PP-7628HF Halogen-Free Prepreg: MLB Green Compliance Guide

If you have been specifying PP-7628 for your multilayer boards and your customer is now asking for a green compliance declaration, you are not alone. The shift from standard brominated FR-4 prepreg to PP-7628HF prepreg — the halogen-free variant of the industry’s most widely used thick-ply glass style — is one of the most common material transitions happening in commercial PCB fabrication right now. Consumer electronics OEMs, automotive Tier 1 suppliers, and enterprise hardware brands are all tightening their material requirements beyond RoHS, and 7628HF is the first material change that many multilayer board programs have to make.

This guide covers everything a PCB engineer needs to know about PP-7628HF prepreg: what makes it halogen-free, the exact compliance thresholds it must meet, how its electrical and thermal properties compare to standard 7628, where it fits in a multilayer (MLB) stackup, and the lamination process adjustments that prevent delamination and voids when switching. Premium halogen-free laminate families from vendors like Doosan Electronic Materials pair specifically with 7628HF prepreg in MLB constructions — knowing the material pairing rules saves re-qualification work down the line.

What Is PP-7628HF Prepreg? Halogen-Free Resin on Heavy Glass Fabric

PP-7628HF prepreg starts with exactly the same IPC-EG-140 glass fabric as standard PP-7628: ECG 75 E-glass yarn, woven at 44 ends per inch in the warp and 31 picks per inch in the fill, producing the heaviest standard commercial prepreg at approximately 203 g/m². Everything visible about the fabric is identical to standard 7628. The difference is entirely in the resin system.

Standard PP-7628 uses tetrabromobisphenol A (TBBPA) as its primary flame retardant. TBBPA is effective, inexpensive, and has been the backbone of FR-4 chemistry for decades. It is also bromine-based — and bromine content in standard FR-4 typically runs 15–20% by weight, far exceeding the halogen-free threshold of 900 ppm (0.09% by weight).

PP-7628HF prepreg replaces TBBPA with a phosphorus-nitrogen (P-N) compound system. The phosphorus-based flame retardant works by a condensed-phase char-forming mechanism rather than the gas-phase radical-quenching mechanism of brominated systems. The result is a UL94 V-0 material — same flame rating as standard FR-4 — with bromine and chlorine content each held below 900 ppm and total halogen below 1,500 ppm, satisfying IEC 61249-2-21 and JPCA-ES-01-2003. The material is also phosphorus-free in some high-performance formulations, where inorganic fillers provide the flame retardant mechanism, but the P-N route is by far the most common in commercial 7628HF products from Shengyi, Ventec, Isola, and ITEQ.

PP-7628HF Prepreg: Compliance Thresholds at a Glance

Substance	Limit (halogen-free definition)	Standard FR-4 typical level	PP-7628HF level	Defining Standard
Bromine (Br)	≤900 ppm	~150,000–200,000 ppm	<900 ppm	IEC 61249-2-21
Chlorine (Cl)	≤900 ppm	<500 ppm (process residual)	<900 ppm	IEC 61249-2-21
Total Br + Cl	≤1,500 ppm	>150,000 ppm	<1,500 ppm	IEC 61249-2-21
PBB (polybrominated biphenyl)	Prohibited	None intentionally	None	EU RoHS
PBDE (polybrominated diphenyl ether)	Prohibited	None intentionally	None	EU RoHS
Antimony trioxide (flame synergist)	Not restricted, but targeted	Sometimes present	Not used	OEM green programs
Halogen-free mark	“HF” designation	Not applicable	IEC 61249-2-21 compliant	IPC-4101 /126, /128

A critical point many engineers miss: a standard FR-4 board can be 100% RoHS compliant while containing 15–20% bromine by weight because RoHS bans PBB and PBDE specifically, not TBBPA. If your customer asks for “halogen-free” compliance, RoHS certification alone does not satisfy it. Always request a specific IEC 61249-2-21 or JPCA-ES-01-2003 material certificate for 7628HF prepreg, not just a RoHS declaration.

