Why dielectric thickness is a design decision
For a controlled-impedance stripline, dielectric thickness, trace width and Dk together set impedance, so an 85–100 Ω differential pair on a thin 0.06–0.13 mm dielectric can use a narrower trace than the same pair on a thicker one. Thickness also sets the board's total height, which sets the aspect ratio of every drilled hole, and it sets how much copper and glass the board contains, which sets its stiffness and expansion.
That makes thickness selection a trade among three goals. Thin dielectrics allow narrow traces, tight pair coupling and more layers in a given board height. Thicker dielectrics allow wider, lower-loss traces for the same impedance and give power planes more separation and current capacity. Total thickness has to stay within what the fabricator can drill and plate reliably. Tecsply classifies cores and prepreg into four thickness bands to make those trade-offs explicit.
Four thickness bands
Tecsply sorts every core and prepreg into four bands, ultra-thin (0.03–0.06 mm), thin (0.06–0.13 mm), standard (0.13–0.50 mm) and thick (0.50–1.60 mm), and each band maps to its own glass styles and applications. Band limits describe the finished dielectric layer, which may be one ply or several.
Ultra-thin
Glass styles 1027, 1037, 1067, 106 (single ply)
- HDI build-up layers and microvia dielectrics
- Substrate-like PCBs and AI accelerator interposer boards
- Tightly coupled 112G/224G stripline pairs where trace width must stay under 3.5 mil
- Spread-glass variants (1067, 1078) to suppress fiber-weave skew
AI Preferred with NE-glass, ultra-low-loss resin
Thin
Glass styles 1078, 1080, 3313, 2113
- Controlled-impedance signal pairs in 12–24 layer boards
- Reference-plane spacing for 85–100 Ω differential lines
- Sub-cores in sequential-lamination stackups
- Thin avionics boards where weight matters
Both Very-low-loss or high-Tg FR-4 depending on data rate
Standard
Glass styles 2116 (two or more plies), 7628 (single or double ply)
- General multilayer cores, 8–16 layers
- Mixed signal and power layers in control electronics
- Aerospace boards built to IPC-6012 Class 3 where 1 oz copper and 0.2 mm+ cores improve PTH reliability
- Hybrid RF stackups (one PTFE layer over a standard core)
Aero Preferred with high-Tg, CAF-resistant resin and low-CTE glass
Thick
Glass style 7628 multi-ply; copper 2–6 oz
- Power distribution planes on AI baseboards and rack power shelves
- Backplanes and midplanes, 30–40+ layers, up to 6.0 mm
- Heavy-copper motor drives and satellite power conditioning
- Bus bars laminated into the board
Both Loss grade follows the adjacent signal layer; T-glass considered above 3.0 mm total
Single-ply thickness by glass style
Single-ply pressed thickness runs from about 0.040 mm for 1027 glass at 74% resin content to 0.178–0.203 mm for 7628 at 42–46%, and finer styles always carry more resin. The table lists the ten styles we use most, with the supplier construction data behind each value.
| Glass style | Typical single-ply thickness | At RC | Weave | Best use |
|---|---|---|---|---|
| 1027 | 0.040–0.044 mm | 74–76% | Ultra-fine, spread | Any-layer HDI, substrate-like PCB |
| 1037 | 0.048–0.053 mm | 74–76% | Ultra-fine, spread | HDI microvia dielectric, 224G pairs |
| 1067 | ≈ 0.051 mm | 65% | Fine, spread | 112G pairs, skew-sensitive routing |
| 106 | ≈ 0.051 mm | 71% | Fine, standard | Thin dielectrics where skew is not critical |
| 1078 | 0.065–0.090 mm | 59–70% | Fine, spread | Default for high-speed stripline in AI boards |
| 1080 | 0.064–0.076 mm | 58–59% | Fine, standard | General high-speed layers, PCIe Gen4/5 |
| 3313 | ≈ 0.089 mm | 51% | Medium, spread | Impedance layers needing 3–4 mil dielectric |
| 2113 | 0.076–0.102 mm | 46–55% | Medium | Balanced impedance control, avionics |
| 2116 | 0.102–0.127 mm | 47–56% | Medium-heavy | General cores, Class 3 aerospace builds |
| 7628 | 0.178–0.203 mm | 42–46% | Coarse | Thick cores, power planes, backplanes |
Resin content and why fine styles carry more resin
Fine glass styles such as 1027 and 1037 carry 74–76% resin by weight while coarse 7628 carries 42–46%, and that difference changes both the laminate's properties and how it behaves in the press. A resin-rich layer has a Dk and Df closer to the resin's own values, which is useful on high-speed layers; it also expands more through its thickness and flows more during lamination.
