Complete engineer’s guide to Arlon 38N — polyimide low-flow prepreg specifications, 38N vs. 37N comparison, rigid-flex bonding applications, vacuum lamination process parameters, and heat sink attachment guidance for military and aerospace PCBs.

Rigid-flex PCB design looks straightforward on paper — you bond rigid layers together using a prepreg, and the flexible sections do their job. The reality is that choosing the wrong bonding prepreg in a high-reliability polyimide rigid-flex assembly is one of the fastest ways to generate field failures, particularly in the plated through-holes and at rigid-to-flex transition zones. Resin that flows too much during lamination bleeds into flex relief areas and stiffens sections that were designed to flex. Resin that doesn’t cure consistently enough leaves interfacial voids that become delamination initiation sites under thermal cycling.

Arlon 38N laminate was designed specifically to solve this problem. It is a second-generation polyimide low-flow prepreg with a 200°C glass transition temperature, improved bond strength to Kapton® polyimide films and copper, and a novel cure chemistry that achieves faster, more uniform resin cure than conventional polyimide low-flow materials. For engineers designing military avionics, aerospace electronics, and high-reliability commercial PCB assemblies where polyimide rigid-flex construction is the standard, understanding Arlon 38N in depth is worth the time.

What Is Arlon 38N Laminate?

Arlon 38N is a second-generation 200°C glass transition temperature polyimide low-flow prepreg system produced by Arlon Electronic Materials Division (Arlon EMD). It represents a significant improvement over first-generation low-flow polyimide prepregs — specifically Arlon’s own 37N — in terms of bond strength, cure uniformity, and adhesion to Kapton polyimide film.

The “low-flow” designation is fundamental to understanding what 38N is for. In multilayer lamination, resin flow is the controlled movement of uncured resin from the prepreg into the surrounding structure under heat and pressure. In standard multilayer boards, moderate resin flow fills interlayer gaps and produces void-free bonds. In rigid-flex assemblies, that same resin flow becomes a liability — excess resin flowing into flex relief areas or via clearance zones stiffens the flex layer, restricts flex radius, and ultimately causes fatigue failures at the flex-rigid interface. A low-flow prepreg minimizes this flow, confining the resin to the bond interface and preventing penetration into areas where it would compromise the flex functionality.

What distinguishes 38N from its predecessors is the novel chemistry that achieves not just low flow, but faster and more uniform cure across the prepreg sheet. Earlier generation low-flow polyimide prepregs were prone to cure non-uniformity — areas of the laminate that cured at different rates produced varying bond line thickness and resin distribution. The 38N formulation addresses this with a cure mechanism that progresses more consistently from the bondline outward, producing a more predictable and dimensionally stable cured bond.

Arlon EMD is the first U.S. laminator recognized under IPC’s Quality Product Listing, and it is the only laminator certified for all three polyimide slash sheets — IPC-4101/40, IPC-4101/41, and IPC-4101/42. Arlon 38N itself meets the requirements of IPC-4101/42, which is the relevant specification for polyimide low-flow bonding materials used in rigid-flex construction.

For a broader context of how Arlon 38N fits within the complete range of Arlon PCB materials, including the full polyimide, epoxy, and PTFE microwave laminate families, Arlon EMD’s product portfolio covers the full spectrum of high-performance PCB material requirements.

Arlon 38N Full Specification Table

The table below presents the typical electrical, thermal, and mechanical properties for the Arlon 38N laminate system. These are typical values from the Arlon EMD datasheet and published sources. Always verify against the current official Arlon EMD datasheet before finalizing a design.

