Arlon 85NT vs 55NT vs 35N: Choosing the Right High-Reliability PCB Laminate

Three Arlon grades — 85NT, 55NT, and 35N — consistently appear together on material shortlists for demanding PCB applications, and they’re frequently confused with each other. Part of that confusion comes from surface similarities: all three are positioned above standard FR-4, all three appear in aerospace and military procurement specifications, and all three target applications where thermal reliability is the primary design constraint. But they solve different problems, and using the wrong one is not a cosmetic issue — it’s a reliability issue that may not show up on the bench but will appear in field service.

This guide cuts through the Arlon high Tg comparison across 85NT, 55NT, and 35N with the specificity that an actual material selection decision requires. It covers the resin chemistry, the reinforcement architecture, the thermal and dimensional property implications of each choice, and the application scenarios where each grade is genuinely the right answer.

Why the Confusion Exists — and What Actually Separates These Three Grades

The naming convention does not help. “85” in Arlon’s system generally indicates pure polyimide resin chemistry with 250°C Tg. “55” indicates high-performance multifunctional epoxy. “35” is a flame-retardant polyimide. The suffix “NT” means nonwoven aramid (Thermount) reinforcement rather than woven E-glass. That decoding gives you the key insight:

35N is a pure polyimide on woven E-glass reinforcement. It is the glass-reinforced equivalent of Arlon’s flagship polyimide range, with the addition of UL94 V-1 flame retardance and a reduced cure cycle compared to traditional polyimides.

85NT is a pure polyimide on nonwoven aramid reinforcement. It combines the thermal performance of polyimide resin with the ultra-low XY-plane CTE that only aramid fiber can provide. The aramid reinforcement is what makes 85NT distinctive — it brings in-plane CTE down to 6–9 ppm/°C, matching silicon and ceramic component CTE values rather than working against them.

55NT is a high-performance multifunctional epoxy on nonwoven aramid reinforcement. The resin is not polyimide. Its Tg of 170°C is solid for lead-free processing but is not in the same class as 85NT’s 250°C. What 55NT shares with 85NT is the nonwoven aramid reinforcement, and therefore the same 6–9 ppm/°C XY CTE. It is the lower-cost, epoxy-based path to CTE matching for SMT-intensive boards that do not need 250°C Tg.

Understanding this three-way split — polyimide glass (35N), polyimide aramid (85NT), and epoxy aramid (55NT) — is the foundation for making the right material selection in the Arlon high Tg comparison.

The Reinforcement Architecture: Why Nonwoven Aramid Changes the CTE Story

Before diving into grade-by-grade property data, it’s worth spending time on the nonwoven aramid reinforcement that 85NT and 55NT share — because this is the feature that most engineers underestimate when they first encounter these materials.

Standard PCB substrates use woven E-glass reinforcement. Woven E-glass has an in-plane CTE of approximately 5–6 ppm/°C and a modulus that dominates the composite’s dimensional behavior. When a polyimide or epoxy resin is applied to woven E-glass, the result is a laminate with XY CTE in the 14–18 ppm/°C range — dominated by the resin contribution, which is much higher than glass.

Aramid fiber (specifically DuPont’s Thermount nonwoven material used in 85NT and 55NT) has an in-plane CTE of approximately 0 ppm/°C — effectively zero or slightly negative. When a polyimide or epoxy resin is combined with nonwoven aramid, the aramid’s near-zero CTE counteracts the resin’s high CTE, producing a composite with XY CTE of 6–9 ppm/°C. That value is in the same range as silicon (2–4 ppm/°C range) and alumina ceramic (6–7 ppm/°C), which is the point: the board’s in-plane expansion now roughly matches the expansion of the devices mounted on it.

For high-I/O BGA packages, fine-pitch SMT components, TSOP and CSP packages, and leadless ceramic chip carriers (LCCCs), this CTE match dramatically reduces the thermal stress on solder joints during power cycling. The CTE mismatch between a standard glass-epoxy PCB (CTE ~18 ppm/°C) and a ceramic BGA package (~7 ppm/°C) is the root cause of solder joint fatigue failures in many high-reliability applications. Aramid-reinforced substrates address that failure mode directly.