Flow matters for prepreg in particular. During lamination, prepreg resin fills the gaps between etched copper features on the adjacent cores, so the pressed thickness of a prepreg layer is less than its nominal thickness when it sits against a heavily etched signal layer. Fabricators calculate that fill for each layer. It is one reason the stackup calculator gives a planning estimate, and why the final stackup comes from the fabricator's own calculation.
Signal layers: thin, fine and spread
High-speed signal pairs sit on thin cores and prepreg of 0.04–0.13 mm built from fine, spread-glass styles (1027, 1037, 1067, 1078) so that trace width and dielectric spacing hit the impedance target. At 112G we default to 1067 or 1078 spread glass; at 224G we move to 1027 or 1037, which lets trace width stay under 3.5 mil while holding impedance on tightly coupled pairs. The spread weave is not optional at these rates, because a coarse weave under a narrow trace produces the local Dk variation that becomes fiber-weave skew.
Power planes and backplanes: thick and heavy
Power planes use standard or thick dielectrics of 0.13–1.6 mm with 2116 or 7628 glass and 2 oz or heavier copper, and backplanes stack multiple 7628 plies and can exceed 5 mm total. Thick dielectrics between power and ground reduce the risk of dielectric breakdown and give heavy copper room to be pressed without voids; the cost is total board thickness, which is why power-heavy boards are where T-glass and low-CTE constructions come into the conversation above 3.0 mm.
Aerospace boards: standard cores for PTH reliability
Aerospace boards built to IPC-6012 Class 3 typically sit in the standard band, where 1 oz copper and cores of 0.2 mm or more improve plated through-hole reliability. 2116 and 2113 cores with 1080 prepreg are our usual starting point for avionics boards below 10 Gbps, and 1078 spread glass is added on signal layers when data rates rise. Thin avionics boards where weight matters move down into the thin band, with the trade-off checked against thermal-cycling requirements.
From plies to total thickness and aspect ratio
A 16-layer board with 2-ply 2116 cores, single-ply 1078 prepreg, 1 oz outer and ½ oz inner copper comes out at roughly 2.5 mm before plating, and with a 0.25 mm finished hole that is an aspect ratio of about 10:1. Aspect ratio, total thickness divided by the smallest drilled hole, is the number that decides whether the fabricator can plate the barrels evenly.
The stackup thickness calculator adds up copper and dielectric plies from this table's typical values, reports total thickness in mm and mil, and flags aspect ratios above 10:1 and 12:1 for confirmation with the fabricator. It also flags boards over 3.0 mm, where we consider T-glass or low-CTE construction for PTH reliability.
How to specify dielectric thickness
A stackup drawing should state 3 things for every dielectric layer: glass style, ply count and target pressed thickness with tolerance. Stating only the target thickness lets the fabricator choose any construction that reaches it, which can put a coarse, skew-prone weave under a 112G pair. Stating only the style leaves the resin content and pressed thickness open. Tecsply stackups state all three, and the fabricator's calculated pressed thickness is recorded on the stackup drawing before release.
Build a stackup, then have it reviewed
Try a construction in the calculator, then send it with your impedance targets for a fabricator-checked stackup.
References
- Resonac, MCL copper clad laminate and prepreg catalog (2025): prepreg thickness after lamination for 1027, 1037 and 1078.
- Isola, 370HR Dk/Df construction tables (rev. C, 2020): core thickness by glass style and resin content.
- Panasonic, MEGTRON 6 R-5775(N) / R-5670(N) data sheet (Apr 2022): core and prepreg thickness by style and resin content.
- IPC-6012 (rigid PCB qualification and performance).
Last reviewed . Values are typical published figures for stackup planning.