PP-7628HF Prepreg Electrical and Thermal Properties

Electrical Property Comparison: 7628HF vs. Standard 7628

The phosphorus-nitrogen resin system in 7628HF has somewhat different dielectric characteristics than brominated epoxy. In most commercial products the Dk of 7628HF is marginally higher — typically 0.1–0.2 units at 1 GHz — due to the higher polarity of phosphorus-containing compounds compared to the non-polar bromine-epoxy system. The impact on a standard multilayer impedance calculation is modest but not zero, and it is the reason you should not simply drop 7628HF into an existing 7628 stackup without recalculating impedance on the inner layers.

Property	PP-7628 Standard (SR)	PP-7628HF SR	PP-7628HF MR	PP-7628HF HR	Test Method
Resin Content	42–44%	42–45%	47–49%	50–53%	IPC-TM-650 2.3.16
Cured Thickness / ply	~170–180 µm	~170–185 µm	~185–195 µm	~195–210 µm	IPC-TM-650 2.4.39
Dk @ 1 GHz	~4.5–4.7	~4.6–4.8	~4.4–4.6	~4.3–4.5	IPC-TM-650 2.5.5.5
Dk @ 2.4 GHz	~4.3–4.5	~4.4–4.6	~4.2–4.4	~4.1–4.3	IPC-TM-650 2.5.5.5
Df @ 1 GHz	~0.014–0.016	~0.013–0.016	~0.013–0.016	~0.012–0.015	IPC-TM-650 2.5.5.5
Tg (DSC) — standard	≥130–140°C	≥150–155°C	≥150–155°C	≥150–155°C	IPC-TM-650 2.4.25
Tg (DSC) — high-Tg grade	N/A	≥170–180°C	≥170–180°C	≥170–180°C	IPC-TM-650 2.4.25
Td (decomposition)	≥300–320°C	≥330–360°C	≥330–360°C	≥330–360°C	IPC-TM-650 2.4.24.6
T-260	>5 min	>10 min	>10 min	>10 min	IPC-TM-650 2.4.24.1
T-288	<1 min (standard)	>5 min	>5 min	>5 min	IPC-TM-650 2.4.24.1
UL 94 Flame	V-0	V-0	V-0	V-0	UL 94
Bromine content	~15–20%	<900 ppm	<900 ppm	<900 ppm	IEC 61249-2-21
Peel strength	≥1.0 N/mm	≥1.0–1.2 N/mm	≥1.0–1.2 N/mm	≥1.0–1.2 N/mm	IPC-TM-650 2.4.8

One important thermal upgrade that comes with 7628HF is Td and T-288 performance. Because phosphorus-nitrogen resin systems require more thermally robust curing agents, commercial 7628HF products almost universally deliver better T-288 and Td values than mid-Tg standard 7628 — this actually makes them better suited for lead-free assembly reflows (peak ~260°C) than the standard material they are replacing.

PP-7628HF Resin Grade Selection for MLB Inner Layers

As with all 7628 prepreg, the HF variant is available in SR (standard resin), MR (medium resin), and HR (high resin) grades. For MLB inner-layer separations between 1 oz inner copper planes, SR 7628HF (42–45% RC) provides adequate resin flow with minimal squeeze-out and the most predictable dielectric thickness. For builds with 2 oz inner copper, shifting to MR or HR ensures full copper topography fill without voids. The dimensional stability advantage of the 7628 glass style — its tight 44×31 weave holds the panel square during lamination — is retained in the HF variant, making 7628HF SR the correct default for large-panel MLB production where registration tolerance is tight.

Where PP-7628HF Prepreg Fits in a Halogen-Free MLB Stackup

Standard 6-Layer and 8-Layer MLB Stackup With 7628HF

In a standard 6-layer 1.6 mm halogen-free board, PP-7628HF prepreg is used in the same structural positions as standard 7628: as the thick inner dielectric separating the layer 2/3 and layer 4/5 core sandwich. The outer build-up uses thinner glass styles (2116 or 1080 in their HF equivalents) to control impedance on the outer signal layers and limit via stub effects.

