PP-106 Ultra-Thin Prepreg: Applications in HDI PCB Design

When board thickness budgets tighten and microvia aspect ratios start dictating dielectric choices, most engineers eventually arrive at the same material: PP-106 ultra-thin prepreg. At roughly 50 µm per pressed ply, the 106 style is the thinnest glass-fabric prepreg in mainstream PCB production — thin enough to support laser-drilled microvias at favorable aspect ratios, thin enough to enable the ultra-compact dielectric sections in HDI build-up layers, and resin-rich enough to bond reliably under standard lamination conditions. Its near-square weave (56×56 threads per inch) also gives it the most uniform Dk distribution of any single-ply thin prepreg, which is why it has a long history as a dual-ply dielectric choice adjacent to power planes and — more recently — as a build-up layer material in HDI smartphone, wearable, and high-speed server board designs.

This guide covers the complete PP-106 ultra-thin prepreg property set — glass construction, SR/MR/HR resin grades, Dk/Df by frequency, pressed thickness — and maps those properties to the specific HDI design situations where 106 is the correct call. It also addresses the material’s genuine limitations: low per-ply stiffness, GWS characteristics at high data rates, and the dual-ply practice that solves most of them. Engineers working with thin, high-reliability laminate families — including the Doosan PCB laminate range — will find 106 prepreg routinely paired with high-Tg and halogen-free cores in sequential build-up constructions.

What Is PP-106 Ultra-Thin Prepreg? Glass Construction and B-Stage Fundamentals

PP-106 ultra-thin prepreg is a B-stage composite built on IPC/NEMA 106 woven E-glass fabric, pre-impregnated with a partially cured epoxy resin. The 106 designation identifies the glass fabric style per IPC-4412B — not a brand name or proprietary product. Any major laminate supplier (Isola, Shengyi, Panasonic, Nan Ya) that produces 106 prepreg starts from the same base fabric specification: ECD-type E-glass yarn, woven at 56 ends per inch in the warp direction and 56 picks per inch in the fill direction.

This 56×56 symmetric thread count is the detail that distinguishes 106 from 1080 and makes it electrically appealing at high data rates. Because the yarn density is equal in both warp and fill directions, the periodic pattern of “glass bundles” and “resin windows” in the cured dielectric is effectively the same regardless of the orientation of the signal trace relative to the weave. The practical consequence is that two legs of a differential pair crossing the 106 fabric in perpendicular orientations will encounter a statistically similar Dk profile — a meaningful advantage over the asymmetric 1080 weave (60×47) for designs where glass-weave skew (GWS) budget is tight.

Base fabric weight is approximately 25 g/m² (0.73 oz/yd²) — about half the weight of the 1080 fabric — and raw fabric thickness from Isola’s manufacturing data is approximately 0.038 mm (1.5 mil). Resin content after impregnation ranges from ~70% to ~76% by weight, the highest resin fraction of any common PCB prepreg style.

PP-106 Ultra-Thin Prepreg in the Glass Style Family

Glass Style	Thread Count (warp × fill)	Fabric Weight (g/m²)	Raw Fabric Thickness	Resin Content Range	Cured Thickness / ply	Dk @ 1 GHz	Weave Symmetry
106	56 × 56	~25	~0.038 mm	~70–76%	~50–65 µm	3.9–4.2	Square
1080	60 × 47	~49	~0.064 mm	~60–66%	~60–90 µm	4.0–4.3	Asymmetric
2116	60 × 58	~109	~0.089 mm	~50–57%	~100–132 µm	4.3–4.6	Near-square
7628	44 × 31	~203	~0.173 mm	~42–52%	~170–200 µm	4.5–4.9	Asymmetric

The 106 style occupies the thinnest position in the standard commercial prepreg family. Nothing between 106 and resin-coated copper (RCC) foil or film dielectrics exists in the glass-fabric world — once you move below ~50 µm per ply, you leave woven glass behind entirely and enter the specialist materials territory of RCC, build-up films, and liquid photo-imageable dielectrics.

PP-106 Ultra-Thin Prepreg Properties: Electrical, Mechanical, and Thermal

Electrical Properties by Resin Grade

The high resin content of the 106 style is why it consistently delivers the lowest Dk of any common glass-fabric FR-4 prepreg. Resin (epoxy) has a lower dielectric constant than E-glass; packing more resin per unit volume reduces the effective Dk of the composite. At 70–76% resin content, 106 prepreg sits 0.2–0.4 Dk units below equivalent 1080 grades and 0.4–0.7 units below 2116.