Property	Value	Test Method
Glass Transition Temperature (Tg) — DSC	200°C	IPC TM-650 2.4.25
Glass Transition Temperature (Tg) — TMA	200°C	IPC TM-650 2.4.24
Decomposition Temperature (Td @ 5% wt loss)	330°C	IPC TM-650 2.4.24.6
Decomposition Temperature (initial)	~311°C	TGA
Dielectric Constant (Dk) @ 1 MHz	4.25	IPC TM-650 2.5.5.3
Dielectric Constant (Dk) @ 1 GHz	4.25	IPC TM-650 2.5.5.3
Dissipation Factor (Df) @ 1 MHz	0.010	IPC TM-650 2.5.5.3
Dielectric Strength	1,600 V/mil (63.0 kV/mm)	IPC TM-650 2.5.6
CTE (X-axis)	17 ppm/°C	IPC TM-650 2.4.41
CTE (Y-axis)	17 ppm/°C	IPC TM-650 2.4.41
CTE (Z-axis, below Tg)	54 ppm/°C	IPC TM-650 2.4.41
CTE (Z-axis, above Tg)	157 ppm/°C	IPC TM-650 2.4.41
Thermal Conductivity	0.3 W/m·K	ASTM E1461
Tensile Strength	32 kpsi (221 MPa)	ASTM D882
Flexural Strength	60 kpsi (414 MPa)	IPC TM-650 2.4.4
Peel Strength (1 oz Cu, after thermal stress)	8.5 lbs/in (1.5 N/mm)	IPC TM-650 2.4.8
Peel Strength to Kapton® (as received)	5.9 lbs/in (1.0 N/mm)	IPC TM-650 2.4.8
Peel Strength to Kapton® (after soldering)	5.2 lbs/in (0.9 N/mm)	IPC TM-650 2.4.8
Water Absorption	< 1.0%	IPC TM-650 2.6.2
Flammability Rating	UL94 V-0	UL94
IPC Compliance	IPC-4101/42	IPC-4101
RoHS / WEEE Compliance	Yes	EU Directive
Lead-Free Process Compatible	Yes	—
Minimum Cure Temperature	350°F (177°C)	—

Three numbers in this table are worth specific attention. The Tg of 200°C is the defining thermal characteristic — it is high enough to withstand lead-free solder reflow processes reliably, and the polyimide expansion characteristics that accompany a 200°C Tg system directly improve PTH barrel reliability compared to standard epoxy Tg systems. The Td of 330°C (5% weight loss) provides a substantial margin above any solder processing temperature, meaning the resin does not begin to decompose during thermal excursions in assembly. And the peel strength improvement to Kapton — up to 50% higher than conventional polyimide low-flow or no-flow products — is the feature that justifies the “second generation” designation.

Arlon 38N vs. Arlon 37N: What Changed in the Second Generation

Engineers who have used 37N or who are comparing the two products frequently ask what specifically changed between the first and second generation. This comparison is important for material qualification decisions.

Parameter	Arlon 37N	Arlon 38N
Generation	First-generation low-flow	Second-generation low-flow
Tg (DSC/TMA)	~200°C	200°C
Decomposition Temp (Td)	~320°C	330°C
Bond Strength to Kapton	Baseline	Up to 50% higher
Cure Uniformity	Standard	Faster, more uniform
Resin Flow Control	Low-flow	Improved low-flow
Heat Sink Bonding Performance	Adequate	Improved
IPC Compliance	IPC-4101/42	IPC-4101/42
Lead-Free Compatibility	Yes	Yes

The key functional improvements in 38N over 37N are in cure chemistry, bond strength, and thermal decomposition. The 38N formulation’s faster and more uniform cure reduces the window during lamination where resin is mobile enough to flow into unintended areas. The improved Kapton adhesion — a genuinely significant 50% increase — reduces the risk of interfacial delamination at flex-rigid transitions during thermal cycling, which is one of the most common failure modes in rigid-flex assemblies in avionics and military electronics.

For new designs that previously specified 37N, 38N is a direct process-compatible upgrade with measurably better reliability margins. The lamination parameters differ slightly (see the fabrication section), but the subsequent processing is identical.

Arlon 38N in the Context of the Full Arlon Polyimide Family

Understanding where 38N sits in the broader Arlon polyimide product line helps engineers make the right material selection decision and avoid over-specifying or under-specifying the resin system.

Product	Tg	Td	Key Feature	Primary Application
Arlon 38N	200°C	330°C	Low-flow, improved Kapton adhesion	Rigid-flex bonding, heat sink attachment
Arlon 37N	200°C	320°C	Low-flow, 1st gen	Rigid-flex bonding (legacy)
Arlon 33N	250°C	389°C	V-0 flame retardant polyimide	High-temp multilayer, avionics
Arlon 35N	250°C	406°C	V-1, fast cure	High Tg multilayer
Arlon 85N	250°C	407°C	Pure polyimide, no flame retardants	Long service life, space, mil
Arlon 84N	250°C	407°C	Filled polyimide prepreg	Copper fill, thermal management
Arlon 47N	135°C	315°C	Modified epoxy low-flow	Lower temperature bonding
Arlon 49N	170°C	302°C	Multifunctional epoxy low-flow	Heat sink bonding (epoxy-based)