Position	Material	Approximate Thickness	Notes
L1 outer Cu	1 oz copper + plating	~50 µm	Outer signal
L1–L2 prepreg	2 × 2116HF MR	~240 µm	Controlled impedance dielectric
L2	0.5 oz core Cu	~18 µm	Ground reference
L2–L3 core	0.3 mm FR-4 HF core	~300 µm	Signal / ground sandwich
L3	0.5 oz core Cu	~18 µm	Signal inner
L3–L4 prepreg	1 × 7628HF SR	~180 µm	Plane separation — 7628HF
L4	0.5 oz core Cu	~18 µm	Power inner
L4–L5 core	0.3 mm FR-4 HF core	~300 µm	Power / signal sandwich
L5	0.5 oz core Cu	~18 µm	Signal inner
L5–L6 prepreg	2 × 2116HF MR	~240 µm	Controlled impedance dielectric
L6 outer Cu	1 oz copper + plating	~50 µm	Outer signal
Total (approx.)		~1.43 mm → pressed to ~1.6 mm

The 7628HF SR ply in the L3–L4 position provides the thick mechanical separation needed between inner power and ground planes. Because signal traces are not routed through this dielectric in a standard 6-layer stackup, the slightly higher Dk of 7628HF relative to standard 7628 does not affect controlled impedance calculations — those are managed by the 2116HF outer dielectric positions.

When to Use 7628HF vs. Thinner HF Prepreg Styles

Design Requirement	Use 7628HF	Use 2116HF or 1080HF Instead
Inner power/ground plane separation in MLB	✓ Yes — low cost, high dimensional stability	✗ Requires multiple thin plies at higher cost
Outer signal layer dielectric	✗ Too thick — Dk too high for fine-pitch impedance	✓ Thinner styles give tighter impedance control
Total board thickness ≥1.6 mm	✓ One 7628HF ply reaches ~180 µm cleanly	✗ Excessive stacking needed
Fine-pitch BGA fanout or HDI	✗ Dense weave deflects small drill bits	✓ 1080HF or 106HF for laser-drillable build-up
Cost-driven commercial MLB	✓ 7628HF is lowest-cost HF prepreg option	✗ Premium thin styles cost 20–40% more
Halogen-free certification required	✓ Confirmed IEC 61249-2-21 compliant	✓ Must also specify HF grades

PP-7628HF Prepreg Lamination: Process Adjustments for HF Materials

The most common fabrication mistake when switching to PP-7628HF prepreg is using the same press profile as standard 7628. Halogen-free resin systems based on phosphorus-nitrogen chemistry have a higher melt viscosity curve and often require a longer time to full crosslink than standard dicy-cured TBBPA epoxy. The practical consequence is that a fast ramp rate that works fine with standard FR-4 can leave HF boards with lamination voids, especially around 2 oz copper topography.

Published process data from Shengyi (S1155 series) and Ventec (VT-441) recommends a slower initial heating rate of 1.0–1.5°C/min compared to the 2.0–3.0°C/min typical for standard FR-4. The high-temperature hold at 180°C should extend to at least 50–60 minutes to ensure full cure. Pressure application should be staged to allow adequate resin flow before gelation locks in the dielectric thickness.

Parameter	Standard 7628 (typical)	PP-7628HF (recommended)	Reason for Adjustment
Initial heat-up rate	2.0–3.0°C/min	1.0–1.5°C/min	HF resin needs slower flow onset
Hold temperature	175–180°C	180–185°C	Higher Tg resin requires higher cure temp
Hold time at peak temp	30–40 min	50–65 min	P-N resin crosslinking is slower
Initial pressure application	At ~100°C gel point	At ~120°C (resin flow confirmed)	Prevents premature viscosity lock
Post-bake before packaging	120°C / 2–4 h optional	125°C / 4–8 h recommended	HF resin more hygroscopic
Desmear chemistry	Standard KMnO₄	May need adjusted concentration	HF resin desmears at lower rate than standard FR-4

Confirm desmear process compatibility with your chemical supplier before first production run. Ventec’s VT-441 process guide explicitly notes that desmear rate is lower for HF materials, meaning standard desmear time may leave resin smear in via barrels. Under-desmear on 7628HF is a common field failure cause — the dense 7628 glass weave generates more drilling heat and resin smear per via than thinner glass styles.

