Multilayer PCB FAQ

E-glass is the standard reinforcement in FR-4, with a fiber Dk of about 6.8 at 1 GHz and a CTE of about 5.6 ppm/°C. T-glass is a high-silica, high-modulus glass with a CTE of about 2.8 ppm/°C, half that of E-glass, and a tensile modulus of about 86 GPa against 75 GPa for E-glass (Nittobo published values). Choose T-glass when warpage and dimensional stability limit the design, such as boards under packages larger than 60 mm or thick boards with many plated through-holes. For the lowest dielectric loss, NE-glass is usually the better reinforcement.

No. T-glass is defined by low CTE (about 2.8 ppm/°C) and high modulus (about 86 GPa), not by its dielectric properties: its Dk is about 5.4 and its Df about 0.0043 at 1 GHz, higher loss than E-glass (Nittobo published values). Low-Dk glass such as NE-glass has a Dk of about 4.8 at 1 GHz against 6.8 for E-glass, with a CTE of about 3.3 ppm/°C but a lower modulus (about 64 GPa). Some suppliers offer low-CTE, low-Dk hybrid cloths, and current AI boards often combine both types in one stackup.

A copper clad laminate core is a cured sheet of glass cloth and resin with copper foil bonded to one or both sides; prepreg is the same glass-and-resin system left uncured, so it flows and bonds the cores together during lamination. In a typical foil-construction multilayer, the outer layers are copper foil pressed onto prepreg, and the inner layers are etched cores separated by prepreg. The resin system sets Dk, Df, Tg and z-axis expansion for both.

Signal pairs use 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. Power planes use standard or thick dielectrics of 0.13–1.6 mm with 2116 or 7628 glass and 2 oz or heavier copper. Backplanes stack multiple 7628 plies and can exceed 5 mm total.

112G PAM4 channels (Nyquist 26.56 GHz) are typically built on M7-grade very-low-loss CCL with Df around 0.002–0.003 at 10–14 GHz (Megtron 7, Tachyon 100G, EM-890K class). 224G PAM4 (Nyquist 53.13 GHz) moves to M8-grade ultra-low-loss laminates below 0.002 (Megtron 8, EM-892K, Astra MT77 class) and, on the longest channels, PTFE-based materials such as RO3003. Plane layers inside the same board can use a lower grade.

A trace running over a glass yarn sees a higher local Dk than one running over resin between yarns. On a differential pair this shows up as skew, which closes the eye at 56G and above. Spread-glass styles (1067, 1078, 3313) flatten the yarn so the Dk is more uniform; low-Dk glass narrows the gap between glass and resin Dk; rotated-panel or zig-zag routing adds a further margin. We specify spread glass by default on any board above 25 Gbps.

At 224G PAM4 the Nyquist frequency is about 53 GHz, where the skin depth in copper is about 0.29 µm, so signal current flows in a layer thinner than the tooth profile of standard or reverse-treated foil. Surface roughness lengthens the current path and raises conductor loss, by up to about 2× for foil much rougher than the skin depth, and at these frequencies conductor loss on a 4–5 mil stripline is usually larger than the dielectric loss of an M8-grade laminate. That is why upgrading the laminate alone recovers less than the Df numbers suggest.

Our loss-budget estimator models this with the Hammerstad–Jensen roughness correction, which grows with frequency, using planning RMS roughness values from about 2.0 µm for standard ED foil down to about 0.25 µm for HVLP3-class foil (specified at Rz ≤ 1.1 µm). We specify HVLP3 or better on 224G signal layers and confirm the foil grade and inner-layer bonding treatment on the stackup drawing, because the bonding treatment adds roughness of its own.

No. HVLP generation numbers are copper foil makers' product designations, not a grade defined by IPC-4562, and published values for the same generation differ between makers: HVLP3 is quoted as Rz ≤ 1.1 µm by some sources and about 0.6 µm by others, and HVLP4 at about 0.5 µm. Two foils sold under the same generation name can therefore behave differently on a 112G or 224G channel.

On a stackup drawing we specify high-speed foil by its maximum treated-side Rz or Rq, or by the named foil product qualified with the laminate, and we record the inner-layer bonding treatment, which adds roughness of its own. The foil product and lot then appear on the material declaration shipped with the boards.

Aerospace and defense multilayer PCBs are usually built and inspected to IPC-6012 Class 3 or 3A, which includes 100 thermal-shock cycles per IPC-TM-650 2.6.7.2 on test coupons. They use high-Tg (170 °C and above) or polyimide laminates, often against program-specified thermal cycling between −55 °C and +125 °C, require CAF-resistant materials, and increasingly need country-of-origin traceability. From January 1, 2027, 10 U.S.C. 4873 bars the Department of Defense from acquiring covered PCBs from North Korea, China, Russia or Iran.

High-Tg FR-4 with Tg of 170 °C or above, Td of 340 °C or above and total z-axis expansion below 3% (50–260 °C) covers most avionics and defense boards, including boards that will see lead-free rework. Polyimide laminates, with Tg of 250 °C and above, are specified when the operating envelope passes 200 °C, and are a common choice for Class 3A satellite and space payload boards. Polyimide costs more and is harder to process, so we recommend it only when the thermal envelope requires it.

From January 1, 2027, 10 U.S.C. 4873 bars the Department of Defense from acquiring a covered printed circuit board from a covered nation: North Korea, China, Russia or Iran. The advance notice of proposed rulemaking of July 2, 2026 (comments closed August 31, 2026) proposes to test where bare and partially manufactured boards are fabricated, backed by an Independent Hardware Assurance Framework built on ISO/IEC 20243, IPC-1782 (traceability) and IPC-1791 (trusted designer, fabricator and assembler), with certifications flowed down to every subcontract tier.

