PP-106 ultra-thin prepreg delivers ~50 µm cured thickness and ~70% resin content — the standard choice for HDI build-up layers, microvia dielectrics, and high-speed PCB stackups.

If you’ve spent any real time working on high-density interconnect (HDI) PCB stackups, you’ve almost certainly run into the question of which prepreg to use for your build-up layers. And if you’ve done the research, the answer keeps coming back to the same material: PP-106 ultra-thin prepreg. At roughly 50 µm cured thickness and resin content sitting around 70–76%, the 106 glass style sits in a class of its own for thin dielectric applications. This article walks through what makes PP-106 tick, where it performs best, and how to design with it effectively in HDI boards.

What Is PP-106 Ultra-Thin Prepreg?

PP-106 (also written as Prepreg 106 or simply 106 prepreg) is a fiberglass-reinforced bonding sheet where the glass weave style is designated “106.” The number doesn’t refer to a thickness directly — it’s actually an industry classification for the glass fabric type used as the reinforcement substrate. The 106 glass weave is notably loose compared to styles like 7628 or 2116, which means the epoxy resin fills more of the volume during impregnation. That’s where its hallmark high resin content comes from.

Key Technical Parameters of PP-106

The specs below represent typical values across mainstream laminate vendors (Isola, Shengyi, Panasonic, Kingboard, and others). Always pull the specific datasheet for your chosen material, because values drift between suppliers and resin systems.

Parameter	Typical Value
Glass Style	106
Cured Thickness	~50–61 µm (0.002–0.0024 in)
Resin Content	70–76%
Dielectric Constant (Dk) @ 1 GHz	3.32–4.10 (varies by resin content & supplier)
Dissipation Factor (Df) @ 1 GHz	0.014–0.019
Glass Weave Pattern	Loose (low fiber density)
Typical Application	HDI build-up layers, microvia dielectrics

One thing that catches designers off guard: the Dk of PP-106 varies noticeably depending on resin content. A 75% resin content 106 prepreg from Isola measures around Dk 3.32 at 1 GHz, while a 72% version might read closer to 4.10 at the same frequency. That 20%+ swing matters if you’re doing controlled impedance work, so always reference the exact datasheet, not generic “FR-4 prepreg” tables.

How PP-106 Fits Into HDI PCB Stackup Design

The Role of Thin Dielectrics in HDI Architecture

HDI boards rely on sequential lamination — you’re not pressing all layers at once. Instead, a core board goes through multiple lamination cycles, each adding thin build-up layers on both sides. The prepreg used for those build-up layers has to be thin enough to support laser-drilled microvias with acceptable aspect ratios.

Here’s the math that drives it: the ideal microvia aspect ratio is ≤1:1, meaning the via depth shouldn’t exceed the via diameter. A typical CO₂ or UV laser drill creates microvias in the 75–150 µm diameter range in volume production. With a 50 µm dielectric (PP-106), you’re landing well inside that comfort zone even with a 75 µm via. Push to a thicker dielectric like PP-2116 (90–110 µm) and you start bumping against aspect ratio limits, forcing larger capture pads or more laser hits.

PP-106 vs. Other Common Prepreg Styles

Glass Style	Cured Thickness	Resin Content	Best Use Case
106	~50–61 µm	~70–76%	HDI build-up, microvia layers
1080	~60–90 µm	~60–68%	Thin multilayer cores, general use
2116	~90–110 µm	~50–57%	Mid-thickness dielectric layers
7628	~170–190 µm	~42–45%	Thick structural cores, power boards

PP-106 is the go-to for build-up layers precisely because you can’t use thicker styles when your microvia depth budget is tight. However, it’s worth noting that the loose weave of 106 prepreg creates a known tradeoff: fiber weave effects. The uneven glass-to-resin ratio across the XY plane can cause Dk variation depending on routing angle relative to the weave. For differential pairs running at multi-GHz speeds, this can introduce skew. Rotating the board 10° or specifying spread-glass or flat-glass weave variants mitigates the problem.