Woven E-glass reinforcement in 35N does not provide this benefit. Its XY CTE follows the standard glass-epoxy profile — better than standard FR-4 due to the polyimide resin, but not in the same neighborhood as the 6–9 ppm/°C that aramid delivers.

Polymeric reinforcement also results in PCBs that are typically 25% lighter in weight than conventional glass-reinforced laminates — a meaningful advantage in weight-critical aerospace and satellite applications.

Full Property Comparison: Arlon 85NT vs 55NT vs 35N

The table below consolidates the key properties for material selection. Always verify against current datasheets before finalizing a specification, as typical properties can vary with laminate thickness and reinforcement style.

Property	Arlon 85NT	Arlon 55NT	Arlon 35N
Resin system	Pure polyimide	Multifunctional epoxy	Pure polyimide
Reinforcement	Nonwoven aramid (Thermount)	Nonwoven aramid (Thermount)	Woven E-glass
Glass Transition Temp (Tg)	250°C (235–245°C practical)	170°C	250°C
Decomposition Temp (Td)	426°C	368°C	406°C
XY CTE (in-plane)	6–9 ppm/°C	6–9 ppm/°C	~14–16 ppm/°C
Z-axis expansion (50–260°C)	~2.3%	~3.5%	Low (similar to 85N)
UL94 Flammability	HB	V-0	V-1
IPC Specification	IPC-4101/53	IPC-4101/55	IPC-4101/40 & /41
Moisture absorption	~0.60%	~0.30%	~0.26%
Weight vs. glass laminate	~25% lighter	~25% lighter	Standard glass weight
Drilling	Laser/plasma ablatable; undercut bits recommended	Laser/plasma ablatable; undercut bits recommended	Standard polyimide drilling
Desmear	Permanganate or plasma (polyimide settings)	Permanganate or plasma (epoxy settings)	Permanganate or plasma
Lamination temp at cure	~218°C (425°F)	~182°C (360°F)	Reduced vs. 85N
Lead-free compatible	Yes	Yes	Yes
Primary application	CTE-matched high-Tg multilayers, BGA, space, CIC replacement	CTE-matched lead-free SMT boards, HDI, commercial BGA	High-temp woven-glass multilayers, downhole, military
CIC replacement capability	Yes (primary use case)	No (Tg too low for extreme duty)	No (wrong CTE profile)

Arlon 35N: Pure Polyimide on E-Glass for Thermal Endurance

H3: What 35N Is Designed to Do

Arlon’s 35N is a 250°C high glass transition temperature polyimide resin system ideal for demanding applications that require low Z-axis directional expansion and resistance to PTH failures during operation in harsh environmental conditions. 35N has reduced temperature and cure times which offers improved throughput during manufacturing compared to traditional polyimide cycles.

35N is the practical choice when you need genuine polyimide performance — 250°C Tg, 406°C Td, low Z-axis expansion, excellent PTH barrel fatigue life — in a woven E-glass construction that processes through a standard high-reliability fabrication line. It does not require aramid-specific handling, laser ablation equipment, or the more demanding drill and routing protocols that aramid fiber introduces.

The UL94 V-1 flame retardance rating distinguishes 35N from Arlon 85N (which is HB) and 33N (which is V-0). V-1 means the material self-extinguishes within 30 seconds when a flame is removed and does not drip burning particles. For applications where fire safety certification is required — commercial avionics, industrial electronics, many military subsystems — a V-1 or V-0 rating is a procurement specification item that eliminates HB-rated materials like 85N and 85NT from consideration.

35N is tougher than conventional polyimides and is less prone to fracture during small hole drilling and profiling. 35N contains no MDA or other potentially carcinogenic diamines — a manufacturing safety consideration that matters for shops running large volumes of polyimide material.

H3: Where 35N Fits in the Arlon High Tg Comparison

35N belongs on the shortlist for any thick multilayer (say, 16 layers and above) destined for military electronics, aerospace avionics, or downhole instrumentation, where woven glass construction is preferred for process compatibility and where XY CTE matching for SMT packages is not the driving requirement. If your board carries primarily through-hole components, press-fit connectors, or SMT packages where solder joint fatigue from CTE mismatch is not the primary failure mode, 35N’s woven glass construction is perfectly adequate — and significantly less demanding to fabricate than an aramid-reinforced system.

