DuPont Pyralux AP-PLUS: Thick Copper All-Polyimide for High-Speed PCB Design

Most flex PCB engineers know the standard Pyralux AP family well — thin dielectrics, light copper, multilayer rigid-flex cores. But there’s a segment of high-speed and high-current design work where the standard AP constructions create a practical manufacturing problem: achieving controlled impedance at 75 ohms or 100 ohms with thin dielectrics forces trace widths so narrow that fabrication yield suffers badly. That’s the exact problem DuPont Pyralux AP-PLUS was designed to solve. This guide covers what AP-PLUS is, how the thick-core dielectric improves both impedance control and fabrication yield, the full construction and material spec, and where this material makes genuine engineering sense.

What Is DuPont Pyralux AP-PLUS? Understanding the Core Concept

DuPont Pyralux AP-PLUS is a family of all-polyimide thick copper-clad laminates — the “PLUS” referring to the extended range of dielectric thicknesses from 7 to 12 mils that go well beyond the standard Pyralux AP range. Like the base AP series, it is an adhesiveless construction: polyimide film bonded directly to copper foil with no intermediate acrylic or epoxy adhesive layer. The defining difference is that AP-PLUS laminates in 7–12 mil all-polyimide thicker base materials provide new design flexibility by allowing the use of greater line widths to achieve desired impedance control while also providing fabricators with higher overall manufacturing yields in standard printed wiring board process equipment and chemistries.

The practical impact of that statement is significant. In high-impedance flex circuit designs — differential pairs at 100 ohms, antenna feedlines at 75 ohms — the trace geometries required on thin dielectrics push fabrication into sub-4 mil line and space territory. Below that threshold, fine-line imaging defects, etch undercut variability, and registration tolerances start accumulating into meaningful yield loss. AP-PLUS sidesteps that problem entirely by moving to a thicker dielectric and proportionally wider traces.

For DuPont PCB design engineers working on high-frequency flex circuits, antenna modules, or power delivery applications combining thick copper with signal integrity requirements, AP-PLUS occupies a unique and valuable position in the material selection matrix.

The Weave-Free Advantage: Why Polyimide Beats Fiberglass for High-Speed Flex

Before getting into the AP-PLUS specifics, it’s worth understanding the fundamental substrate advantage that motivates the choice of all-polyimide dielectric in the first place.

Unlike typical printed circuit boards that are constructed from woven fiberglass fabrics strengthened and bound in an epoxy matrix, AP-PLUS is a “weave-free” all-polyimide profile providing a smoother surface and a homogeneous medium for improved signal performance. It gives designers a consistent dielectric constant for controlled impedance circuit requirements and minimized signal degradation.

The fiber weave effect in FR4 is a documented signal integrity problem at high data rates. The dielectric constant varies spatially depending on whether a trace runs over a glass fiber bundle or over an epoxy-rich resin pocket — and this non-uniformity introduces differential phase delay between the two legs of a differential pair, which shows up as common-mode noise, skew, and ultimately degraded eye diagrams. On a flex circuit built from polyimide, this mechanism simply doesn’t exist. The dielectric constant is spatially uniform across the entire substrate.

AP-PLUS does not contain glass, which gives it exceptional dielectric isotropy. Signals are routed in any direction on the circuit board and see the same dielectric constant regardless of routing angle. For antenna flex circuits where trace routing follows curved geometries, this is not a theoretical benefit — it’s a real design constraint eliminated.

DuPont Pyralux AP-PLUS Construction and Available Configurations

AP-PLUS is primarily a single-sided copper-clad laminate in its standard offering, supplied in sheet form. The key differentiator from standard AP is the dielectric thickness range:

Dielectric Thickness Options

Dielectric Thickness (mil)	All-Polyimide	Adhesiveless
7	Yes	Yes
8	Yes	Yes
9	Yes	Yes
10	Yes	Yes
11	Yes	Yes
12	Yes	Yes

Thickness values are available in 1 mil increments from 7 to 12 mil — a granularity that standard AP doesn’t offer in this thickness range. The tight thickness control minimizes impedance variations of signal lines. In contrast to other products that run at 15–20% thickness tolerance, AP-PLUS’s tighter tolerance directly translates to tighter impedance prediction accuracy during PCB design simulation.

Copper Configurations

Standard AP-PLUS is supplied as a single-sided copper-clad laminate, with thick copper foil options extending well beyond the standard 2 oz (70 µm) cap of the base AP series. Custom sizing up to 100 inches in length (2.54 meters) is available through a DuPont representative, as are additional copper types and double-sided constructions. Specialty dielectric thickness constructions can also be arranged through direct engagement with DuPont.

Sheet Sizes

AP-PLUS laminates are supplied in standard sheet sizes suitable for processing on standard printed wiring board equipment. Non-standard panel sizes up to 100 inches in the length dimension are available on request — useful for long flex cable assemblies that exceed standard panel dimensions.