The choice between 38N and the higher-Tg systems like 33N, 85N, or 35N is primarily driven by operating temperature requirements. If your assembly will experience sustained temperatures above 200°C — which is relatively unusual in electronics outside of down-hole oil and gas or some specific aerospace applications — the 250°C Tg polyimides are appropriate. For the large majority of rigid-flex designs in avionics, military electronics, and commercial aerospace operating to MIL-PRF-55110 or IPC-6013 standards, 38N’s 200°C Tg provides adequate thermal margin with lead-free processes while offering the improved bonding performance that makes rigid-flex construction more reliable.

Primary Applications for Arlon 38N Laminate

The application profile for Arlon 38N follows directly from its combination of low-flow behavior, 200°C Tg, improved Kapton adhesion, and lead-free compatibility.

Application Category	Specific Use Cases
Military Electronics	Avionic multilayer rigid-flex assemblies, cockpit display boards, weapon system electronics
Aerospace	Aircraft flight computer boards, satellite bus electronics, rigid-flex harness replacement
Space Electronics	Spacecraft electronics needing reliable thermal cycling performance
Heat Sink Bonding	Attaching aluminum or copper heat sinks to polyimide multilayer boards in power circuits
High-Layer-Count Multilayers	Bonding core-to-core in complex multilayer polyimide MLB structures
Industrial High-Reliability	Down-hole electronics, harsh-environment industrial controls, medical imaging
HDI and Microvia PCBs	Bonding ply in HDI designs requiring polyimide materials for thermal performance

The heat sink bonding application is worth elaborating. In high-power military and aerospace electronics, it is common to bond an aluminum or copper heat spreader directly to the back of a polyimide MLB to provide a low-thermal-resistance path for heat from power devices. The bond between the metal heat sink and the polyimide MLB must survive the same thermal cycling profile as the board itself — often -55°C to +125°C or wider in defense applications. Arlon 38N’s improved bond strength to metals — specifically engineered for heat sink bonding — makes it the right material for this application over a standard polyimide prepreg.

Why Low-Flow Behavior Matters in Rigid-Flex Design

This is the design concept that justifies the existence of a product like Arlon 38N, and it is worth spending time on for engineers who don’t work with rigid-flex regularly.

A rigid-flex PCB consists of alternating rigid sections (where components are mounted) and flexible sections (which allow the assembly to bend). The flexible sections typically use a polyimide film like Kapton as the base material, with copper traces etched on it. The rigid sections bond multiple layers of copper-clad polyimide laminate together using prepreg.

At the transition between rigid and flex sections, the rigid cover layers stop and the flex layer continues. This transition zone — called the flex relief area — must not have resin from the prepreg flowing into it, because cured resin in the flex relief would stiffen the flex and cause crack initiation at the rigid edge during bending cycles. The flex relief is specifically designed to be resin-free so the flex layer can freely bend without a stress concentration at the resin-laminate boundary.

A standard prepreg flows enough during lamination to infiltrate the flex relief area. A low-flow prepreg like Arlon 38N does not. The 38N formulation’s faster cure kinetics — reaching gelation before significant flow occurs — confine the resin to the intended bondline area and leave the flex relief zone clean. This is not a minor processing benefit; it is a fundamental reliability requirement for the product.

The same principle applies to via clearance areas. In rigid-flex assemblies, blind and buried vias often have specific geometry requirements around their clearance areas. Standard prepreg resin flowing into via clearance zones creates reliability problems during thermal excursion. 38N’s low-flow behavior prevents excessive flow into these areas, maintaining the designed via geometry after lamination.

Arlon 38N Fabrication and Lamination Process Guidelines

Pre-Lamination Drying

Because of varying storage conditions and the moisture sensitivity of polyimide prepregs generally, Arlon specifies that 38N prepreg should be dried at 29″ (736 mm Hg) vacuum for 12 to 24 hours before use. Moisture in the prepreg at the time of lamination creates two problems: it produces steam voids under press conditions, and it affects the cure kinetics of the resin, leading to non-uniform bond quality. This drying step is not optional — it is a process prerequisite for reliable void-free lamination.

