The RF-35HTC is a single-sided, thru-hole plated dielectric laminate with a silver (Ag) surface and aluminum oxide ceramic substrate. The Ag surface on the RF-35HTC laminate provides an effective heat dissipation material. At the same time, it maintains low thermal resistance. This combination makes this product suitable for high-power/high-performance applications. You enjoy this where we would require larger heatsink areas without this board. The RF-35HTC has a pre-expanded copper pad on one side to allow for larger pads on the other side of the PCB. Established manufacturers such as RayMing PCB and Assembly plate this product through a single gold (Au) layer, then exposed and plated on both sides with aluminum oxide (Al2O3). The RF-35HTC board is non-metallic, electrically conductive, and is available in many grades from 15 µin (0.058mm) to 4 in (100mm). This laminate is available in both FR and CFR material types.
Copper Layer Information
The Taconic RF-35HTC surface plating combines an oxygen plasma etch and a tin (Sn) electroplating process. The US industrial standard (AIM) specification limits board thickness to less than 500µin via dry film. We should consider the aluminum oxide “cap” on the RF-35HTC laminate part of the overall board thickness for FR and CFR materials. Then we plate the RF-35HTC laminate with an oxygen plasma etch to pass the surface. Afterward, we plate it by an Sn electroplating process, which provides Ag anodes for the Au plating process.
They plate the RF-35HTC laminate with a tin (Sn) electroplating process. This process builds up the Ag (Ag + Sn2+) anode layer by adding another layer of Ag and Sn to the surface. The final Au layer on this laminate does not contain Ag since we anneal it. However, it does provide both electrical conductivity and a barrier against copper oxidation.
This surface is then exposed to a 400 Watt Oven bake at 50C (122F) to evaporate the Sn, leaving behind Ag. They apply a final Au plate over the entire laminate. You cannot directly bond the AU layer to the Ag layer because of the annealing process, so the Au traces and pads must electrically connect it. This Au layer is then exposed to a passive anneal.
This passive anneal softens the Au conductors, allowing them to “flow” into the grain boundaries of the Ag plating. The Au will not enter the Sn grain boundaries as it is incompatible. This flow creates electrical continuity between the Au surface and the underlying Ag plating.
The final step in this process is a second 400WO bake at 50C (122F). This second bake “evapuates” the Au out of the grains.
Copper Epoxy Layer Information
The RF-35HTC laminate is a dual-sided double conductive epoxy board. They plate both sides with copper via Ag electroplating. You can accomplish this by creating a copper (Cu) layer on one side and the remaining copper epoxy (CE).
We plate the Cu side similarly to the Tin side, with an Ag electroplating process and a 400WO bake.
After creating the Cu layer, we expose it to a dry film deposition (DFD) to reduce its thickness. This reduces surface impedance, spacing, and cost by reducing the thickness of the conductor. The final step in this process is a second 400WO bake at 50C (122F). This second bake “evaputes” the copper out of the grains.
They then expose the CE layer to a 400WO bake. This process evaporates the CE out of the grains, leaving behind Cu. Then we apply a final Au layer over the entire laminate. You cannot directly bond the CU layer to the CE layer since we anneal it. However, it does provide both electrical conductivity and a barrier against copper oxidation.
The conductivity of the RF-35HTC laminate
The Copper Epoxy Layers are non-metallic, electrically conductive, and are available in many grades from 15 µin (0.058mm) to 4 in (100mm). This laminate is available in both FR and CFR material types.
The RF-35HTC laminate is relatively conductive. However, it will not be an ideal board for high-power applications without a highly conductive surface layer. This is because the RF-35HTC laminate does not have the electrical resistance of a conventional PCB (the “R’ term). However, it has an effective resistance of 100Ω or less. This, combined with the high thermal resistance of this laminate, means that heat transfer and dissipation can be slow. They design this material with this in mind.
Impact Resistance of the RF-35HTC Laminate
The laminate composition is not a very good heat transfer compound. Instead, it is primarily an electrical insulator. To become a suitable heat transfer medium, it must have multiple skin effects. It must also have diffusion layers that increase its conductivity and decrease its resistance (the “T’ term). The RF-35HTC has an effective thermal coefficient of expansion (E) of 6.7×10/°C, an absorption coefficient (A) of 1.0×10/Wm, and a dielectric constant (K) of 0.36 to maximize its heat transfer. To increase this thermal expansion coefficient, the laminate consists of non-brittle materials. It has no voids or voids that will cause buckling or void formation when the temperature increases:
The E value for copper is 5.4×10/°C.
