Rush PCB Inc. makes hybrid PCBs, otherwise commonly known as mixed material lamination. We combine two different material such as FR4 and Aluminum base, FR4 and Teflon, FR4 and Ceramic, and FR4 and Polyimide.
We offer hybrid PCBs with their combination of different material so that designer can work with two types of functionality on the same board. For instance, the combination of FR4 and PTFE materials allow designers to have power and high-frequency functionality in the same PCB, thereby reducing the footprint and cost of the device.
The greatest challenge in manufacturing a hybrid PCB is managing the different CTE or coefficient of thermal expansion of the dissimilar materials both during fabrication and PCB assembly.
Rush PCB Inc. has vast experience in handling different laminates for their physical properties and the capabilities in our equipment. We bank on this critical experience when producing PCBs with dissimilar material.
As the CTE values of all the materials in the PCB are different, they expand and contract at different rates with thermal exposure. This is a serious issue since one material may shrink while the other is expanding, resulting in significant registration issues. In fact, this may also cause delamination between the copper and its substrate interfaces. Therefore, we must be extremely selective when combining different material for hybrid applications, as most combinations are not suitable for fabrication even if they demonstrate the desired performance.
We typically combine a low-loss material like Rogers or Nelco with a core material like FR4. Typical combinations are FR4 with Aluminum base, Rogers 4003C, or Rogers 4350B.
Stackup for Hybrid PCB
At Rush PCB Inc., we use Hybrid PCB technology to improve the signal integrity of signals on select layers. Our hybrid PCB stackup uses low-loss dielectrics for some layers, while we fabricate the others with the traditional FR4.
Rather than making all layers with the low-loss dielectrics, the cost of a hybrid stack up is lower, but it offers the same advantages as only the layers with high speed signals have the low-loss dielectrics, thereby optimizing the cost to performance ratio and improving the signal integrity performance for select routing layers. We can further improve the hybrid stack up by mixing low-loss material and ultra-low-loss materials, which will significantly reduce losses in routing. Therefore, with a suitable stackup, hybrid PCBs offer two major advantages:
- Significant improvement in performance over standard FR4 boards
- Significant cost reduction over all low-loss stack up
We have different options and associated costs for creating hybrid PCB stackup. For instance, we can use the same high-loss or low-loss material for all the pre-preg layers, and different materials for the core layers.
Materials for Hybrid PCBs
For specific applications, we can use two completely different laminates for the core and pre-preg, such as FR4 and PTFE sandwiched in the same PCB. We can also use different laminates from the same material family in the layers, since manufacturers usually create a variety of specifications with minor changes to the formula of the material.
The dissimilar materials that we use for a hybrid PCB depend on the application, environmental factors, power needs, and other design aspects. Some common materials that we use are:
This is a standard material all PCB manufacturers use. FR is an acronym for Fire Retardant, implying the material does not allow flames to propagate if it catches fire. The figure 4 denotes that the material complies with UL standard UL94V-0. The material is glass-reinforced epoxy laminate.
FR4 is inexpensive, has high dielectric strength, can withstand high temperatures, resists moisture ingress, and has high electrical insulation. Therefore, we use it for a wide range of applications.
However, FR4 is not particularly suitable for high-speed digital circuits and high-frequency operation as necessary for 5G. This is because at microwave frequencies FR4 suffers high dielectric loss, causing signal integrity problems.
Commonly known as Teflon, PTFE is the acronym for Polytetrafluoroethylene. This material is woven fiberglass or ceramic filled plastic. The main advantage of PTFE is it is a suitable laminate for high-frequencies or high-speed circuits. High frequency signal needs low signal loss and better impedance control, and the low dielectric constant ratio of PTFE offers the ideal choice. PTFE also has good mechanical stability, making it suitable for operations at higher temperatures.
However, at higher temperatures, PTFE exhibits increased dielectric loss, which limits its use somewhat. As it is a softer material, at higher temperatures it exhibits dimensional changes. Finally, PTFE is more expensive than FR4 is.
We typically use the Polyimide material for making flexible and rigid-flex PCBs. However, as a flexible polymer film, this material is suitable for use in hybrid, multilayer PCB applications.
Together with a high level of thermal and chemical stability, Polyimides also exhibit good electrical properties. Being lightweight and flexible, Polyimides are suitable for making PCBs for portable applications.
The only disadvantage of Polyimides is they are more expensive than FR4 and PTFE are.
How Different Materials Help Hybrid Designs?
The main reason we use a variety of materials for creating multi-layered hybrid PCBs is to reinforce the poor structural and mechanical properties of some of the high-speed laminates. By combining them with the superior mechanical characteristics that some other materials offer, we can build a stronger PCB with high-speed functionality.
For instance, although PTFE offers excellent high-speed performance, it starts to exhibit structural integrity problems at high temperatures. By combining PTFE with Polyimide in alternate layers, we improve the mechanical properties of the PCB.
Another reason for creating a hybrid PCB is to cut down on the cost while retaining the high performance necessary. For instance, the combination of FR4 and PTFE in a hybrid design is of substantially lower cost compared to a PCB formed from PTFE alone, while offering practically the same performance. To achieve this, the designer must make sure to route the high-speed circuits in the PTFE layers only, while routing the low-speed circuits in the FR4 layers.
We also use the FR4 to even out the thickness of the laminate layers in the PCB. In some cases, we use it to act as glue for holding things together and providing the PCB with an even thickness.
Not many companies have the capability and the knowledge necessary to create hybrid PCBs. Rush PCB Inc. has been making hybrid PCBs for a long time, mainly as a part of our family of flexible and rigid-flex boards. We are aware of the specific issues related to etching and fabricating multi-layered hybrid PCBs, including material shrinkage as we etch copper out, and the impact of the laminate construction process.