PP-2116 Prepreg: Why It’s the Standard Choice for Build-Up Layers

Meta:PP-2116 prepreg explained: glass style, SR/MR/HR resin grades, Dk/Df data, standard 4–8L stackup use, and why 2116 is the default build-up layer choice. Engineer’s guide.

Ask a PCB fabricator what prepreg they reach for first when building a standard multilayer board, and the answer is almost always the same: 2116. It is not a coincidence. PP-2116 prepreg occupies a middle ground in the glass fabric style family that makes it the most versatile bonding sheet in everyday multilayer PCB manufacturing. It is thicker than 1080 but thinner than 7628, has a moderate resin content that handles most copper topographies without excess squeeze-out, drills cleanly at standard via sizes, and its Dk sits in a range that is compatible with the majority of controlled impedance designs. If your fabrication notes simply say “FR-4 prepreg” without a glass style designation, 2116 is almost certainly what you are getting.

This article breaks down exactly what PP-2116 prepreg is — the glass construction, the resin content grades, the key electrical and mechanical properties, and where it fits correctly in standard 4–8 layer FR-4 stackups. Engineers comparing it against 1080 and 7628 will find the direct comparison tables useful. For a look at the high-performance laminate materials that partner with PP-2116 prepreg in more demanding build-ups, the Doosan PCB material range covers relevant options across Tg grades and halogen-free classes.

What Is PP-2116 Prepreg?

PP-2116 prepreg is a B-stage composite made of woven E-glass fabric with IPC/NEMA glass style designation 2116, pre-impregnated with a partially cured epoxy resin. The “B-stage” state means the resin has been partially polymerised during manufacturing — hard enough to handle on a roll, not yet cross-linked to its final rigid state. During PCB lamination, heat takes the resin through its gel point and into full cure, fusing adjacent cores and copper foils into a single solid dielectric.

The 2116 fabric is woven from ECE 225 E-glass yarn at approximately 60 ends per inch in the warp direction and 58 picks per inch in the fill direction. That near-square thread count — nearly equal density in both directions — gives the 2116 style its characteristic dimensional stability and explains why it is less prone to directional warping than fabrics with very asymmetric weave counts. Finished cloth weight is approximately 109 g/m² (3.2 oz/yd²), placing it squarely in the mid-weight category between the lighter 1080 style (~49 g/m²) and the heavier 7628 style (~203 g/m²).

PP-2116 Prepreg in the Glass Style Family

To understand why 2116 is the default build-up layer material, it helps to see how it compares to the other common glass styles in context.

Glass Style	Thread Count (warp × fill per inch)	Fabric Weight (g/m²)	Nominal Cured Thickness	Typical Resin Content	Dk @ 1 GHz	Best Use Case
106	~56 × 56	~25	~50 µm	~70–76%	3.9–4.1	HDI thin dielectrics, high-frequency layers
1080	~60 × 47	~49	~60–73 µm	~60–70%	4.0–4.3	Thin build-up layers, fine-pitch inner layers
2116	~60 × 58	~109	~100–132 µm	~50–57%	4.3–4.6	Standard build-up for most 4–8 layer MLBs
7628	~44 × 31	~203	~170–200 µm	~42–52%	4.5–4.9	Power plane separation, thick dielectrics

The 2116 style hits a balance point that no other single fabric matches: thick enough to give useful dielectric spacing without stacking multiple plies, thin enough to keep total board thickness manageable in 4–8 layer designs, and with sufficient resin content to fill standard 1 oz inner copper topography reliably. The near-square weave pattern also produces a more uniform Dk across the panel than the coarser, asymmetric 7628 weave — a meaningful advantage for controlled impedance consistency.

PP-2116 Prepreg Resin Content Grades: SR, MR, and HR

PP-2116 prepreg is available in three resin content grades. Each serves a different role, and specifying the wrong one on your fabrication notes is the kind of error that shows up as impedance out of tolerance or unexplained lamination voids.

Standard Resin (SR) — ~50–52% Resin Content

SR is the lowest resin content variant. It produces the minimum pressed thickness per ply and the lowest resin flow during lamination. SR 2116 is appropriate where inner copper coverage is high and fairly uniform — adjacent power plane layers with minimal etching, for example — and where you want maximum dimensional stability with minimum resin movement. It is also the grade to specify when tight pressed-thickness tolerances matter more than generous copper fill, such as plane-layer separations in a controlled impedance stripline configuration.

Medium Resin (MR) — ~53–55% Resin Content

MR is what most fabricators supply when a design specifies “2116” without a resin grade suffix. The flow is balanced — enough to fill standard 1 oz etched inner-layer topography without producing excessive bleed or squeeze-out at panel edges. Pressed thickness with MR 2116 is approximately 115–125 µm per ply against 1 oz copper at 80% coverage, which is the working number most stackup calculators use as their baseline. For straightforward FR-4 4–8 layer designs with 1 oz inner copper, MR is correct.

