PP-7628 Prepreg: Glass Style, Resin Content & MLB Applications

If you have spent any time reviewing multilayer PCB stackup documentation, you have almost certainly seen the designation 7628 in a prepreg call-out. It is one of the most widely used glass fabric styles in the industry, and it shows up everywhere from industrial power supply boards to automotive control modules. Yet it is also the prepreg style that gets misapplied most often — specified on layers where it is too thick, or assumed interchangeable with 2116 without accounting for the Dk shift and drilling impact. This guide covers everything a PCB design or process engineer needs to know about PP-7628 prepreg: the glass construction, the three resin content grades, how it performs in multilayer board (MLB) stackups, and where its limits are.

For engineers sourcing laminate and prepreg materials for production builds, the Doosan PCB materials range offers a relevant comparison point in the same performance class as standard PP-7628 prepreg.

What Is PP-7628 Prepreg?

PP-7628 prepreg is a B-stage composite material consisting of woven E-glass fabric with the IPC glass style designation 7628, pre-impregnated with a partially cured epoxy resin system. The “B-stage” description means the resin has been partially cured during manufacturing — it is solid and handleable at room temperature, but not yet cross-linked to its final state. During PCB lamination, heat and pressure drive the resin through its gel point and into full cure, bonding adjacent cores or core-and-foil assemblies into a single rigid multilayer structure.

The 7628 fabric is defined under IPC-EG-140 (formerly NEMA specification) and is one of the heaviest standard glass weave styles used in PCB manufacturing. The cloth is woven from E-glass yarns — yarn designation ECG 75 (twisted) for warp and ECG 75 for fill — in a plain weave pattern at approximately 44 ends per inch (warp) and 31 picks per inch (fill). The result is a dense, heavy fabric that gives the 7628 style its defining characteristics: thick cured section, excellent dimensional stability, and relatively low resin-to-glass ratio compared to lighter styles like 1080 or 2116.

How PP-7628 Differs From Other Prepreg Glass Styles

Understanding PP-7628 prepreg requires placing it clearly in the family of common PCB glass fabric styles. The numbering system — 106, 1080, 2116, 7628 — is not a thickness hierarchy in sequence; it is an IPC fabric designation code. The practical differences between them matter significantly for stackup design.

Glass Style	Warp × Fill (threads/in)	Nominal Cured Thickness	Resin Content (typical)	Dk @ 1 GHz (typical)	Best Application
106	56 × 56	~50 µm (0.002 in)	~70–76%	3.9–4.1	HDI thin dielectrics, high-frequency layers
1080	60 × 47	~60–73 µm (0.0029–0.0035 in)	~60–70%	4.0–4.3	Fine-pitch inner layers, thin stackups
2116	60 × 58	~100–132 µm (0.0040–0.0052 in)	~50–57%	4.3–4.6	General-purpose signal layers, most 4–8L designs
7628	44 × 31	~170–200 µm (0.0066–0.0078 in)	~42–52%	4.5–4.9	Power plane separation, thick dielectrics, MLB core layers

The 7628 style is the heaviest in this group by far. At ~170–200 µm cured thickness per ply, a single sheet of PP-7628 prepreg gives roughly 1.5–2× the dielectric thickness of a 2116 ply and 3–4× that of a 106 ply. That thickness is its primary value — it allows larger dielectric gaps to be built efficiently using fewer plies — but it is also the reason 7628 is the wrong choice when dielectric thinness, surface smoothness, or fine-drill compatibility are required.

PP-7628 Prepreg Resin Content: SR, MR, and HR Grades

Within the 7628 glass style, PP-7628 prepreg is available in three resin content grades. This is one of the most important variables in prepreg selection and one that is frequently omitted from design stackup documents — with consequences that show up as impedance errors, lamination voids, or delamination in production.

Standard Resin (SR) — 42–44% Resin Content

The SR grade has the lowest resin content in the 7628 family, producing the thinnest pressed dielectric section. It has lower resin flow during lamination, which makes it the preferred choice when dimensional stability and thickness control are the priority. SR is typically used between power and ground plane pairs where the function is voltage isolation rather than signal routing, and where tight impedance control is not required. The reduced flow minimises resin squeeze-out into drilled holes.