In practice a prime contractor needs to show where the bare board was fabricated, trace the laminate and copper foil lots behind it, and hold certifications from every tier. Tecsply's partner fabricators are located in Taiwan, Japan, Thailand, Vietnam and Malaysia, outside the four covered nations, and because ownership and control may also be examined we document each partner's ownership as well as its location. Each aerospace or defense order ships with an origin package for the board, the laminate and the foil. Whether a specific program is a covered system, and the final certification requirements, are set by the statute and the final DFARS rule; confirm them with your contracting officer.

It can. 10 U.S.C. 4873 and the July 2026 advance notice of proposed rulemaking focus on where the bare board is fabricated, but prime contractors increasingly ask for traceability of the laminate and copper foil behind the board, and much of the world's standard-grade foil capacity is in mainland China. High-end HVLP foil comes mainly from Japanese, Taiwanese and Luxembourg producers.

For aerospace and defense orders we record the foil product, lot and country of origin on the material declaration alongside the laminate lot, so the origin package covers the board, the laminate and the foil. Whether a specific program requires material-level origin is set by the contract; confirm it with your contracting officer.

No. Tecsply commits that every board it supplies for a US defense or aerospace program is fabricated, in whole or in part, outside North Korea, China, Russia and Iran, and only by partner fabricators that are not owned or controlled by an entity in those countries or by an entity on the Department of Defense Section 1260H list. The fabricator and fabrication site are named on every certificate of conformance, and the same conditions are written into our purchase terms with each partner. Raw material for metal parts is not melted in those four countries, and for DFARS 252.225-7009 programs titanium and nickel alloys are melted in the United States or a qualifying country.

Tecsply Inc. itself is a Delaware corporation owned by Smilden Inc, whose ultimate parent is a Taiwan company with no investment from any covered nation; beneficial-ownership details are available on request in SF-328 format. This is a contractual sourcing commitment, not a government certification; program certifications under the final 10 U.S.C. 4873 rule will be provided when that rule takes effect.

AI server demand for high-layer-count boards is consuming glass cloth and copper foil capacity. Electronic-grade glass cloth prices rose by more than 100% year on year in 2026 and CCL by roughly 70%, with high-speed laminate production lead times stretching from 12–18 days to 18–25 days. Industry outlooks expect capacity relief to begin only in late 2027. We lock material allocation with our fabricators on framework agreements rather than order by order.

As of September 2026, PCBs of Taiwan origin enter the US at a combined rate of about 10%, Japan origin at about 12.5% (the Section 301 duty tops the MFN rate up to 12.5%), and Thailand origin at MFN plus 12.5%. China-origin boards carry the 25% Section 301 duty plus a further 12.5%, and the remaining Section 301 exclusions covering bare boards expire on November 10, 2026. Rates change frequently; we quote DDP with a documented tariff-adjustment clause. Vietnam- and Malaysia-origin rates are confirmed on each quote.

Tecsply works with partner fabricators in Taiwan, Japan, Thailand, Vietnam and Malaysia so that a US program has more than one qualified source located outside the four 10 U.S.C. 4873 covered nations. Taiwan partners form the core of the network for high-layer-count and HDI builds; Japanese partners add an origin next to the makers of much of the high-end laminate, glass cloth and HVLP copper foil; Southeast Asian partners add capacity and a further origin when AI server demand is absorbing high-layer-count capacity and tariff rates move between origins. Which partner builds a given board is decided by its qualified capability for that stackup, not by price alone.

An engineering query is a written question a PCB fabricator sends the customer during the pre-production CAM review when the files conflict, are incomplete, or contain a feature that cannot be built as drawn. The fabricator does not guess the designer's intent: typical EQs cover a missing outline or drill file, unclear plated and non-plated holes, a stackup that does not match the drill spans, or features below the fabricator's limits. Each open EQ delays the quote or the build until it is answered, so a complete first submission saves days.

No. The pre-check reads the files with your browser's File API and analyses them on your own computer; no file content is sent to Tecsply or to any other server. That keeps ITAR-controlled and CUI design data under your control. To get a quote, send the files through your normal controlled channel; the pre-check report can travel with them.

Yes. Under DFARS 252.225-7009, titanium and titanium alloys, and nickel alloys with more than 10% alloying metals, are specialty metals that must be melted or produced in the United States or a qualifying country when they are delivered in US defense contracts. The qualifying-country list in DFARS 225.003 includes Japan, Germany, the United Kingdom and 25 other countries, but not Taiwan.

Tecsply therefore buys titanium and nickel-alloy material for DFARS programs from mills in the United States or a qualifying country, and the mill certificate showing the melt location travels with each lot; machining can be done by a qualified partner elsewhere. Aluminum-lithium alloys are not specialty metals under this clause. Exceptions such as the 2% de minimis threshold are confirmed program by program.

Use aluminum-lithium when weight or stiffness limits the design. Each 1 wt% of lithium lowers density by about 3% and raises elastic modulus by about 6%; alloy 2050, for example, matches or exceeds 7050-T7451 in tensile and fracture properties with about 4% lower density and up to 5% higher modulus. Al-Li costs more than 2xxx and 7xxx alloys, so it is usually reserved for weight-critical structure such as launch-vehicle tanks, fuselage and wing structure, and spacecraft primary structure.

It depends on tolerance and cost. Solution-treated Inconel 718 (AMS 5662) is softer and machines faster, but the part changes dimension slightly when it is aged afterwards, so tight features may need finishing after aging. Solution-treated-and-aged material (AMS 5663) is delivered at service hardness and holds its dimensions, but machining is slower and tool wear is higher. Tecsply agrees the condition and the machining and heat-treat sequence with the customer before quoting.