Complete PP-7628HF prepreg guide: specs, halogen limits (IEC 61249-2-21), MLB stack-up examples, green compliance checklist, and processing tips for halogen-free PCB production.

If you’ve been quoting multilayer boards for EU, Japanese, or major US OEM supply chains lately, there’s a good chance someone on the sourcing side has asked the question: “Is this halogen-free?” The answer usually comes down to one material decision made early in the stack-up process — the prepreg. For thick structural interfaces in MLB (multilayer board) construction, PP-7628HF prepreg is the halogen-free workhorse that satisfies both mechanical requirements and green compliance mandates without forcing you into exotic resin systems.

This guide covers what PP-7628HF prepreg actually is, how its specifications compare to standard 7628 and other glass styles, what “green compliance” requires in real regulatory terms, and how to specify and validate it correctly for production.

What Is PP-7628HF Prepreg and Why the “HF” Designation Matters

The “7628” in the name refers to the IPC glass fabric style — a tightly woven E-glass construction that produces one of the heaviest, thickest, and most dimensionally stable prepreg layers available in standard PCB fabrication. The tight weave gives 7628 excellent mechanical strength and keeps dimensional variation in the X-Y plane well below 0.5%, which matters a lot in large-panel MLB manufacturing where registration tolerance across 20+ layers is cumulative.

The “HF” suffix stands for Halogen-Free. It signals that the epoxy resin system infused into the glass fabric does not rely on tetrabromobisphenol A (TBBPA) — the brominated flame retardant used in conventional FR-4 — for its flame retardancy. Instead, PP-7628HF prepreg uses phosphorus-nitrogen (P-N) compound systems or inorganic flame retardants that achieve UL 94 V-0 flammability ratings without introducing chlorine or bromine above threshold limits.

This distinction matters for compliance. Standard FR-4, while legal under RoHS as currently written, still contains brominated epoxy resin that releases hydrogen bromide (HBr) when burned. Multiple OEMs — particularly in consumer electronics, automotive, and telecom — now require total bromine content below 900 ppm at the laminate and prepreg level, which standard FR-4 cannot meet.

PP-7628HF Prepreg: Key Technical Specifications

PP-7628 has a cured thickness of 170–190 μm and Er values in the range of 4.1–4.6, with resin content around 45%. The HF variant shares the same glass construction but substitutes the resin chemistry. Key differences between the standard and HF variants are highlighted in the table below.

PP-7628HF Specification Table

Parameter	Standard PP-7628	PP-7628HF (Halogen-Free)	Notes
Glass Style	IPC 7628	IPC 7628	Tight woven E-glass
Resin System	Brominated epoxy (TBBPA)	Phosphorus-nitrogen / modified epoxy	HF uses P-N flame retardant
Cured Thickness	170–200 μm (6.7–7.9 mil)	175–200 μm (6.9–7.9 mil)	Slightly higher RC possible
Resin Content	40–45%	42–48%	HF resin may carry slightly more filler
Dk @ 1 GHz	4.4–4.6	4.3–4.5	P-N resin slightly lowers Dk
Df @ 1 GHz	~0.020	~0.018–0.021	Comparable; check supplier data
Tg (standard)	130–150°C	140–170°C	HF variants often run higher Tg
Bromine (Br)	18,000–22,000 ppm	< 900 ppm	HF meets IEC 61249-2-21
Chlorine (Cl)	< 200 ppm	< 900 ppm	Both well within total limit
Total Halogens	~20,000 ppm	< 1,500 ppm	Key compliance threshold
UL 94 Rating	V-0	V-0	Same flame class
Moisture Absorption	< 0.5%	< 0.4%	HF often shows slight improvement
Shelf Life	6 months @ ≤23°C	6 months @ ≤23°C	Same storage requirements

Engineer’s Note: PP-7628HF prepreg from different suppliers — Shengyi, Isola, Ventec, Doosan — will show different specific Dk/Df values due to their proprietary P-N resin systems. Always pull the engineering-grade Dk/Df table from the supplier, not the marketing sheet. One supplier’s 7628HF at 42% RC will have a different pressed Dk than another’s at 46% RC.

