DuPont Interra HK04: Embedded Capacitance Material for HDI PCB — How It Works

If you’re working on a high-density server board, a GPU PCB, or any design where power delivery integrity and EMI suppression keep eating into your BOM cost and board real estate with bypass capacitors, DuPont Interra HK04 is a material you should understand cold. It’s not glamorous — it doesn’t show up in signal layer traces or impedance specs — but it’s the buried layer that fundamentally changes how your power distribution network behaves at high frequency. Done right, it can eliminate dozens or even hundreds of surface-mount bypass capacitors, reduce PDN impedance, and shrink your board. Done without understanding, it adds fabrication complexity and cost that outweighs the benefit.

This guide covers what the DuPont Interra HK04 is, how embedded capacitance physically works, the two main variants (HK04J and HK04M), full material specifications, PCB stackup integration, and what applications actually benefit from using it.

What Is DuPont Interra HK04?

DuPont Interra HK04 is a thin copper-clad laminate specifically designed for use as an embedded capacitance material in multilayer rigid printed circuit boards. It functions as an ultra-thin power/ground plane pair buried inside the PCB stackup — a planar capacitor that distributes capacitance uniformly across the board rather than concentrating it at discrete surface-mount locations.

DuPont Interra thin copper-clad laminates are specifically designed for use as embedded capacitance materials in multilayer rigid printed circuit boards, offering the best mechanical strength, reliability and capacitance stability on the market.

By utilizing Interra laminates between the power and ground planes in a Power Distribution Network (PDN), designers can reduce the modal resonances and lower the inductance between the power and ground planes. This has the effect of reducing the impedance in the system and decreasing the number of required surface mount capacitors.

The “HK04” product family currently comprises two main variants for rigid multilayer boards: the HK04J (the original all-polyimide embedded capacitor laminate) and the HK04M (the next-generation version with thinner dielectric options for even lower inductance). Both share the same fundamental construction — thin polyimide dielectric bonded to low-profile electrodeposited copper — but differ in dielectric thickness options and maximum capacitance density achievable.

For a broader view of how Interra HK04 fits within DuPont’s full electronics materials portfolio, DuPont PCB covers the complete range from laminates to specialty materials.

How Embedded Capacitance Works in PCB Power Distribution

Before getting into the material specifics, it’s worth understanding the physics, because that’s what drives material selection decisions.

The Problem with Surface-Mount Bypass Capacitors

A surface-mount bypass capacitor placed between a power pin and ground has to deal with:

• The via inductance from the surface pad down to the power plane (~1–2 nH per via)
• The trace inductance from the pad to the via
• The package inductance of the capacitor itself

At frequencies above a few hundred MHz, these inductances dominate over the capacitor’s actual capacitive impedance. The capacitor stops doing its job — it’s effectively an inductor at the frequencies where fast digital ICs are switching and generating noise. You end up needing more capacitors, placed closer to the IC, with shorter via stubs, which takes up pad real estate and adds assembly cost.

How Interra HK04 Solves This

All Interra thin copper-clad laminates utilize low-profile electrodeposited copper laminated to thin polyimide-based dielectric. This dielectric is engineered to have superior adhesion to copper than traditional glass-reinforced materials utilized in rigid boards. The polyimide dielectric is based on DuPont Kapton technology, but is specifically engineered to have low dielectric loss, high dielectric isolation and strength, and tight thickness tolerance.

When the HK04 laminate is placed between the power and ground planes in a multilayer stackup, it creates a distributed planar capacitor across the entire board area. Every square centimeter of board that has power and ground above it contributes capacitance. There is no via inductance to fight — the capacitance exists everywhere simultaneously, at zero inductance relative to through-hole capacitors. This is what “power bus decoupling” means in practice.

Interra HK04M provides very low impedance at high frequency, power bus decoupling, and electromagnetic interference reduction. It replaces surface-mount bypass capacitors and their plated-through-holes, which improves the reliability, design flexibility, packaging size and cost of the PWB.

DuPont Interra HK04 Variants: HK04J vs. HK04M

The HK04 family has two current production variants, each suited to slightly different design requirements:

HK04J — Original All-Polyimide Embedded Capacitor Laminate

DuPont Interra HK04J Capacitor Laminate is designed to function as a thinner and more efficient power and ground plane in printed wiring boards. This double-sided copper-clad laminate features an adhesive-less, all-polyimide dielectric layer that affords excellent resilience during processing and fabrication.