Medium Resin (MR) — ~47% Resin Content

MR is the general-purpose grade for MLB applications. It offers balanced flow — enough resin to fill the surface topography around 1 oz inner copper without voids, while keeping thickness variation manageable. When a stackup specification simply calls for “7628 prepreg” without a resin grade suffix, MR is typically what the fabricator supplies. For 4–8 layer boards with standard 1 oz inner copper, MR 7628 is a reasonable default.

High Resin (HR) — ~50–52% Resin Content

HR has the highest resin content and the highest flow during lamination. It is specified when inner layers carry 2 oz or heavier copper, where the deeper copper topography requires additional resin to achieve void-free fill. HR also produces a slightly thicker pressed dielectric section than MR at the same ply count. The higher flow means more potential for resin bleed into drill entry holes and potentially tighter press parameter control is needed. Not all fabricators stock HR 7628 as a standard item — confirm availability early.

PP-7628 Prepreg Property Summary Table

Property	SR (42–44% RC)	MR (~47% RC)	HR (50–52% RC)	Test Method
Resin Content	42–44%	~47%	50–52%	IPC-TM-650 2.3.16
Cured Thickness (1 ply)	~170 µm (0.0067 in)	~185 µm (0.0073 in)	~200 µm (0.0078 in)	IPC-TM-650 2.4.39
Dk @ 1 GHz	~4.7–4.9	~4.5–4.7	~4.3–4.6	IPC-TM-650 2.5.5.5
Df @ 1 GHz	~0.014–0.015	~0.014–0.016	~0.014–0.016	IPC-TM-650 2.5.5.5
Resin Flow	Low	Medium	High	IPC-TM-650 2.3.17
Primary Use	Power plane separation	General MLB bonding	Heavy copper fill	—
UL 94 Flame Rating	V-0	V-0	V-0	UL 94

Note: Dk values vary with frequency. At 10 GHz, 7628-based prepregs typically measure Dk ~4.2–4.5. Always use the supplier’s frequency-specific Dk/Df tables for impedance modelling — do not use a single 1 MHz or 1 GHz value across your entire frequency range of interest.

PP-7628 Prepreg in Multilayer Board (MLB) Stackup Design

Where 7628 Is Correctly Used in a Standard MLB Stackup

PP-7628 prepreg earns its place in multilayer boards by doing something no thinner prepreg style can do as cost-effectively: building thick dielectric sections in a single ply. In a standard FR-4 MLB, the most appropriate positions for 7628 prepreg are between voltage-differentiated plane pairs (e.g., 5V plane and ground), between groups of layers where signal isolation matters more than controlled impedance, and in the outer foil-to-core bond-up on boards where a relatively thick outer dielectric is acceptable.

A typical 6-layer FR-4 stackup for an industrial control board illustrates this well:

Layer	Content	Dielectric Material	Approximate Thickness
L1	Signal (outer)	— (1 oz copper)	—
Dielectric 1	—	2 × 1080 MR	~145 µm
L2	Ground plane	0.2 mm core	—
Dielectric 2	—	1 × 7628 MR	~185 µm
L3	Signal	0.1 mm core	—
Dielectric 3	—	1 × 7628 MR	~185 µm
L4	Power plane	0.2 mm core	—
Dielectric 4	—	2 × 1080 MR	~145 µm
L5	Signal	—	—
Dielectric 5	—	—	—
L6	Signal (outer)	1 oz copper	—

In this arrangement, PP-7628 prepreg handles the central dielectric separations — positions where the dielectric gap needs to be relatively large for voltage isolation and where signal impedance on L2/L3 or L4/L5 is not the critical variable. The thinner 1080 layers are used adjacent to the outer signal layers where impedance control is tightest.

Mixing PP-7628 With Thinner Prepreg Styles

It is standard practice — and often necessary — to mix 7628 with thinner styles like 1080 or 2116 in the same MLB stackup. A few principles make this work without problems. CTE compatibility is not normally an issue within the same resin system (e.g., all standard epoxy FR-4), so mixing 7628 and 2116 from the same supplier poses no delamination risk if the lamination cycle is correct. Dk mismatch between layers is a real consideration for impedance-controlled stackups: the 7628 layers will have a higher Dk (~4.5–4.9) than adjacent 1080 layers (~4.0–4.3), so each layer’s impedance must be calculated using that layer’s specific Dk, not a blanket FR-4 average. Always provide the fabricator with a full stackup document that specifies material and resin content per dielectric layer, not just total board thickness.

