Semiconductor device including a thermal fuse encapsulated in a droplet of silicone rubber

A semiconductor device of this invention comprises a semiconductor chip including a high power consumption portion, an internal lead wire connecting an electrode portion of the semiconductor chip with an external terminal, a soft material adhered to a substantially middle portion of the lead wire, and a moulding material for integrally moulding the semiconductor chip, lead wire, and soft material. Further, the semiconductor device includes a thermal fuse to prevent flaming of the device by cutting off the lead wire within the soft material in case the device is subjected to overcurrent.

This invention relates to a semiconductor device including a thermal fuse 
for preventing flaming hazards peculiar to moulding type semiconductor 
devices. 
In semiconductor devices including a high power consumption portion, such 
as discrete power transistors and integrated circuits or large scale 
integrated circuits for power amplifier, active and passive elements of a 
semiconductor chip are separately connected with external terminals by 
means of internal lead wires within the package. In this type of 
semiconductor device the semiconductor chip is mounted on a metallic 
mounting member, a part of which may be used for the internal lead wire. 
The general methods for integrating the external terminals, internal lead 
wires, and semiconductor chip include a method in which they are enclosed 
by a metal can and a method in which they are enclosed by resin moulding. 
The semiconductor device with the metal can type package, however, costs 
higher as compared with one employing the resin moulding type package, so 
that the latter is more often used. The semiconductor chip used with such 
semiconductor device may be prepared by the conventional semiconductor 
manufacturing technique. 
Referring now to FIG. 1, there will be described an example of the 
construction of the prior art moulding type semiconductor device. On a 
mounting member 10 is mounted a semiconductor chip 12 including a high 
power consumption portion. Fixed electrode portions on the chip 12 are 
separately connected with external terminals 16 by means of internal lead 
wires 14. The chip 12 is covered with a soft resin 18 for preventing the 
degeneration of the chip affected by the moulding resin and the like. 
Thereafter, the member 10, chip 12, wire 14, terminal 16 and resin 18 are 
integrally moulded by a moulding resin 20. The aforementioned moulding 
structure is applicable to any of the discrete transistors and integrated 
circuits or large scale integrated circuits. However, the coefficient of 
thermal expansion of the wire 14 is substantially different from that of 
the resin 18. Therefore, if the semiconductor device is subjected to 
thermal cycles accompanying the repeated start and stop of its operation, 
bonding between an electrode portion of the chip 12 and the wire 14 will 
often be broken by the stress caused due to the different coefficients of 
thermal expansion and contraction. 
In order to avoid the aforesaid contact failure, there is proposed a 
semiconductor device with construction such as that shown in FIG. 2. In 
this construction the resin 18 may be omitted by making use of such a 
resin that can mould the member 10, chip 12, wire 14 and terminal 16 while 
maintaining the chemical stability of the surface of the semiconductor 
chip 12. Thus, there may be avoided the above-mentioned contact failure 
attributable to the difference between the coefficients of thermal 
expansion of the wire 14 and resin 18. Either of the constructions shown 
in FIGS. 1 and 2, however, are subject to the following significant 
shortcoming. In those semiconductor devices which include a portion to 
deal with a power consumption of approximately 1 watt or higher, if any 
abnormal operating condition (e.g., a condition with ordinary power supply 
in which an input signal is applied from the output terminal of a short 
circuited power amplifier circuit) is brought about, then a large current 
will flow from the power circuit into the semiconductor chip 12. This 
large current, i.e., overcurrent, extraordinarily overheats the chip 12 
and wire 14 inside the semiconductor device. The moulding resin 20 would 
be carbonized by the heat produced at that time, and at last caused to 
flame. Such flaming trouble is quite hazardous, damaging other electronic 
components used with the semiconductor device or leading to a fire, 
depending on the circumstances. 