High Resin (HR) — ~56–57% Resin Content

HR provides the highest flow in the 2116 family. It is called for when inner layers carry 2 oz copper, where the deeper etched features require more resin volume to fill voids and achieve consistent dielectric thickness. HR 2116 also slightly increases the pressed dielectric section compared to MR — which can be used intentionally to push the stackup toward a target thickness when thickness budget is tight. Not every fabricator stocks HR 2116 as a standard item; confirm with your fab house before designing around it.

PP-2116 Prepreg Property Table

Property	SR (~50–52%)	MR (~53–55%)	HR (~56–57%)	Test Method
Resin Content	50–52%	53–55%	56–57%	IPC-TM-650 2.3.16
Cured Thickness / ply (1 oz Cu, ~80% coverage)	~100–110 µm	~115–125 µm	~125–132 µm	IPC-TM-650 2.4.39
Dk @ 1 GHz	~4.5–4.6	~4.3–4.5	~4.3–4.4	IPC-TM-650 2.5.5.5
Dk @ 10 GHz	~4.2–4.4	~4.0–4.3	~4.0–4.2	IPC-TM-650 2.5.5.5
Df @ 1 GHz	~0.016–0.018	~0.016–0.018	~0.016–0.018	IPC-TM-650 2.5.5.5
Resin Flow	Low	Medium	High	IPC-TM-650 2.3.17
Primary Application	Power plane separation, controlled thickness	General signal and mixed-use MLB layers	Heavy copper fill (2 oz inner layers)	—
UL 94 Flame Rating	V-0	V-0	V-0	UL 94

Always use your specific supplier’s Dk/Df frequency tables for impedance modelling — not a generic FR-4 default. Isola, Panasonic, Shengyi, and Nan Ya all have slightly different cured Dk values for nominally equivalent 2116 products, and those differences matter when you are holding ±5% impedance tolerance on a production run.

Why PP-2116 Prepreg Is the Standard Build-Up Layer Choice

The Thickness Sweet Spot for 4–8 Layer FR-4 MLB Designs

A standard 1.6 mm (0.063 in) 4-layer FR-4 board with a 0.8 mm central core needs approximately 230–280 µm of total dielectric on each side to hit its thickness target after pressing. One ply of 2116 MR at ~120 µm plus one ply of 1080 MR at ~75 µm gives 195 µm — close enough for many designs, or add a second ply of 2116 for a thicker outer build. Achieving the same dielectric in a single ply would require 7628 (~185 µm), but the 7628’s higher Dk and heavier glass weave compromise the outer signal layers. Achieving it with 1080 alone would require two to three plies, adding lamination complexity and cost. Two plies of 2116 MR gives approximately 230–250 µm, which is the standard outer build on most commercial 4-layer 1.6 mm FR-4 boards.

A typical standard 4-layer 1.6 mm stackup using PP-2116 prepreg looks like this:

Layer	Content	Material	Approximate Thickness
L1	Signal (outer foil)	0.5 oz Cu + plating	~35 µm
Dielectric 1-2	—	2 × 2116 MR	~230 µm
L2	Ground plane	—	—
Core	—	FR-4 0.8 mm	800 µm
L3	Power plane	—	—
Dielectric 3-4	—	2 × 2116 MR	~230 µm
L4	Signal (outer foil)	0.5 oz Cu + plating	~35 µm
Total			~1.60 mm

This is the most common standard 4-layer stackup shipped worldwide. The fact that it is built on 2116 MR in every dielectric position is not arbitrary — it reflects the practical reality that 2116 MR delivers the right pressed thickness in a commercially available ply count at a cost that is rational for the majority of commercial PCB production.

PP-2116 Prepreg for Controlled Impedance Designs

The 2116 style earns its place in impedance-controlled designs through three properties that lighter and heavier fabric styles cannot simultaneously match: a moderate Dk that keeps trace widths manageable for standard track-to-track spacing; a pressed thickness per ply (~100–132 µm) that allows practical 50 Ω microstrip geometries without excessively narrow or excessively wide traces; and a weave density (60 × 58 threads/inch) that is uniform enough to keep fiber weave effects manageable on signal layers running standard digital speeds.

For a 50 Ω controlled impedance microstrip on a 4-layer board with 2 × 2116 MR outer dielectric, the required outer trace width on L1/L4 is approximately 4.5–5.5 mil (0.11–0.14 mm) — a dimension that virtually every volume PCB fabricator can hold to ±0.5 mil etching tolerance. The same impedance on a 2 × 1080 MR outer dielectric would require a narrower trace (~3.5–4.5 mil), which some fab houses cannot reliably produce at volume. On a single-ply 7628 outer dielectric, the required trace width would be wider and the higher Dk creates more Dk variability across the board surface.