PP-7628 Prepreg and Drilling Considerations

The heavier glass weave of the 7628 style creates more resistance to drill penetration than lighter styles, and it is more prone to causing drill deflection on small-diameter drills. For drill diameters at or above 0.3 mm (12 mil), this is generally not a problem with modern carbide drills and proper drill file parameters. Below 0.3 mm — microvias, HDI stub removal, or fine-pitch via grids — 7628 PP is the wrong material. The stiff, heavy cloth deflects small-diameter bits, leading to hole position errors and breakage. For HDI layers requiring drilled holes below 0.3 mm, specify 1080 or 106 prepreg. PP-7628 is not an HDI material.

Fiber Weave Effect and Signal Routing on 7628 Layers

All woven glass fabrics create a periodic Dk variation across the board surface — the “fiber weave effect” — because resin-rich zones between yarns have a different Dk than the glass-reinforced zones. In the 7628 style, this periodic variation is larger in spatial scale than in 106 or 1080 styles due to the coarser weave. For signal layers directly adjacent to a 7628 dielectric, the fiber weave effect can cause trace-to-trace phase variation on differential pairs that are routed at certain angles to the glass grain. This is why 7628 prepreg is better suited to power plane separation and non-critical signal layers. If the dielectric adjacent to a controlled-impedance or differential-pair layer must use 7628 for thickness reasons, routing differential pairs at 45° to the board grain is one mitigation, as is using spread-weave glass variants available from some suppliers.

When Not to Use PP-7628 Prepreg

Knowing where 7628 is the wrong choice saves production headaches. The table below summarises the constraints clearly.

Application / Requirement	7628 Suitable?	Better Alternative	Reason
HDI microvia layers, drill < 0.3 mm	No	106, 1080	Drill deflection, hole quality
Controlled impedance adj. to high-speed diff. pairs	Caution	2116, 1080	Fiber weave effect, higher Dk
Ultra-thin dielectric (<100 µm) target	No	106, 1080	Minimum ply thickness ~170 µm
Power plane separation, thick dielectric	Yes	—	Primary application
General 4–8L industrial MLB	Yes (MR/SR)	—	Cost, dimensional stability
Outer foil bond-up, standard FR-4 MLB	Yes	—	Adequate for non-HDI outer layers
RF/microwave signal layers	No	Specialist laminates	Dk too high, fiber weave effect
Heavy copper (2 oz) inner fill	Yes (HR)	—	HR grade provides adequate fill

Useful Resources for PP-7628 Prepreg Specification and Design

Resource	Description	Link
Isola IS420 Dk/Df Data Tables	Comprehensive frequency-specific Dk and Df data for 7628 and other glass styles	isola-group.com
Panasonic R-1755M Prepreg Datasheet	Construction tables with 7628 ply Dk/Df @ 1 GHz and 2–10 GHz for MLB design	industrial.panasonic.com
IPC-4101 Base Materials Standard	Qualification and acceptance specification for glass-reinforced laminates and prepregs	ipc.org
IPC-EG-140 Glass Fabric Specification	Defines glass style numbering, yarn designations, and fabric parameters including 7628	ipc.org
Isola — Making Sense of Laminate Dielectric Properties	Technical white paper covering resin content vs. Dk/Df relationships	isola-group.com/knowledge
PCBSync Prepreg Guide	Practical engineer-focused breakdown of glass styles, resin grades, and selection criteria	pcbsync.com
Shengyi Technology SY-1141 Prepreg Data	Chinese-market supplier data for 7628 standard/medium/high resin grades	sytech.com.cn
Z-zero Stackup Planner	Free PCB stack-up tool that imports supplier-specific Dk/Df data per glass style and resin content	z-zero.com

5 FAQs: PP-7628 Prepreg in PCB Design and Fabrication

Q1: What is the actual pressed thickness of a single ply of PP-7628 prepreg, and why does it vary between suppliers?