Property	SR (~70–72%)	MR (~73–74%)	HR (~75–76%)	Test Method
Resin Content	70–72%	73–74%	75–76%	IPC-TM-650 2.3.16
Cured Thickness / ply (0.5 oz Cu)	~50–56 µm	~55–60 µm	~58–65 µm	IPC-TM-650 2.4.39
Dk @ 1 GHz	~4.0–4.2	~3.9–4.1	~3.8–4.0	IPC-TM-650 2.5.5.5
Dk @ 5 GHz	~3.8–4.0	~3.7–3.9	~3.7–3.8	IPC-TM-650 2.5.5.5
Dk @ 10 GHz	~3.7–3.9	~3.6–3.8	~3.5–3.7	IPC-TM-650 2.5.5.5
Df @ 1 GHz	~0.014–0.018	~0.014–0.018	~0.014–0.018	IPC-TM-650 2.5.5.5
Resin Flow	Low	Medium	High	IPC-TM-650 2.3.17
UL 94 Flame Rating	V-0	V-0	V-0	UL 94

As always, the numbers above are indicative ranges — pull the frequency-resolved Dk/Df table from your specific supplier’s datasheet before running impedance calculations. Isola, Panasonic, and Shengyi 106 products will differ slightly, and the difference matters at ±5% impedance tolerance.

Mechanical and Thermal Properties

Property	Typical Value	Notes
Tg — standard epoxy	≥130–150°C (DSC)	Mid-Tg FR-4 epoxy system
Tg — high-Tg grade	≥170–180°C (DSC)	Phenolic-cured or dicy-free system
Decomposition Temp (Td)	≥300–340°C	Higher in high-Tg grades
T-260 (standard)	>5 min	IPC-TM-650 2.4.24.1
Z-axis CTE (below Tg)	~55–65 ppm/°C	Higher than 2116/7628 due to high resin content
Peel Strength (0.5 oz Cu)	≥0.8–1.0 N/mm	IPC-TM-650 2.4.8
Water Absorption	≤0.40%	Higher than heavier fabrics; humidity control important
Shelf Life (≤23°C, <50% RH)	~3–6 months	In original sealed packaging

The higher Z-axis CTE of 106 prepreg — driven by the same high resin fraction that gives it low Dk — means more thermal expansion per degree than a 2116 or 7628 dielectric in the same position. In standalone use as a thin build-up layer, this is manageable because the overall thickness contribution of a single 106 ply is small and the adjacent copper provides constraint. In a dual-ply 106 position where total dielectric thickness reaches 100–130 µm, Z-axis expansion is broadly comparable to a single 1080 ply.

HDI PCB Design Applications for PP-106 Ultra-Thin Prepreg

#### Microvia Build-Up Layers: Matching Dielectric to Laser Aspect Ratio

The most critical application for PP-106 ultra-thin prepreg in HDI design is as the dielectric layer through which laser-drilled blind microvias are formed. IPC guidelines cap microvia aspect ratio at 1:1 (depth:diameter) for reliable copper plating. A single ply of 106 HR at 60–65 µm allows a minimum laser drill diameter of 60–75 µm to satisfy a 1:1 aspect ratio target. This is exactly the diameter range used in smartphone, tablet, and wearable device HDI boards where 0.4–0.5 mm pitch BGA fanout is required.

For comparison: if you use a single 1080 MR ply (~73 µm) in the same position, the minimum microvia diameter rises to ~75 µm — workable, but leaving less margin. A 2116 ply at ~120 µm would demand a minimum 120 µm microvia diameter, which is too large for the via-in-pad geometry under a 0.4 mm pitch BGA. PP-106 ultra-thin prepreg is therefore not just preferred but often necessary in the outermost dielectric of fine-pitch HDI build-ups.