Intel’s FPGA PCB design guidelines explicitly recommend 2116 as a medium glass weave compromise for high-speed signal layers in production designs where cost must be balanced against fiber weave uniformity. It is the glass style the PCB industry converged on for mainstream controlled impedance work — and that consensus is well-earned.

PP-2116 Prepreg in Multi-Layer Build-Ups Beyond 4 Layers

In 6, 8, and 10-layer boards, PP-2116 prepreg is equally dominant as the standard dielectric between inner layer pairs. The dielectric spaces between inner signal layers in a 6-layer commercial FR-4 design are typically held to 100–130 µm, which one ply of 2116 MR delivers naturally without stacking. The near-square thread count of the 2116 cloth produces consistent dielectric thickness across the full panel width — an important factor in high-layer-count builds where press-out uniformity directly affects interlayer registration and plated through-hole aspect ratio reliability.

Where thicker separation between inner layers is required — such as between a power and ground plane pair used for low-inductance decoupling — a 2116 + 1080 combination or a single 7628 ply handles the thicker requirement, while 2116 continues as the standard on signal-adjacent positions.

PP-2116 Prepreg vs. 1080 and 7628: Direct Comparison

Property	PP-1080	PP-2116	PP-7628
Cured thickness range	~60–90 µm	~100–132 µm	~170–200 µm
Resin content	~60–70%	~50–57%	~42–52%
Dk @ 1 GHz	~4.0–4.3	~4.3–4.6	~4.5–4.9
Drill compatibility	Excellent	Good	Caution <0.3 mm drill
Fiber weave uniformity	Good	Good	Moderate
Controlled impedance suitability	Good (thin layers)	Excellent (standard layers)	Limited
Resin fill for 1 oz Cu	Good	Excellent	Good (SR/MR)
Relative cost per ply	Low	Moderate	Low–moderate
Standard default?	Sometimes	Yes — most common	Occasional

Handling, Storage, and Shelf Life for PP-2116 Prepreg

PP-2116 prepreg is sensitive to the same environmental conditions as all B-stage materials, but engineers who send out fabrication jobs are not always aware of what the fab house needs to manage on their end. Understanding these constraints matters if you are evaluating fab house quality or specifying materials for in-house lamination.

PP-2116 prepreg should be stored at temperatures below 23°C (73°F) and at relative humidity of 50% or lower. Exposure to ambient humidity causes the resin to absorb moisture, which has two consequences: reduced resin flow during lamination (because moisture occupies surface sites that would otherwise contribute to bonding) and potential void formation as trapped moisture converts to steam under press heat. PP-2116 prepreg stored under correct conditions has a shelf life of approximately three months from manufacture. Material approaching or past its shelf date will exhibit reduced tack, lower flow on gel time tests, and is likely to produce interlayer voids or delamination in production. Any reputable fab house runs incoming flow and gel time tests on prepreg before it enters production.

During handling, PP-2116 prepreg should not be exposed to direct UV light, which can advance the cure state of the resin. Rolls or sheets should be kept wrapped in their original moisture-barrier packaging until immediately before use.

Useful Resources for PP-2116 Prepreg Specification

Resource	Description	Link
Isola IS420 Prepreg Dk/Df Data Tables	Frequency-specific dielectric data for 2116 and other glass styles across 100 MHz–10 GHz	isola-group.com
Panasonic R-1755M Datasheet	Detailed construction tables for 2116 + 7628 hybrid cores used in commercial MLB designs	industrial.panasonic.com
IPC-4101F Base Materials Standard	The qualification and acceptance specification covering glass-reinforced FR-4 prepregs including 2116	ipc.org
PCBSync Prepreg Selection Guide	Practical engineer-focused breakdown of glass styles, resin grades, and stackup decision-making	pcbsync.com
Intel FPGA PCB Stackup Design Guidelines (AN613)	Recommended glass weave styles for high-speed FPGA designs, including 2116 for signal layers	intel.com/programmable
Altium Designer Stackup Manager	Integrated PCB stackup tool with prepreg Dk/Df import for controlled impedance design	altium.com
Shengyi Technology S1000H Datasheet	High-Tg FR-4 product using 2116 glass in standard MLB constructions, with Dk/Df by resin content	sytech.com.cn
Z-zero Stackup Planner	Free web-based stackup tool that uses supplier-specific Dk/Df values per glass style and resin content	z-zero.com

5 FAQs: PP-2116 Prepreg in PCB Design and Fabrication

Q1: Why is PP-2116 prepreg the most common default prepreg in standard FR-4 4-layer designs rather than 1080 or 7628?