Green Compliance Standards That PP-7628HF Prepreg Must Satisfy

This is where a lot of engineers get confused. “Green compliance” is not a single standard — it’s a stack of overlapping regulatory frameworks and OEM requirements, each with slightly different scope and thresholds. Here’s what actually applies to PP-7628HF prepreg selection:

Regulatory Compliance Framework Table

Standard / Directive	Scope	Halogen Limit	PP-7628HF Status
IEC 61249-2-21	PCB laminates and prepregs	Cl ≤ 900 ppm, Br ≤ 900 ppm, total ≤ 1500 ppm	✅ Compliant
JPCA-ES-01-2003	Japanese electronics industry	Same 9-9-15 rule	✅ Compliant
EU RoHS Directive	Restricts PBB and PBDE specifically	No explicit 900 ppm rule — bans specific BFRs	✅ Compliant (note: RoHS ≠ halogen-free)
EU REACH	Chemical registration and restriction	Substance-of-very-high-concern (SVHC) reporting	✅ Compliant with P-N systems
WEEE Directive	End-of-life recyclability	Facilitates safer e-waste processing	✅ Supports WEEE goals
IPC-WP/TR-584	Industry white paper defining halogen-free	Codifies 9-9-15 rule, references IEC 61249-2-21	✅ Defines the baseline
UL 94 V-0	Flammability	Not a halogen standard — flame performance only	✅ P-N systems achieve V-0

RoHS compliance does not equal halogen-free. The EU RoHS Directive banned two specific brominated flame retardants — polybrominated biphenyls (PBB) and polybrominated diphenyl ethers (PBDE) — but does not ban all brominated flame retardants. Many engineers confuse this and assume RoHS-compliant standard FR-4 is also halogen-free. It is not.

According to standards such as JPCA-ES-01-2003 and IEC 61249-2-21, a printed board is considered halogen-free if chlorine is no more than 900 ppm, bromine is no more than 900 ppm, and combined Cl + Br is no more than 1500 ppm. PP-7628HF prepreg is specifically engineered to meet all three thresholds across the entire laminated structure.

Why PP-7628HF Is the Go-To for MLB Structural Layers

Mechanical Strength and Dimensional Stability

Heavyweight styles like 7628 cost less and provide excellent dimensional stability, but their rougher surface and higher Dk make them less suitable for controlled impedance or high-speed designs. That’s actually the point — PP-7628HF prepreg is not selected for signal integrity layers. It’s specified for power plane separation, board-thickening interfaces, and structural bonding layers in 8–20+ layer MLB designs where dimensional stability and mechanical integrity under thermal cycling take priority over Dk precision.

In a 16-layer MLB, you might have four or five interfaces between inner cores where the only design requirement is mechanical bond strength, Z-axis CTE control, and fire-rating compliance. Using PP-7628HF at those interfaces delivers the required thickness per ply (~185 μm), reduces layer count relative to using multiple thin plies, and costs less than 2116 or 1080 alternatives — all while keeping the board halogen-free throughout.

Thermal Performance Advantage of HF Resin Systems

One underappreciated benefit of PP-7628HF prepreg is that the phosphorus-nitrogen resin systems used in HF materials typically deliver higher Tg than conventional DICY-cured FR-4 at similar cost. Modern halogen-free systems deliver high-Tg of 170–200°C and Df of 0.009–0.012 at 1 GHz, enabling reliable performance across multiple reflow cycles.

For lead-free assembly — where peak reflow temperatures hit 255–260°C — having your structural prepreg at Tg ≥ 150°C (ideally 170°C) provides meaningful safety margin against delamination. PP-7628HF in phenolic-cured or P-N cure systems often reaches Tg 150–175°C, which is 20–40°C better than DICY-cured standard FR-4.

Cost Position Relative to Other HF Options

Not all halogen-free prepregs are created equal in cost. PP-7628HF sits at the lower end of the HF prepreg cost curve because the 7628 glass fabric itself is the lowest-cost style, and high-volume availability of HF resin systems for 7628 has brought pricing down significantly over the past decade. The cost premium over standard 7628 runs approximately 8–18% depending on supplier and volume, which is far more palatable than switching to halogen-free 1080 or 2116 variants for every layer in a thick MLB.