Core Applications of PP-106 Ultra-Thin Prepreg

HDI Build-Up Layers for Smartphones and Wearables

The clearest application is consumer electronics. Modern flagship smartphones use HDI constructions like 2+n+2 or 3+n+3 (where n is the standard core), meaning two or three sequential build-up layers on each side of a central core. Every one of those build-up dielectric layers needs to be thin enough for laser microvia formation, and PP-106 is the near-universal choice at that tier. The 50 µm dielectric thickness enables microvia diameters down to 75–100 µm in high-volume manufacturing.

High-Speed Digital Designs: 5G, AI, and Server Boards

For PCBs running DDR5, PCIe Gen 5, or high-speed SerDes above 25 Gbps, the dielectric between routing layers directly affects trace impedance, propagation delay, and insertion loss. PP-106’s relatively low Dk (~3.3–3.5 with standard resin systems) translates to wider traces for a given impedance target, which can be helpful in fine-pitch BGA fanout scenarios. Its thinner profile also reduces parasitic via capacitance — a real concern when vias are packed densely in BGA escape routing.

A common approach on server boards is to use PP-106 for the outer HDI build-up layers where microvias are needed, then transition to PP-2116 or PP-1080 for the inner core layers where mechanical stiffness matters more than thinness.

RF and Microwave PCBs

While dedicated RF laminates like Rogers 4350B or Taconic materials dominate the sub-6 GHz and mmWave world, there’s a segment of applications — particularly in 5G sub-6 GHz infrastructure and Wi-Fi 6E antenna feed networks — where cost pressure drives engineers toward enhanced FR-4 or low-loss halogen-free stackups. In those designs, PP-106 layers positioned under surface routing layers help minimize dielectric layer thickness while keeping the overall Dk reasonably consistent.

Medical Electronics and Implantable Devices

Miniaturization is critical in medical devices, and the thin dielectric stack that PP-106 enables allows designers to hit aggressive overall PCB thickness targets (sometimes under 0.4 mm for flexible-rigid constructions) while maintaining enough layer count for signal routing and power distribution. The halogen-free variants of PP-106 also align with the materials compliance requirements common in medical device manufacturing.

Automotive ADAS and LiDAR Boards

Advanced driver assistance systems (ADAS) PCBs increasingly use HDI constructions to manage component density in space-constrained ECUs. PP-106 appears here as the dielectric between high-density routing layers, particularly in camera processor boards and radar frontend modules. Automotive-grade versions must pass thermal cycling and Tg requirements — always verify the specific Tg specification of the PP-106 variant you’re using, since standard-Tg FR-4 106 prepreg tops out around 130–140°C, while high-Tg versions push to 170°C+.

PP-106 Stackup Design: Practical Engineering Guidance

Combining PP-106 with Other Prepreg Styles

You can stack multiple prepreg sheets together to hit a target dielectric thickness. A common example is combining PP-106 + PP-1080 to achieve roughly 110–150 µm — useful when you need a thicker dielectric than one PP-106 ply provides, but want more resin fill than PP-2116 offers. When doing this, make sure you account for combined resin flow during pressing; too much resin and you risk void-free fill problems, too little and microvia voids become a concern.

Impedance Control with PP-106

Controlled impedance traces over a PP-106 dielectric follow the same physics as any other prepreg, but the thin dielectric creates some specific engineering challenges:

Impedance Target	Trace Width over PP-106 (~50 µm)	Trace Width over PP-2116 (~100 µm)
50Ω microstrip	~55–65 µm	~90–110 µm
50Ω stripline	~30–45 µm	~65–85 µm
100Ω differential stripline	~40–55 µm (w/ ~50 µm gap)	~70–90 µm (w/ ~90 µm gap)

Values are approximate and depend on copper weight, Dk, and stack geometry. Always run through your fab’s impedance calculator.

The thin dielectric means trace widths get very narrow, especially for embedded stripline geometries. At 50 µm dielectric, a 50Ω stripline might require traces narrower than 50 µm — which pushes into advanced fab territory (sub-2 mil). Factor this into your DFM checklist early.

Storage and Handling of PP-106

PP-106 prepreg is sensitive to its environment, and the high resin content makes it more susceptible than thicker styles:

Requirement	Guideline
Storage temperature	0–10°C (refrigerated)
Storage humidity	<50% RH
Packaging	Sealed moisture-barrier bag until use
Shelf life (sealed)	Typically 3–6 months from manufacture date
UV exposure	Avoid direct light; can initiate premature curing

Moisture absorption is the biggest killer. A PP-106 sheet that has absorbed humidity before lamination can produce voids, blistering, or delamination after the press cycle. If you’re working with material that’s been open for any length of time, bake it per the manufacturer’s spec before use.