Key Electrical and Material Properties of Pyralux AP-PLUS

AP-PLUS shares the core all-polyimide material properties of the standard Pyralux AP family. The following data applies to the AP-PLUS construction based on DuPont’s published specifications and the AP material family baseline.

Electrical Properties

Property	Typical Value	Notes
Dielectric Constant (Dk) at 1 MHz	3.4	Consistent vs. frequency (isotropic)
Loss Tangent (Df) at 1 MHz	0.002	Adhesiveless — no acrylic penalty
Dielectric Strength	200 V/µm (5,000 V/mil)	ASTM D-149
Surface Resistivity	>10¹³ Ω	IPC-TM-650
Volume Resistivity	>10¹⁴ Ω·cm	IPC-TM-650
Spatial Dk Uniformity	Excellent	Weave-free, no fiber effect

Mechanical and Thermal Properties

Property	Typical Value	Notes
Glass Transition Temperature (Tg)	220°C	TMA measurement
In-Plane CTE (T < Tg)	~25 ppm/°C	Low — good for rigid-flex
Tensile Strength	345 MPa	Method 2.4.19
UL Flammability	V-0 / VTM-0	UL 94
IPC Certification	IPC-4204/11	—
Refrigeration Required	No	4–29°C, <70% RH storage
Product Warranty	2 years	From date of manufacture

The Dk of 3.4 with loss tangent of 0.002 is the same exceptional figure that defines the full AP family. At multi-GHz frequencies, AP-PLUS maintains this low loss characteristic — and because it’s weave-free, the Dk remains flat across the spatial dimensions of the substrate in a way that even the best woven laminates cannot match.

The Controlled Impedance Case: Why Thick Dielectric Improves Fabrication Yield

This section is specifically for engineers who have lived through the pain of trying to hit 100-ohm differential impedance on a thin flex substrate. The math is straightforward, but its consequences for yield are worth spelling out.

For a microstrip trace on a polyimide substrate, characteristic impedance scales with the ratio of dielectric thickness to trace width. To achieve 75-ohm or 100-ohm impedance on a thin dielectric — say 4 mils — you end up needing trace widths in the 3–4 mil range. At that geometry, fabrication variability in etch chemistry, imaging resolution, and registration all contribute to impedance spread that can exceed ±10%, which is often outside the ±5–7% impedance tolerance called out in high-speed design specs.

AP-PLUS enables better manufacturing yields not only at 50 ohms, but at higher impedance values including 75 and 100 ohms. Designers and fabricators also have the flexibility of choosing thickness values in 1 mil increments to better optimize designs. As a concrete example: a 75-ohm impedance design that requires 4 mil line/space on a 4 mil thick substrate can instead be realized with 8–9 mil line/space traces on an 8 mil thick AP-PLUS substrate — providing substantially higher manufacturing yield while achieving identical electrical output.

That 2x wider trace geometry is not a minor improvement. In flex circuit fabrication, 4 mil traces run close to the practical yield limit for many shops’ photoimaging and etching capabilities. 8–9 mil traces run comfortably within the process window of virtually any flex shop worldwide. The yield difference on a high-layer-count panel can be substantial enough to change the economics of the design entirely.

Impedance vs. Dielectric Thickness: Design Trade-off Summary

Target Impedance	Approx. Trace Width on 4 mil PI	Approx. Trace Width on 8 mil PI	Yield Impact
50 Ω (microstrip)	~8–9 mil	~18–20 mil	Moderate improvement
75 Ω (microstrip)	~4 mil	~8–9 mil	Significant improvement
100 Ω differential	~3–4 mil / 4 mil space	~7–8 mil / 8 mil space	High improvement

Values are approximate and depend on coverlay thickness and copper weight. Always verify with stack-up modeling.

Attenuation, Skin Effect, and Signal Integrity at High Frequency

For high-speed design engineers, the two dominant sources of signal loss at GHz frequencies are dielectric loss and conductor (skin effect) loss. AP-PLUS addresses both.

Dielectric loss is minimized by the low loss tangent of the polyimide dielectric (0.002), which stays low across the GHz range unlike acrylic adhesive-based systems where the loss tangent increases significantly above 1 GHz. Attenuation is a significant concern at high frequencies, and it can be reduced through the use of thicker AP-PLUS laminates to create wider lines.

Conductor loss / skin effect is reduced by using low-profile, smooth copper foil, which minimizes losses from the skin effect. At high frequencies, current flows in an increasingly thin skin depth at the copper surface. Rough copper surface (as found in standard electrodeposited foils) creates additional surface area in the current path and increases effective resistance. Using low-profile copper foil in combination with AP-PLUS’s smooth polyimide surface gives the lowest possible conductor loss for a given copper weight.