The answer comes down to a combination of pressed thickness, drilling behaviour, and resin fill performance that 2116 balances better than its neighbours. A single ply of 2116 MR at ~115–125 µm gives close to the right dielectric spacing for 50 Ω microstrip on a standard 1.6 mm board without stacking multiple plies. Achieving similar thickness with 1080 alone requires two plies — adding lamination material cost and introducing more potential for ply registration variation. Using 7628 gives a thicker dielectric, higher Dk, and a heavier glass weave that causes drill deflection risk on fine via pitches. The 2116 style also drills cleanly at via diameters from 0.2 mm up, handles standard 1 oz copper fill without excessive squeeze-out, and is the most universally stocked material across commercial fab houses — meaning shorter lead times and a lower risk of supplier substitution surprises.

Q2: What is the actual Dk I should use for PP-2116 prepreg in my impedance calculations?

This is where many engineers make errors. The generic “FR-4 Dk = 4.2” value that appears in online impedance calculators is derived from a historical mid-frequency measurement and does not accurately describe PP-2116 prepreg at the specific resin content, frequency, and post-lamination state of your actual board. For most PP-2116 MR prepreg at 1 GHz, the post-lamination Dk is in the range of 4.3–4.5. At 5 GHz it typically drops to 4.1–4.3, and at 10 GHz to approximately 4.0–4.2. The correct approach is to use the Dk/Df table from your specific supplier’s datasheet at the target frequency, then confirm with your fab house that their press-out data for that material matches the published figures. Many experienced fabricators have empirically-derived pressed-Dk values for their standard prepreg lots that differ from raw datasheet values — the raw material is measured before lamination, and the PCB lamination process shifts the effective Dk slightly. For IPC-6012 Class 3 controlled impedance boards, always specify TDR testing on impedance coupons at the target RF frequency to verify the actual pressed result.

Q3: Can I mix PP-2116 prepreg with 1080 or 7628 in the same board stackup?

Yes — and this is standard practice in commercial MLB fabrication. The most common combination is 2116 + 1080, where 2116 provides the main dielectric layer thickness and 1080 is added to fine-tune the total dielectric to hit a specific target thickness or impedance. The combination 2116 + 1080 gives approximately 190–200 µm pressed thickness, compared to ~120 µm for 2116 alone or ~75 µm for 1080 alone — a useful middle thickness that cannot be achieved with a single ply of either style. Mixing 2116 with 7628 is also done in larger boards where cost and thickness build-up are priorities on non-impedance-critical layers. CTE compatibility within the same resin system (all standard FR-4 epoxy from the same or equivalent suppliers) is not an issue for mixed glass style stackups. The critical requirement is that each dielectric layer’s Dk and pressed thickness must be calculated individually — you cannot average across layers or use a single Dk value for the whole stackup when styles are mixed.

Q4: How does PP-2116 prepreg perform in high-Tg and halogen-free versions, and does the Dk change?

High-Tg 2116 prepreg — Tg ≥ 170°C by DSC — uses a modified epoxy chemistry such as a dicy-free or phenolic-cured system. The resin change affects both Dk and Df values relative to standard mid-Tg 2116. Phenolic-cured high-Tg systems typically show Dk values approximately 0.1–0.2 units lower than their standard mid-Tg equivalents at 1 GHz, and Df that is broadly similar or slightly higher. Halogen-free 2116 prepreg replaces brominated flame retardant with phosphorus-based or inorganic compounds, which can also shift Dk slightly. The practical implication is the same as for any material grade change: do not carry over the Dk values from your standard 2116 impedance model to a high-Tg or halogen-free version of the same glass style. Request the supplier’s specific frequency-resolved Dk/Df table for the exact product grade before re-running your impedance calculations. This is particularly important on Class 3 IPC-6012 boards where impedance tolerance is ±5%, because a 0.1–0.2 unit Dk difference translates directly to a 1–3 Ω impedance shift.

Q5: How many plies of PP-2116 prepreg can I safely stack between two cores in a multilayer design?

The industry guideline is a maximum of three plies of prepreg stacked between adjacent core layers. Beyond three plies, void risk increases significantly — the trapped volatiles from the resin system have less opportunity to escape during the press cycle, and the cumulative thickness of stacked uncured resin creates pressure and temperature gradients within the stack that can result in non-uniform cure. Additionally, stacking more than three plies of 2116 would give a dielectric section of ~360–390 µm, which is approaching the point where a core laminate would provide better mechanical reliability and dimensional stability than a prepreg stack. If your stackup needs a dielectric section thicker than ~350 µm at a single position, the correct approach is to specify a thicker core at that position rather than stacking additional prepreg plies. For PP-2116 specifically, the most common practical maximum used in commercial MLB production is two plies per dielectric position, giving ~230–250 µm pressed thickness with 1 oz copper.