Heat actuated fuse apparatus with solder link

An apparatus for protecting circuitry includes a heat actuated fuse which opens at a predetermined temperature and a resistive device disposed proximate the fuse for heating the fuse in response to an electrical current passed through the device. The fuse includes first and second electrodes bridged by a solder link which melts at the predetermined temperature and retreats to the electrodes. A quantity of flux-containing material is disposed on the link to promote rapid and complete retreat of the solder when the fuse is actuated.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to a heat actuated fuse and to an apparatus which 
incorporates such a fuse for protecting electrical circuitry. The 
invention is particularly useful for interrupting current flow to thick 
film circuits and components. 
2. Description of Related Art 
A heat actuated fuse has an important advantage over the more common 
current actuated fuse. Although both types of fuses ultimately open at a 
predetermined temperature, a heat actuated fuse can be positioned such 
that it responds directly to the temperature of a protected component or 
circuit, and temperature is often a better indicator of imminent failure 
than current. Examples of known heat actuated fuses are described in Great 
Britain Patent 2,145,295A and U.S. Pat. No. 4,533,896. 
Great Britain Patent 2,145,295A discloses a thermal fuse disposed on a 
substrate nearby a resistor, which is also disposed on the substrate. The 
fuse includes a pair of electrodes defining a gap between them, a gold 
fuse link extending across the gap to electrically interconnect the 
electrodes, and a film of solder overlying the gold fuse link and 
overlapping at least one of the electrodes. When the temperature of the 
substrate exceeds the melting point of the solder film, the solder is 
intended to melt, dissolve the gold fuse link, and then retreat from the 
gap with the dissolved gold to sever the electrical connection between the 
electrodes. 
This fuse is relatively simple to manufacture, but its reliability depends 
on thorough dissolution of the gold fuse link by the process of leaching. 
This is a gradual process which begins after the film of solder melts. The 
gold fuse link then dissolves and is attracted to tin in the solder. The 
speed of opening of this fuse is relatively slow, and the solder material 
is limited to a composition containing tin or whatever material will 
effectively leach the gold or other metal chosen for the fuse link. 
U.S. Pat. No. 4,533,896 discloses a fuse for protecting thick film devices 
deposited on a substrate. The fuse includes two terminal blocks, each 
including a hole, which are mounted to the substrate in close proximity to 
each other and with the holes aligned. An electrically conductive fusible 
link (e.g. solder) is suspended across a space between the terminal blocks 
and extends into the holes to complete an electrical circuit. The holes 
are larger than necessary to accept the fusible link so that, when the 
fusible link melts because of excessive heat, molten material from the 
fusible link will be drawn into the holes. 
Manufacture of this fuse is a complicated and labor intensive process. 
Manufacture requires boring of holes in the terminal blocks, coating the 
inner surfaces of the holes and the end surfaces of the terminal blocks 
with solder, assembling the blocks and fusible link, and then positioning 
and soldering the assembled fuse onto electrodes provided on the 
substrate. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide a heat actuated fuse which 
opens reliably and quickly. 
It is another object of the invention to provide such a fuse which can be 
manufactured simply to open at any predetermined temperature within a wide 
range. 
It is yet another object of the invention to provide apparatus 
incorporating one or more of such fuses for protecting circuitry including 
one or more electrical elements. 
In accordance with the invention, a heat actuated fuse for opening at a 
predetermined temperature comprises an arrangement of materials which can 
be deposited on an electrically insulating substrate by commonly used 
printed circuit fabrication techniques. First and second electrodes of 
electrically conductive material are disposed on the substrate in a spaced 
apart relationship to define a gap between the electrodes. A layer of a 
first solder material, having a melting temperature which is higher than 
the actuation temperature of the fuse, is disposed on at least one of the 
electrodes. A conductive link disposed on at least a portion of each 
electrode and on the substrate extends across the gap to provide an 
electrical connection between the electrodes. 
The conductive link comprises a predetermined quantity of a second solder 
material, which is in contact with the solder layers on the electrodes. 
The second solder material has a melting temperature which substantially 
corresponds to the actuation temperature of the fuse, and the 
predetermined quantity of this material is containable on the first and 
second electrodes. 