The combination of these two effects — wider traces reducing edge roughness effects, and smooth copper reducing skin effect losses — makes AP-PLUS a strong candidate for flex circuits operating above 5 GHz, particularly in antenna modules, radar front-end flex interconnects, and 5G mmWave feed structures.

Applications Where DuPont Pyralux AP-PLUS Delivers Real Value

Application	Why AP-PLUS Works Here	Key Technical Driver
Antenna-flex circuits	Weave-free Dk uniformity for consistent antenna performance	Isotropic dielectric constant
5G mmWave flex modules	Low loss at multi-GHz with wider, yieldable trace geometries	Low Df + thick dielectric
High-current flex busbars	Thick copper capability in an all-polyimide, thermally stable substrate	Copper weight + Tg 220°C
EV battery management flex	Combines signal traces with high-current conductors in one laminate	Multi-function thick copper
Radar front-end interconnects	Controlled impedance at 75/100 ohm with high fab yield	Thick-core impedance geometry
Rigid-flex multilayer cores	Low CTE, adhesiveless, compatible with standard rigid-flex process	CTE + process compatibility
High-speed backplane flex cables	100-ohm differential pairs at manufacturable trace widths	Impedance + yield
Defense / avionics flex	All-polyimide thermal resistance, IPC-4204/11 certified	Reliability certification

The antenna-flex application is worth a specific note. DuPont’s published AP-PLUS datasheet includes a 250X cross-section image of an antenna-flex circuit to illustrate the material’s use in this context. The smooth surface, homogeneous dielectric, and isotropic Dk make it particularly well-suited for planar antenna structures where dielectric non-uniformity would cause radiation pattern distortion.

DuPont Pyralux AP-PLUS vs. Standard Pyralux AP: When to Choose Which

Engineers moving from standard AP to AP-PLUS should understand where the transition makes sense and where it doesn’t.

Factor	Standard Pyralux AP	Pyralux AP-PLUS
Dielectric range	0.5–6 mil (standard); up to 20 mil (special order)	7–12 mil (standard)
Copper range	6–140 µm (standard)	Thick copper focus; custom double-sided available
Impedance target	50 Ω standard; higher impedances require fine lines	50–100 Ω at manufacturable widths
Fabrication yield at 75/100 Ω	Low (narrow traces)	High (wider traces possible)
Dynamic flex capability	Excellent (thin, light)	Not recommended (thick, stiffer)
Best use	Thin multilayer flex, dynamic flex, fine-pitch	High-impedance signal, thick copper, antenna-flex
Process compatibility	All conventional flex processes	Standard PWB processes

The dynamic flex column is important. AP-PLUS, because of its thicker dielectric, has higher bending stiffness than standard thin AP constructions. It is not appropriate for circuits that require repeated flexing in service. The material is intended for static flex applications — installed and bent once, or used in rigid-flex constructions where the flex zone is only used during assembly and not during operation.

Fabrication, Processing, and Quality Notes

Process Compatibility

AP-PLUS copper clads are fully compatible with all conventional flexible circuit fabrication processes including oxide treatment and wet chemical plated-through-hole desmearing. Fabricated circuits can be cover coated and laminated together to form multilayers or bonded to heat sinks using polyimide, acrylic, or epoxy adhesives. Critically, AP-PLUS offerings are fully compatible with existing printed wiring board processes and material handling systems — standard PWB shop equipment, not specialized flex-only lines.

Storage and Handling

AP-PLUS does not require refrigeration and should be stored in the original packaging at temperatures of 4–29°C (40–85°F) and below 70% humidity. The product should not be frozen and should be kept dry, clean, and well protected. As with all copper-clad laminates, sharp foil edges are a handling hazard — appropriate cut-resistant gloves should be worn by fab personnel. Lamination areas require adequate ventilation to manage trace quantities of residual solvent that can volatilize during press cycles.

Quality and Certification

AP-PLUS copper clads are manufactured under a quality system registered to ISO 9002 (older certification reference in published datasheet) and certified to IPC-4204/11. Complete material and manufacturing records, including archive samples of finished product, are maintained by DuPont. Each manufactured lot is identified for reference and traceability via the packaging label, which includes the product name, batch number, size, and quantity.

