Antenna boards are a different breed of PCB. On a digital board, the substrate is largely a mechanical carrier. On an antenna board, the substrate directly shapes how the circuit radiates, how much signal it loses, and how reliably it performs in the field. Get the material wrong, and no amount of layout skill will save the design.

This article breaks down a recent 2-layer AD250C antenna board we manufactured—compact, ultra-thin, and finished with pure gold. Beyond the construction and stackup, we want to dig into a question that often gets overlooked: why did this board use pure gold instead of the more common ENIG, immersion silver, or OSP?

What the Board Looks Like

Let's start with the full specification.

The base material is Rogers AD250C, a 2-layer rigid PCB measuring 55.36mm × 76.2mm, held to a tolerance of ±0.15mm. Minimum trace and space is 4/6 mils, and minimum hole size is 0.4mm. There are no blind or buried vias—all connections are through-hole.

Finished board thickness is 0.6mm, with 1oz (35 μm) copper on the outer layers and a via plating thickness of 20 μm. The surface finish is pure gold. Top silkscreen is yellow, with no bottom silkscreen. Green solder mask is applied on both sides. Every board undergoes 100% electrical testing prior to shipment, built to IPC-Class-2, with artwork supplied in Gerber RS-274-X format.

A few parameters stand out immediately: the 0.6mm ultra-thin thickness, the pure gold finish, and 49 vias on a board with only 2 nets.

The Stackup: Thin by Design

The stackup is straightforward: 35 μm copper, a 0.508mm (20mil) AD250C substrate, and 35 μm copper.

The finished thickness is 0.6mm—roughly half that of a typical 1.6mm FR-4 board. Thin antenna boards serve two purposes: they reduce overall module height in space-constrained devices, and in certain patch and planar antenna designs, reduced dielectric loading can improve radiation efficiency.

But thin boards bring manufacturing challenges. At 0.6mm, the board is more flexible, demanding greater care during drilling, plating, and assembly. Warpage control is especially critical with a core that is only 20mil thick.

Why AD250C

AD250C is a glass-reinforced, PTFE-based antenna laminate from Rogers, designed specifically for the wireless antenna market—cellular infrastructure base stations, automotive telematics, and commercial satellite radio.

Its key electrical specs: dielectric constant of 2.5 ± 0.04 at 10GHz, dissipation factor of 0.0013 at 10GHz, and PIM of −159 dBc at 30 mil, 1900 MHz.

Low Dk and antenna geometry. A Dk of 2.5 is far lower than FR-4's roughly 4.3. In antenna design, a lower Dk means the electromagnetic field extends further into the air rather than concentrating in the substrate. This generally improves radiation efficiency and reduces energy trapped in the dielectric. It also allows antenna elements to be sized closer to free-space wavelengths, simplifying matching and broadening bandwidth.

Low Df and loss. A dissipation factor of 0.0013 is exceptionally low. FR-4 ranges from 0.012 to 0.020—roughly ten times higher. At antenna frequencies, this difference translates directly into reduced insertion loss and improved gain. The material's loss tangent stays below 0.002 across typical wireless bands, making it suitable for multi-band designs.

Low PIM and antenna performance. Passive intermodulation is a critical spec for cellular infrastructure antennas. When two or more signals mix in a passive component, they generate intermodulation products that interfere with receive bands. AD250C's −159 dBc helps minimize this effect, reducing yield loss from PIM-related failures in production.

Dimensional stability. CTE is 47 ppm/°C in X, 29 ppm/°C in Y, and 196 ppm/°C in Z. The glass-reinforced construction constrains in-plane movement, which matters for antenna arrays where element spacing must be precise.

Surface Finish: Why Pure Gold, Not ENIG, Immersion Silver, or OSP

This is the most interesting design decision on this board.

Surface finish on an antenna board is not a casual choice. It directly affects three things: RF loss, PIM performance, and long-term reliability. Let's walk through each option.

ENIG: The Nickel Layer Is a High-Frequency Killer

ENIG's structure is copper → nickel (3–6 µm) → gold (0.05–0.1 µm). The gold layer is thin; the nickel beneath it does the real work. But nickel is ferromagnetic, with resistivity more than four times that of copper.

At high frequencies, skin effect pushes current to the conductor surface. At 28 GHz, copper's skin depth is only about 0.4 µm—and ENIG's nickel layer is more than ten times that. The signal is effectively trapped in a highly lossy nickel layer. In measurements, a 28 GHz antenna using ENIG shows 0.7–1.2 dB lower efficiency than bare copper.

