Hybrid Multilayer Printed Circuit Boards for Microwave and RF Applications

During the past years a new multilayer printed circuit board (MLB) technology has evolved from the two different sectors of the electronics industry. Complex MLBs are being designed and fabricated which incorporate two or more dissimilar laminate materials. The most common design involves the bonding of PTFE/glass and epoxy/glass to form a four layer MLB.
Referred to as "hybrid" or "composite" MLBs, this type of construction offers RF designers interesting low-cost options for commercial projects. The hybrid concept yields several advantages in the reduction of overall assembly size, reduction or elimination of cables, connectors and wiring harnesses, integrated EMI shielding and a more rigid assembly. In most cases, the cost of the hybrid MLB is lower than the costs associated with two or more conventional printed circuit boards and their related hardware.

The cost savings associated with conversion to hybrid MLB construction is application dependent, however, the analysis based on printed circuit board is shown below.


Assembly Type Cost
Discrete PTFE & FR-4 PCBs 2.5 X
PTFE MLB PCB-4 layer 2.0 X
FR-4 MLB PCB-4 layer 1.0 X
Hybrid PTFE/FR-4 MLB-4 layer 1.6 X


Traditionally, multilayered PTFE boards were rarely used because of their high cost and poor plated-through-hole reliability. The PTFE substrates used are considered to be very high cost and their exotic nature created many problems for PCB fabricators.

A newer class of PTFE/glass materials was introduced to support the burgeoning commercial RF marketplace. Taconic´s offerings, TLC and TLE substrates, were designed to offer electrical properties similar to traditional PTFE laminates and mechanical properties matched closely to those of polyimide materials. The per square foot costs of the new materials are forty to sixty percent lower than those of traditional substrates.


Property Typical Value
Dielectric Constant @ 10 GHz 3.20 +/- 0.05
Dissipation Factor @ 10 GHz 0.003
Moisture Absorption < 0.02%
Flexural Strength PSI and N/mm² X > 276
Y > 241
Peel Strength 12 Ibs./ inch = 2.1 N/mm
Coefficient of Thermal Expansion
Z-ppm (25-250°C) 70
% (25-250°C) 1.7
X-Y ppm (50-150°C) 9 - 12


TLC materials are available from a minimum thickness of 0.0145" = .37 mm to a maximum thickness of 1.00" = 25 mm. TLE materials are available from a minimum thickness of 0.004" = .1 mm to a maximum of 0.250" = 6.26 mm. Our expectation was for TLC materials to be used for traditional double-sized PCBs and TLE to support new MLB designs. These markets developed quickly, however, new hybrid designs utilizing epoxy/glass (FR-4) substrates have become dominant in the multilayer market place.

Traditionally, RF functions are segregated to the PTFE layer and digital functions to the FR-4 layer. A ground plane is utilized on the 2nd layer or EMI shielding purposes. In most designs, the FR-4 layer also acts as a mechanical stiffener allowing the part to be fabricated and assembled in an 18 inch x 24 inch (457 mm x 610 mm) format.

The fabrication scheme is outlined below:

  1. Cut material to size

  2. Chemically clean copper surfaces

  3. Image circuit layers 2 & 3

  4. Etch circuit layers 2 & 3

  5. Microtech/oxide coat inner layers

  6. Collate laminate & bonding materials

  7. Laminate

  8. Drill

  9. Condition hole plate

  10. Electroless plate

  11. Electroplate

  12. Image outer layers

  13. Etch

  14. Solder mask

  15. Profile

  16. Test & Ship


Several bonding materials can be used in the hybrid MLB concept and their selection is based on operating frequency, assembly conditions, and cost considerations. The most poular material in current use is epoxy prepeg. This material is UL 94V-O flame retardant, readily available, and of epoxy prepegs are based in their thermosetting resin systems. Epoxies cure at a relatively low temperature (typically 350°F for one hour) therefore, the number of fabricators with the equipment to process these materials is much greater. Additionally, epoxy materials exposed to soldering temperatures do not exhibit a noticeable mechanical phase change. Thermal defects such as delamination and blistering are greatly reduced.

Thermal management or heat sinking of high power devices is accomplished with two methods, a grid of closely spaced via holes placed adjacent to and below components and soldered, and cutouts which expose inner layer ground planes.

Epoxy prepegs are used in devices which usually operate below 5.8 GHz. Devices which operate at higher frequencies may require improved layer-to-layer impedance matching. In these situations, thermoplastic films and fiberglass reinforced thermoplastic prepeg materials can be used. Hybrid multilayer printed circuit board technology is currently being used to build power amplifiers to support cellular and paging base stations and KU band low noise block down convertors. The technology is also used for producing low frequency microwave radio transmit/ receive units, satellite antenna systems nd large format planer phased array antennas. This concept is also used in high-speed digital electronics for high performance work stations and integrated circuit probe cards.
Corporate Homepage | ©2007 by Taconic | Update: 11/11/2008