Applications for 35N include military, aerospace, downhole oil and gas drilling, commercial, and industrial electronics — exactly the application space where polyimide’s 250°C Tg and resistance to repeated thermal cycling is the specification driver, not CTE matching for fine-pitch SMT packages.

Arlon 55NT: Epoxy Meets Aramid for CTE-Controlled Lead-Free SMT

H3: The Role Epoxy Plays in 55NT

Arlon’s 55NT is an epoxy laminate and prepreg system, reinforced with a nonwoven aramid reinforcement. This system combines compatibility with lead-free processing, using a high-temperature multifunctional epoxy resin, with the low in-plane (X,Y) expansion of 6–9 ppm/°C and outstanding dimensional stability of nonwoven aramid reinforcement.

The resin in 55NT is multifunctional epoxy at 170°C Tg — not polyimide. This is the most important single fact in the Arlon high Tg comparison for anyone considering 55NT. A Tg of 170°C is well above what is needed for lead-free assembly (peak around 260°C, but the board is above Tg for only seconds — what matters more is that the Td of 368°C is well above peak reflow, which it is). But 170°C Tg means 55NT is not appropriate for sustained high operating temperatures or designs that require multiple rework cycles at elevated temperature over their service life. The board will not catastrophically fail at 171°C, but above Tg the resin transitions from glassy to rubbery and dimensional stability degrades.

What 55NT does exceptionally well is provide CTE-matched substrates for high-density SMT-intensive boards where BGA, TSOP, CSP, and fine-pitch SMT packages are the predominant interconnect style. The Tg of 170°C, decomposition temperature of 368°C, and Z-expansion of 3.5% between 50–260°C ensures compatibility with most lead-free processes. The XY CTE of 6–9 ppm/°C is where 55NT earns its specification position — matching the CTE of the components on the board, reducing solder joint fatigue in applications that will undergo significant thermal cycling during field operation.

H3: HDI, Microvia, and Registration Benefits of Nonwoven Aramid in 55NT

The nonwoven aramid reinforcement in 55NT brings a fabrication benefit beyond CTE control: outstanding dimensional stability and enhanced registration for improved multilayer yields. In high-density interconnect (HDI) boards with blind and buried vias and fine-pitch registration requirements, the aramid reinforcement resists the panel expansion and contraction during etching and lamination that woven glass laminates undergo. The registration improvement can meaningfully reduce scrap in high-layer-count HDI production.

Nonwoven aramid reinforcement is also laser and plasma ablatable, enabling microvia formation by CO₂ laser without the glass fiber shadowingEffects that create irregular via walls in woven-glass substrates. For HDI boards with laser-drilled microvias as small as 0.004″ (100 µm), the ablation quality on aramid is significantly more consistent than on woven E-glass.

55NT is well-suited for applications including BGA, TSOP, fine-pitch SMT, CSP packages, and HDI designs in commercial avionics, telecom, and industrial electronics where lead-free assembly and CTE matching for solder joint reliability are the design constraints — but where sustained high operating temperature above 150–170°C is not required.

Arlon 85NT: The Convergence of Polyimide Performance and Aramid CTE Control

H3: What Makes 85NT the Most Demanding Grade in the Family

Arlon’s 85NT is a pure polyimide laminate and prepreg system with a Tg of 250°C, reinforced with a nonwoven aramid substrate. 85NT combines the high-reliability features of polyimide — improved PTH reliability and temperature stability — with the low in-plane (X,Y) expansion of 6–9 ppm/°C and outstanding dimensional stability of the aramid reinforcement.

85NT is the material you specify when you need everything that 55NT offers from its aramid reinforcement, plus the full thermal performance of pure polyimide chemistry. The Td of 426°C is the highest of the three grades and reflects pure polyimide resin with no flame retardant additives that would compromise thermal stability. The Z-axis expansion of approximately 2.3% from 50–260°C is lower than 55NT’s 3.5% — a direct result of the polyimide resin’s superior thermomechanical properties above the epoxy’s 170°C Tg.

