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Embedded Thin-Film Resistors
for Medical Electronics
Introduction
The use of embedded passive components in the printed circuit board industry have expanded exponentially over the past several years, primarily in response to a need for smaller, thinner and denser electronic circuitry, and nowhere has this demand appeared to have become greater than in the medical electronics industry.
In this article, we will review some of our own experiences
with the application of one specific embedded passive material
known as OhmegaPly®,
a thin-film resistive material manufactured by
Ohmega Technologies,Inc.
Thin-Film Embedded Resistive Materials
OhmegaPly® is the trade name
for a Thin-Film embedded resistive material
produced by electroplating a Nickel-Phosphorous compound (NiP),
onto a copper foil substrate.
This plated foil substrate may then be bonded to a variety of dielectric materials.
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Utilizing conventional subtractive print-and-etch techniques,
a single circuit layer may be created,
comprising conductive copper foil traces integrally connected
with the desired resistive elements.
The subsequent circuit layer may be used as an inner layer
of a multilayer printed circuit board, or as a surface layer
in conventional circuit construction.
We have successfully bonded this OhmegaPly® material to
PTFE (Teflon), Polyamides (rigid and flexible), FR-4 epoxy-glass,
along with several other exotic materials.
The OhmegaPly®
material is currently available in ½ oz (18 micron) and 1 oz (35 micron)
copper foils, with sheet resistivities of 10, 25, 50, 100 and 250 Ohms per square inch
being currently available.
Reliability
As medical electronic circuit reliability is of the utmost importance,
the utilization of OhmegaPly® materials can substantially enhance
circuit board reliability by the reduction or elimination of discrete resistors
as well as the problems associated with fluxes, soldering and washing.
Additionally, reducing the area required for discrete resistors
yields a smaller and/or thinner circuit board, and in addition,
double-sided surface-mount circuit boards may be reconfigured as single-sided.
Such resistive materials have been utilized extensively
for more than 30 years in a variety of critical applications,
repeatedly exhibiting outstanding long-term reliability.
Applications
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One such medical application we encountered required a multilayer
flexible circuit cable utilizing the OhmegaPly® resistive film.
We obtained an OhmegaPly® resistive copper material comprised of
½ oz copper with a 10 ohm per square inch resistive coating,
and with the use of a press, bonded the film
to one side of a flexible Kapton substrate.
The remaining side of the Kapton substrate was laminated
with conventional ½ oz copper foil.
Applying standard imaging and etching processes to both sides
of the flexible Kapton strip yielded the required copper foil traces
on one side of the strip, and a combination of copper and resistive traces
on the remaining side. The strips were then electrically tested
to ensure compliance with customer specifications.
The next step was to apply a protective coating to the resistive elements
in order to prevent possible mechanical damage during handling and application.
We completed this process with an additional resistance test
to verify the value and integrity of the resistive film.
Another specific application utilizing the OhmegaPly resistive film
involved a medical imaging memory controller,
where the film provided a series of termination resistors
within 5 internal logic planes of a 14-layer multiplayer FR4 glass epoxy circuit board.
The termination resistors
were designed to be in close proximity to the integrated circuits
in order to improved impedance matching and reduce propagation delay.
Additionally, the embedded resistors serve to reduce EMI, which is
often associated with chip or through-hole resistors.
Conclusion
The stringent reliability goals set forth by the medical industry and others,
comes on the heels of increasing demands for more dense, cost-competitive, compact and sophisticated printed circuit boards.
Our expertise is exhibited in our ability to adapt to
and incorporate advanced manufacturing technologies,
and it is of paramount importance that we remain ahead
of these key drivers in an ever-evolving industry.
Embedded passive technologies, such as OhmegaPly®,
represents just one part of the growing arsenal of tools being made available
in order to meet or exceed customer specifications, designs and expectations.
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Considerations for the
Thermal Management
of Printed Wiring Boards
By
Brigitte Lawrence
,
BRIGTFLEX
, Inc.
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The drivers in the electronic packaging market continue to push for
higher density, better performance, and improved reliability.
When performance includes thermal management,
generic substrate and board processing options may not be sufficient.
The design hurdles are ever increasing power density, as packages get smaller and hotter.
Higher frequencies and data rates are reducing efficiency and more energy
is lost to heat.