Useful Resources for Engineers Specifying Pyralux AP-PLUS

Resource	Description	Link
Pyralux AP-PLUS Datasheet (PDF)	Full specifications, construction options, impedance yield data	Available via Cirexx or pyralux.dupont.com
DuPont Pyralux AP Official Page	AP family overview and latest datasheet downloads	dupont.com/pyralux-ap
IPC-4204 Specification	Industry standard for flexible metal-clad dielectrics	ipc.org
Cirexx AP-PLUS Datasheet	Distributor-hosted AP-PLUS PDF download	cirexx.com
DuPont Flex Materials Portfolio	Complete Pyralux family overview including AP-PLUS	dupont.com/laminates
DuPont Safe Handling Guide	Processing safety and storage guidelines	pyralux.dupont.com
PCBSync DuPont PCB Hub	DuPont material flex PCB manufacturing guidance	pcbsync.com/Dupont-pcb

Frequently Asked Questions About DuPont Pyralux AP-PLUS

Q1: What is the key difference between Pyralux AP-PLUS and standard Pyralux AP?

The fundamental difference is dielectric thickness range and design intent. Standard Pyralux AP covers dielectric thicknesses from 0.5 to 6 mils in its off-the-shelf range, optimized for thin multilayer flex and dynamic flex applications. AP-PLUS covers 7 to 12 mil dielectric thicknesses in 1 mil increments, specifically designed for high-impedance controlled-impedance designs (75 Ω, 100 Ω differential) where thick dielectric enables proportionally wider trace geometries and significantly improved fabrication yield. Both are adhesiveless all-polyimide constructions with the same Dk/Df performance, but AP-PLUS is a static-flex or rigid-flex material, not a dynamic flex material.

Q2: Can DuPont Pyralux AP-PLUS be used for dynamic flex applications?

No — AP-PLUS should not be specified for dynamic flex applications. The thicker dielectric increases bending stiffness substantially compared to standard thin AP constructions. AP-PLUS is designed for static-flex and rigid-flex installations where the circuit is bent once during assembly and then remains in a fixed position, or where it functions as the inner core of a rigid-flex panel. For continuously flexing applications — hinges, robotic arm cables, wearable flex loops — the standard thin Pyralux AP series with RA copper is the correct material.

Q3: How does the weave-free construction of Pyralux AP-PLUS benefit antenna and high-frequency designs?

Glass fiber-reinforced substrates like FR4 create a spatially varying dielectric constant because the local Dk depends on whether a given point in the substrate is dominated by glass fiber (higher Dk ~6) or epoxy resin (lower Dk ~3.5). This fiber weave effect causes differential phase velocity between parallel traces, introducing skew in differential pairs and distortion in tightly spaced antenna elements. AP-PLUS, being a pure polyimide dielectric without any glass reinforcement, has a uniform and isotropic Dk of 3.4 across the entire substrate surface. For antenna-flex circuits where trace routing follows curved paths and consistent electrical length is critical to radiation pattern accuracy, this isotropy is a genuine design requirement, not just a nice-to-have.

Q4: What IPC specification should I reference when calling out Pyralux AP-PLUS on a fabrication drawing?

Pyralux AP-PLUS is certified to IPC-4204/11: “Flexible Metal-Clad Dielectrics for Use in Fabrication of Flexible Printed Wiring, Sheet 11.” Reference this specification in your fabrication notes when calling out the base laminate. Your flex fabricator’s approved materials list should confirm AP-PLUS as an approved source for IPC-4204/11 compliance. If AP-PLUS is not on their approved list, request a material qualification — the processing compatibility with standard flex and PWB equipment typically makes this a straightforward qualification task.

Q5: What copper thicknesses are available for Pyralux AP-PLUS, and can I get a double-sided construction?

Standard AP-PLUS configurations are single-sided copper-clad laminates with thick copper foil options. Double-sided constructions and additional copper types are available by request through a DuPont representative. Custom sizing up to 100 inches in length is also available for large-panel or long flex cable applications. For specific copper weight requirements — particularly in high-current applications like EV battery management flex where conductors need to carry tens of amperes — contact your DuPont representative or authorized distributor to confirm availability of your required construction.

Wrapping Up: Where Pyralux AP-PLUS Fits in Your Material Selection

DuPont Pyralux AP-PLUS is a specialist material that solves a specific, well-defined problem: it makes high-impedance flex circuits manufacturable at reasonable yields without sacrificing the dielectric quality that makes the AP family worth specifying in the first place.

If your design has 75-ohm or 100-ohm traces on a flex substrate and your fab shop is struggling with fine-line yield, AP-PLUS is the answer. If your design has thick copper conductors running alongside signal traces on an all-polyimide flex substrate — think EV battery sensing or RF power modules — AP-PLUS gives you the thermal headroom and copper capacity in a format that standard thin AP simply can’t provide.

The weave-free polyimide substrate, tight thickness tolerance, and full compatibility with standard PWB equipment make it practically accessible. It doesn’t require a specialized fab process. It doesn’t require exotic materials handling. What it does require is a design decision to move to a thicker core, and in the applications it’s built for, that trade is almost always worth making.

For flex PCB manufacturing guidance using DuPont materials including Pyralux AP-PLUS, visit PCBSync’s DuPont PCB resource page.