Duct Tape and Test-Time Bandages
Despite thorough engineering, surprises do happen during compliance testing. Inevitably,
while you are still giving the last touches to your design, the marketing department has
already sold a few dozen units to their most prestigious customers and eagerly awaits getting
their hands on the very prototype you are testing for the next trade show. “This is not
good,” you think. Next comes the calculation of how long you can live on your credit cards
before finding another job in a distant corner of the world where no one would have heard
about your mistake.
In reality, vulnerabilities in well-designed devices can often be patched up at the
test site. Understand, however, that any modifications you make to pass the tests will
have to be implemented in the actual product. Don’t go overboard implementing every
possible solution at once. Although adding a capacitor here, a ferrite there, and some
shielding to a cable may sound trivial, each of these modifications can turn into a logistic
nightmare during production. Moreover, unbudgeted additions multiplied by the number
of units to be produced over time may end up reducing the profit margin to intolerable
levels.
In addition, and before applying any corrective measures to a medical device prototype,
however, make sure that by filtering or shielding a specific line, component, or assembly,
you do not violate insulation or leakage requirements. With that said, let’s look at the
essential elements of a first-aid kit.
Ferrites Ferrites are the first thing most people use to try to clean the signals from a suspect
line. A huge variety of ferrites are available; they come in all sizes and provide attenuation
at diverse frequency ranges. Ferrites also come with various installation options:
Some require you to pass the cable or lead component through them, while some have
clamps that make it easy to retrofit equipment without disconnecting wires. Ferrites are
also available to specifically treat differential or common-mode problems. Ferrite diagnostic
kits are available from various vendors (e.g., Fair-Rite), and test houses usually have
plenty of ferrite beads and chokes available for their customers.
Ferrite is even available in tape form (actually, it is a ferrite-coated tape) and can be
applied to PCBs and as a shield on the chassis, enclosure, or other surfaces. Before buying
tape specifically for this application, try the magnetic backing sold in arts and craft
stores to make refrigerator magnets out of photographs. It works wonders!
Connector Filters If a signal line on a connector is a suspect for noncompliance, the
easiest and cleanest way to apply in-line filtering is either to exchange the original connector
for a filtered one or to insert a filtered adapter between the original plug and its target socket.
D-type filtered connectors and filtering adaptors are widely available and come with a
feedthrough capacitor in the range 50 to 2000 pF with or without ferrite inductors. You should
also consider using ferrite plates with holes to match your connectors’ pinout configuration.
Connector Shields Very often, shielded wires are used to protect sensitive or noisy lines,
only to terminate at an unshielded connector. Connector backshields can easily be
retrofitted onto many cables. Make sure, however, that by connecting the cable’s shield to
an exposed metallic connector you do not violate insulation requirements.
Capacitors and In-Line Filters Small capacitors can be tacked on to suspect lines. These
are especially useful when dealing with problems that may be occurring on a PCB. It is
always a good idea to take an assortment of pF- and nF-range chip capacitors to the test
house. If additional filtering is needed, some surgery can be done to a PCB to retrofit inline
filter modules. Beware, however, that the addition of capacitors to the circuit of a medical
device may change its leakage characteristics, causing the device to fail leakage and/or
hipot safety tests.
Cable Shielding Cable shielding is the next step in troubleshooting and fixing EMC
problems. Mesh with and without zip-on sheaths, as well as conductive foils with or without
adhesive backing, are available and can be applied to cables with ease. You will need
to decide how thick a shield you use and how you connect the shield to ground.
Enclosure Gaskets Metallic enclosures are often assumed to be bulletproof barriers
against EMI, both incoming as well as outgoing. However, no instrument is perfectly
sealed, since cables, displays, and controls couple the inside of the instrument to the outside
environment. In fact, a metallic case can sometimes act as a resonator, guiding EMI
to (or from) the vulnerable (or offensive) circuit.
Assuming that filtering and shielding of cables and controls has not sufficed to control
an EMI problem, the next step is looking for gaps in the enclosure which may require
shielding. Here, you can use metallic foils and tapes to improve contact along enclosure
seams and determine if a permanent solution could be achieved through the use of conductive
EMC gaskets. In addition, temporary application of conductive foils to display
windows and ventilation apertures is a very useful diagnostic to determine if conductive
transparent screens (e.g., very fine wire mesh) need to be applied to these openings. Many
of these shielding materials are available from Chomerics.
If the problem is ESD, consider even tiny gaps or joints in enclosure shields which are
weak spots, because they divert very large fast currents as they flow around the enclosure,
causing current density hot spots that emit strong EMI through the shield and into the
enclosure. In fact, for the very high frequency components of ESD pulses, gaps and joints
may act as slot antennas that help get EMI into the enclosure.
On a different front, consider that metallic shields can sometimes be completely transparent
to offensive fields that have a dominant magnetic components. In these cases, vulnerable
parts of the metallic enclosure may require further shielding with a material that
provides a low-reluctance magnetic path for the interference field. The idea is to make the
shield attract flux lines to itself and divert the magnetic field away from the sensitive component.
The magnetic sheets mentioned above are a good start for solving these problems.
If nonmagnetic shields are needed (e.g., to form a shield close to a CRT or a magnetic sensor),
you may try one of the shielding alloys produced by Magnetic Shield Corporation.
Magnetic Shield sells a $150 engineering kit that includes various 10 in. × 15 in. sheets as
well as some braided sleeving made of their CO-NETIC and NETIC alloys. The kit even
includes an ac magnetic field probe that can be used with a DVM or oscilloscope to measure
magnetic fields from 10 Hz to 3 kHz.
Conductive SprayPaint Many medical devices are not built with metallic enclosures. If
extensive shielding of the case becomes necessary, an alternative to changing the design to
use a conductive enclosure is to spray-paint the enclosure using conductive paint. EMI
spray paints are available to provide varying degrees of EMI shielding, all the way from
light, graphite-based paints to provide mild shielding against ESD through nickel/chrome-
loaded sprays that can divert strong magnetic fields away from sensitive components.
Shielding Components If you got this far down the list of quick patches, chances are that
you may need to go back to the drawing board (or more likely, the PCB layout station).
Before going back home, though, you may try to shield individual components. You could
apply one of various available conductive foils and tapes directly to PCBs (to shield tracks)
or to components. There are even precut conductive cardboard boxes that can be used to
shield entire sections of a circuit. However, if you need to build a complete village of protective
housings on your PCB, it may be worthwhile biting the bullet and going back to the
lab to reengineer the product.
Duct Tape and Test-Time Bandages
Despite thorough engineering, surprises do happen during compliance testing. Inevitably,
while you are still giving the last touches to your design, the marketing department has
already sold a few dozen units to their most prestigious customers and eagerly awaits getting
their hands on the very prototype you are testing for the next trade show. “This is not
good,” you think. Next comes the calculation of how long you can live on your credit cards
before finding another job in a distant corner of the world where no one would have heard
about your mistake.
In reality, vulnerabilities in well-designed devices can often be patched up at the
test site. Understand, however, that any modifications you make to pass the tests will
have to be implemented in the actual product. Don’t go overboard implementing every
possible solution at once. Although adding a capacitor here, a ferrite there, and some
shielding to a cable may sound trivial, each of these modifications can turn into a logistic
nightmare during production. Moreover, unbudgeted additions multiplied by the number
of units to be produced over time may end up reducing the profit margin to intolerable
levels.
In addition, and before...
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