A 0.25mm CLTE RF Board: Why Dimensional Stability Matters More Than the Loss Number

A double-sided board at 0.25mm is already very thin. The core is only 0.135mm (about 5.3mil), and with 35 um copper on both sides, the finished board lands at 0.25mm.

At this thickness, the board feels soft in your hand and has almost no rigidity. Handling, drilling, and assembly all require care. But the designer did not choose this thickness just to make the board thin—it serves applications like ADAS radar, patch antennas, and phased array antennas, where size, weight, and electrical performance are all sensitive.

The material is Rogers CLTE. The board itself is not complicated, but there is one thing worth discussing behind the material choice: at this thickness, CLTE's dimensional stability matters more than its low loss.

What the Board Looks Like

Dimensions are 130.6mm × 40mm, held to ±0.15mm. Finished thickness is 0.25mm, minimum trace and space is 6/8 mils, minimum hole size is 0.25mm, no blind vias. Outer copper is 1oz, via plating is 20 um, and the surface finish is immersion tin. Top silkscreen is white with blue solder mask; the bottom has no silkscreen and no solder mask. The board has 168 components, 93 pads, 42 vias, and 2 nets. It is 100% electrically tested before shipment, built to IPC-Class-2.

168 components against 93 pads suggests many components are densely packed or share pads—mostly passives.

What CLTE Is Really Worth

CLTE's loss tangent is 0.0012 at 10GHz. That is low, but there are lower numbers on the market. What gives CLTE its standing among PTFE-based laminates is in-plane CTE consistency—its planar CTE variance is among the lowest in this material class.

That matters for two types of designs.

One is embedded resistors. CLTE has a long history with resistor foil. Stable in-plane CTE means resistor values do not drift with temperature changes or during processing. For RF circuits that need precise resistance, this is more practical than the loss number.

The other is attaching ceramic active devices. CLTE's dielectric constant changes little with temperature. When the board is attached to ceramic filters or ceramic antennas, low Dk drift means lower interface stress and higher reliability.

So CLTE's value is not "lowest loss." It is that electrical and mechanical performance does not move much when temperature changes or after processing.

The Real Hassles of 0.25mm

Drilling. Minimum hole size is 0.25mm on a 0.25mm board—roughly a 1:1 aspect ratio. The ratio itself is not extreme, but on a thin board, support and chip evacuation during drilling need control. All 42 vias are through-hole, and the 20 um plating must cover the hole walls evenly.

Warpage. A 130.6mm-long board at only 0.25mm thick makes warpage the main process challenge. CLTE's low Z-axis CTE (20 ppm/°C) helps here—the material expands less, so deformation during lamination and reflow is easier to control.

Handling and assembly. Thin boards deform easily during SMT and need carriers or vacuum support. This is not a material issue; it is determined by thickness.

Immersion Tin, and What to Watch For

The surface finish is immersion tin. On RF boards, immersion tin is less common than immersion gold or ENIG, but it offers good flatness, low cost, and good solderability, meeting the assembly needs of the 34 top-side SMT pads and 59 through-hole pads.

Immersion tin has a known issue: tin whiskers. Whiskers can grow during long-term storage, which is a risk for high-reliability applications. If this board is used in ADAS or phased array antennas, storage conditions and assembly timelines need to be confirmed within the safe window.

This trade-off is worth stating clearly: if the board needs long-term storage in humid environments, or if the assembly cycle is long, immersion gold or ENIG is more reliable. Immersion tin suits short assembly cycles with controlled storage.

42 Vias, 2 Nets

High via count with low net count is common in RF boards, usually indicating a grounded structure—via fencing, ground plane stitching, or a shielded transmission line. These vias are not routing signals; they provide grounding and shielding. 59 through-hole pads and 34 top-side SMT pads indicate mixed assembly; no bottom SMT pads and no bottom solder mask, consistent with the bottom side serving as a ground plane or mounting surface.

Where This Board Fits

CLTE's typical applications are ADAS, patch antennas, phased array antennas, and power amplifiers.

They share one thing: size and weight are constrained, and electrical performance cannot be compromised. The 0.25mm thickness suits compact modules, and CLTE's dimensional stability and low loss suit antennas and power amplifiers. If a design does not need to be this thin, or is more cost-sensitive, a standard-thickness board with conventional RF material may be more sensible.

The Bottom Line

The core of this board is not "0.25mm is thin." It is "at 0.25mm, CLTE's dimensional stability and low Z-axis CTE keep a thin board controllable."

Thin is not an advantage in itself. It only becomes one when size is constrained and the material can absorb the process risks that thinness brings.

If you are working on ADAS, patch antenna, or phased array designs and need to balance thinness, dimensional stability, and low loss, send us your frequency, thickness constraints, and reliability requirements. We will tell you whether CLTE is the right fit—or whether a more economical alternative will do the job.

Have you built 0.25mm-thick RF boards? What drilling or assembly issues did you run into?