MLB Green Compliance: How to Specify and Certify PP-7628HF Prepreg

For a multilayer board to carry a valid halogen-free declaration, the 7628HF prepreg must be backed by a traceable material certificate. The complete documentation chain for a green-compliant MLB typically requires the following.

Document Required	What It Proves	Who Provides It
Material datasheet with IEC 61249-2-21 designation	Base material meets Br/Cl/total halogen thresholds	Laminate supplier (Shengyi, Ventec, Isola, ITEQ)
IPC-4101 slash sheet reference (/126 or /128)	Material passes IPC qualification for HF epoxy	Laminate supplier
UL 94 V-0 certificate with HF-grade UL file number	Flame retardancy without halogens confirmed	UL certification on file
Third-party halogen test report (IEC 61189-2 / oxygen bomb IC method)	Measured Br and Cl in finished prepreg	Accredited lab or supplier QC
RoHS compliance declaration	PBB and PBDE absent	Supplier declaration
REACH SVHC declaration	No substances of very high concern	Supplier declaration

A standard RoHS declaration from your fabricator is not sufficient. You specifically need the IEC 61249-2-21 material certificate naming the 7628HF product. If your fabricator is unsourced on this, Sierra Circuits, JLCPCB, and major Asian volume fabs routinely supply this documentation for MLB green programs.

PP-7628HF Prepreg: Key Suppliers and Reference Products

Several major laminate vendors offer a 7628-style halogen-free prepreg that matches the dimensional and electrical profile described above. The specific product designations differ but the IEC 61249-2-21 compliance standard is shared. Always verify that the specific prepreg product — not just the core laminate — carries the HF designation, since some suppliers offer HF cores with standard (brominated) prepreg as the default unless the HF prepreg is explicitly specified.

Supplier	Halogen-Free Laminate System	7628HF Prepreg Designation	IPC-4101 Type
Shengyi Technology	S1155 / S0455	S0455 (prepreg for S1155)	/126
Ventec International	VT-441	VT-441 prepreg	/126
Isola	370HR (low-halogen) / IS420 (HF)	IS420 PP	/126, /128
ITEQ	IT-180A HF	IT-180A PP	/126
Taiwan Union (TUC)	TU-862 HF	TU-86P HF	/127, /128
Nan Ya Plastics	NP-175HF	NP-175HF PP	/126

Note: Isola’s 370HR is sometimes described as “low-halogen” rather than fully halogen-free by IEC 61249-2-21 — verify the specific product certificate before using it in a declared halogen-free MLB program.

Useful Resources for PP-7628HF Prepreg Compliance and Specification

Resource	Description	Link
IEC 61249-2-21 Standard	Defines halogen-free thresholds for PCB laminates and prepregs (Br <900 ppm, Cl <900 ppm, total <1500 ppm)	iec.ch
IPC-4101E Specification	Qualification spec for PCB base materials including HF slash sheets /126 and /128	ipc.org
IPC-WP/TR-584	IPC white paper: “Halogen-Free Materials for Printed Boards” — addresses 9-9-15 rule, RoHS vs HF confusion, and test methods	pcbsync.com
Ventec VT-441 Process Guide	Detailed lamination parameters for HF 7628-style prepreg including desmear adjustments	ventec-group.com
Shengyi S1155/S0455 Datasheet	Dk/Df tables, resin content grades, T-260/T-288 data for commercial halogen-free 7628 system	sytech.com.cn
Isola Halogen-Free Overview	Covers IS420 and 370HR system properties and compliance documentation	isola-group.com
CALCE UMD: Halogen-Free Electronics	Research-level analysis of HF material transitions, T-260/T-288 data, and moisture effects	calce.umd.edu
PCBSync IEC 61249-2-21 Guide	Plain-English explanation of halogen-free standards, 9-9-15 rule, and common RoHS/HF misconceptions	pcbsync.com

5 FAQs: PP-7628HF Prepreg in Multilayer Green Boards

Q1: Is PP-7628HF prepreg the same as RoHS-compliant 7628? Can I use a RoHS certificate to satisfy a halogen-free requirement?