Because different solder materials having a large range of respective 
melting temperatures are readily available, the fuse can be constructed to 
open at any temperature within a correspondingly large range. A quantity 
of flux-containing material is disposed in contact with the conductive 
link to effect a rapid and reliable flow of the second solder material to 
the first and second electrodes, when the fuse is heated to its actuation 
temperature. 
In a preferred embodiment of the invention, a resistive device is disposed 
proximate the fuse for effecting heating of the fuse in response to an 
electrical current passed through the device. Various advantages can be 
achieved by utilizing such means, independent of the protected circuit 
element(s), for heating the fuse. For example, by electrically connecting 
and physically disposing the resistive device such that it is subjected to 
the same electrical and environmental conditions as the circuitry 
protected by the fuse, freedom is achieved in the placement of the circuit 
element(s). Also, types of circuits elements which cannot, without 
destruction, produce sufficient heat to melt the fusible link may be 
protected.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIGS. 1 and 2 illustrate in top view and cross section, respectively, a 
preferred embodiment of a heat actuated fuse constructed in accordance 
with the invention. The fuse is formed by a combination of deposits 
supported on a portion of a substrate 10 of an electrically insulating 
material. The particular substrate material must be compatible with the 
fuse fabrication method employed and must have heat conductivity 
characteristics compatible with the manner of actuating the fuse. For 
example, if the fuse and an actuating device are provided on the same 
substrate, and the fuse senses the temperature of the component primarily 
by means of heat conduction through the substrate, a material having 
substantial thermal conductivity, such as alumina, should be chosen. 
Alternatively, if lower heat conductivity is satisfactory for a particular 
application, a material such as phenolic may be used. 
The substrate 10 supports two thick film metallic electrodes 12, 14 having 
respective projecting tab portions 12a, material, such as a 
palladium-silver alloy, and are spaced apart such that a gap of width w 
exists between the tab portions. The width w of the gap determines the 
volume of solder which must flow onto the electrodes to open the fuse. In 
a practical embodiment of the fuse, each of the electrodes 12, 14 has a 
main portion measuring 0.100.times.0.100 in., a respective tab portion 
measuring 0.020.times.0.040 in., and the gap has a width w=0.015 to 0.020 
in. These dimensions may be adjusted from one embodiment to another to fit 
the fuse in the amount of available space. 
Disposed on the electrodes 12, 14 are respective layers 16, 18 of a first 
solder material having a melting temperature which is substantially higher 
than the actuation temperature of the fuse. For example, a 96.5Sn/3.5Ag 
solder having a melting temperature of 221.degree. C. was used for a fuse 
having an actuation L temperature of 179.degree. C. The layers 16, 18 have 
respective portions 16a, 18a disposed on the corresponding tab portions 
12a, 14a of the electrodes. 
A conductive link 20 of a second solder material extends across the gap, 
electrically connecting the electrodes 12, 14. The conductive link is 
disposed in globular form covering the tab portions 12a, 14a, the 
respective overlying solder layer portions 16a, 18a, and the surface area 
of the substrate lying between and immediately adjacent to the tab 
portions. The second solder material has a melting temperature 
substantially corresponding to the actuation temperature of the fuse. For 
example, a 62Sn/36Pb/2Ag, solder having a melting temperature of 
179.degree. C. was used for the above mentioned fuse having the same 
actuation temperature. 
A small quantity of flux-containing material 22 covers the solder link 20. 
Alternatively, this material may be disposed on the substrate immediately 
adjacent the link or as a dab on some substantial portion of the link. The 
primary requirement is that the material 22 be in physical contact with 
the link. 
Suitable flux-containing materials include solder paste and simple flux 
itself. Either of these materials will, when the fuse link is heated to 
its melting temperature, promote the flow of the melted solder link 20 
from the gap and onto the solder layers 16, 18 covering the electrodes 12, 
14. Solder paste is a convenient material, since it may also be used to 
form the link. However, flux is a less expensive material and may be used 
in a smaller quantity. Not only does the use of flux offer a cost 
advantage, but also it lessens the total volume of solder material which 
must flow from the gap during opening of the fuse, thereby improving 
reliability of opening of the fuse. 
