Note: Descriptions are shown in the official language in which they were submitted.
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' ' CA 02343660 2001-04-11
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A CIRCUIT DEVLCE HAVING A PROTECTIVE COATING FORMED BY
INJECTION MOLDING A REACTIVELY CLfRED MATERIAL
The Applicants hereby claim the benefit of their provisional application,
Serial
Number 60/206626 filed May 24, 2000 for "A Circuit Device Having A Protective
Coating Formed By Injection Molding A Reactively Cured Polyurethane."
1. Technical Field
The invention relates to electronic circuit device construction and
particularly to
protective coatings for electronic circuit devices. More particularly the
invention is
concerned with an electronic circuit device coated by a reactive injection
molding
process.
2. Background Art
I5 Encapsulating circuit devices in an electrically in ulating material is a
known
method to protect a circuit device from water and other substances, and to
distribute
heat evenly in the circuit device. Other desired results are electrical
insulation, physical
damage protection, packaging ease and reduced total package cost. The
selection and
application of coating materials must be done carefully to compliment the
circuit
device's mechanical, electrical and environmental conditions. There are three
commonly applied methods to encapsulate electronic circuit devices. The first
is
potting wherein one forms a container (a pot) including th.e circuit device
and then fills
the container with an insulating material that hardens around the circuit. The
potting
materials are commonly silicone based rubbers. The problem is the container is
typically a metal casting or similar container resulting in a final circuit
device that is
bulky, and heavy. The cost of the casting is also significant. There is
therefore a need
to protect a circuit device from water and other materials without potting it
in a metal
container.
There is little exposed surface to release solvents or other outgased
components
by using either method. The potting material is poured on the circuit device,
enabling it
to flow by the force of gravity over, around and between the circuit device
elements.
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Care must be taken that there are no voids (bubbles) left in the pot which
alter the
performance of the circuit device with respect to a sta~~dard, provide an
electrical
discharge path or act as a water or similar access channel or reservoir. To
achieve this,
the potting material is necessarily quite fluid, but that l;enerally means the
potting
material takes a longer time in curing. Potting a circuit: device in a
container then
requires a relatively long cure time. There is then a need to reduce the
coating time for
the circuit device. There is a similar need for a coating pro<:ess with a
short curing time.
Another encapsulation method is injection molding. More recently the use of
high-pressure plastic injection molding for encapsulation has been used where
the
circuit device characteristics allow. The key difference between potting and
the current
injection molding methods is pressure. Potting is done at: atmospheric
pressure while
injection molding is done at a very high pressure. Depending upon the
electrical
application, voids are the significant problem in encapsulation. Voids result
from the
inability of the encapsulant to flow and fill the entire cavity. Additionally
air may be
trapped while mixing the potting components. Voids can cause electrical
breakdown,
particularly under high voltages. Voids can cause poor thf:rmal conductivity
leading to
circuit component failures. Potting materials can have poor adhesion, thereby
providing
conduction or water retention channels that limit circuit device integrity.
Voids can also
be an aesthetic issue in some circuit devices.
Techniques to minimize and ideally eliminate voids have been developed,
including: preheating the materials and circuit device to workable limits,
thereby
reducing viscosity, vibrating the potted circuit device to induce air
migration, applying
vacuum to the mixed materials or the potted circuit device to assist in air
migration, and
heating the potted circuit device to its full cure. The general observation is
that
injection molded circuit devices have a greater susceptibility to voids due to
high
material viscosity and low flow rates.
The ability of the encapsulant to adhere to circuit device components reliably
is
of great value, particularly where the coated sections are thin and the
coating increases
mechanical integrity. Surface arc tracking is a significant problem in high
voltage
applications. The encapsulant formulation is key to blocl;ing surface arc
tracking, and
has been demonstrated particularly in the injection molded circuit devices.