Standard HK04J Product Offerings:

Product Code	Capacitance Density (pF/cm²)	Copper Thickness µm (oz)	Dielectric µm (mil)
HK04J2518E	125	18 (0.5 oz)	25 (1.0 mil)
HK04J2536E	125	36 (1.0 oz)	25 (1.0 mil)
HK04J2572E	125	72 (2.0 oz)	25 (1.0 mil)

HK04J is offered in balanced and unbalanced copper constructions. Available with ½ oz (18 µm), 1 oz (35 µm), and 2 oz (70 µm) copper thicknesses in balanced and unbalanced constructions, offered with IPC 4562 Grade 3 reverse treated electrodeposited copper.

HK04M — Next-Generation Embedded Capacitance Laminate

Interra HK04M is DuPont’s next-generation embedded capacitance laminate, which has been optimized to handle thinner dielectric layers and further reduce the impedance between power and ground planes in a PWB.

The key advancement in HK04M is the addition of a 12 µm (0.5 mil) dielectric option alongside the existing 25 µm (1.0 mil) dielectric. Thinner dielectric = higher capacitance density = lower inductance. This matters a great deal at GHz operating frequencies where even small inductance reductions translate to measurable PDN impedance improvements.

Standard HK04M Product Offerings:

Product Code	Cap. Density (pF/cm²)	Cu1 µm (oz)	Dielectric µm (mil)	Cu2 µm (oz)
HK04M351235E	240	35 (1.0)	12 (0.5)	35 (1.0)
HK04M701270E	240	70 (2.0)	12 (0.5)	70 (2.0)
HK04M182518E	125	18 (0.5)	25 (1.0)	18 (0.5)
HK04M352535E	125	35 (1.0)	25 (1.0)	35 (1.0)
HK04M702570E	125	70 (2.0)	25 (1.0)	70 (2.0)
HK04M182535E	125	18 (0.5)	25 (1.0)	35 (1.0)
HK04M352570E	125	35 (1.0)	25 (1.0)	70 (2.0)

Available with ½ oz (18 µm), 1 oz (35 µm), and 2 oz (70 µm) copper thicknesses in balanced and unbalanced constructions, offered with IPC 4562 Grade 3 reverse treated electrodeposited copper. VLP and rolled annealed copper is available upon request. Available with ½ and 1 mil dielectric thicknesses. 1/3 mil (8 µm) thickness is under development.

Key Electrical and Material Properties

Both HK04 variants share the same fundamental polyimide dielectric, which gives them consistent electrical properties across the product family:

Property	Value	Notes
Dielectric Constant (Dk) @ 1 MHz	3.5	Stable across frequency
Dielectric Constant (Dk) @ 2 GHz	3.5	Flat Dk curve — no frequency drift
Dissipation Factor (Df) @ 2 GHz	0.004	Low loss for power distribution
Capacitance Density (25 µm dielectric)	125 pF/cm²	Per unit board area
Capacitance Density (12 µm dielectric)	240 pF/cm²	HK04M only
IPC Certification	IPC-4821/1	Embedded capacitance materials standard
UL Rating	94V-0	Flame retardant compliant
Halogen-Free	Yes	RoHS compatible
Storage Temperature	4–29°C (40–85°F)	≤70% relative humidity
Shelf Life	2 years from Certificate of Analysis date	In original packaging

The flat Dk curve (3.5 at both 1 MHz and 2 GHz) is notable. Most FR-4 dielectrics show Dk variation from 4.5 at low frequencies down toward 4.0 at GHz range. The Interra HK04 polyimide dielectric’s Dk stability means that capacitance density is predictable across a wide frequency band — a critical requirement for accurate PDN simulation.

How Interra HK04 Is Integrated Into an HDI PCB Stackup

Understanding the physical integration is where this material goes from theory to fabrication practice.

Basic Stackup Concept

The HK04 laminate is placed between the two closest power and ground planes in the stackup. In a standard 8-layer HDI board with the configuration S/GND/S/PWR/GND/S/GND/S, you would replace the PWR/GND layer pair in the center with an HK04 laminate — the two copper layers of the HK04 become the new power and ground planes, separated by 12–25 µm of polyimide instead of the 100–200 µm of prepreg that would otherwise occupy that space.