In order to analyze the flaming phenomena as mentioned above, the inventor 
has obtained the following test results. The inventor employed as samples 
power amplifiers with a moulding type power transistor with a collector 
dissipation of 3-watt class and moulding type integrated circuits for 
audio power amplifiers with the nominal output power at 3.5 watts. The 
output circuit and the ground circuit were short-circuited, and there was 
conducted a trial examination with 300 samples corresponding to respective 
supply voltages ranging from 12.5 Vdc to 17.5 Vdc. Approximately 2 sec 
after the application of the short-circuit, the semiconductor devices 
produced flames 40 mm to 50 mm high, and the flaming continued for a few 
seconds after the samples were cut off from the power supply. As a result 
of the analytical investigation of the samples after the trial 
examination, it was found that the moulding resin at portions in contact 
with the internal lead wire was carbonized. From this the inventor 
concluded that if the internal lead wire was cut off by an overcurrent, a 
current would flow through the carbonized portion to allow such portion to 
function just as a heater, thereby encouraging the flaming phenomenon. 
On the other hand, if any mechanical strain stress remains in the moulding 
resin after moulding, no flaming trouble may be caused for any flow of 
overcurrent. Instead of the occurrence of such flaming trouble, a thermal 
expansion stress yielded by the production of heat produced by the 
overcurrent might combine with the remaining strain stress to break and 
disperse the moulding resin. Though the troubles attributable to such 
breakage is insignificant when compared with the flaming hazards, they are 
still harmful because any broken pieces of the resin scattered about would 
possibly damage the peripheral electronic components. 
As described above, the semiconductor devices of the prior art construction 
as shown in FIGS. 1 and 2 are provided with no measure to counter those 
hindrances attributable to the overheating with the flow of overcurrent. 
These prior devices therefor have been subject to serious troubles, such 
as flaming. 
An object of this invention is to provide a semiconductor device including 
a thermal fuse capable of securely preventing at low cost the package of 
the device from flaming or scattering due to any troubles, such as 
overheating due to an overcurrent, caused in the semiconductor device. 
In order to attain the above object, the device of the invention is 
composed as follows. The invention comprises at least one semiconductor 
chip including a high power consumption portion, at least one thin lead 
wire connecting an external terminal with the semiconductor chip, the lead 
wire being possibly subjected to a large current, an electrically 
insulating soft material being adhered to at least one portion of the lead 
wire between the semiconductor chip and the external terminal, and an 
electrically insulating moulding material for moulding the semiconductor 
chip, lead wire, and soft material, the moulding material having a higher 
coefficient of thermal expansion than that of the soft material. In such 
construction, a thermal fuse is formed as a combination of the moulding 
resin, the soft material contained in the resin, and a portion of the thin 
lead wire encased in the soft material. Thus, the package of the device 
may securely be prevented from flaming or scattering due to overheating 
caused by overcurrent, and may be made at low cost because of the simple 
construction.

Referring now to FIGS. 3 to 5, there will be described in detail an 
embodiment of this invention. Those parts which are used in common with 
the prior art devices will be denoted by the same numerals. In FIG. 3, a 
semiconductor chip 12 with a transistor structure or electrical circuit 
structure formed thereon is mounted on a mounting member 10 by means of 
solder or a mounting means composed of conductive epoxy cement. 
predetermined electrode portions 22 on the semiconductor chip 12 are 
connected with external terminals 16 by means of internal thin lead wires 
24 respectively. Each electrode portion 22 is an aluminium-evaporated 
layer ohmic-contacted with a predetermined portion on the semiconductor 
chip 12. For the lead wires 24 ther may be used aurum (gold) or aluminium 
thin wires, or metal thin wires covered with aurum or aluminium, usually 
having a diameter of 25 .mu.m to 100 .mu.m. Each wire 24 is bonded and 
electrically connected with the aluminium layer of each electrode portion 
22 and with a prescribed portion of each external terminal 16. The 
prescribed portion of the external terminal 16 corresponds to the free end 
portion of a connecting lead 32 as described hereinafter. 