Lamination Process Parameters

38N is described as process-tolerant: it can be laminated with either a cold platen press start or a hot start. This flexibility is significant in production environments where multiple board types share press equipment. The critical parameters are:

• Vacuum draw down to <29″ (736 mm Hg) for 30 minutes before applying press pressure
• Maintain vacuum through the resin set point (above 160°C / 320°F)
• Platen temperature range: 182°C–193°C (360°F–380°F)
• Heat rise rate: 4°C–6°C per minute (8°F–12°F per minute) between 93°C–149°C (200°F–300°F)
• Cure time: 90 minutes at temperature

The vacuum lamination requirement is especially important for 38N and other low-flow prepregs. Because low-flow materials do not displace air voids as effectively as standard flowing prepregs, the vacuum must do the work of removing air from the bondline before resin gelation. Skipping vacuum or using inadequate vacuum draw reduces the vacuum’s effectiveness and leads to interlaminar voids that appear as delamination under thermal or mechanical stress.

Post-Lamination Processing

Once cured, subsequent processing of Arlon 38N laminated assemblies follows the same procedures used for conventional polyimide rigid-flex PCBs. Drilling parameters, plasma desmear (particularly important for polyimide, which desmears differently from epoxy), electroless copper deposition, and electroplating are all standard polyimide rigid-flex processes. No special post-cure bake beyond the 90-minute cure cycle is required for 38N.

Storage and Shelf Life

Store 38N prepreg rolls or panels in a cool, dry environment. Vacuum-sealed or foil-packed packaging should be maintained until immediately before use. The pre-lamination vacuum dry step is designed to recover prepreg that has been exposed to ambient humidity during handling; however, prepreg that has been exposed to high humidity for extended periods may not fully recover through drying alone. Monitor out-time (time outside refrigerated or sealed storage) against Arlon’s recommended limits and work to your fabricator’s incoming inspection procedure for moisture content.

Design Considerations When Using Arlon 38N

Dk and Df in the Rigid Section

With a Dk of 4.25 at 1 MHz and 1 GHz, and a Df of 0.010 at 1 MHz, Arlon 38N behaves as a standard polyimide material electrically. It is not a high-frequency low-loss material — it is a structural bonding prepreg where the primary performance metrics are thermal, mechanical, and adhesion-related rather than electrical. For the rigid sections of a rigid-flex PCB where signal integrity at microwave frequencies is required, the core laminate choice (typically 33N, 35N, or 85N for high-Tg polyimide laminates) drives electrical performance. The 38N bond ply in the stackup contributes its Dk and Df to the overall structure, but it represents only the thin bondline rather than the bulk of the signal layer dielectric.

PTH Reliability and Z-Axis CTE

The Z-axis CTE of 54 ppm/°C below Tg and 157 ppm/°C above Tg must be considered in PTH barrel reliability calculations for vias that span the 38N bond ply. The thermal conductivity of 0.3 W/m·K is typical for polyimide-based systems and is relevant for heat flow calculations in heat sink bonding applications. When designing the thermal model for an assembly that uses 38N as a heat sink bonding ply, use 0.3 W/m·K as the through-board thermal resistance contribution from the bond ply.

Bond Strength Verification

For critical applications — particularly military and aerospace programs with qualification and traceability requirements — verify bond strength by testing coupons from production panels. Arlon’s specified peel strength values (8.5 lbs/in to copper after thermal stress; 5.9 lbs/in to Kapton as received) are typical values and should be used as minimum acceptance criteria targets. Testing per IPC TM-650 2.4.8 provides a direct comparison against the datasheet values.

Useful Resources for Arlon 38N Engineers

Resource	Description	Link
Arlon EMD 38N Official Product Page	Official product description and application overview	arlonemd.com
Arlon 38N Official Datasheet PDF	Complete datasheet with lamination process parameters	arlonemd.com PDF
Arlon Laminate Guide (10th Edition)	Comprehensive Arlon laminate selection guide	arlonemd.com PDF
Cirexx 38N Datasheet PDF	Mirror datasheet with lamination parameters	cirexx.com PDF
LookPolymers 38N Entry	Material summary with key specifications	lookpolymers.com
Insulectro Arlon EMD Page	Distributor perspective on full Arlon EMD product range	insulectro.com
UL Prospector 38N Entry	Full property database entry for Arlon 38N	ulprospector.com
MatWeb 38N Entry	Engineering database with converted property units	matweb.com
IPC-4101 Standard	Specification for base materials for rigid/multilayer boards	ipc.org
RayPCB Arlon PCB Resource	Practical guide to Arlon PCB materials and manufacturing	RayPCB Arlon PCB