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Cure time and overall cycle time issues are insignificant problems in standard
injection molded circuit devices. Potting compounds typically have long cure
times and
need to be "accelerated" or "catalyzed" to shorten their cure time. The cure
times are
usually very long, necessitated by having to have a reasonable process
"working time"
prior to curing. Therefore the option is to have a long ambient cure time of
up to a day
or more, or to bake the circuit device at a non-destructive temperature for
several hours
to permit in-process handling and then allowing the circuit device to finish
cure at
ambient temperature. In either case, the circuit device, the' molds and other
equipment
are dedicated for an extended time period. This is expensive.
With any encapsulant, the physical shrinkage of the encapsulating material
must
compliment the physical limits of the circuit device and its components.
Current injection molding methods use high durometer materials leading to high
forces on the circuit components either during injection or subsequent
shrinkage. The
forces resulting from injection molding of these materials, either during the
initial
molding or during shrinkage, can destroy circuit board components. There may
be
implosions of void-containing components such as capacitors; thermal damage to
insulation, or coatings; fracture of sintered structures such as magnetic
cores; distortion
or tearing of interconnections such as foils, or leads; and mechanical
dislocation of
components. There may be large voids throughout the cavity. There may be poor,
or
no adhesion to components or substrate thereby forming electrical discharge
paths along
these internal channels. The entire package may be distorted due to shrinkage
differentials.
The Applicants have developed a molding process using reaction injection
molding or RIM molding. RIM molding simultaneously rnixes and injects two
viscous
materials that react to form a solid or resilient substance. RIM Molding, was
initially
developed to produce moderate-to-large, low-to-medium density parts for the
automotive trim and interior market; it has since expanded to include
relatively large
consumer and commercial items. It has however not been widely applied to small
items, presumably because of material arid process alternatives. Notably it
has not been
used with electronic products, and specifically not as a substitute for
potting.
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Disclosure of the Invention
A protected electronic circuit device can be formed from a circuit device
carrier
supporting an electronic circuit device with two or more connection leads; and
an
electrically insulating, coating material closely covering and adhering to the
circuit
device. Connections for electrical input and output from the circuit are left
extending
through the coating. The coating material comprises a reactive combination of
two or
more fluid components. It is not necessary for the coating material to be
further
enclosed by an adjacent container. The protected circuit can be formed by
positioning a
carrier body with at least one electronic circuit device composed of two or
more
electronic components in a mold; supplying the mold with a curable, insulating
coating
material to coat and cover the electronic circuit device; curing the coating
material; and
ejecting the coated circuit device carrier body from the mold.
An object of the invention is to provide an overall snnaller coated circuit
device.
Another object of the invention is to provide an overall lighter weight coated
circuit
device. An object of the invention is to speed up the manufacturing process in
making
coated circuit devices, and to enable a mass production process for the
protective
coating of circuit devices. Another object of the invention is to enable the
use of low
viscosity materials) (under 300 centipoise), to minimize void formation in the
coating.
Other objects of the invention are to enable a reasonably :rapid coating
process time of
potentially less than two minutes, to increase production speed, and to enable
high
material flow rates with injection times of less than a few seconds while
eliminating
mechanical damage to circuit devices. A further object of the invention is to
enable low
molding temperatures (under 150°F). An object of the invention is to
enable molding
with a low (under 6.895 x 104 Pascals (10.0 pounds per square inch)) cavity
pressure to
reduce or eliminate component damage. An object of the invention is to reduce
overall
production coating cost.
Brief Description of the Drawings
FIG. 1 shows a coated circuit device.
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FIG.s 2, 3 and 4 show respectively top, side and bottom views of a preferred
circuit
device.
FIG. 5 shows a group of circuit devices grouped in a ladder form.
FIG. 6 shows an open mold.
FIG. 7 shows an open mold with two circuit sets inserted unto position.
FIG. 8 shows a partial, cross sectional, end view of a mold closed around two
circuit
devices.
FIG. 9 shows a top view of a molded set of two groups of circuit devices prior
to prior
to flash trimming and circuit device separation.
FIG. 10 shows a side view of the molded set of circuit devi<;es from FIG. 9.
FIG. 11 shows a coated circuit device with an exterior EMI barrier coating.