PP-7628HF Prepreg in a Typical MLB Stack-Up

Below is a representative 10-layer MLB stack-up showing where PP-7628HF prepreg is used for structural interfaces:

10-Layer MLB Stack-Up with PP-7628HF (Total ~2.4 mm)

Layer	Material	Thickness	Purpose
L1	1 oz Cu foil	35 μm	Signal / ground
PP	2× PP-7628HF (42% RC)	~370 μm	Outer structural bond
Core	L2–L3 FR-4 HF core	100 μm	Inner signal pair
PP	1× PP-7628HF (46% RC)	~190 μm	Power plane separation
Core	L4–L5 FR-4 HF core	100 μm	Power / ground pair
PP	1× PP-7628HF (46% RC)	~190 μm	Central structural
Core	Central FR-4 HF core	800 μm	Mechanical backbone
PP	1× PP-7628HF (46% RC)	~190 μm	Lower power separation
Core	L6–L7 FR-4 HF core	100 μm	Power / ground pair
PP	1× PP-7628HF (46% RC)	~190 μm	Inner signal separation
Core	L8–L9 FR-4 HF core	100 μm	Inner signal pair
PP	2× PP-7628HF (42% RC)	~370 μm	Lower structural bond
L10	1 oz Cu foil	35 μm	Signal / ground
Total		~2.37 mm	±10% lamination tolerance

In this construction, PP-7628HF handles all structural bonding interfaces while the tight-tolerance signal layers use thin HF prepreg (1080HF or 2116HF) to maintain impedance control. This mixed approach gives you full green compliance on every layer while keeping material cost under control.

How to Specify PP-7628HF Prepreg Correctly

Getting the spec wrong on your fab drawing is one of the most common ways halogen-free compliance fails in the supply chain. Here’s what your fabrication notes need to include:

PP-7628HF Stack-Up Specification Checklist

Specification Field	What to State	Why It Matters
Glass Style	IPC 7628	Locks in fabric construction
Resin System	Halogen-free (P-N or phosphorus-based)	Prevents substitution with TBBPA resin
Compliance Standard	IEC 61249-2-21	States the 9-9-15 threshold requirement
Bromine Limit	Br < 900 ppm	Explicit measurement threshold
Chlorine Limit	Cl < 900 ppm	Explicit measurement threshold
Combined Halogens	Cl + Br < 1500 ppm	Total threshold per IEC standard
Tg Minimum	≥ 150°C (or 170°C for lead-free)	Thermal reliability requirement
Test Method	EN 14582 or ICP-MS	Specifies how halogen content is verified
Material Certificate	Required per lot	Supplier CoC with actual test values

For engineers specifying materials, the key takeaways are: reference IPC-WP/TR-584 or IEC 61249-2-21 thresholds explicitly, include both laminate and solder mask in your requirements, specify the EN 14582 test method, and require supplier certification. Following these practices ensures your halogen-free specifications are clear, achievable, and verifiable.

Known Processing Challenges with PP-7628HF Prepreg

PP-7628HF is not a drop-in replacement for standard 7628 in every fab environment. Be aware of these production-level issues:

Phosphorus-based flame retardants can increase material brittleness, leading to handling difficulties during PCB assembly. Higher Tg values may also demand increased lamination and drilling temperatures, impacting throughput and equipment calibration. Some halogen-free resins exhibit lower flow, complicating prepreg bonding and layer uniformity.

Specifically for PP-7628HF: the tight weave of the 7628 fabric already limits resin flow relative to lighter styles. In HF variants with P-N resin systems, flow characteristics can shift further depending on cure profile. Working with your fabricator’s actual press cycle data — rather than assuming standard FR-4 lamination parameters — is mandatory. Confirm minimum press temperature, hold time, and cooldown rate with the specific lot of PP-7628HF prepreg you’re using.

Drill wear also increases slightly with 7628 glass style due to the dense weave, and HF resin systems with inorganic fillers can accelerate this further. Budget for more frequent drill bit replacement if you’re running small-diameter vias through thick PP-7628HF stacks.

