating bath chemistry should be monitored more frequently because polyimide residues can contaminate the bath.

Lamination

Arlon 85N prepreg requires higher lamination temperatures than epoxy—typically 220–230 °C peak with a 60–90 minute dwell at full pressure. Ramp rates of 3–5 °C/min prevent resin starvation. The press cycle is longer and hotter, which increases energy cost and reduces press throughput—one of the reasons polyimide boards cost more. Use vacuum-assisted lamination to eliminate voids, especially in boards with eight or more layers.

Solder Mask and Surface Finish

Standard liquid photo-imageable (LPI) solder masks are compatible with 85N, but choose a mask rated for the board’s operating temperature. For boards operating above 200 °C, silicone-based conformal coatings may replace solder mask entirely. ENIG, ENEPIG, and electrolytic hard gold are all compatible surface finishes; OSP and immersion tin are not recommended for high-temperature service because they degrade or oxidize rapidly above 150 °C.

Design Tips for Arlon 85N Multilayer Boards

• Minimize layer count if possible. Each additional layer adds lamination cost and cycle time. Polyimide’s higher resin cost means the per-layer price premium over FR-4 is amplified in thick stackups.
• Use generous annular rings. Polyimide’s higher drilling difficulty and the need for aggressive desmear make registration tighter. An annular ring of 5 mil (0.127 mm) minimum after plating is a practical target for Class 3 boards per IPC-6012.
• Specify plasma desmear on the fabrication drawing. Do not leave it to the fabricator’s discretion—call it out explicitly.
• Account for Dk in impedance calculations. At Dk 4.20, trace widths for 50 Ω microstrip on 85N are narrower than on PTFE laminates (Dk ~2.1–3.5). A 50 Ω microstrip on 10 mil (0.254 mm) 85N core with ½ oz copper requires roughly 18 mil (0.46 mm) trace width—verify with your stackup tool.
• Consider hybrid stackups. If only the RF layers need low loss, bond a PTFE RF layer to 85N structural layers. This approach captures polyimide’s thermal resilience for power and digital layers while giving the RF signal path the low Df it needs.

Cost Drivers for Arlon 85N PCBs

Arlon 85N boards typically cost three to five times more than equivalent FR-4 boards. The main cost drivers are:

1. Raw material price. Polyimide resin is inherently more expensive than epoxy. Arlon 85N laminate sheets cost significantly more per square foot than standard FR-4 or even high-Tg FR-4.
2. Longer press cycles. The 220–230 °C lamination temperature and 60–90 minute dwell reduce press throughput.
3. Plasma desmear. Not every fabricator has plasma desmear capability, limiting the supply base and increasing per-board cost.
4. Drill wear. Shorter drill life means more frequent tool changes and higher consumable cost.
5. Lower yield. Polyimide is less forgiving of process variation; scrap rates are higher than FR-4, especially in high-layer-count builds.

Despite the premium, the cost is justified when field failure of a lower-grade board would be catastrophic or when replacement is physically impossible—as in a downhole tool or a satellite.

Moisture Sensitivity and Handling

Polyimide absorbs more moisture than epoxy—Arlon 85N’s 0.35 % absorption is typical for the class. Absorbed moisture can cause delamination or blistering during reflow or rework. Pre-bake bare boards at 120 °C for 4–6 hours before soldering if they have been stored in an uncontrolled environment for more than 72 hours. Store laminates and prepregs in sealed, desiccated packaging per J-STD-033D moisture sensitivity guidelines.

Frequently Asked Questions

What is the maximum operating temperature of Arlon 85N?

Arlon 85N is rated for continuous use at 250 °C. Short excursions to 280–300 °C during soldering are acceptable, but sustained operation above 250 °C will gradually degrade peel strength and mechanical properties.

Can Arlon 85N be used for RF circuits?

Yes, but with caveats. At Dk 4.20 and Df 0.014, 85N is adequate for RF circuits below about 3 GHz where thermal survival is the primary concern. Above 3 GHz, insertion loss becomes significant compared to PTFE or low-loss thermoset alternatives. For higher frequencies, consider a hybrid stackup that pairs 85N structural layers with a low-loss RF laminate.

Is Arlon 85N lead-free compatible?

Absolutely. With Tg above 250 °C and Td above 390 °C, 85N has wider thermal margins than any lead-free reflow profile requires. It is one of the safest choices for lead-free assembly of high-reliability boards.

How does Arlon 85N compare to DuPont Pyralux for flex circuits?

They serve different roles. Arlon 85N is a rigid laminate for multilayer rigid boards. DuPont Pyralux is a flexible polyimide film system for flex and rigid-flex circuits. Both use polyimide chemistry, but the form factor, reinforcement and application space are distinct.

What IPC slash sheets cover Arlon 85N?

Arlon 85N is qualified under IPC-4101 slash sheets /40 (woven E-glass, polyimide resin, standard Tg) and /42 (same system, high-performance grade). Always confirm the specific slash sheet with your Arlon distributor for the construction you need.