Best Mode for Carrying Out the Invention
The preferred coated circuit device 10 consists of a circuit device 12
populated
with circuit components that is then coated with a protective coating 14.
FIG.s 2, 3 and
4 show respectively, top, side and bottom views of a preferred uncoated
circuit device
12. In the preferred embodiment a common circuit device support (board 16)
holds' a
series of similarly constructed circuit devices 12. Each circuit device 12 is
supported on
a separable portion of the circuit device support board 16. The separable
circuit device
12 sections may be divided later one from another. In this manner, a plurality
of coated
circuit devices can be molded at once, thereby sharing common set up, mold and
cure
times. FIG. 5 shows a group of six circuit devices 12, similar to the one seen
in FIG.s 2,
3 and 4, grouped on a common circuit board 16 in the form of a ladder. Six
examples
of the preferred circuit device 12 are positioned parallel arnd adjacent one
another on the
common circuit board 16 base forming the rungs of the ladder. Two side rails
18, 20
complete the ladder by coupling the ends of the six circuit devices 12. Slots
22 through
the circuit board 16 extend between each of the adjacent circuit devices 12.
In the
preferred embodiment, the electrical input contacts 24 and output contacts 26
for the
circuit are formed on ends of the circuit devices 12, adjacent the side rails
18, 20. The
contact points 24, 26 lie in a region that is not protectively coated in the
final coating
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' CA 02343660 2001-04-11
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process. This leaves the contact points 24, 26 free of the coating material l4
and ready
for electrical connection. In the preferred embodiment, foamed on the common
circuit
board 16 between the side rails 18, 20 and the contact points 24, 26 are score
lines 28,
30. The score lines 28, 30 enable subsequent, rapid removal of the side rails
18, 20.
The circuit board 16 may also be formed with registration holes) 32 to enable
accurate
location of the circuit board 16 assembly in the mold 34.
Depending on the geometric form of the preferred circuit, and supporting
circuit
board, a mold is designed to enclose the circuit device within a chamber to
receive,
shape and hold the coating material in contact with those portions of the
circuit device
that are desired to be coated. Once the circuit device is coated, and the
coating material
is cured, the mold is opened and the coated circuit device is removed. Excess
coating
material, if any, is then trimmed, and any final circuit completion steps are
taken.
A preferred mold was designed and built to hold two sets of the six rung
ladders
of circuit devices 12 as shown in FIG. 5. FIG. 6 shows an open view of the
mold 34 for
the two sets of six step ladders. The mold 34 had an upper half 36 hinged to a
lower
half 38. The mold halves 36, 38 included respective surface walls 40, 42 that
when
properly closed, define therebetween two retention chambers 44, 46 to receive
and hold
the two sets of the six step ladder structures (twelve circuiit device
boards), and related
coating material delivery channels 48, 50 and exit cha~lnels 52, 54. The
delivery
channels 48, 50 (also termed runners) for the coating material 14 lead to the
two
retention chambers 44, 46. The lower mold half wall 42 defines two cavities
each
including five lower baffle walls 56. The lower baffle walls 56 define six
separate
subchambers 58 in each of the retention chambers 44, 4Ei. Each subchamber 58
then
encloses a volume surround a respective side of one of the circuit devices 12.
The
upper mold half 36 is similarly formed with two cavities 62, 64 each with five
similar
upper baffle walls 66. The upper mold half 36 further includes four parallel
seal
channels 68. The four seal channels 68 extend perpendicular to the ladder
rungs and
run parallel and adjacent the ladder side rails 18, 20, overlapping the ends
of each of the
sets of the circuit devices 10: As shown, in each seal channel 68 is an
inserted,
compressible rubber end seal 70. Elongated rectangular blocks of rubber may be
used
as the end seals 70.