No, and this is probably the most consequential misconception in green material compliance. RoHS bans two specific brominated flame retardants — polybrominated biphenyls (PBB) and polybrominated diphenyl ethers (PBDE) — but does not restrict tetrabromobisphenol A (TBBPA), which is the primary flame retardant in standard FR-4 prepreg. A standard 7628 board can contain 15–20% bromine by weight, be 100% RoHS compliant, and still fail a halogen-free audit by an order of magnitude. PP-7628HF prepreg requires a separate IEC 61249-2-21 or JPCA-ES-01-2003 certificate confirming that bromine and chlorine content each fall below 900 ppm. Always specify “halogen-free” explicitly in the purchase order and request the IEC certificate — RoHS alone does not cover it.

Q2: Will switching from standard PP-7628 to PP-7628HF prepreg change my controlled impedance on existing MLB designs?

Likely yes, by a small but measurable amount. The phosphorus-nitrogen resin in 7628HF typically produces a post-lamination Dk that is 0.1–0.2 units higher at 1 GHz than the standard brominated version at the same resin content grade. For a 50 Ω stripline routed over a 7628HF dielectric, this shifts characteristic impedance by approximately 1–2 Ω depending on trace geometry. In most practical 6–8 layer MLB designs, 7628HF is used only in inner plane-to-plane positions where signal traces are not present, so the Dk change does not affect controlled impedance routing. However, if your stackup routes controlled impedance signals directly adjacent to a 7628HF position, recalculate trace widths using the supplier’s measured post-lamination Dk before signing off on the artwork.

Q3: Why does my fab need a slower press profile for PP-7628HF prepreg? What happens if I use the standard FR-4 press cycle?

The phosphorus-nitrogen resin system in 7628HF behaves differently in the press than standard TBBPA-epoxy. The melt viscosity at the initial ramp phase is higher, meaning the resin needs more time to flow and fill copper topography before it begins to gel and lock thickness. Running a standard FR-4 ramp rate of 2–3°C/min with 7628HF compresses the flow window — the resin starts to gel before it has fully filled the 7628 glass weave valleys around inner copper features. The result is micro-voids in the dielectric, most commonly around 2 oz inner copper traces, which show up as delamination blisters or reliability failures in thermal cycling. Ventec’s VT-441 data recommends a ramp rate of 1.0–1.5°C/min and a minimum 50-minute hold at 180°C. Post-baking finished boards at 125°C for 4–8 hours before packaging is also recommended because HF resin is more hygroscopic than standard TBBPA epoxy.

Q4: Does PP-7628HF prepreg work with lead-free (SAC305) assembly? Is T-288 performance adequate?

Yes, and in fact this is one area where 7628HF is genuinely better than standard mid-Tg FR-4. Standard 7628 with a 130–140°C Tg typically shows T-288 of less than 1 minute — marginal for aggressive lead-free reflow profiles that keep boards above 250°C for 60–90 seconds. Commercial 7628HF products from Shengyi, Ventec, and ITEQ consistently deliver T-288 of greater than 5 minutes and Td of 330–360°C, providing comfortable margin against the 260°C peak temperatures of SAC305 soldering. The switch to 7628HF is therefore often driven not just by green compliance, but by the need to pass the T-288 and Td thresholds required for lead-free process qualification — the two requirements tend to be solved together by moving to 7628HF.

Q5: My BOM calls for PP-7628. Can I substitute PP-7628HF from a different supplier without a re-qualification?

This depends entirely on your customer’s material control requirements and the end-use specification of the board. For commercial consumer electronics where the only requirement is RoHS + halogen-free, substituting any IEC 61249-2-21 compliant 7628HF prepreg from a qualified supplier is generally acceptable with a material change notice and updated documentation. For automotive (IATF 16949 governed programs), medical (IPC-6012 Class 3), or aerospace applications, any material substitution — even within the same glass style — typically requires formal design qualification testing including Tg confirmation, thermal cycling, and IST (interconnect stress test). Always check your customer’s “approved materials list” and PPAP requirements before making a drop-in substitution. The physical dimensions of 7628HF from different suppliers are functionally identical, but Dk differences of 0.1–0.2 units between suppliers can affect impedance in sensitive stackup positions.