A representative 6-layer 1+N+1 HDI stackup with PP-106 outer build-up:

Layer	Material	Approximate Thickness
L1 outer signal	0.5 oz Cu (18 µm) + plating	~35 µm
Build-up dielectric (L1→L2)	1 × 106 HR (laser-drillable)	~62 µm
L2 ground plane	0.5 oz Cu	~18 µm
Inner core (L2–L5)	FR-4 high-Tg 0.8 mm core	~800 µm
L5 power plane	0.5 oz Cu	~18 µm
Build-up dielectric (L5→L6)	1 × 106 HR (laser-drillable)	~62 µm
L6 outer signal	0.5 oz Cu + plating	~35 µm
Total (approx.)		~1.03 mm

For 0.8 mm pitch BGAs on thicker boards, a dual-ply 106 SR build-up (~100–110 µm combined) allows a slightly larger microvia diameter with equivalent aspect ratio performance while adding mechanical robustness compared to a single ultra-thin ply.

Dual-Ply PP-106 for Power Plane Separation

PP-106 ultra-thin prepreg has a long-established role as a dual-ply dielectric between inner power and ground planes — a practice that predates HDI design and continues in standard multilayer FR-4 boards where tight power plane coupling is needed. Two plies of 106 SR give approximately 100–112 µm of dielectric between inner planes, with a combined Dk lower than a single 1080 ply of similar thickness. The lower Dk means slightly higher impedance between the planes, which is typically acceptable for power distribution networks. This is also cost-effective: two plies of 106 typically cost less per unit area than one ply of 1080 or 2116 of comparable thickness, making it the economical default for inner power/ground separation in commercial-grade multilayer boards.

As Z-zero’s Bill Hargin noted in his PCDandF series on glass weave skew, the 56×56 near-square weave of the 106 style also gives it a modest glass-weave skew advantage over 1080 in this dual-ply configuration. For designs running below 5 Gbps where power plane dielectrics neighbour signal layers, dual-ply 106 is a defensible choice. For designs above 10 Gbps, where the signal layers adjacent to the power plane need tighter GWS control, upgrading to dual-ply 1067 or mechanically spread glass is the more rigorous approach.

Ultra-Thin Boards and Compact Consumer Device Designs

In boards targeting total thickness below 0.8 mm — smart cards, certain wearable modules, high-density memory modules — every dielectric layer needs to be as thin as practically possible. A single 106 HR ply at 60 µm replaces an 80 µm 1080 ply, saving 20 µm per dielectric position. Across four dielectric positions in a 6-layer board, that is 80 µm total recovered — the difference between a 0.8 mm and a 0.72 mm finished board can determine whether a product fits in its housing.

PP-106 Ultra-Thin Prepreg GWS Characteristics: The Nuanced Reality

At low to moderate data rates (below 5 Gbps), the 56×56 square weave of 106 prepreg is a genuine advantage over 1080’s asymmetric 60×47 construction. Because the glass bundle pitch is equal in both directions, differential pairs are less sensitive to orientation relative to the weave. However, published research including an Applied Sciences fiber weave study shows that at 25 Gbps, even square-weave fabrics experience GWS through their large resin windows. The 106 fabric’s resin windows — the open areas between glass bundles not covered by glass — represent a significant fraction of the total dielectric area because the yarn is fine and the weave is relatively open.

Z-zero’s glass-weave skew data series demonstrates that for the same target dielectric thickness, single-ply 106 glass shows larger resin windows than alternative fabrics like 1067 (70×70 thread count). For designs firmly above 10–25 Gbps where the build-up dielectric directly neighbours SerDes or PAM4 signal layers, upgrading to mechanically spread 106 glass or substituting a dedicated HDI film dielectric for the signal-adjacent positions is the rigorous path. Retaining 106 prepreg for the non-critical inner plane positions while using spread glass on signal-adjacent build-up layers is a practical hybrid approach that most volume HDI fabs support.