5 Frequently Asked Questions About Arlon 38N Laminate

1. What is the difference between Arlon 38N and Arlon 37N, and should I upgrade?

Arlon 38N is the second-generation version of the Arlon 37N polyimide low-flow prepreg. Both are 200°C Tg systems that meet IPC-4101/42 and are used for bonding multilayer polyimide rigid-flex assemblies and heat sink attachment. The key improvements in 38N are faster and more uniform resin cure, improved bond strength to Kapton polyimide film (up to 50% higher), higher decomposition temperature (330°C vs. ~320°C), and better performance in heat sink bonding applications. For new designs, 38N is the recommended current product. For existing 37N-qualified assemblies, upgrading to 38N requires a lamination parameter adjustment and a re-qualification cycle, which may or may not be warranted depending on program requirements.

2. Can Arlon 38N be used with lead-free solder reflow processes?

Yes. Arlon 38N is fully compatible with lead-free solder processing and is RoHS/WEEE compliant. The 200°C Tg and 330°C Td provide adequate margin above lead-free reflow peak temperatures (typically 250–260°C for SAC alloys). The PTH reliability benefits from the polyimide expansion characteristics are particularly relevant in lead-free assemblies, where multiple reflow cycles place higher thermal demands on barrel integrity than traditional tin-lead processes.

3. Why is vacuum lamination required for Arlon 38N?

Low-flow prepregs like Arlon 38N do not displace air voids during lamination the way standard flowing prepregs do. In a standard prepreg, resin flow during lamination physically displaces trapped air from the bondline. With 38N, the controlled low-flow behavior prevents this displacement mechanism. Vacuum must therefore remove air from the bondline before the resin gels. Insufficient vacuum during lamination leaves interlaminar air voids that appear acceptable on cross-section inspection initially but become delamination nucleation sites under thermal cycling. The vacuum draw-down before applying pressure — 30 minutes at less than 29″ Hg — is a non-negotiable process step.

4. Is Arlon 38N suitable for space and aerospace applications?

Yes. Arlon 38N is listed by Arlon EMD for military, aerospace, and space applications, in addition to commercial and industrial use. Its lead-free compatibility, UL94 V-0 flammability rating, 200°C Tg, and polyimide chemical resistance make it appropriate for demanding aerospace programs. For space programs with specific outgassing requirements, verify the TML and CVCM values for your specific lot against the applicable outgassing threshold (NASA SP-R-0022A). Arlon EMD can provide outgassing test data for qualification purposes.

5. What surface finish is recommended for PCBs fabricated with Arlon 38N?

For polyimide rigid-flex assemblies bonded with Arlon 38N, the choice of surface finish is driven by the core laminate and application requirements rather than the 38N bonding ply specifically. ENIG (Electroless Nickel Immersion Gold) is commonly used for polyimide rigid-flex boards in avionics and military applications because of its flat, solderable, and oxidation-resistant surface. HASL is generally not recommended for polyimide assemblies because the high-temperature solder bath can stress the rigid-flex transition zones. For assemblies with long in-service life requirements, ENEPIG is increasingly preferred as it provides better wire bondability and resistance to nickel corrosion compared to standard ENIG.

Final Thoughts on Arlon 38N Laminate

Arlon 38N laminate is a well-engineered solution to a specific and important manufacturing problem: how do you reliably bond multilayer polyimide rigid-flex assemblies with a prepreg that won’t flow into the flex relief areas it must leave clean, while still achieving the bond strength and thermal performance the finished assembly needs across its service life?

The second-generation chemistry in 38N — faster, more uniform cure, 50% higher Kapton adhesion, improved heat sink bond strength — represents meaningful engineering progress over conventional polyimide low-flow materials. For military, aerospace, and space programs where rigid-flex construction is standard and where field failures are never acceptable, these improvements translate directly into more reliable finished assemblies.

The fabrication requirements are not particularly exotic by polyimide rigid-flex standards — vacuum lamination, pre-use drying, and standard polyimide subsequent processing are all routine for shops experienced with this material class. For engineers and procurement teams evaluating bonding prepreg options for their next polyimide rigid-flex program, Arlon 38N deserves to be the default first choice at the 200°C Tg level.