The wires 24 are each usually connected in a bent or curved state as shown 
in FIG. 3, though it may be connected in a substantially rectilinear shape 
depending upon the mechanism of the bonding tool. Then, a soft material 26 
is adhered to a substantially middle portion of the wire 24, so 
constructed as described above. The material 26 encases the middle portion 
of the wire 24. This material 26 is to be in compliance with the following 
requirements. It is required that the material 26 has a relatively small 
coefficient of thermal expansion and an electrically insulating, 
non-carbonizing property. The material should neither prevent the 
expansion and contraction of the wire 24 encased by the material 26, i.e., 
the conglobation of the fused portion of the wire 24 by its surface 
tension when the wire 24 is fused, nor give off any substances adversely 
to affect the semiconductor chip 12 during the normal operation. The 
materials to comply with these requirements include, besides silicone 
rubber, two-liquid mixture type rubber (e.g. RTV-11 from GE, USA) and 
varnishes. The silicon rubber should preferably be used because of its 
easiness to handle. Since this silicon rubber is liquid before it is 
solidified, the adhesion of the silicon rubber or the soft material 26 to 
the thin lead wire 24 may be achieved by forming a droplet of the liquid 
silicon rubber on the tip of an injector needle and dripping the droplet 
onto a substantially middle portion of the wire 24 by means of an 
injector, for example. 
Subsequently, the member 10, chip 12, wires 24, terminals 16, and materials 
26 are moulded by the moulding resin 20. This resin 20 is to be in 
compliance with the following requirements. It is required to be formed of 
material that has a relatively large coefficient of thermal expansion and 
electrically insulating property, will never give off any substances 
adversely to affect the semiconductor chip 12 during the normal operation, 
and is suitable for moulding. The materials found to comply with these 
requirements include, for example, epoxy resin and silicon resin. 
The mounting member 10, which is a metallic material with such structure as 
shown in FIG. 4, for example, before the semiconductor device is 
completed, is composed of a thick mounting portion 28 having a function to 
transmit the heat produced from the semiconductor chip 12 to the exterior, 
a plurality of parallel comblike connecting leads 32 each having one end 
connected at a right angle to a frame 30. One lead 32 is connected to the 
portion 28, and the leads 32 are left as free ends. The transverse 
connecting line 34 electrically and mechanically connects the ends of the 
connecting leads 32 i.e., opposite to the frame 30. Each of the connecting 
leads 32 is separated from the transverse connecting line 34 and the frame 
30 after being moulded by the moulding resin 20, thereby forming the 
external terminals 16 as shown in FIG. 3. 
In the aforementioned construction the combination of the moulding resin 
20, soft materials 26 encasing a portion of the resin 20, and the thin 
lead wires 24 encased in the materials 26 forms the thermal fuses for the 
semiconductor device. 
FIG. 5 shows schematically an example of the state of the thin lead wire 24 
of aurum cut off inside the soft material 26 by overheating due to the 
overcurrent. According to the results of the trial examination, the 
inventor noticed that each tip end portion of the lead wire 24, as shown 
in FIG. 5, had been cut off and formed into a ball with a diameter 
somewhat larger than that of the lead wire 24. That is, the function as a 
thermal fuse is achieved inside the soft material 26. 
Thus, one preferred embodiment of the semiconductor device according to the 
invention may be accomplished. 
The aforementioned preferred construction resulted from ideas obtained 
according to the results of the various experiments conducted by the 
inventor in order to prevent the flaming troubles of the moulding type 
semiconductor device. Namely, the inventor found that the moulding resin 
around the internal lead wire carbonized to form a conductive path if the 
lead wire was fused by overheating due to a fixed current level or 
overcurrent. Accordingly, the inventor assumed that the conductive path 
should only be prevented from being formed by the carbonization. Then, the 
inventor believed that a porous moulding resin should be employed in order 
to prevent the formation of the conductive path due to the carbonization. 