85NT is commonly used to replace boards containing Copper-Invar-Copper (CIC) in traditional CTE-matched environments. That is a significant application statement: CIC is an expensive metallic CTE-matching approach that adds weight and complex processing. 85NT achieves comparable in-plane CTE control through material chemistry and reinforcement selection, with the weight reduction benefit of polymeric (rather than metallic) construction. In space and aerospace applications where every gram matters, this is a compelling specification argument.

H3: Practical Processing Requirements for 85NT

85NT requires more process discipline than either 55NT or 35N because of the combination of polyimide resin and aramid reinforcement. The polyimide cure cycle (product temperature at start of cure = 218°C, or 425°F) is higher than 55NT’s epoxy cycle. The aramid reinforcement demands attention to drill and routing parameters: undercut drill bits are recommended for small vias, router bits with chip-breaker geometry are not recommended, and de-smear must use permanganate or plasma with settings appropriate for polyimide rather than epoxy.

Vacuum desiccation of prepreg for 8–12 hours immediately prior to lamination is mandatory for 85NT — as it is for all of Arlon’s high-performance polyimide systems. Moisture absorbed during storage will form steam during the lamination press cycle, creating voids and delamination that are very difficult to detect visually but will cause failures under thermal cycling in service.

The moisture absorption of 85NT (~0.60%) is the highest of the three grades in this comparison, reflecting the hygroscopic character of aramid fiber. Pre-bake inner layers in a rack for 60 minutes at 107°C–121°C immediately prior to lay-up. This step is not optional; it is the difference between a well-bonded laminate and one that contains latent moisture-induced voids.

Application Decision Matrix: Arlon High Tg Comparison

Design Scenario	Best Choice	Reason
20-layer military avionics backplane (V-1 required)	35N	250°C Tg, V-1 flame rating, woven glass fab familiarity
20-layer space/military backplane (no flame rating required)	85N or 85NT	85N for standard glass; 85NT if weight reduction needed
Downhole electronics, >200°C operating temp	35N or 85N	Woven glass, polyimide Tg, proven high-temp service life
High-density BGA board, lead-free, commercial avionics	55NT	CTE match for BGA solder joint reliability; V-0; lead-free compatible
Fine-pitch CSP/TSOP board with HDI microvias	55NT	Laser-ablatable aramid; CTE match; dimensional stability for registration
Aerospace HDI module with 250°C Tg requirement	85NT	Aramid CTE match + polyimide Tg in same material
CIC replacement for satellite electronics	85NT	Primary CIC replacement grade; polyimide + aramid + weight savings
High-layer-count board with heavy BGA + high operating temp	85NT	Only grade that combines CTE matching (aramid) with 250°C Tg
Commercial industrial board, V-0 required, moderate temp	55NT	V-0 epoxy/aramid; adequate Tg for most commercial environments
Rigid-flex bonding in polyimide MLB	37N or 38N (not these three)	Low-flow polyimide prepregs for rigid-flex bonding — different product family

Fabrication Comparison: What the Shop Needs to Know

Process Step	Arlon 85NT	Arlon 55NT	Arlon 35N
Inner layer oxide treatment	Brown oxide	Brown oxide	Brown oxide
Pre-lamination moisture removal	60 min bake 107–121°C + 8–12 hr vacuum desiccation	60 min bake 107–121°C + 8–12 hr vacuum desiccation	60 min bake 107–121°C + 8–12 hr vacuum desiccation
Lamination cure start temperature	~218°C (425°F)	~182°C (360°F)	Reduced vs. 85N
Heat ramp rate	Controlled (polyimide)	4.5–6.5°C/min (82–138°C)	Controlled (polyimide)
Drill speed	350–400 SFM	350–400 SFM	Standard polyimide
Drill bit style	Undercut bits for vias ≤0.023″	Undercut bits for vias ≤0.023″	Standard; no chip-breaker
Router bits	No chip-breaker style	No chip-breaker style	No chip-breaker style
Desmear chemistry	Permanganate or plasma (polyimide)	Permanganate or plasma (epoxy)	Permanganate or plasma
Laser/plasma microvia formation	Yes (CO₂ laser ablatable)	Yes (CO₂ laser ablatable)	Not standard — E-glass is not laser-ablatable cleanly
Pre-HASL bake	1–2 hr at 121°C	1–2 hr at 121°C	1–2 hr at 121°C
Plating	Conventional plating processes	Conventional plating processes	Conventional plating processes