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The upper mold half 36 includes an inlet 72 for the coating material 14. The
inlet 72 leads to a mixing labyrinth 74 formed between the mold halves, and
designed to
assure good mixture of the injected coating materials 34. The serpentine
labyrinth 74
mixer is comprised of a series of turbulence inducing corners. Thereafter the
gates and
channels guiding the injected coating material 14 are generally formed so as
to not
induce turbulence and cavitation in the circuit device retention chambers 44,
46. The
outlet of the mixing labyrinth 74 leads by two equal delivery channels 48; 50
to the two
retention chambers 44, 46. The delivery channels 48, 50 include low
turbulence, fan
type entrances 76, 78 coupled along the side lengths of the first two circuit
device
subchambers 58. The fan type entrances 76, 78 help provide a turbulence free
flow of
the coating material 14 as it enters the retention chambers 44, 46. At the
opposite end
of the circuit device retention chambers 44, 46 are similar fan shaped low
turbulence
exits leading to exit channels 52, 54. The exit channels 52, 54 receive the
overflow of
any excess coating material 14. The exit channels 52, 54 lead to a vacuum
coupling
(not shown) on the exterior of the upper mold half 36. The final exit channel
is
designed with an extended length to act as a volumetric accumulator (cushion)
for any
excess injected material. The two mold halves 36, 38 further include
registration pins
80 (two located at the top of each for each retention chamber 44, 46) to mate
with the
registration holes 28, and thereby properly locate the sets of circuit device
12 in the
retention chambers 44, 46. The two mold halves 36, 38 nnay also include
registrations
and locking features known in the art to assure the two mold halves 36, 38
stay in
proper alignment during the molding process. The preferrc°d mold 34 is
also electrically
heated.
FIG. 7 shows an open mold with two circuit sets inserted in place. The
registration holes 28 mate with the registration pins 80 to assure proper
positioning.
When the mold compresses around the sets of the circuit device 12 ladders, the
four
rubber end seals 70 press against the ladder side rails 18, 20 and the ends of
each circuit
device 12 to cover the circuit device 12 contacts 24, 26. FIG. 8 shows a
partial, cross
sectional, end view of a mold 34 closed around two circuit devices 12. With
the mold
34 closed, the lower half baffle walls 56 and the upper half baffle walls 66
approach
each other between each of the circuit devices 12, but do not close with one
another, or
CA 02343660 2001-04-11
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the circuit devices 12; thereby leaving an open channel extending along the
length of
the adjacent subchambers 58. Since the lower baffle walls 56 and the upper
baffle walls
66 do not close with the circuit board slots 22 between adjacent circuits
sets, the coating
material 14 is free to flow from one subchamber 58 into the: next.
The coating material 14 is selected to be sufficiently fluid so that it may be
injected safely around the circuit components, and thereafter cure to provide
an
insulating and protective coating. For practicality, it is convenient to form
the coating
material 14 by mixing two or more reactive fluid components just prior to
injection into
the mold chambers 44, 46. During injection the mixture coats or fills all of
the relevant
areas, and volumes. By using reactive,. fluid components, the mixture starts
reacting
during the injection stage, which may be quite rapid, arnd when in final
position, the
components finish reacting to cure as a solid. It is understood that fluid
here means
capable of flowing, and solid means either a rigid or a resilient substance
that does not
flow at the normal operating conditions of the circuit device. The coating
material
should have a sufficiently high dielectric constant, given the operating
voltages and
separations of the circuit components to resist discharge between circuit
components
through the coating material. The coating material should also have sufficient
heat
tolerance as to not deteriorate under the heating load o:f the operating
circuit. The
preferred coating material is a reactively cured polyurethane based rubber
(RIM).
These materials are typically formed by combining an isomer component and a
resin
component. The Applicants' preferred coating material was a reactive
combination of a
resin material, ELASTOLIT # M50872R (BASF Corporation), and isomer material, .