Processing Compatibility

Interra HK04M can be processed as a thin flexible circuit laminate through the develop/etch/strip process steps. The HK04M dielectric is flexible and can be imaged and etched to remove copper on both sides of the dielectric at the same time.

This is practically important: the HK04 can be processed through standard develop-etch-strip (DES) lines used for flex circuit fabrication. It does not require exotic equipment — shops with flex circuit capability can integrate HK04 into rigid multilayer builds without adding new process equipment. The flexible nature of the polyimide dielectric means it handles the thin construction without the brittleness issues that would plague thin glass-reinforced cores.

Interra HK04M Capacitor Laminate is supplied in sheet form, with standard dimensions of 18 × 24 in (457 × 610 mm). It integrates into standard panel-based rigid multilayer lamination cycles.

Fabrication Considerations for HDI Integration

Working with HK04 in an HDI multilayer build introduces some handling and process disciplines that differ from standard core layers:

Thin material handling: at 12–25 µm dielectric thickness with copper total stack of 30–70 µm per side, these are fragile sheets. Panel handling should use tacky roller cleaning to remove particles (which can cause pinholes in such thin dielectrics) and flatness support during transport to the lamination press.

Etching both copper layers simultaneously: the HK04 dielectric is thin enough that conventional heavy-handed etchant conditions can cause dielectric attack if both sides are etched at the same time without careful spray balance control. Use well-balanced spray pressure across top and bottom and verify etch uniformity on first articles.

Registration precision in multilayer layup: the HK04 pair, once imaged and etched, must be registered against the adjacent inner layers during the lamination layup. The tolerances on power and ground plane openings (clearance pads, anti-pads) need to account for the reduced dielectric thickness and its impact on voltage isolation.

Lamination cycle: the HK04 laminate participates in the standard multilayer vacuum lamination cycle. Because it is polyimide-based, not epoxy-based, it does not contribute to prepreg flow during lamination — it is treated as a rigid core layer in the stackup, sandwiched between prepreg layers.

Application Cases That Justify Using DuPont Interra HK04

Not every board needs embedded capacitance. The HK04 adds material cost and fabrication complexity. Here are the application profiles where the tradeoff consistently makes sense:

Application	Why HK04 Helps
High-speed servers and routers	PDN at GHz+ frequencies; 100s of bypass caps eliminated
GPU PCBs	Dense power delivery to many cores; board area for routing recovered
Telecom backplanes	Low-inductance power delivery across large boards
Military and aerospace PWBs	High reliability, reduced solder joint count improves MTBF
Boards with >4 SMT bypass caps per in²	BOM and assembly cost justification threshold
Compact IoT/mobile designs	Board thickness and area reduction from fewer caps

Testing confirmed that 0.001-inch unreinforced copper-clad polyimide provides benefits to power supply PCB packaging, validated by Teradyne in production server power supply boards. The same paper noted that 156 surface-mount 100 nF capacitors could be reduced significantly through Interra HK04 integration in a power plane application.

HK04J vs. HK04M: Which Variant for Your Design?

Criteria	HK04J	HK04M
Dielectric thickness	25 µm (1.0 mil) only	12 µm (0.5 mil) and 25 µm (1.0 mil)
Max capacitance density	125 pF/cm²	240 pF/cm²
PDN inductance reduction	Good	Better — thinner dielectric = lower inductance
Extreme environment reliability	High (Mars Rover qualified)	High
Fabrication complexity	Standard flex processing	Requires tighter handling for 12 µm variant
Best fit	Established HDI designs, defense, aerospace	Next-gen server, high-frequency power delivery

If your design is pushing above 3–5 GHz operating frequencies, the 12 µm HK04M variant with 240 pF/cm² is the right path. For most HDI server and telecom work at 1–3 GHz, the 25 µm HK04J at 125 pF/cm² is proven, well-understood by qualified fabricators, and carries the field reliability history that matters for MIL/AERO qualification.