Nominal pressed thickness for one ply of 7628 MR is approximately 185 µm (0.0073 in), but this is a guideline rather than a guaranteed number. Actual pressed thickness in a laminated board depends on the inner copper coverage of the adjacent layers, the lamination press cycle parameters (pressure ramp rate, hold pressure, temperature profile), and the specific resin content of the lot. When inner copper coverage is high — a dense power plane — the resin has less topography to fill and the pressed dielectric runs slightly thicker than nominal. When copper coverage is low (etched-away signal layer), the resin fills deeper valleys and the pressed section runs thinner. Different suppliers — Isola, Panasonic, Shengyi, Nan Ya — also produce slightly different nominal thicknesses for the same 7628 style and resin content because the base fabric weight and resin coating parameters vary between production lines. The correct approach is to obtain pressed-thickness tables specific to your supplier’s material and have your impedance calculations confirmed by the fabricator using their measured press-cycle data, not just datasheet nominals.

Q2: Can I substitute PP-7628 prepreg for PP-2116 in a stackup without re-calculating impedance?

No — and this is one of the most common stackup errors in production design handoffs. A single ply of 7628 MR is roughly 55–85 µm thicker than a single ply of 2116 MR, and its Dk is approximately 0.2–0.4 higher. Both changes affect controlled impedance calculations. On a standard 50 Ω microstrip referenced to the adjacent layer, swapping 2116 MR for 7628 MR without recalculating trace width will shift your characteristic impedance by 4–8 Ω depending on the layer pair geometry — enough to push an IPC Class 3 impedance requirement from within spec to out of spec. If the substitution is made for board thickness adjustment purposes, run new impedance calculations for every affected layer using the 7628 supplier Dk at the target frequency, and update the stackup document with the revised trace widths. Do not treat 7628 and 2116 as interchangeable materials on impedance-controlled layers.

Q3: How does PP-7628 prepreg perform in high-Tg and halogen-free versions, and are the electrical properties the same?

High-Tg 7628 prepreg (Tg ≥ 170°C DSC) uses a modified epoxy resin system — typically a dicy-free formulation or a phenolic-cured system — that shifts the resin chemistry sufficiently to affect Dk and Df values compared to standard mid-Tg epoxy. Halogen-free 7628 prepreg replaces the brominated flame retardant with phosphorus-based or inorganic-filler compounds, which also changes the dielectric behaviour. Df in particular tends to be slightly higher in some halogen-free 7628 products than in their standard-epoxy equivalents, though not dramatically so for the sub-GHz applications where 7628 is most commonly used. The practical implication is that you cannot use the standard-Tg Dk/Df table from your impedance calculator when you have switched to high-Tg or halogen-free material. Always request the supplier’s specific Dk/Df data for the exact product grade — standard, high-Tg, or halogen-free — before running impedance calculations for production.

Q4: Why does PP-7628 prepreg cause problems with small-diameter drills, and what is the safe minimum drill size?

The 7628 glass fabric has a coarser, heavier weave than 1080 or 2116 — the glass yarns are thicker and spaced further apart, and the fabric weight is approximately 6.0 oz/yd² (203 g/m²), compared to 3.2 oz/yd² (109 g/m²) for 2116. When a drill bit encounters the glass yarn at an angle, the heavier 7628 fabric exerts more lateral force on the bit, causing deflection off the programmed centre-line. On bits below 0.3 mm diameter, this deflection is sufficient to cause hole position errors outside IPC-6012 Class 3 tolerances, and drill breakage risk increases significantly. The practical safe minimum for reliable drilling through 7628 layers is 0.3 mm (about 12 mil). For drill diameters between 0.3 mm and 0.5 mm, ensure the drill parameters (hit count, infeed rate, chip load) are correct for the specific 7628 material — 7628-reinforced laminates require lower infeed rates and more frequent bit replacement than 2116-based stacks.

Q5: Should I specify resin content grade (SR/MR/HR) on my fabrication drawing, or is it the fabricator’s call?

You should specify it when it matters — and for most MLB designs it does. The fabricator’s default grade when you write “7628 prepreg” without qualification is typically MR, which is correct for most 1 oz inner copper applications. But if your inner layers carry 2 oz copper, specifying HR ensures the fabricator uses a grade with sufficient resin flow to fill without voids. If your stackup targets a specific dielectric thickness for impedance control and you have modelled with SR parameters, specify SR — otherwise the fabricator may press with MR and your dielectric thickness will be thicker than calculated, shifting impedance out of tolerance. The clearest practice is to document stackup tables in your fabrication notes that list glass style, resin grade (SR/MR/HR), ply count, and target pressed thickness per dielectric layer. This removes ambiguity from the fabricator’s interpretation and makes it clear what was modelled versus what they are expected to build.