ENIG is good for soldering. It is not good for RF.

OSP: Great RF, Too Short-Lived

OSP forms a film directly on copper, with no nickel layer, giving excellent RF performance—loss second only to ideal copper. But it has two hard problems.

First, OSP is an insulator. If an antenna board has test points or requires probe contact, OSP causes poor contact and false open-circuit reports.

Second, shelf life is only 3–6 months, and every reflow consumes the film. On a double-sided assembly or a board undergoing multiple thermal cycles, the OSP on the bottom side begins degrading after the first reflow, and the second soldering pass becomes risky. Antenna boards are often long-deployment products, and OSP's short life does not match that reality.

Immersion Silver: The Sweet Spot for Antenna Boards

Immersion silver deposits directly on copper, with no nickel barrier, at a thickness of only 0.15–0.4 µm. Silver's resistivity is only slightly higher than copper, so high-frequency signals travel on a surface that is close to bare copper. In measurements, immersion silver's antenna efficiency is only 0.1–0.2 dB below bare copper—far better than ENIG.

The trade-off is storage. Silver tarnishes and sulfides, requiring sealed, moisture-controlled packaging. Shelf life is 6–12 months. Immersion silver's position: RF performance close to bare copper, moderate cost, suitable for most antenna applications.

Pure Gold: Why This Board Chose It

Pure gold also avoids the nickel-layer loss problem, but its value goes further.

Extremely low PIM. One of AD250C's core selling points is PIM as low as −159 dBc. PIM is highly sensitive to surface condition: metal oxides, rough surfaces, and dissimilar metal interfaces all become nonlinear sources. Pure gold is chemically inert, smooth, and resistant to oxide formation, reducing PIM at the source. ENIG's nickel layer is itself a dissimilar metal that can contribute additional nonlinearity; tarnished silver can also degrade PIM.

Long-term reliability. Base station antennas operate for years, deployed outdoors through thermal cycling and humidity. Pure gold does not oxidize, sulfide, or discolor, and contact resistance stays stable over time. OSP films degrade over time, and silver tarnishes. For an antenna that is installed and left alone, gold's reliability premium is worth paying.

Compatibility with soldering and bonding. This board has 27 top-side SMT pads and 24 through-hole pads. Pure gold supports reliable soldering and also supports wire bonding. If a design includes direct chip attachment, pure gold is one of the few finishes that handles both.

The trade-off is cost—the highest of the options. What it buys is the lowest PIM risk plus the longest reliability.

Other Construction Details

4/6 mil trace and space. For an antenna board, this resolution is typically used for matching networks, feed lines, and tuning structures. AD250C's glass reinforcement improves processability and dimensional stability during etching, making this precision achievable.

49 vias, 2 nets. This is one of the more interesting details. A high via count with few nets in an antenna design usually indicates a grounded structure—a via fence, ground plane stitching, or an edge-grounded slot antenna. These vias are not routing signals; they provide shielding and grounding.

The board carries 24 through-hole pads and 27 top-side SMT pads, mixing connector and component types. There are no bottom-side SMT pads, consistent with a design where the bottom serves primarily as a ground plane or mounting surface.

0.4mm minimum hole size. This is moderate rather than the finest possible, but it is a sensible choice for a 0.6mm thin board. Smaller holes on thin material risk drill breakage and uneven plating. The 20 μm via plating ensures barrel integrity.

Solder mask and silkscreen. Green solder mask on both sides, with yellow silkscreen on the top only. No bottom silkscreen is typical when the bottom side is a ground plane or mounting surface where markings serve no purpose. Yellow on green provides good contrast for assembly and inspection.

Quality and Delivery

This board was manufactured to IPC-Class-2 standards, with 100% electrical testing prior to shipment to confirm net integrity and isolation. Artwork was supplied in Gerber RS-274-X, and the capability is available worldwide.

What This Board Tells Us

This AD250C build is a good example of how antenna boards differ from general-purpose PCBs. The material choice—driven by Dk, Df, and PIM requirements—sets the electrical foundation. The 0.6mm thin construction supports compact module integration. The pure gold finish protects surface conductivity while minimizing PIM risk. And the high via count on a low-net design reflects the grounded structures typical of antenna layouts.

If you are working on an antenna design and need a manufacturer experienced with Rogers AD250C and similar PTFE-based laminates, we would be glad to review your requirements. Whether it is a simple patch antenna or a multi-element array, the conversation starts with your electrical and mechanical constraints.

Contact us to discuss your next antenna PCB project.