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Low Cost Materials as Heat Sinks and Stiffeners in High Power Multi-layer
Applications
Low cost materials, such as FR-4 epoxy-based laminates, used stiffeners and
heat sinks in PTFE printed circuit boards (PCBs) provide several advantages
over traditional thick metal backed materials. The advantages of this
approach have quickly established PTFE/FR-4 multi-layer hybrids as a design
standard for commercial microwave power amplifiers.
Advantages
Reduced Cost: Manufacturing costs for producing PTFE/FR-4 multi-layer
hybrids are significantly lower than using thick metal backed PTFE substrates.
Cost reductions are achieved as a result of considerable reductions in material
and fabrication expenses.
Greater Design Flexibility: The laminations on design flexibility imposed by
thick metal backed substrates are eliminated. The PTFE/FR-4 multi-layer
hybrid approach allows the designer to mount lower frequency components
on the FR-4 side of the multi-layer, thus saving space and reducing overall
package size.
Ease of fabrication/Expanded Fabricator Base: The PTFE/FR-4 multi-layer
hybrid approach allows the use of a much larger fabricator base as the highly
specialized technologie of fabricating thick metal backed substrates is
eliminated.
Reduced Weight: The significantly lower weight of the PTFE/FR-4 hybrid
approach is extremely useful when overall component or system weight
is a consideration.
Questions and Answers
Question:
Can your project afford the additional expense and potential problems
associated with using thick metal backed laminates?
Answer:
Replacing thick metal backed materials with a PTFE/FR-4 hybrid approach can
produce savings in material costs, assemly weight, fabrication and lead-times.
Question:
What sort of cost reduction can I expect by replacing thick metal backed
substrates with PTFE/FR-4 hybrid multi-layer technology?
Answer:
The cases we evaluated suggest that an overall savings of 30% - 50% over
the thick metal backed PTFE approach can be achieved.
Question:
Can you dissipate hest generated from a few components or from an entire
assembly using the PTFE/FR-4 hybrid approach?
Answer:
Yes. By soldering a power component directly to a grid of plated through
holes, excellent heat dissipation can be achieved.
Question:
What is the mechanical advantage of the PTFE/FR-4 hybrid approach?
Answer:
By bonding the PTFE to a stiffener (typically FR-4) that is 2 to 3 times thicker
than the PTFE substrate, the entire assembly will become much more rigid
and is less likely to bow during infrared, convection or wave soldering. In
many applications, thin PTFE substrates can be made self-supporting using
this technology.
Methods and Material
Unlike thick metal backed PTFE materials, PTFE/FR-4 hybrid PCBs can be
drilled, machined and plated using standard printed circuit board fabrication
equipment, processes and techniques. Using the PTFE/FR-4 Hybrid approach
allows the designer to move lower frequency circuitry and components
off the more expensive PTFE substrate and onto the less expensive
heatsink/stiffener. Space savings are also a result of this approach.
Bonding of the PTFE substrate to the FR-4 heat sink/stiffener, can be done
either prior to plating and fabrication or after final fabrication. If bonding is
done using conventional low-flow epoxy prepregs prior to drilling and plating,
the board can be procesed as a conventional multilayer. If bonding of the two
substrates is done as a part of final fabrication, the PTFE printed circuit board
and FR-4 heat sink/stiffener may be bonded using a filled adhesive that is
thermally and/or electrically conductive.
The methods outlined above offer the designer maximum flexibility in design
while reducing the cost of the finished asembly. By combining PTFE and other
low cost materials in the same assembly, it is possible to maximize the
advantages of both materials while eliminating the need for costly and
complicated metal backed PTFE materials or metal carries.
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