Learn everything about PP-7628 prepreg — glass style construction, resin content grades (SR/MR/HR), key dielectric specs, and how to use it correctly in multilayer PCB stackups. Written from a PCB engineering perspective with spec tables, storage tips, and application guidance.

If you’ve spent any time reviewing PCB stackup documentation, you’ve almost certainly run into PP-7628 prepreg. It’s one of the most widely used bonding materials in multilayer board fabrication, and for good reason — it’s thick, cost-effective, and mechanically robust. But if you’re just pulling it into your stackup because “everyone uses it,” you might be leaving performance on the table or running into trouble you don’t expect.

This guide breaks down exactly what PP-7628 prepreg is, what its glass style and resin content actually mean for your board, and where it fits — and doesn’t fit — in multilayer PCB (MLB) applications.

What Is PP-7628 Prepreg?

PP-7628 prepreg (also written as 7628 prepreg or prepreg 7628) is a B-stage composite material made of woven E-glass fabric pre-impregnated with a partially cured epoxy resin system. The designation “7628” refers specifically to the IPC-recognized glass fabric style — the weave pattern, yarn density, and construction of the fiberglass cloth.

In PCB manufacturing, prepreg sits between core layers or between a core and copper foil. During the lamination press cycle, heat activates the resin, which flows, fills in around etched copper features, and then fully cures — permanently bonding the stack into a solid multilayer structure. Once cured, it becomes the dielectric layer separating your copper planes and signal layers.

The “PP” simply stands for pre-preg or pre-impregnated, and the number tells you which glass cloth was used. Other common types include PP-106, PP-1080, and PP-2116. Each has its own weight, thickness, and resin pickup characteristics.

PP-7628 Glass Style: What the Number Tells You

The 7628 glass style is one of the heaviest and most tightly woven fabrics used in standard FR-4 PCB production. The weave is dense and uses thicker yarns compared to lighter styles like 1080 or 106.

The glass weave in 7628 prepreg is tight — controlled with a loom during manufacturing — and the final electromagnetic properties, including dispersion, losses, and fiber weave effects observed by signals in the board, are determined by the gaps and homogeneity of that weave.

What this means in practice: 7628 has less resin relative to glass compared to lighter styles, which gives it excellent dimensional stability and mechanical stiffness. However, the coarser surface texture is less forgiving for fine features and high-speed signals.

PP-7628 Weave Characteristics vs Other Glass Styles

Glass Style	Weave Type	Fabric Weight (g/m²)	Typical Application
106	Very loose	~24	HDI, ultra-thin, high-frequency
1080	Loose	~48	Fine-pitch, high-frequency
2116	Medium	~104	General multilayer, balanced performance
7628	Tight/Heavy	~203	Thick MLB, power layers, cost-sensitive designs

PP-7628 Resin Content: SR, MR, and HR Explained

Resin content (RC%) is the weight percentage of resin relative to the total prepreg weight. For PP-7628, the standard resin content sits at around 42–48% — noticeably lower than lighter styles like 1080 (which can reach 60–70%).

Typical resin content ranges for heavy glass cloth like 7628 run approximately 40–50%, while lighter styles like 106 and 1080, typically used to bond inner layer cores together, run 65–75%.

Within the 7628 style, you’ll encounter three resin content tiers:

PP-7628 Resin Content Variants

Variant	Resin Content	Flow Behavior	Best Use Case
SR (Standard Resin)	~42%	Low flow	Plane-to-plane bonding, flat surfaces
MR (Medium Resin)	~45%	Moderate flow	General MLB dielectric layers
HR (High Resin)	~48–50%	Higher flow	Thick copper (2oz+) inner layers with deep etching

Choosing the right resin content class ensures full copper filling without excessive squeeze-out, helping maintain dielectric consistency and prevent lamination voids.

Increasing resin content in 7628 also shifts the dielectric constant slightly. Higher RC means more resin and less glass, which generally lowers Dk — important if you’re running impedance-controlled traces through layers using 7628 as the dielectric.

PP-7628 Prepreg Key Specifications

Here’s a reference table for PP-7628 based on standard FR-4 epoxy systems:

Parameter	Typical Value	Notes
Cured Thickness	170–190 µm (0.0067–0.0075 in)	Varies by RC and lamination pressure
Dielectric Constant (Dk) at 1 GHz	4.1–4.6	Higher than lighter styles
Dissipation Factor (Df) at 1 GHz	~0.02	Standard FR-4 range
Resin Content	42–48%	Depends on SR/MR/HR grade
Glass Fabric Weight	~203 g/m²	Heaviest standard PCB style
Tg (Standard FR-4)	130–170°C	High-Tg and halogen-free grades available

PP-7628 has a thickness range of 170–190 µm and an Er (dielectric constant) of 4.1–4.6. That’s the thickest single-ply prepreg in the standard FR-4 lineup, which makes it extremely useful when you need to hit a target board thickness without stacking multiple thinner plies.

