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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:
- Cut material to size
- Chemically clean copper surfaces
- Image circuit layers 2 & 3
- Etch circuit layers 2 & 3
- Microtech/oxide coat inner layers
- Collate laminate & bonding materials
- Laminate
- Drill
- Condition hole plate
- Electroless plate
- Electroplate
- Image outer layers
- Etch
- Solder mask
- Profile
- 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
and 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.
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