Higher-wattage components are raising junction temperatures and reducing component reliability.
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Thermal Management
To address these thermal management concerns, we need to
understand the material's thermal impedance.
This value is the sum of a material's inherent resistance to
heat transfer or thermal resistance, plus the imperfections at
material interfaces which further impede heat transfer, or interfacial resistance.
System reliability is absolutely critical, especially for applications such as
medical technology and flight hardware, where safety is paramount.
System failures, regardless of the application, will equal lost revenue
and affect a company's bottom line and reputation.
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The use of heavy metal backplanes, thermal vias, thermal coins, heat
spreaders, heat risers, and conductive adhesives are all proven ways in which
materials can be used to reduce junction temperatures.
Additionally, active cooling and water-cooling are also effective.
Any or all of these methods can, however, adversely affect the
cost, size, weight, reliability, and electrical performance
of RF circuits.
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The
Arrhenius Chart
tells us that, with every 10°C increase in temperature,
failure rates at the component-level will double:
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HV HBT Reliability - Arrhenius Chart
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Thermal Management Solutions for Digital & High-output LED Designs
One
solution for digital and LED designs that would complement all of
these approaches would be the use of substrates materials with high
thermal conductivity. These epoxy based substrate materials like
Arlon® 91ML, will help limit the maximum temperature the
component will see by dissipating and spreading the heat in plane.
These substrates will also dissipate heat faster to metal planes
like a heatsink or thermal coin.
Materials with in plane thermal conductivity of 2 –
4 W/mK can increase the ability of the board to spread heat 10 to 20
times more efficiently than conventional epoxy materials with in
plane thermal conductivity in the range of 0.2 W/mK. The use of thin
dielectrics with good electrical strength and low thermal
resistance, provide the perfect solution for the LED and other
high power applications
Thermal
Management Solutions for Power RF Designs
The same thermal management concerns exist in the RF design arena.
This is particularly true in active component designs like power
amplifiers. The use of thermal vias to transfer heat is common on
digital designs but may impact signal integrity on RF designs.
While thick heatsinks and copper coins can be effective in reducing board
temperatures, they also add additional cost and weight.
By designing with materials that have high thermal conductivity,
the materials and fabrication costs could be reduced
by the use of thinner heatsinks or coins.
Reducing the maximum surface temperature of the boards should
increase component reliability.
Reducing surface temperature is achieved by spreading the heat away
from components and reducing hot spots.
In addition to reducing junction temperatures
thermally conductive PTFE materials like Arlon’s TC600, will
provide a thermally stable dielectric constant over temperature.
This property insures that the dielectric constant will not shift over
temperature and impact the impedance values of the circuit.
A stable dielectric constant will reduce reflections and dead bandwidth.
Conclusion
Using thermally-conductive materials can increase component reliability
by reducing junction temperatures.
This helps engineers reduce the limitations to improved designs in
high-power LED illumination, high-speed logic circuits and power RF applications.
Brigitflex has developed an expertise with many of the materials designed for
thermal management concerns of high-power LED, digital, and RF designs,
using single-sided, multilayer and rigid flex constructions.
The use of high thermal conductivity materials to improve heat transfer
in both RF and digital designs is now a common solution, with both the
epoxy-based and PTFE based materials.
The consensus of many of the engineers in these markets has been positive
regarding the use of these ceramic-filled 'prepregs' and laminates.
Brigitflex is working with suppliers of these types of substrates, like the
Materials for Electronics Division of Arlon, to meet the thermal
requirements in variety of applications.
Vendor relationships are important; our collaboration with Arlon, and our other suppliers,
has been key to our successes, from prototyping through production.
Our ability to provide conceptual engineering and total-in house manufacturing capability
here in the United States has been important for both our military and commercial customers.
All materials at Brigitflex are lead-free solder process-friendly.
Special thanks to:
Russ Hornung,
Technical Marketing Manager,
Arlon, Materials for Electronics Division
References:
Russ Hornung, Arlon Materials
"Expanding the Thermal Management Tools
for RF Infrastructure & Power Amplifiers"
IWPC, October 18, 2007
Eli Reese, TriQuint Semiconductor
IWPC, November, 2005
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copyright © 2009 Brigitflex, Inc.
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