People often denote the effective thermal expansion coefficient for a laminate as E.
Also, we define it in terms of the coefficients for each material in the laminate.
The RF-35HTC through-hole copper thicknesses are 0.00254in, 0.00638in, 0.06060in and 0.125in or 2.54µin, 6.38µin and 12.7µm (0.00255mm, 0.00639mm and 0.06065mm). The RF-35HTC surface layer thickness is approximately 500µin (0.05 mm), 1/97in (1.3 µm) for FR and 400µin (0.04 mm) or 1/96in (1.2 µm) for CFR. It has a surface resistance of 100Ω or less. The entire assembly thickness is approximately 3oz (85g/m).
Benefits of using Taconic RF-35HTC PCB
The key to the low thermal resistance of the RF-35HTC surface is the increase in conductivity resulting from high-quality surface treatments applications. To increase conductivity, we use substrate materials with lower thermal expansion coefficients. We also use special techniques to create through-holes so that conduction can conduct heat transfer.
1. “Best in Class” Thermal Conductivity
The RF-35HTC is the only copper PCB with a thermal coefficient of expansion (E) less than or equal to 5.4×10/°C.
All the other copper PCBs in production use materials with a higher thermal coefficient of expansion. It increases the effective thermal resistance.
2. “Best in Class” Surface Conductivity
The RF-35HTC is the only PCB laminate with a surface resistance of 100Ω or less. Other copper PCB laminates have a higher surface resistance because of the copper thickness. The RF-35HTC laminate has a surface conductivity of 10-12W/mK. It is 30x higher than other copper laminates with similar thermal resistances available on the market.
3. Low Loss Tangent
A PCB’s weakest point is often at the edge of the copper or aluminum layer as it must interface with a device. This interface is often sharp at the copper alloy surface, causing a high angle loss and poor signal transfer. The RF-35HTC tapers along its edges to reduce its thicknesses progressively over its entire surface area. It reduces high angle loss.
4. No Glass Reinforcement
The RF-35HTC is the only copper laminate requiring glass fiber to provide support. This results in a lower thermal impedance. The glass fibers are only helpful for mechanical support. We do not need them for heat transfer.
5. Resists Thermal Oxidation
There are many materials in the RF-35HTC laminate that resist thermal oxidation. To prevent an oxidation layer from forming on the surface of the laminate, we plate it with an As (arsenic) based alloy for a thin base layer. Then, we apply additional surface treatments to further protect against thermal oxidation.
6. High Dimensional Stability
The RF-35HTC laminate has two strong surface layers limiting its dimensional instability at high temperatures. Since these surface layers are electrically conductive, they provide an insulating interface. It increases the strength of the laminate and limits dimensional changes in the copper layer.
Application
We use the RF-35HTC in Aerospace, Avionics, Military/Navy, Medical, and High-Performance Systems. Its use in commercial industries is on the rise due to the significant advances made in its thermal properties. The RF-35HTC was previously not considered for most commercial applications. This is because of its high thicknesses and large thermal resistance (R). However, the Surface Resistance technology has reduced this thermal resistance by up to 50%. It makes it an ideal material for high-performance systems.
Satellites
We use this material for satellites as its thermal conductivity needs to be as high as possible. Its dimensional stability must also be very high. The RF-35HTC is also used in earth-to-satellite applications as it is a highly flexible PCB, and we can laminate it to any shape.
Aircraft
We use the RF-35HTC in aircraft because it has equal or better electrical properties than gold-plated copper laminates. The RF-35HTC is less electrically resistant and more flexible. It allows surface mount applications on curved surfaces without using a flat flex cable.
Military Systems
Due to the increased demand for military systems, from helicopter and UAV platforms, to warships, this material quickly becomes a critical component for military applications. This is because the RF-35HTC’s thermal conductivity needs to limit the need for high electrical insulation. It becomes more important at higher temperatures and in light of increased power dissipation.
High power applications such as filters, couplers, dividers & power amplifiers
We use the RF-35HTC in high-power applications that have rapid power changes. The RF-35HTC material is the only electrical component to handle over 1,000V of DC current. It allows for higher power consumption.
Conclusion
The RF-35HTC laminate is a premium board for high power and high heat applications. However, it does not provide a conventional board’s resistance or heat dissipation. We can use this product in any system with 100% reliability and all components attached to the PCB. It is common for systems requiring USB, Ethernet, analog, or digital signals because only this material will allow that. The through-hole construction of this laminate provides better signal integrity into ultra-thin printed circuit boards. For instance, some mobile phones and small laptop computers.