The volumes of the solder forming the conductive link 20 and of the 
flux-containing material in contact with the link are determined by a 
combination of factors. For example, the combined volumes should be 
containable on the electrodes 12 and 14 after melting of the solder link. 
Also, the cross-sectional area of the conductive link 20 should be 
sufficiently large to limit fuse resistance to whatever maximum value is 
allowable for the circuit in which the fuse is connected. Further, the 
length of the conductive link 20 should be sufficient to extend across the 
width w of the gap and to make good electrical contact with the electrodes 
12, 14 and substantial physical contact with the solder layers 16, 18. 
A protective covering 24 of a material such as latex or silicon rubber may 
be provided over at least the flux-containing material 22 to prevent 
damage to or deterioration of this material. The protective covering 
should be of an electrically insulating material and should be flexible to 
facilitate flow of the solder link 20 and flux-containing material 22 onto 
the electrodes upon opening of the fuse. The protective covering shields 
the flux-containing material 22 from potentially damaging process steps 
performed subsequent to making of the fuse and provides protection against 
removal of the material 22 during handling. Covering 24 also contains the 
solder when it melts during opening of the fuse and ensures that the 
melted solder does not come into contact with other components that might 
be mounted on the substrate. Advantageously, the covering 24 may be 
provided over the entire exposed surface of the fuse, as is illustrated in 
FIGS. 1 and 2. 
The process steps involved in the manufacture of a typical heat actuated 
fuse in accordance with the invention is illustrated sequentially in FIGS. 
3a-3h and described in the following corresponding paragraphs a-h: 
a. A layer of palladium silver, thick film ink is screen printed on a 96% 
(by weight) alumina substrate 10 and fired to form each of the electrodes 
12, 14 having a thickness of about 12 to 25 microns. 
b. A layer of 96.5Sn/3.5Ag solder paste is screen printed over each of the 
electrodes and then heated to its melting temperature by infrared or other 
available means to form solder layers 16, 18 having a thickness of about 
75 to 150 microns. After these layers are formed, any flux residue is 
cleaned away by washing the substrate with a suitable solvent. 
c. Layers 30 and 32 of solder resist material are screen printed over the 
respective layers 16, 18, except for substantial areas of the tab portions 
16a, 18a. The resist material will prevent all but these areas of the tab 
portions of layers 16, 18 from taking solder in the next step. 
d. A layer 20' of solder paste is screen printed over the areas of the tab 
portions 16a, 18a which are not covered with the solder resist material 
and over the adjacent area of the substrate. Type 62Sn/36Pb/2Ag solder 
paste worked well in a practical embodiment of the fuse. The layer 20' is 
printed to a height and volume which effects formation of a bridge between 
the tab portions. 
e. Layer 20' is then heated by infrared or other means to a temperature 
which is above the melting point of layer 20' but below the melting point 
of solder layers 16,18. After melting, layer 20' flows to form the 
globular solder link 20. 
f. The layers 16, 18 are uncovered by removing the solder resist material 
printed in step c. The solder resist material is removed by cleaning. For 
example, the substrate can be washed with a suitable solvent (such as 
water) for the type of resist material used. Any flux residue, from the 
melting of layer 20' in step e, is cleaned from the substrate and the 
solder link 20 by this rinsing step, or by an additional rinsing step if a 
different solvent is required. 
g. One or more drops of flux material 22 is deposited by a dispenser on the 
solder fuse link 20. The amount of flux material should be sufficient to 
contact a substantial area of the link, and may cover the entire fuse 
link. 
h. The protective layer 24 of latex or silicon rubber is deposited by a 
dispenser over the entire fuse and the surrounding surface area of the 
substrate 10 to a thickness of about 0.005 to 0.010 in. 