PP-106 Ultra-Thin Prepreg vs. Other Thin Dielectric Options

Property	PP-106 (glass prepreg)	PP-1080 (glass prepreg)	RCC Film	ABF Film
Cured thickness range	~50–65 µm	~60–90 µm	~30–60 µm	~15–40 µm
Glass reinforcement	Yes (56×56 E-glass)	Yes (60×47 E-glass)	No	No
Laser drillability	Good (CO2/UV)	Good (CO2/UV)	Excellent (CO2)	Excellent (UV)
Dk @ 1 GHz	~3.9–4.2	~4.0–4.3	~3.3–3.8 (resin dependent)	~3.0–3.4
Mechanical strength / ply	Moderate	Good	Low (no glass)	Low (no glass)
Dimensional stability	Good	Good	Lower	Lower
Standard FR-4 process compatible	Yes	Yes	Requires modified process	Requires modified process
Relative cost	Low–moderate	Low–moderate	Moderate–high	High
Best HDI use	1+N+1 to 2+N+2 outer build-up	Thin standard build-up	RCC-specific HDI	ELIC/ultra-HDI

For standard HDI constructions (1+N+1 or 2+N+2) built on FR-4-compatible lamination processes, PP-106 ultra-thin prepreg is the lowest-cost, most universally available thin dielectric. RCC and ABF films are reserved for constructions that require sub-50 µm dielectric and those processes require specialist equipment investment — not practical for general-purpose commercial HDI fabrication.

Storage, Handling, and Shelf Life

PP-106 ultra-thin prepreg is the most moisture-sensitive of the common glass-fabric styles because its very high resin content means more resin surface area exposed to ambient humidity. Store at temperatures below 23°C and at relative humidity below 50%, sealed in original moisture-barrier packaging. Shelf life under these conditions is typically three to six months from manufacture.

The thin, high-resin character of 106 prepreg also makes it the most prone to handling damage — a 50 µm ply has approximately half the flexural stiffness of a 100 µm 1080 ply. Sheets should be handled flat and not subjected to bending during layup. Fab houses with automated layup equipment typically handle 106 without problems; manual layup on tight panels requires care to avoid creasing, which creates thickness irregularities that directly affect microvia depth control during laser drilling.

UV exposure and excessive heat at the layup station will advance the B-stage cure and reduce resin flow. Incoming gel time and flow testing should be performed on every production lot before use.

Useful Resources for PP-106 Ultra-Thin Prepreg Specification

Resource	Description	URL
Isola IS410 Prepreg Datasheet	Full Dk/Df frequency tables for 106 style at multiple resin content grades, 100 MHz–10 GHz	isola-group.com
Panasonic R-1755C/W Datasheets	Construction tables for 106 and 106 + 1080 dual-ply HDI build-up constructions	industrial.panasonic.com
IPC-2226 HDI Design Standard	The design standard for HDI PCBs — defines 1+N+1 through ELIC stackup types and microvia aspect ratio requirements	ipc.org
Z-zero: “How to Avoid Getting Totally Skewed” Series	Quantitative glass-weave skew analysis comparing 106, 1067, and other thin glass styles in dual-ply configurations	z-zero.com
Intel AN528: PCB Dielectric Material Selection and Fiber Weave Effect	Detailed skew analysis at high data rates across glass styles including 106 and 1080	intel.com
Altium Designer Stackup Manager	PCB layer stack tool with HDI build-up support and laser-drillable prepreg Dk/Df import	altium.com
Applied Sciences: “Effect of Fiber Weave Structure in PCB” (MDPI, 2019)	Peer-reviewed paper including 106 fiber weave Dk variation and GWS skew data at 25 Gbps	mdpi.com
PCBSync HDI Prepreg Guide	Practical selection guide for thin prepreg styles in HDI and high-speed applications	pcbsync.com

5 FAQs: PP-106 Ultra-Thin Prepreg in HDI PCB Design

Q1: Why is PP-106 ultra-thin prepreg preferred over 1080 in HDI microvia build-up layers?

The primary reason is microvia aspect ratio. A laser-drilled blind via must obey the IPC ≤1:1 depth-to-diameter ratio to achieve reliable sidewall plating. For a build-up dielectric of ~60 µm (one ply 106 HR), the minimum microvia diameter is approximately 60 µm — directly targeting the 60–75 µm range used under 0.4–0.5 mm pitch BGAs. If you use a 1080 MR ply (~73 µm) instead, the minimum diameter rises to ~75 µm, which may not fit within the via-in-pad pad geometry at fine pitch. Beyond aspect ratio, 106 prepreg’s higher resin content (~70–76%) produces a smoother surface that bonds cleanly to thin 0.5 oz copper foil and minimises micro-roughness in the via sidewall prior to desmear. The lower Dk of 106 (~3.9–4.1 at 1 GHz) compared to 1080 (~4.0–4.3) is a secondary benefit — slightly lower effective impedance per trace width unit, which can relax trace width requirements marginally on the outer build-up layer.