WUC 3092 T Isocyanate (BASF Corporation). These two materials combine to form
a
Shore-A 70 durometer urethane material. The resulting coating material has a
specific
gravity of 1.05 grams per cubic centimeter, a flexural modulus at 23°
Celsius of 1100
pounds per square inch, a tensile strength of 1060 pounds Viper square inch,
an elongation
of 220 percent at breakage, and an instant hardness of 70 Shore A. The cured
coating
material 14 had a measured dielectric value of about 1.772 x 104 volts per
millimeter
(450 volts per mil). The preferred material at the time of mixing has a warm
viscosity
of about 200 or 300 centipoise, and a cold viscosity of about 700 centipoise
at 35° to
40.5° Celsius (95° to 105° Fahrenheit). The preferred
F~IM material components are
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CA 02343660 2001-04-11
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heated to about 35° to 40.5° Celsius (95° to 105°
Fahrenheit) prior to mixing and
injecting the combination into the mold. Depending on the final use of the
coated
circuit, care may have to be used to avoid coating material formulations
including
components that may outgas over time and interfere with the final operational
environment. The Applicants found that lamp ballasts otherwise properly coated
with a
material including a small portion of triethlenediamine (tertiary amine)
negatively
effected the aluminized polycarbonate lamp housings, and the formulation had
to be
changed.
The chosen circuit device is placed in an injection mold. The mold may be
heated to an appropriate temperature: The mold in the preferred embodiment is
heated
to 65.5°C (150°F). Controlled heating helps assure a consistent
product, and speeds the
curing process. In the preferred embodiment the delivery channels and exit
channels
are positioned in the mold so as to be protected from being; coated, or
rapidly cleared if
filled. In the preferred method, a vacuum is drawn on the mold to remove air
from
around the circuit devices, and to speed the flow of the coating material
through the
mold. The two or more reactive components forming 'the reactively cured
coating
material are mixed and rapidly fed into the mold to fill. the space around and
coat
portions of the circuit device components as desired. In the preferred method,
the
coating material components are each fed under pressure (injected) into a
mixing head
and then into a mixing labyrinth where the coating mateo~ial components
(isomer and
resin) are adequately mixed. The emerging mixture of the components (isomer
and
resin) forms the raw RIM material that reactively cures to form the final
coating
material.
The raw coating mixture then enters the main mold cavity to spread around the
enclosed circuit device. Coating material is then simultaneously introduced
through a
divided channel to the two lead ballasts via "butterfly venturi's". These
regulate the
rates of flow of coating materials by proportioning them to compliment the
circuit
device restrictions and providing complete and simultaneous filling of the
entire
retention chambers. The coating material flows laterally across the paralleled
circuit
devices via the inter-connecting webs and exits via similarly configured
retention
chambers and commonly exit via a long perimeter exit channel. The pressure
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differential between the injection pressure and the ambient exit or applied
vacuum
pressure (as the case may be) causes the curing coating material to flow
through, over
and around the circuit device. Preferably the circuit device is configured
provide
reduced flow resistance. Flow resistance can result in injection turbulence in
the
coating material leading to coating defects such as bubbles, unfilled
cavities, poorly
adhered coating, and uncoated sections. In the prefewed embodiment, the curing
coating material is pushed or drawn across the circuit device to an exit port
allowing the
escape of entrained or enclosed gases. The long coating material exit channel
has
vacuum drawn on it (and effectively the entire mold) by an external pump. The
exit
charnel allows for low mold pressures (typically less than 6.895 x 104 Pascals
(10.0
pounds per square inch)) that assists the coating material inflow. A long exit
channel is
preferred as it compensates for possible variations in coating material
injection volume
(purposely or otherwise) while allowing the material to cure in place. The
long exit
channel negates the possibility of excess coating material reaching the mold
exterior
and being subsequently ingested into the vacuum pump, if any, a potential
operational
difficulty for regular production.
The cure time for the injected coating material should be greater than the
time
needed to fully inject the mold cavity, but thereafter may have any
conveniently short
cure time. After filling the mold, the coating material is allowed to cure.
Preferably,
this is rapid, and may even be completed within only a few seconds after
injection is
completed. The mold is opened and the coated circuit devices) are ejected from
the
mold. FIG. 9 shows a top view of a molded set 70 of two groups of circuit
devices 12
prior to separation. FIG. 10 shows a side view of the molded set of circuit
devices of
FIG. 9, prior to separation.