FIG. 4 illustrates an alternative embodiment of a heat actuated fuse 
constructed in accordance with the invention. This embodiment is 
substantially identical to the embodiment of FIG. 1, except that the 
electrodes 12, 14 and the overlying solder layers 16, 18 do not include 
tab portions. Rather, the solder link 20 bridging the gap between the 
electrodes is disposed on inwardly extending portions of the electrodes. A 
fuse of this type can be made to occupy less space than that of the FIG. 1 
embodiment. However, less space will also be available for holding the 
melted solder link after actuation of the fuse. 
FIGS. 5 and 6 illustrate a practical circuit protection apparatus in 
accordance with the invention. This apparatus comprises a circuit board 
including the substrate 10 supporting a heat actuated fuse F1 constructed 
in accordance with the invention, a thick film resistor R1, and conductive 
tracks 26, 28 and 30. In this illustrated embodiment, the substrate should 
be of a material having a substantial thermal conductivity, such as 
alumina. The resistor and conductive tracks are deposited on the substrate 
in accordance with conventional printed circuit fabrication techniques. 
Only a portion of the circuit board is illustrated in FIG. 5, and 
additional electrical elements such as circuit components and/or 
electrical terminals may be disposed on the remainder of the board located 
beyond the illustrated broken edge 32. 
Conductive track 26 begins at an end thereof which is formed into an 
electrical terminal T1, and forms a conductive path connecting this 
terminal to one end of resistor R1 and to electrode 12 of fuse Fl. Track 
26 also forms a conductive path leading onto the non-illustrated portion 
of the circuit board for connection to one or more of the electrical 
elements disposed thereon. Similarly, conductive tracks 28 and 30 begin at 
respective ends thereof, forming terminal T2 and T3, and form conductive 
paths connecting these terminals to an opposite end of resistor R1 and to 
electrode 14 of fuse Fl. 
Manufacturing processes for connecting the conductive tracks 26 and 28 to 
respective ends of the resistor R1 are well known in the art. With regard 
to the process for connecting the conductive tracks 26 and 30 to the 
respective electrodes 12 and 14 of the fuse F1, this may be done 
conveniently by simultaneously forming these electrodes as integral 
extensions of the respective tracks. This is illustrated in FIG. 5, with 
solder layers 16, 18 being disposed on these integral extensions which 
form the electrodes 12, 14. Alternatively, these tracks and electrodes may 
be conductively joined by means such as a conductive epoxy, a wire 
connection, or by overlapping one conductive layer onto another. 
By appropriately connecting the conductive track 26 and/or the terminals 
T1, T2 and/or T3 to other electrical elements, the protective circuit 
apparatus can be utilized in a variety of circuit configurations. For 
example, in a practical embodiment which is illustrated schematically in 
FIG. 6, terminal T1 is connected to circuitry located off the circuit 
board, conductive track 26 is connected to circuitry located on the 
circuit board, terminal T2 is connected to ground, and terminal T3 is 
connected to an electrical power source 34 such as a low voltage telephone 
line. Both the off-board circuitry and the on-board circuitry receive 
their electrical energy through fuse F1 from power source 34, and thus 
both will be protected against damage caused by overvoltage conditions 
which might occur at the power source. 
In the illustrated embodiment of FIG. 6, the temperature of resistor R1 is 
determined primarily by the voltage applied across terminals T2 and T3 by 
the power source 34. The resistance of resistor R1 and its placement on 
the substrate 10 relative to heat actuated fuse F1 are predetermined to 
effect heating of the fuse to its actuation temperature at a specified 
power source overvoltage condition. If the fuse is actuated, the 
electrical connection of the off-board and on-board circuitry to terminal 
T3 will be permanently opened, but this circuitry can be reconnected to a 
power source by means of terminal T1 through a substitute protective 
apparatus. 
A variety of other embodiments of the protective circuit apparatus in 
accordance with the invention are also possible. For example, a plurality 
of the series connected resistor/fuse combinations illustrated in FIG. 5 
may be disposed on a single substrate, with similar or dissimilar terminal 
and conductive track arrangements. Alternatively, the substrate may bear 
only one of such combinations, with no additional circuit elements being 
disposed on the substrate. As another alternative, the current passed 
through resistor R1 may be controlled by an independent circuit or sensory 
device, such as a circuit or device for sensing a critical physical or 
electrical characteristic of a protected circuit, motor etc.