Q2: Can I use PP-106 ultra-thin prepreg with standard FR-4 lamination cycles, or does HDI require special equipment?

A 1+N+1 HDI board using 106 prepreg as the outer build-up can be fabricated with standard FR-4 press cycles using existing FR-4 compatible equipment, provided the fab house has a laser drill for microvia formation. The 106 prepreg itself does not require any non-standard press temperature or pressure — it cures under the same conditions as 1080 or 2116. The difference from standard MLB fabrication is the process sequence: the inner core is laminated and plated first, then the 106 build-up layer and copper foil are laminated in a second cycle, and laser drilling and plating of the microvias follow. This sequential lamination adds fabrication steps and cost compared to a standard single-lamination multilayer board. For 2+N+2 constructions requiring two build-up cycles on each side, the process requires even tighter layer registration control. Confirm your fab house’s sequential lamination capability before committing to HDI stackup in your design.

Q3: What Dk value should I use for PP-106 ultra-thin prepreg in my impedance model?

Do not use the default “FR-4 Dk = 4.2–4.4” in your impedance calculator. For PP-106 MR prepreg at 1 GHz, the post-lamination Dk is typically 3.9–4.1. At 5 GHz it drops to approximately 3.7–3.9, and at 10 GHz to approximately 3.6–3.8. These values are 0.2–0.4 units lower than equivalent 2116 values, and the difference is significant enough to affect trace width on the outer build-up layer by 0.5–1.5 mil at 50 Ω. The correct workflow is to use the supplier’s specific frequency-resolved Dk/Df table for the exact 106 product and resin grade, enter it into an impedance calculator that supports layer-by-layer Dk input, and confirm against your fabricator’s empirical press-out data. Also note that the Dk of 106 prepreg is more sensitive to resin content grade than the heavier glass styles — the difference between SR and HR 106 at 1 GHz is approximately 0.2 Dk units, which is non-trivial. Always specify both the glass style and the resin grade (SR/MR/HR) in your stackup notes.

Q4: How does dual-ply PP-106 ultra-thin prepreg compare to single-ply 1080 for the same total dielectric thickness?

Two plies of 106 MR give approximately 110–120 µm pressed thickness — similar to one ply of 1080 MR (~73 µm) + one thin ply of 106 (~60 µm), or one ply of 2116 SR (~100–110 µm). The dual-ply 106 construction has a lower combined Dk than single-ply 1080 of comparable thickness because the overall resin fraction is higher. Mechanically, dual-ply 106 provides better panel stiffness than single-ply because two independent glass plies in the Z-stack offer some delamination resistance even when the individual plies are ultra-thin. Dual-ply 106 also averages out glass-weave periodicity across two physically shifted ply positions — if the plies are not registered in the same orientation, resin window positions statistically offset across the two layers, slightly reducing GWS impact compared to a single ply. This is the original reason dual-ply 106 became a standard power-plane technique before explicit GWS awareness: it was empirically better than single-ply for interlayer dielectric consistency, even if the mechanism was not well understood at the time.

Q5: When should I upgrade from PP-106 ultra-thin prepreg to a non-glass dielectric like RCC or ABF film in my HDI build-up?

The triggers are either sub-50 µm dielectric requirement, data rates above 25 Gbps, or component pitch below 0.3 mm that demands via diameters below 50 µm. A single ply of 106 HR can be pressed to ~58–65 µm but cannot reliably achieve below 50 µm in a glass-fabric construction — the glass bundle diameter in the 106 style is approximately 5–7 µm per filament, and below 50 µm total cured thickness, glass fiber density begins to create local thickness irregularities that affect laser drilling depth control. RCC foil (30–50 µm dielectric thickness) or ABF build-up film (15–40 µm) step below this limit, but they require process modifications: RCC uses a modified lamination approach bonding directly to inner copper, and ABF film requires a vacuum laminator rather than a conventional press. Both are significantly more expensive than 106 prepreg per unit area and require a fab house with specific ABF or RCC capability. For the vast majority of commercial HDI designs targeting 0.4–0.5 mm BGA pitch with 60–75 µm microvia diameters, PP-106 ultra-thin prepreg remains the correct and most cost-effective build-up material.