However, the semiconductor chip must be completely enclosed against the 
open air to avoid adverse effects of the surroundings. Therefore, when 
using a porous moulding resin, it should be enclosed by other high-density 
material. To enclose the porous resin with the high-density material, 
however, is troublesome and not pragmatically feasible because it would 
reduce the distinctive feature of the method for manufacturing 
semiconductor devices according to the moulding method which has an 
advantage in mass-production at low cost. 
Thereupon, in order to attain the aforesaid object, the inventor 
constructed the semiconductor device as follows. A droplet of the 
electrically insulating soft material with a small coefficient of thermal 
expansion is adhered to a substantially middle portion of each thin lead 
wire between each electrode portion of the semiconductor chip and the 
external terminal. Theoretically, the adhering position of the droplet may 
be located at any portion on the lead wire excepting the bonding parts of 
the external terminal and electrode portion. In order fully to give the 
lead wire the thermal stretch stress attributable to overheating, the 
adhering position of the droplet should preferably be located at the 
middle portion of the lead wire. Next, the semiconductor chip, one portion 
of the external terminals, the thin lead wires, and the soft materials are 
moulded by the moulding resin with a relatively large coefficient of 
thermal expansion. According to such construction, the flaming troubles 
and the dispersion of the package may securely be prevented if the 
semiconductor device is subjected to an overcurrent and the semiconductor 
chip and thin lead wire are overheated. 
The theory of operation is as follows. The coefficient of thermal expansion 
of the soft material is smaller than that of the moulding resin, so that 
the soft material is compressed by the surrounding moulding resin when the 
semiconductor device is heated. The resultant compression of the soft 
material applies a stretch stress to the lead wire portion disposed inside 
the soft resin. Then, the thin lead wire, which has been softened or 
melted by the heating due to the overcurrent, is fused inside the soft 
material by the stretch stress produced inside the soft material before 
the moulding resin is carbonized, before flames develop, and before the 
moulding resin is broken and dispersed. On that occasion, the cut ends of 
inside the soft material are conglobated by the surface tension at fusion. 
Meanwhile, the moulding resin in contact with the thin lead wire at 
portions other than the soft material is carbonized by the overheating of 
the thin lead wire to form a carbonized layer around the thin lead wire. 
The carbonized layer is compressed by the thermal-expanded moulding resin 
and formed into a tubular electrical conductive layer. Since the soft 
material has an electrically insulating property, however, the 
semiconductor device will completely be cut off from the power supply at a 
point of time when the thin lead wire is cut off by the overheating, thus 
prohibiting further overheating. 
The semiconductor device according to the invention includes the thermal 
fuse with the aforesaid construction, so that it may securely prevent the 
above-mentioned flaming hazards and dispersion of the package. Further, as 
the components additionally required for the effectuation of the invention 
include the soft material alone, the manufacturing cost may be 
substantially the same as that of the prior art moulding type 
semiconductor device. 
As regards the flaming troubles, the inventor conducted the following trial 
examination. With 300 samples prepared for the respective test and with 
supply voltages ranging from 12.5 Vdc to 17.5 Vdc, the semiconductor 
devices with the constructions as shown in FIGS. 1 and 2 were subjected to 
an examination under fixed conditions. As a result, the incidence of 
flaming of these devices proved to be 1% and 70% respectively. Although 
the incidence of flaming of the construction of FIG. 1 at 1% may seem to 
be rather low, it still indicates the possibility of the flaming hazards 
which will lead to fires and other serious accidents, exhibiting quite a 
significant defect. On the other hand, according to the results of a trial 
examination conducted on the construction according to the invention as 
shown in FIG. 3 under the same conditions as aforesaid, the incidence of 
flaming was revealed to be 0%. 
Variations and modifications may be made within the scope of this invention 
and portions of the improvements may be used without others.