Note: Descriptions are shown in the official language in which they were submitted.
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FUSIBLE LINK
This invention relates to d fusible link which acts to
break an electrical circuit of which the link forms part in the event
of a high fault voltage.
Battery feed resistors serve in part to protect
sensitive electronic telephone line circuits from external fault
conditions such as intermittent or steady state fault voltages. Such
voltages may cause the resistors to dissipate power levels which are
hazardous to the system. It is desirable to have a controlled "fail
safe" mechanism which, in effect, disconnects the line circuit from
such fault voltages.
In order to implement the overvoltage protection,
various approaches have been tried, such as thermistors, gas tubes,
bimetallic switches, and solid state devices. Generally such
techniques have not been fully successful owing variously to expense,
bulkiness and questionable reliability and performance.
A further method of providing overvoltage protection is
by fusible links.
Known fusible link technologies suffer from several
deficiencies. Firstly, to be effective the fusible link must be
subjected to a temperature which is accurately related to the power
dissipated within the electricdl circuitO Frequently the heating of
the fusible link by the resistive film and associated substrate is
indirect and difficult to control because they are not in intimate
2~ contact with the power dissipating element.
Metallic fusible links are prone to oxidation if they
are maintained at high temperature during normal circuit operation.
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To avoid the problems of oxidation~ it is necessary to provide Firstly
a coating of flux to remove the oxides during operation and then a
second coating of solvent protection over the -Flux coating to avoid
working off the flux during various cleaning processes.
In other types of fusible link, if the fuse is
positioned externally of the circuit board and reaches high
temperature, emission of hot particles may occur. Ho-t particle
emission can result in other system failures due to the fire hazard
and/or shorting of circuit elements.
Generally known fusible links require a specially
tooled lead frame to connect the fusible link and much additional
manual labor to assemble the link to the lead frame.
According to the invention, there is provided a fusible
link for an electrical circuit, the fusible link formed at an aperture
through a ceramic printed circuit board, the link being a length of
fusible metal having a close fit within said aperture wherein one end
of said fusible metal length is electrically connected to a circuit
lead on one side of the board and the other end of said length is
electrically connected to a circuit lead on the opposite side of the
board, and a resistive film located adjacent said link and together
with said link and said circuit leads being connected in-to said
circuit, said resistor adapted to heat the fusible metal to its
melting point when the circuit is subjected to an overvoltage
condition.
Preferably the fusible metal length is swaged into
position within the aperture to establish a close fit and thereby good
thermal coupling between the circuit board and the fusible metal. The
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fusible metal link may additionally be soldered at opposed ends to the
circuit leads.
Preferably the circuit board is made of d ceramic
material having a high thermal capacity. The resistive film can be
located closely adjacent to the fusible link whereby the heat
generated by current passing through the resistive film raises the
temperature of an adjacent part of the circuit board and thereby
raises the temperature of -the fusible metal or solder link. As a
threshold power is dissipated within the resistive film, the solder
link melts, flows out of the aperture and so breaks the electrical
circuit.
Preferably the length of solder projects beyond the
aperture by an amount sufficient that when the solder melts, surface
tension forces act to force a part of the melted solder outside the
aperture into a ball and so draw further solder from inside the
aperture. The aperture can be a circular hole and can be tapered to
encourage the action of surface tension in drawing the molten solder
from the aperture.
A metal pad can be formed on the circuit board
surrounding the fusible link whereby when molten solder flows from the
aperturet it flows initially over the metal pad and is thereby
encouraged to solidify.
An embodiment of the invention will now be described by
way of example, with reference to the accompanying drawings, in
which:-
Figure 1 shows a first stage in the formation of afusible 1ink according to the invention;
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Figure 2 shows a subsequent stage in the formatiorl of a
fusible link;
Figure 3 shows a further stage in the formation of the
fusible link somewhat different from that shown in Figure 2;
Figure 4 is a sectional view through a printed circuit
board showing the board having a fusible link according to the
invention; and
Figures 5 and 6 are sectional views -through a fusible
link as the link is broken.
Referring to the drawings in detail, Figure 1 shows
part of a circuit board 10, the circuit board being a 0.1 inch thick
ceramic substrate with a 0.055 inch diameter laser drilled hole 12
extending therethrough. Into the hole is inserted a 0.05 inch
diameter rosin flux core 60/40 tin/lead solder wire 14. The solder
wire is swaged into place to spread the solder ends 16 over respective
electrical leads 18 at opposed surfaces of the circuit board (Figure
2).
As an al-ternative, or in addition to, swaging the
solder in place within an aperture through the circuit board, the
solder lin~ can be held within the aperture and solder 20 applied to
edch end of the link to solder the link ends 16 to the adjacent
conducting leads 18 (Figure 3). The second method is clearly somewhat
slower than the first although thermal coupling achieved by the second
method is better.
As shown in Figure 4, in operation, the fusible link
lies close to a resistive film 22 which forms part of the electrical
circuit which is to be protected by the fusible link. The film is
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deposited over the conducting lead 1~ to establish electrical
continuity between the resistive film 22 and the solcler o-f the -Fusible
link 14. When current is passed through the electrical circuit, the
resistive film 22 is heated and thermal coupling takes place from the
resistive film 22 -to the ceramic substrate 10 and then to the solder
of the fusible link 14.
Eventually, when a high enough temperature is reached,
the solder melts. As shown in Figure 5, by using an amount of solder
which causes the solder link 14 to project slightly beyond the ends of
the hole 12, outflow of molten solder from the hole is encouraged by
surface tension forces which form as soon as the solder melts. The
surface tension tends to reshape the solder at the surface of the
circuit board into a ball. As a result of the surface tension forces,
solder is drawn From inside the hole. Eventually, the continuity o-F
solder through the hole 12 is broken and the ball of solder drops away
from the mouth ~f the aperture and onto the metallization 18
surrounding the hole. To encourage outflow of molten solder, the hole
12 can be tapered somewhat as shown in Figure 5.
The molten solder tends to flow along the outside
surface of the board 10 once it exits the hole 12. To ensure that the
molten solder does not travel to a location where it could cause a
short or otherwise damage the electrical circuit, the board has a
metal pad closely adjacent -to the fusible link site. As the molten
solder exits the hole 12 it flows along the surface of the metal pad
and quickly solidifies.
Typically the solder used is a 60/40 tin/lead solder
having a melting point of 1~3~C. However by choosing different
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fusible alloys, the fusible link can be made to fuse at higher or
lower tempera-tures. Thus to achieve low temperature melting the
fusible alloy can include indium. The thermal capacity of the circuit
board acljdcent to the fusible link site is sufficiently high that as
the fusible link melts, the solder stays molten long enough for it to
flow out of the aperture to break the electrical circuit.
As previously indicated, a particular advantage of this
type of fusible link is that since the solder wire is swaged into the
hole, good thermal coupling results between the ceramic or other
circuit board body and the solder, this thermal coupling being
minimally influenced by external drafts, heat sinks, etcetera.
Moreover the solder within the hole is protected from oxidation as it
is sealed off from the air.
