A common misconception in RF material selection is the assumption that higher Dk is always better. High Dk does shrink circuits, but it comes at a cost.
Push Dk from 3.48 to 9.80, and the wavelength shortens by roughly 1.68 times. Patch antennas, filters, and couplers can shrink to about one-third of their original size. But at the same time, impedance control gets harder, sensitivity to dielectric thickness tolerance increases, the pool of available materials shrinks, and cost and lead time become harder to manage.
We recently built a 2-layer TMM10i board with a 2mm finished thickness and a Dk of 9.80. This board is a good starting point to discuss what a Dk near 10 actually buys you—and what it takes away.
The Board Itself
Double-sided, TMM10i base material, 1.905mm (75mil) core, 35 μm copper, 2mm finished thickness. Minimum trace and space is 5/6 mils, minimum hole size is 0.4mm, no blind vias. Via plating is 20 μm, surface finish is immersion gold, and there is no solder mask or silkscreen on either side. 48 components, 62 pads, 39 vias, 2 nets.
The spec list is short, but every line points to the same use case: high frequency, high reliability, not a general-purpose board.
What Dk 9.80 Buys You
Wavelength is inversely proportional to the square root of the dielectric constant. Raising Dk from 3.48 to 9.80 shortens the wavelength by about 1.68 times. At the same frequency, antennas, filters, and couplers can shrink to roughly one-third their physical size. For designs where space is tight, this is not “optimization.” It is the difference between fitting and not fitting.
TMM10i has one easily overlooked detail: its Dk is isotropic. Many PTFE-based materials have different Dk values in X/Y versus Z, which makes the electrical behavior of vias and vertical structures hard to predict. TMM10i is consistent in all three directions, which gives more confidence when designing high-aspect-ratio vias and three-dimensional structures. On thick boards, this is especially noticeable.
Boundary One: Impedance Is Harder to Control
On a Dk 9.80 material, dielectric thickness variation has a more direct effect on impedance than on lower-Dk materials. On this board, that meant tighter incoming thickness tolerance and tighter lamination control than a standard board.
The minimum trace is 5/6 mils, and the top layer requires 50Ω single-ended impedance. We performed stricter incoming inspection on the core and prepreg thickness before lamination, and verified dielectric thickness by cross-section after lamination to keep impedance within the design window.
This is not unique to TMM10i. Any high-Dk material faces the same issue. The difference is whether the manufacturer has the process controls to handle it.
Boundary Two: Fewer Materials to Choose From
There are plenty of RF materials with Dk between 3 and 4: RO4350B, RO4003C, AD250C, F4BTME350. Engineers have enough room to compare cost, loss, and processability.
Materials with Dk near 10 are far fewer. TMM10i is basically the default option in that range. Fewer choices means less bargaining power, more exposure to material supply lead times, and limited alternatives. High Dk gives design freedom, but it takes away supply chain freedom.
Boundary Three: Loss and Dk Usually Trade Off
High-Dk materials usually come with higher loss. TMM10i’s Df is 0.0020—low for a high-Dk material, but still higher than pure PTFE.
If a design needs both high Dk and extremely low loss, the available range narrows further and cost goes up. Dk 9.80 is a position that requires trade-offs: high enough for meaningful miniaturization, but not so high that loss becomes the main problem. Beyond that, loss starts to dominate design decisions.
How This Board Handles Those Boundaries
The 2mm thickness is not arbitrary. TMM10i’s CTE is matched to copper (19 ppm/K in X/Y, 20 ppm/K in Z). A thick board with high-aspect-ratio vias demands more from plating quality, but once plated properly, the barrel is mechanically stronger and less likely to crack under thermal cycling. The 2mm thickness also keeps an 87mm-long board from warping during reflow.
Neither side has solder mask. On a Dk 9.80 material, this matters more than on a low-Dk material. Solder mask’s dielectric constant differs sharply from TMM10i’s, and covering RF traces with it changes the effective dielectric constant, which affects impedance and tuning. Removing the mask exposes copper directly and makes impedance more predictable.
Immersion gold surface finish. No ENIG nickel-layer loss problem, good flatness, suitable for 5/6 mil traces and 0.4mm holes.
39 vias, 2 nets. High via count with low net count usually means a grounded structure—via fencing, ground plane stitching, or a shielded transmission line. These vias are not routing signals; they provide grounding and shielding. 43 through-hole pads and 19 top-side SMT pads indicate mixed assembly.
When Dk 9.80 Is Worth It
When miniaturization is a hard constraint—antennas, filters, and couplers are space-constrained, and Dk 9.80 can shrink them to roughly one-third the size of low-Dk materials.
When plated through-hole reliability is critical—CTE matched to copper, plus isotropic Dk, makes high-aspect-ratio vias and three-dimensional structures more reliable.
When wire bonding is required—the thermoset resin supports reliable wire bonding, suitable for direct chip attachment.
When temperature stability matters—thermal coefficient of Dk is −43 ppm/°K, giving consistent performance across temperature.
When Dk 9.80 Becomes a Liability
When size is not a constraint—if antennas and filters have enough space, miniaturization has nowhere to pay off, and you are left with added impedance control difficulty and cost.
When loss is the primary metric—TMM10i’s Df is 0.0020, good, but still higher than pure PTFE. If loss is tight, a low-Dk PTFE material may be more suitable.
When supply chain flexibility matters—materials with Dk near 10 are few, and lead times and costs are more exposed to material supply. If a project needs multiple sources or fast switching, lower-Dk materials offer more room.
When cost is the first constraint—TMM10i is a high-Dk ceramic-filled material, priced above FR-4 and RO4350B. In applications with modest performance requirements, there is no reason to pay for performance you will not use.
The Bottom Line
TMM10i’s value is not that “Dk is high.” It is that “Dk is high enough to solve the problem, without being so high that other metrics spiral out of control.” At Dk 9.80, miniaturization is achievable, and loss and processability are still manageable. Beyond that, design freedom starts to decline.
If you are working on an RF or microwave design and need to balance miniaturization, plated through-hole reliability, and processability, we would be glad to review your requirements. Material selection should start from frequency, size constraints, and reliability targets—not from which material has the highest Dk.
Have you worked with materials near Dk 10? What design or process issues did you run into?
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