PP-7628 Prepreg in Multilayer Board (MLB) Applications

Where PP-7628 Excels

PP-7628 earns its place in multilayer stackups primarily because of its mechanical properties and cost efficiency. In production, it’s a go-to choice for:

Thick MLB constructions (6–16+ layers): When you need board thickness approaching 1.6mm or beyond, 7628 lets you reach target dimensions with fewer plies than 2116 or 1080. This reduces material cost and press time.

Power plane separation: Separating power and ground planes doesn’t require premium dielectric performance. 7628 SR at low cost fills this role well — good adhesion, stable thickness, no need for low-Dk or low-Df properties.

Industrial and commercial-grade electronics: Telecom backplanes, industrial control boards, and general computing hardware where signal frequencies stay under 1–2 GHz are well-served by 7628 prepreg.

Cost-sensitive designs: Heavyweight 7628 prepreg costs less and provides excellent dimensional stability, making it a practical choice for non-critical layers where controlled impedance or high-speed designs are not the priority.

Doosan PCB laminates, for example, often utilize 7628-based prepreg systems in standard MLB stackups, combining it with matched core materials to maintain consistent dielectric properties through the press cycle.

Where PP-7628 Prepreg Falls Short

The thicker glass fibers in 7628 can deflect small drill bits, causing hole quality issues in HDI applications. For any board with laser vias, fine drill patterns under 0.2mm, or HDI build-up layers, 7628 is not the right call.

For high-speed digital signals above 2–3 GHz, 7628’s higher Dk and coarser weave introduce signal integrity challenges including fiber weave effect, which causes differential skew between closely routed pairs. Lighter styles like 1080 or 2116 with tighter resin control are better suited there.

Mixing PP-7628 with Other Prepreg Styles

It’s common practice in MLB fabrication to combine prepreg types within the same stackup. A typical approach:

• Use 1080 or 2116 for signal-sensitive dielectric layers where Dk consistency and smooth surface matter
• Use 7628 for outer dielectric layers or plane separations where cost and thickness matter more

Just ensure CTE compatibility and verify Dk values for each layer in your impedance calculations when mixing prepreg types.

PP-7628 Prepreg Storage and Handling

Prepreg shelf life matters more than most engineers think. Expired or improperly stored material has advanced its cure state, reducing resin flow during lamination and causing adhesion failures, voids, or inconsistent pressed thickness.

Keep PP-7628 prepreg:

• In a temperature-controlled environment (typically 5–23°C)
• Wrapped in moisture-proof packaging at all times
• Away from UV light and strong artificial light sources
• Within the manufacturer’s stated shelf life (usually 3–6 months from production date)

Before production runs with questionable material, always conduct a flow test on sample coupons.

Useful Resources for PP-7628 Prepreg

Resource	Description	Link
IPC-4101	Standard specification for base materials used in rigid PCBs, including prepreg	IPC.org
Isola Prepreg Data Sheets	Dk/Df tables for IS410, IS420 across glass styles including 7628	Isola-Group.com
Ventec VT-47 / VT-441 Data	Process guides and prepreg specs for halogen-free 7628 variants	Ventec-Group.com
Shengyi S1141 Data Sheet	Widely used FR-4 with 7628 prepreg specs, common in Asia-based fabs	Shengyi-tech.com
JLCPCB Impedance Calculator	Stackup tool referencing 7628 layer thicknesses for impedance estimation	JLCPCB.com/impedance

FAQs About PP-7628 Prepreg

Q1: Can I use a single ply of PP-7628 as the only prepreg between two cores?

Yes, a single ply of 7628 is actually one of its advantages — at 170–190 µm cured thickness, one sheet gives you meaningful dielectric separation without stacking multiple thinner plies. However, for very heavy copper layers (3oz+), consider HR grade or adding an additional ply of a thinner style like 2116 to ensure full void-free resin fill around the copper features.

Q2: What is the Dk of PP-7628 prepreg, and does it vary?

The nominal Dk of standard FR-4 7628 prepreg runs from about 4.1 to 4.6 at 1 GHz, depending on resin content, supplier, and test method. Higher resin content slightly lowers Dk. Always use your fab’s actual measured Dk values — not just datasheet nominals — when running impedance calculations.

Q3: Is PP-7628 suitable for lead-free assembly processes?

Standard Tg 7628 prepreg (Tg ~130°C) can be marginal for lead-free reflow profiles, which peak at 260°C. For lead-free processes, use high-Tg (Tg 170°C+) or mid-Tg 7628 versions with a higher decomposition temperature (Td > 300°C) to avoid measling, delamination, or CAF failures after assembly.

Q4: Why does my board warp when using PP-7628 in the outer layers?

7628’s heavy glass construction provides good in-plane stability, but asymmetric stackups — more 7628 on one side than the other — can cause warping as the board cools from the press. Always design your stackup with symmetric prepreg placement around the center core to balance thermal expansion and minimize bow and twist.

Q5: How does PP-7628 compare to PP-2116 for a standard 4-layer board?

For a 4-layer, 1.6mm board, 7628 is a common and cost-effective choice for the outer prepreg layers. It easily achieves target thickness in a single ply. PP-2116 gives slightly better signal integrity (lower Dk, smoother surface) and is preferred when layer 1 or 4 carry high-speed signals. If your signals are under 1 GHz and cost matters, 7628 SR or MR is a practical default.

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