Once the ladder is cured, and ejected from the mold, the side rails 18, 20 are
broken off at the score lines 28, 30 and removed. The web of cured RIM
material is cut
adjacent where the subchambers are segmented by the pairs of baffle walls to
separate
the individual circuit devices 12. The individual circuit devices 12 may also
be
conductively coated (metallized, for example flash aluminized, or other
conductive
coating, for example an Indium Tin Oxide coating). The conductive coating 74
can
then be electrically grounded in a fixture to form an electromagnetic
interference (EMI)
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radiation barrier. FIG. 11 shows a coated circuit device 76 with an exterior
EMI barrier
coating 74 overcoating the RIM injected coating material.
In production of an actual example, a circuit device mold assembly was
oriented
on a support stand and coupled to receive the injected raw material. Raw
Material
Canisters, containing the first part of the reactive coating material (isomer)
and the
second part of the reactive coating material (resin) was coupled to a mixing
head.
External gas pressure was supplied to the raw material sources to assist
material
delivery from the canisters. The raw material mixture ways supplied under
pressure to
two hydraulic pump-driven tandem cylinder units that ingested and then metered
the
raw materials from the canisters. As known in the art, a control system that
inter-relates
all of the process components and elements may be coupled to the system.
The preferred mold attitude was set at an inclination of approximately
45°
upwards from the material inlet end (mixing chamber) toward the exit channel.
The
upward sloping mold enhanced the purging of any entrapped cavity air by
flowing
coating material through the upper ends of the top mold ballast cavities. The
45°
inclination of the mold had a further ergonomic benefit to the operator in
materials
handling and visibility.
The empty, closed circuit device mold was preheated to its preferred operating
temperature, which in one example was 65.5°C (150°F). The
preheated mold was
opened and two six ballast circuit device board assemblies were placed into
position
and the mold was closed. A hydraulic pump system vvas initiated to evacuate
the
retention chambers through the exit channels. A vacuum was then drawn on the
mold
to induce a negative pressure. With the gas assist, the two tandem hydraulic
cylinders
are retracted, ingesting a capacity charge of both moldin;~ material
components. The
vacuum pump was started. The molding cycle was first started without a circuit
device
in position. This was to initially fill (prime) the material delivery system
and verify
molding integrity.
After the initial priming, two sets of six circuit devices 12 were positioned
in the
mold 34 with the circuit device 12 lead contacts facing up. The upper mold
half was
closed, thereby pressing the rubber seals along the lengths; of the circuit
device 12 lead
contacts. A vacuum of approximately 6.895 x 104 Pasc;als (10.0 pounds per
square
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inch) was drawn on the exit channel 46. The RIM materials were then mixed
immediately adjacent the input in a mixing head, and them injected under
pressure into
the mold. The hydraulic control circuit device was then started, forcing the
two
material components through the serpentine mixing cJhamber in the correct mix
proportions. The mixed (and now curing) coating material was forced through
the two
butterfly venturi (at either ends of the mold) and is proportionally
distributed laterally
and upwardly through the two mufti-ballast mold cavities. The coating material
passed
through the serpentine mixing labyrinth 74 and then into the first circuit
device
subchambers to flow below, through and over the first two circuit devices. The
coating
material then passes between the ends of the lower baffle walls and upper
baffle walls
and through the circuit board slots 22 between the first a.nd the second sets
of circuit
devices. The coating material 14 then flowed into the second subchamber. The
coating
material continued until it reached the exit channels. Aided by the vacuum;
the coating
material accumulates and moves through the exit butterfly chambers and into
the long
perimeter exit channel. During this period the coating material delivery cycle
ceases,
the vacuum to the mold is interrupted and the coating material is allowed to
cure. The
mold was opened and the two molded ballast panel assemblies were removed. The
average molding cycle time was approximately 1.5 minutes total. The individual
circuit
devices were then cleaned of any excess coating material, and divided one from
another. Mold release, if any is necessary, may be applied to the mold as
required
between operational cycles. Delivery cylinder replenishment is automatic
within the
control cycle. These production steps may be repeated as necessary.