Useful Resources for PP-7628HF Prepreg Selection and Compliance

Resource	URL	What It Covers
IEC 61249-2-21 Standard	iec.ch	Definitive halogen-free base material specification
IPC-WP/TR-584 White Paper	ipc.org	IPC’s official halogen-free definition and threshold rationale
CALCE Halogen-Free Guide	calce.umd.edu	Academic reference on HF material performance and failure modes
Isola Engineering Data Sheets	isola-group.com	Dk/Df tables for 7628 style at multiple resin contents
Sierra Circuits HF PCB Guide	protoexpress.com	Practical HF design and fabrication guidance
Ventec VT-441 Process Guide	ventec-group.com	Specific halogen-free laminate processing parameters
Doosan PCB Materials	Doosan PCB	Doosan’s HF CCL and prepreg product line data, widely used in Korean MLB supply chains
PCBSync IPC-WP/TR-584 Guide	pcbsync.com	Practical breakdown of halogen-free specification for engineers

5 Frequently Asked Questions About PP-7628HF Prepreg

1. Does PP-7628HF prepreg actually perform differently than standard 7628 in a press cycle?

Yes, with some nuances. The P-N resin systems in PP-7628HF typically have a slightly different viscosity profile during lamination. Flow at peak temperature may be lower than TBBPA-based 7628, which is generally a good thing for thickness control but can be a problem at power plane interfaces with heavy copper if your press cycle isn’t tuned. Ask your material supplier for the recommended press profile — specifically the heat-up rate, hold temperature, hold time, and cool-down ramp — and don’t assume standard FR-4 parameters carry over.

2. Is PP-7628HF prepreg mandatory for a board to be called “halogen-free”?

Not specifically 7628HF — what’s mandatory is that all prepreg layers in the stack-up carry HF-grade material. The halogen-free definition per IEC 61249-2-21 applies to the entire board, including laminate, prepreg, and solder mask, not just the outer layers. A board is only halogen-free if every dielectric layer — including the core, every prepreg interface, and the solder mask — independently meets the 900 ppm / 1500 ppm limits.

3. Can I mix PP-7628HF with standard (TBBPA) 1080 or 2116 prepreg in the same stack-up and still claim halogen-free?

No. If any prepreg in the stack-up is standard brominated FR-4, the total halogen content of the assembled board will exceed the IEC 61249-2-21 threshold. Green compliance is not a layer-by-layer claim — it applies to the complete laminated structure. The entire stack, including outer copper foils and solder mask, must use HF-qualified materials.

4. How do I verify that a delivered lot of PP-7628HF prepreg actually meets the halogen limits?

Require a Certificate of Conformance (CoC) from the supplier referencing specific lot testing against IEC 61249-2-21. The preferred test method is EN 14582 (ion chromatography after combustion), which measures actual Cl and Br concentrations independently. Some suppliers offer lot-level reports; if yours doesn’t, you can send samples to a third-party laboratory. Do not rely solely on material type designation — misidentified or mixed resin rolls have been documented in Asian supply chains. For automotive or medical MLB applications where the halogen claim carries safety implications, third-party lot verification is strongly recommended.

5. What is the typical cost premium for PP-7628HF prepreg versus standard 7628?

Halogen-free materials typically cost 8–20% more than standard FR-4, but the added safety and compliance value typically justifies the cost increase. For PP-7628HF specifically, the premium tends to sit at the lower end of that range (8–12%) because of the high production volume of 7628-based HF materials among major suppliers like Shengyi, Isola, and Panasonic. Higher-layer-count MLBs with thick intermediate stacks can achieve nearly the same cost efficiency as standard FR-4 builds since fewer plies of 7628HF are needed to meet thickness targets compared to thinner HF styles.

Summary: When to Specify PP-7628HF Prepreg

PP-7628HF prepreg earns its place in an MLB stack-up at every structural interface where green compliance is a hard requirement and the layer’s primary job is mechanical bonding and thickness contribution rather than impedance-sensitive signal routing. Its thick pressed profile (~185–200 μm), dimensional stability, improving Tg performance from P-N resin systems, and full compliance with IEC 61249-2-21 make it the practical default for halogen-free MLB production in regulated markets.

Specify it with explicit halogen limits (Br < 900 ppm, Cl < 900 ppm, total < 1500 ppm), reference IEC 61249-2-21 on your fab drawing, require lot-level CoC documentation, and confirm lamination parameters directly with your fabricator. Do all that, and PP-7628HF prepreg gives you a cost-effective route to green compliance without sacrificing board integrity.