Useful Resources for PCB Engineers

Resource	Link
Interra HK04J Official Datasheet (EI-10134)	DuPont PDF
Interra HK04M Official Datasheet (EI-10135)	DuPont PDF
DuPont Interra Product Page (Qnity)	qnityelectronics.com — Interra HK04M
Insulectro — Interra HK04M and HK04J	insulectro.com/products/interra
DuPont PCB Materials Overview	pcbsync.com/Dupont-pcb
Northwest Engineering — HK04M Material Guide	nwengineeringllc.com
GlobalSpec — Interra HK04J Datasheet & Specs	globalspec.com
IPC-4821 — Specification for Embedded Passive Devices	ipc.org
IPC-2316 — Design Guide for Embedded Passive Printed Boards	ipc.org

5 FAQs About DuPont Interra HK04

Q1: How do I calculate how many bypass capacitors I can eliminate with Interra HK04? The baseline calculation is: total board capacitance = capacitance density (pF/cm²) × active board area (cm²). For a 200 cm² board using HK04J at 125 pF/cm², that’s 25,000 pF = 25 nF of distributed capacitance. At high frequency this distributed capacitance is more effective than concentrated SMT capacitors because it has near-zero inductance. Typically, one HK04 layer can replace the highest-frequency bypass capacitors — often 100 nF 0402 capacitors in the 100 MHz to 1 GHz range. The exact count depends on your IC’s switching frequency, target PDN impedance, and current delivery requirements. Accurate PDN simulation using S-parameter models of the HK04 layer will give you the best answer. DuPont provides simulation models through their technical support team.

Q2: Is Interra HK04 compatible with standard FR-4 multilayer fabrication? Yes, with caveats. The HK04 laminate itself is fully compatible with standard PWB processes including etching, lamination, drilling, and plating. However, because it is a polyimide-based laminate rather than an epoxy/glass laminate, it behaves differently during lamination (it does not reflow) and requires flex-circuit-style handling for the thin dielectric variants. Fabricators who are qualified and experienced with HK04 integration are listed by DuPont. Using an unqualified fab for your first HK04 build is a risk — process audits or DuPont technical support engagement before committing to a production run is strongly recommended.

Q3: Does Interra HK04 affect controlled impedance traces on adjacent layers? Yes, the HK04’s close power/ground plane spacing has an effect on adjacent signal layers. The thinner power/ground pair acts as a better reference plane for signals on the layers immediately above and below it — this actually tends to improve impedance control consistency on those layers. However, the dielectric constant of polyimide (Dk 3.5) differs from FR-4 prepreg (Dk ~4.0–4.5), so the thickness of prepreg between the HK04 and adjacent signal layers needs to be accounted for in your impedance calculations. Most impedance calculators handle mixed dielectric stackups — just input the correct Dk and thickness for each layer in the stackup.

Q4: What is the voltage isolation capability of the Interra HK04 thin dielectric? The HK04 polyimide dielectric provides very high voltage isolation relative to its physical thickness. Polyimide has a dielectric strength of approximately 300–400 V/µm — a 25 µm HK04J layer provides isolation well above the typical 3.3V–12V power planes used in digital systems. Even the 12 µm HK04M variant at typical power voltages is not at risk for dielectric breakdown in normal digital applications. For high-voltage applications (>48V), consult DuPont’s technical data for breakdown voltage specifications of the specific variant.

Q5: How does embedded capacitance with HK04 affect CAF (Conductive Anodic Filament) performance? CAF testing was specifically conducted on Interra HK04 in production boards by Teradyne. The key finding was that at a constant test voltage, the HK04 core is subjected to twice the electrical stress per mil of material compared to standard cores (because it is half the thickness). Despite this higher stress per unit thickness, the all-polyimide dielectric’s CAF resistance was comparable to standard glass-reinforced laminates in tests conducted at both 10V and 100V DC. The polyimide dielectric does not have the glass fiber/resin interface that is typically the initiation site for CAF in traditional FR-4 materials, which actually gives it an inherent CAF resistance advantage in some test modes.

Engineering Perspective

The DuPont Interra HK04 family is mature, production-proven technology that has been qualified on boards ranging from enterprise server power supplies to the Mars Rover. The decision to use it comes down to a straightforward tradeoff: the material and fabrication premium for adding a thin HK04 core to your multilayer stackup versus the assembly cost, board area, and reliability cost of the bypass capacitors it replaces. For any design carrying more than 4 bypass capacitors per square inch in the power delivery network, or operating at frequencies above 500 MHz where SMT bypass capacitor effectiveness starts degrading, the Interra HK04 analysis is worth running. The HK04M’s availability in 12 µm dielectric with 240 pF/cm² pushes the effectiveness into the 1–5 GHz frequency range where modern high-speed digital designs are increasingly demanding better power delivery solutions.