The
origins of radial box film capacitor technology lie in
radial dipped film technology, which was first developed
by WIMA and other European manufacturers in the
early-middle 1960s. The radial design offered obvious
advantages over the axial design. These included board
real estate savings as well as improved electrical
performance (lower ESR and self inductance) and did not require the leads to
be bent in order for the part to be inserted on the
printed circuit board.
1962 Radial dipped
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Improved electrical performance. |
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Lower ESR and self-inductance. |
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Less space required on PC-board in
comparision to axial leaded devices. |
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Easy plug-in mounting. |
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1963
Moulded case
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Defined case-size. |
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Humidity protection. |
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Robotic insertion. |
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Less space required on PC-board. |
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| 1970
Resin potted case
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Improved humidity protection. |
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Reduced mechanical stress on leads/
solder joints due to support on edge of box. |
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Good self-healing properties due to
encapsulation without pressure. |
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1974
Box type
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Increased humidity protection. |
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Reduced outside dimensions. |
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Easy robotic insertion. |
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Seating plane defined through
"standoff-feet". |
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Excellent self-healing properties due to
encapsulation without pressure. |
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Standardized sizes. |
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Humidity Protection
The
poreous epoxy coating of radial dipped film capacitors
cannot be expected to provide reliable humidity
protection where moderate to severe humidity conditions
exist, even when potting compound such as bitumen is
used. In addition, since there is no clearly defined
seating plane, components rest on the "laquered
pants" that can and do develop fissures at the lead
exit points when parts are automatically inserted, thus
further compromising their ability to resist the effects
of humidity.
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Box type |
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Two casting steps guarantee high humidity
protection. |
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Homogenous encapsulation without air enclosures. |
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Case and lead exit points secure from high
humidity. |
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Additional protection by cast sealing is not
necessary. |
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Dipped version |
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Very porous coating with visible air enclosures. |
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Mechanical stress during insertion leads to
cracks around the lead exit points. |
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Insufficient humidity protection due to thin and
irregular encapsulation. |
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Bitumen or tar casting does not provide the
expected protection. |
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Uniform Dimensions
Radial
box film capacitor provides uniform dimensions for
purposes of optimizing space and second sourcing.
Further, it allows for greater flexibility in automatic
insertion including robotic insertion of larger parts.
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Box
type |
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All dimensions clearly defined. |
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Allows for close placement of parts. |
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Easy second source because of standardized box
size. |
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Dipped
version |
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Only lead spacing is defined. |
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All other dimensions are undefined. |
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No standardized body sizes (second source). |
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Need of additional space between the parts due
to variations of lead exit points and body
dimensions. |
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Exact Setting on PC-Board
Since
radial dipped film capacitors rest on their leads rather
than on the case or "standoff-feet", any
vibration that the capacitor element may be experiencing
will be transmitted through the leads to the solder
joints. This is of particular concern in AC applications
where self-generated electro-mechanical vibrations
accelerate the ageing of the solder joints.
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Box
type |
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Capacitors rest on "standoff-feet". |
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No stress on leads. |
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Electro-mechanical vibrations do not impact
solder joints. |
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Exact setting on PC-board. |
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Small footprint on PC-board. |
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Dipped
version |
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Capacitors rest on solder joints. |
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Electro-mechanical vibrations can lead to
accelerated ageing of solder joints. |
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More space required to avoid short circuits. |
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Flammability Resistance
The
extremely thin covering, plus the presence of air pockets
contained within the coating, makes the dipped film
packaging of doubtful value with regard to passive and
active flammability. This is especially the case when the
applications does not use any potting compound. On the
other hand, the mechanical integrity of the box and cast
resin technology provides a high level of flammability
resistance when the capacitor is used in AC applications
such as in a power line filter or in series with the
lamp.
Basically
all plastic film dielectrics are flammable. Only
encapsulation protects the capacitor against fire.
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Box
type |
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Uniform thickness of encapsulation. |
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High flammability protection in accordance with
UL 94 V-0. |
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No cast sealing necessary. |
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Dipped version |
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Epoxy material may or not be passively
flammable. |
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Thin coating gives quick access to winding
element after short application of flame. |
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Air bubbles in coating stimulate the flames. |
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Higher fire risk when tar is used. |
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When
radial box film capacitors are used, the need for potting
disappears, since the box capacitor provides sufficient
humidity and flammability protection, as well as
mechanical integrity, in the face of externally or
internally generated shock and vibration.
The need
for qualified capacitor applications to operate for many
years without the maintenance or replacement in often
times difficult environmental and operating conditions,
makes it critical that the components meet the highest
standards of packaging technology. Without such
standards, safe and reliable operation over the life of
the product cannot be guaranteed. The box
film technology is
designed to meet this challenge both in radial and SMD
version. |