Doosan PCB and PP-106 Material Options

One of the well-regarded suppliers in the HDI laminate and prepreg space is Doosan. Their CCL and prepreg product lines cover the 106 glass style across multiple resin systems — standard FR-4, high-Tg, and halogen-free variants. If you’re sourcing materials for a high-density design, it’s worth reviewing Doosan PCB alongside other tier-1 laminate suppliers to compare Dk/Df performance, Tg rating, and laser drillability specifications. Doosan’s prepreg products are used in HDI multilayer constructions for consumer electronics, server infrastructure, and automotive ECUs.

Useful Resources for PCB Engineers

The following resources are directly relevant to PP-106 material selection and HDI stackup engineering:

Resource	What You’ll Find
Isola Dk/Df Tables (IS415)	Measured Dk and Df for 106 and other glass styles across frequency sweep
Panasonic PCB Materials Selector	Halogen-free prepreg options including 106-style products
IPC-4101 Standard	Specification for base materials for rigid and multilayer PCBs
IPC-2226 (HDI Design)	Design standard specifically for HDI PCBs including microvia rules
IPC-6012 Class 3	Qualification and performance for rigid PCBs in high-reliability applications
Sierra Circuits HDI Material Guide	Practical guidance on laminate and prepreg selection for HDI
Altium Designer Layer Stack Manager	Software tool for modeling prepreg thickness and Dk in stackups

Frequently Asked Questions About PP-106 Ultra-Thin Prepreg

Q1: Can PP-106 be used as the sole prepreg in all layers of an HDI board?

Not typically. PP-106 is best suited for the thin build-up layers in HDI where microvias need to be drilled. The loose glass weave provides less mechanical strength than 2116 or 7628 styles, so most designs use PP-106 only on the outer build-up layers and transition to stiffer prepreg types for the inner core laminations.

Q2: Why does PP-106 have such a high resin content compared to other prepreg styles?

The 106 glass fabric has a loosely woven structure with relatively low fiber density. During the prepreg impregnation process, the large gaps in the weave get filled with resin, resulting in resin content typically ranging from 70% to 76% by weight. This high resin fill is actually beneficial for microvia reliability, since it helps prevent voids around the via land during pressing.

Q3: How does PP-106 behave during laser drilling?

PP-106’s high resin content is generally favorable for CO₂ laser drilling. The resin ablates efficiently under the laser beam, and the thin dielectric (50 µm) means the laser doesn’t need to penetrate deeply, producing cleaner hole walls. However, the loose glass weave can still cause slight variation in hole shape if the laser parameters aren’t optimized for the specific material. Always get confirmed laser parameters from your fabricator.

Q4: What’s the difference between PP-106 and PP-1067?

PP-1067 is a flat-weave variant of the 106 glass style with a modified weave structure designed to reduce fiber weave effects. It has slightly denser fiber distribution, making Dk more uniform across the material plane. This matters for differential pairs and skew-sensitive routing at high data rates. If you’re working above 10 Gbps, PP-1067 or spread-glass variants are worth considering over standard PP-106.

Q5: Is PP-106 RoHS and halogen-free compliant?

Standard FR-4 PP-106 contains bromine-based flame retardants and is not halogen-free. Most major laminate manufacturers offer a halogen-free version of 106 prepreg (sometimes designated HF or HFR in their product codes) that meets IEC 61249-2-21 requirements. For automotive, medical, and certain consumer markets, always specify the halogen-free variant and verify RoHS compliance documentation from your material supplier.

Conclusion

PP-106 ultra-thin prepreg isn’t the most glamorous material conversation in PCB engineering, but it’s genuinely foundational to how modern HDI boards get built. The combination of ~50 µm cured thickness, high resin content, relatively low Dk, and excellent laser drillability makes it the default choice for build-up layer dielectrics in any HDI design requiring microvias. Whether you’re routing a 5G modem chipset, a high-speed server switch fabric, or a compact medical device, understanding how to specify, characterize, and design around PP-106 is core PCB engineering knowledge that pays dividends on every multilayer HDI project you tackle.