The most critical process discipline for 85NT and 55NT is moisture management of the aramid reinforcement. Aramid fibers are inherently more hygroscopic than E-glass — this is a fundamental property of the aramid polymer, not a manufacturing defect. Strict storage (sealed packaging, climate-controlled environment) and mandatory pre-lamination desiccation are non-negotiable for both grades. Shops that run standard E-glass polyimide without these controls will produce defective 85NT and 55NT boards.

Useful Resources

Resource	Description	Where to Find It
Arlon 85NT Datasheet	Tg, Td, XY CTE, Z-expansion, fabrication guidelines	arlonemd.com → Products → 85NT; storage.googleapis.com/shipwebassets/85NT.pdf
Arlon 55NT Datasheet	Full epoxy/aramid specs, lamination cycle, drill parameters	arlonemd.com → Products → 55NT; ship.ie/assets/pdfs/55NT.pdf
Arlon 35N Datasheet	Polyimide/glass 250°C Tg, V-1 flame, process guide	arlonemd.com → Products → 35N; pcbmay.com/wp-content/uploads/2021/06/Arlon-35N.pdf
Arlon Laminate FAQ Guide	“Everything You Wanted to Know” — polyimide processing, CTE, moisture management	arlonemd.com/wp-content/uploads/2020/05/Laminate-Guide.pdf
Arlon Controlled CTE/SMT Page	Overview of 85NT, 55NT, 45NK as a family for SMT applications	arlonemd.com/applications/controlled-cte-smt/
Arlon PCB Material Overview	Grade selector and application guide for full Arlon portfolio	pcbsync.com/arlon-pcb/
IPC-4101/40 & /41	Specification for polyimide/glass laminates (covers 35N, 85N)	ipc.org
IPC-4101/53	Specification for polyimide/aramid laminates (covers 85NT)	ipc.org
IPC-4101/55	Specification for epoxy/aramid laminates (covers 55NT)	ipc.org
IPC-TM-650	Standard test methods for Tg, Td, CTE, Z-expansion	ipc.org

5 FAQs: Arlon 85NT vs 55NT vs 35N

FAQ 1: Can I substitute 35N for 85NT in a board design where weight matters?

They are not substitutable if weight reduction and CTE matching are both requirements. 35N uses woven E-glass reinforcement, which gives it approximately the same weight as standard high-performance polyimide/glass boards. 85NT’s nonwoven aramid reinforcement results in a board that is typically 25% lighter — a meaningful reduction in aerospace and satellite applications where component weight contributes to launch cost or structural loading. More importantly, 35N’s XY CTE is in the standard glass-reinforced range (~14–16 ppm/°C), while 85NT delivers 6–9 ppm/°C. If your design requires CTE matching for fine-pitch SMT packages, 35N will not deliver it regardless of how the rest of the board is specified. If your design is all through-hole or press-fit and CTE matching is not a concern, 35N is an excellent high-temperature multilayer material that is easier to fabricate than 85NT.

FAQ 2: Is 55NT a good choice for military avionics boards?

55NT is qualified for aerospace and commercial avionics applications where its 170°C Tg is sufficient for the design’s operating temperature range, and where its V-0 flame rating and CTE-matching capability are the primary specification drivers. For boards in avionics bays where operating temperatures remain well below 150°C and the design is dense with BGA and fine-pitch SMT packages, 55NT is a technically sound specification. Where 55NT is not the right choice is any application requiring sustained operation above 150°C, multiple rework cycles at elevated temperature, or long service life in harsh thermal environments. For military electronics where 20+ year service life under demanding thermal cycling is the norm, the 170°C Tg of 55NT provides insufficient margin, and 85NT or 35N should be specified instead. The distinction between commercial avionics and military long-life electronics is the key decision point.

FAQ 3: Why does 85NT have a lower practical Tg than its nominal 250°C rating?