The two circuit device retention chambers had a total volume of approximately
180 cubic centimeters (I 1 cubic inches). The mold fill time was approximately
2.0 or
3.0 seconds for a flow rate of approximately 130 grams per second. The mixing
head
pressure was approximately 1.723 x 10' Pascals (2500 pounds per square inch).
The
coating material was fed under a pressure of approximate°1y 6.895 x 104
Pascals (10.0
pounds per square inch). Peak gauge pressure on the retention chamber during
filling
has been measures at approximately 3.447 x 104 Pascals (5.0 pounds per square
inch).
The gel time for the coating material was approximately 30 to 70 seconds. Once
the
coating material was injected, it was allowed to cure for about one minute.
The mold
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was then opened and the circuit devices surrounded by t:he cured RIM material
were
released. The circuit devices were gang sliced with rotary blades plunged
along the
length of the web formed between the adjacent circuit devices where the upper
and
lower baffle walls formed deep grooves. The ladder rail portions were then
separated
by cutting or breaking them away, leaving the individual coated circuit units.
No clean
up of the mold was necessary. It is suggested that a mold release such as a
silicon spray
or an aerosol wax spray be intermittently used. Because o:Fthe rapid mold
filling, it has
been found that the circuit device components can cause turbulence in the
flowing
coating material. This turbulence can lead to down stream voids in the cured
coating
material that are cosmetically undesirable, and may lead to reduced shielding
and
insulation. It is suggested that the circuit device components be located on
the circuit
device board so that there are few or no flat surfaces on the; forward and
trailing sides of
components with respect to the coating material flow. ;3uch flat surfaces have
been
found to induce turbulence and therefore result in unfilled cavities. The
Applicants
I S found that in general round surfaces should face directly towards the
inflow or outflow,
and that flat surfaces were best oriented to parallel the coating material
flow direction.
The coated circuit devices have been tested by initially weighing them and
then
soaking them in water for a 24 hour period. An insignificant increase in
weight of 1
percent was noted. The circuit devices were then operal:ed, and no change in
circuit
device operation was detected.
The resulting circuit device has a protective coating covering all of the
circuit
device elements except the input and output connections. The coating was found
to fill
virtual the entire mold cavity, and regions around the circuit device
components with no
significant voids detected to date. The coating provided a~ homogeneous
material, with
no apparent visual or physical variations detected to date. The coating had a
shrinkage
compatible with circuit device components with no significant distortion. The
coating
is resilient, offering excellent impact protection and some mounting
flexibility. The
coating is aesthetically acceptable with a consistent matte finish. The
coating is
resistant to water and other materials, insulates the circuit device elements
and helps
distribute heat. The assembly is small and light weight in comparison to
similar circuit
device constructions potted in a can. The construction avoids the cost,
increased size,
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CA 02343660 2001-04-11
00-1-226 PATENT
weight, and other limitations of using a pot to hold the circuit device. The
conductively
coated format allows for EMI noise reduction by electrical grounding. The
assembly is
inexpensive with regard to both materials and production time in comparison to
potted
circuit devices.
The RIM molding process yielded an economical manufacturing process
alternative with high capacity, good repeatability and high yield. The process
equipment has relatively low equipment and tooling costs. The process yields
an
aesthetically acceptable, uniform and repeatable circuit device. The urethane
sheathing
provides excellent circuit device impact and deformation properties with some
mounting resilience, as well. The integral mold seals and the use of low
molding
pressure result in little or no mold flash. No significant internal voids have
been
observed in any circuit device. An occasional external upper mold corner has
shown air
entrapment, but this problem has been solved by process adjustment. No
mechanical
damage to the circuit components has occurred, including any injury to fine
wires and
foil features. While there have been shown and described what are at present
considered to be the preferred embodiments of the invention, it will be
apparent to those
skilled in the art that various changes and modifications cain be made without
departing
from the scope of the invention defined by the appended claims.
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