The Arlon 85NT datasheet notes that the resin system has a Tg of 250°C but the combination of the resin and the nonwoven aramid reinforcement develops a Tg more in the range of 235–245°C with conventional polyimide cure cycles. This is a well-documented characteristic of polyimide/aramid composites: the aramid fiber’s interaction with the resin during curing slightly modifies the effective crosslink density of the finished composite. The 235–245°C practical Tg is still dramatically higher than FR-4 or 55NT, and is sufficient for all lead-free assembly processes and most high-temperature operating environments. If the additional margin to 250°C is critical for your application, ensure the cure cycle is fully optimized per Arlon’s recommendations — the full 250°C Tg is achievable with the correct temperature profile and dwell time.

FAQ 4: Why would I choose 55NT over standard high-Tg epoxy (like Arlon 49N) for a BGA-intensive board?

The answer is the XY CTE. Standard high-Tg epoxy systems on woven E-glass — including Arlon’s 49N and comparable materials — deliver XY CTE values in the 14–18 ppm/°C range. This is roughly 2–3× higher than the CTE of ceramic packages, LCCCs, and many high-I/O BGA substrates. Under thermal cycling, the CTE mismatch generates cyclic shear stress on solder joints that accumulates as fatigue damage. Fine-pitch packages (0.5 mm BGA pitch and below, LCCCs with many I/Os) are particularly vulnerable because their solder joint geometry constrains the accommodation of the expansion difference. 55NT’s aramid reinforcement brings XY CTE to 6–9 ppm/°C, reducing the differential to a level that dramatically extends solder joint fatigue life. For boards populated predominantly with standard through-hole or coarser-pitch SMT packages, the CTE mismatch with a standard high-Tg epoxy glass substrate is not severe enough to drive solder joint failures — in which case 49N or similar is a more cost-effective and easier-to-fabricate choice. Specify 55NT when fine-pitch CTE mismatch is actually the identified failure risk in your thermal cycling analysis.

FAQ 5: How do 85NT and 55NT handle moisture compared to standard polyimide/glass laminates?

Both aramid-reinforced grades are more hygroscopic than their glass-reinforced equivalents, and this requires stricter materials handling in the fab shop. Aramid fiber absorbs more moisture than E-glass — it’s a fundamental property of the Kevlar/Thermount polymer chemistry. Arlon 85NT has a moisture absorption of approximately 0.60%, and 55NT approximately 0.30%, both measurably higher than 35N’s 0.26% and standard polyimide/glass materials. The practical implication is that vacuum desiccation of prepreg for 8–12 hours before lamination is mandatory (not optional) for both grades. Panels should be stored in sealed packaging in climate-controlled environments. Any shop that runs these materials without the pre-bake and desiccation step will produce boards with moisture-induced voids and compromised bond strength at the aramid/resin interface — defects that often survive initial electrical test but fail catastrophically under thermal cycling. For more context on the full range of Arlon’s high-reliability materials and how this family fits into the broader portfolio, the Arlon PCB guide at PCBSync is a useful reference.

Choosing Between 85NT, 55NT, and 35N: The Decision Framework

The Arlon high Tg comparison across these three grades resolves cleanly once you answer two questions about your application.

First, does your board require CTE matching for fine-pitch SMT packages? If yes, you need aramid reinforcement — either 85NT or 55NT. If no, 35N’s woven E-glass construction is adequate and significantly easier to fabricate.

Second, within the aramid-reinforced family, does your application require 250°C polyimide Tg, or is 170°C multifunctional epoxy Tg sufficient? If your operating temperature, service life, rework requirements, or procurement specification demands polyimide performance, 85NT is the answer. If the application is CTE management for lead-free SMT reliability in commercial or commercial-avionics electronics with moderate operating temperatures, 55NT delivers the same aramid CTE benefit at lower material cost with simpler processing.

35N fills the space where polyimide’s thermal performance is mandatory but the application geometry and component mix do not require CTE matching — thick military multilayers, downhole electronics, industrial control systems running at sustained elevated temperatures. Its reduced cure cycle compared to Arlon 85N makes it a practical choice for high-volume fabrication environments where throughput matters.

Each of these grades represents a genuine engineering solution to a specific reliability problem. Knowing which problem your design actually has is how you make the right choice.