Semiconductor device having a thermal resistance detector in the heat radiating path

In a semiconductor device comprising a semiconductor chip on which semiconductor elements are formed, the semiconductor device further comprises a thermal resistance detector for detecting an increase of thermal resistance of a heat radiating path which is provided to radiate the heat generated in the semiconductor device during operation, and a thermal resistance detection result output circuit for outputting a result of a detection by the thermal resistance detector to an output of the semiconductor device. The semiconductor device can detect at the early stage the increase of the thermal resistance of the heat radiating path, and the deterioration of the semiconductor device due to the crack in the solder layer bonding the chip mounting insulation substrate and heat sink during the operation of the device.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a semiconductor device, particularly to a power 
semiconductor element such as an IGBT (Insulated Gate Bipolar Transistor), 
in which an insulation substrate holding a semiconductor chip and a heat 
sink are soldered each other to improve the efficiency of radiating the 
heat generated in the power semiconductor element. 
2. Description of the Related Art 
FIG. 7 is a sectional view of the conventional power semiconductor device. 
As shown in FIG. 7, the device comprises a semiconductor chip 11 
incorporating power semiconductor elements (not shown), a chip holding 
insulation substrate 12 having high thermal conductivity, a heat sink 13 
formed of, for example, Cu-based material, and a solder layer 14 bonding 
the substrate 12 and the heat sink 13 together. 
The chip holding insulation substrate 12 comprises an insulating plate 15 
made of material such as ceramics or the like. Metal patterns 16 made of 
metal such as Cu are formed on the chip mounting surface of the insulating 
plate 15. A metal layer 17 made of a material such as Cu is formed on the 
back surface of the insulating plate 15. The metal patterns 16 are used as 
chip mounting patterns and terminal connecting patterns. 
The chip 11 is bonded on the chip mounting pattern 16 by a first solder 
layer 18. The terminal connecting patterns 16 are bonded to pad sections 
(not shown) of the chip 11 by bonding wires 19, and the metal layer 17 is 
bonded to the heat sink 13 by a second solder layer 14. 
Shown in FIG. 7 are main terminals 101 of the power system and signal 
processing system. The main terminals are connected to the patterns 16 
with solder. 
The chip holding insulation substrate 12 having high thermal conductivity 
is interposed between the power semiconductor chip 11 and the heat sink 
13. The power semiconductor chip 11 and the chip holding insulation 
substrate 12 are bonded by the first solder layer 18. The chip holding 
insulation substrate 12 and the heat sink 13 are bonded by the second 
solder layer 14. By virtue of this structure, the heat generated in the 
chip 11 can be radiated efficiently, without adversely affecting the 
insulation of the power semiconductor chip 11. 
Incidentally, the power semiconductor device generates a large amount of 
heat. Therefore, the thermal resistance of the heat radiating path 
provided to radiate the heat generated in the device during the operation 
of the device is increased. Consequently, the device may have some 
deficiency such as the increase of the heat loss or the thermal 
destruction. 
In the structure in which the chip holding insulation substrate 12 having 
the ceramic insulating plate 15 bonded on its surfaces to the Cu films 16 
and 17 is soldered to the heat sink 13 made of Cu-based material, 
expansion and contraction are repeated through the temperature cycles due 
to the difference in thermal expansion coefficient of the chip holding 
insulation substrate 12 and the heat sink 13. As the result, the second 
solder layer 14 bonding the substrate 12 and the heat sink 13 together in 
the heat radiating path is fatigued, and the solder layer 14 eventually 
has cracks which will weaken the solder layer 14. When the solder layer 14 
has the cracks, its thermal resistance will increase. This results in an 
increase of the thermal resistance of the entire heat radiating path, and 
efficient heat radiation can no longer be possible. The heat rapidly 
accumulates in the semiconductor device having such a deficiency, 
resulting in the thermal destruction. 
As described above, the conventional power semiconductor device having the 
chip mounting insulation substrate soldered to the heat sink, may have 
some deficiency due to the increase in the thermal resistance of the heat 
radiating path for radiating the heat generated in the device during the 
operation of the device. 
SUMMARY OF THE INVENTION 
The present invention intends to solve the above-mentioned problem and has 
its object to provide a power semiconductor device in which the increase 
of the thermal resistance of the heat radiating path which is provided to 
radiate the heat generated in the device is detected, thereby detecting 
the deterioration of the semiconductor device due to the crack in the 
solder layer which bonds the chip mounting insulation substrate and the 
heat sink, during the operation of the device. 
A semiconductor device according to an aspect of the present invention 
comprises: 
a semiconductor chip on which semiconductor elements are formed; 
a heat sink; 
an insulation substrate having high thermal conductivity, being provided 
between the semiconductor chip and the head sink so as to hold the 
semiconductor chip; 
a solder layer for bonding the insulation substrate and the head sink; 
thermal resistance detection means for detecting an increase of thermal 
resistance of a heat radiating path which is provided to radiate the heat 
generated in the semiconductor device during operation; and 
thermal resistance detection result output means for outputting a result of 
a detection by the thermal resistance detection means to an output of the 
semiconductor device. 
The thermal resistance detection means may determine on the basis of 
temperature detection output of one or a plurality of temperature 
detection element/elements arranged at least on a periphery of the 
insulation substrate that the thermal resistance in the heat radiating 
path is higher than a reference value. 
The thermal resistance detection means may determine that the thermal 
resistance in the heat radiating path is higher than a reference value on 
the basis of an electric characteristics output depending on a thermal 
resistance of one or a plurality of temperature detection element/elements 
arranged at least on a periphery of the insulation substrate. 
The thermal resistance detection means may determine that the thermal 
resistance in the heat radiating path is higher than a reference value by 
use of temperature detection element/elements arranged on a periphery of 
the semiconductor chip. 
A semiconductor device according to another aspect of the present invention 
comprises: 
a semiconductor chip on which semiconductor elements are formed; 
an insulation substrate having an element mounting surface on which a metal 
pattern is formed, and a back surface on which a metal layer is formed; 
a first solder layer for bonding the insulation substrate to the 
semiconductor chip in a state where the semiconductor chip is mounted on 
the metal pattern for mounting the chip; 
bonding wires provided between the metal pattern on the insulation 
substrate for mounting the semiconductor chip and a pad section of the 
semiconductor chip; 
a heat sink; 
a second solder layer for bonding the metal on the back surface of the 
insulation substrate to the heat sink in a state where the metal on the 
back surface of the insulation substrate contacts on the heat sink; 
thermal resistance detection means for detecting an increase of thermal 
resistance of a heat radiating path which is provided to radiate the heat 
generated in the semiconductor device during operation; and 
thermal resistance detection result output means for outputting a result of 
a detection by the thermal resistance detection means to an outside of the 
semiconductor chip. 
Additional objects and advantages of the invention will be set forth in the 
description which follows, and in part will be obvious from the 
description, or may be learned by practice of the invention. The objects 
and advantages of the invention may be realized and obtained by means of 
the instrumentalities and combinations particularly pointed out in the 
appended claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
This invention can be more fully understood from the following detailed 
description when taken in conjunction with the accompanying drawings. 
FIG. 1 shows a section of the power semiconductor device according to a 
first embodiment of the present invention. 
In the power semiconductor device shown in FIG. 1, a chip holding 
insulation substrate 12 with high thermal conductivity is provided between 
a semiconductor chip 11 on which the power semiconductor elements (not 
shown) are formed and a heat sink 13 formed of Cu-based material, for 
example. The semiconductor chip 11 and the chip holding insulation 
substrate 12 are bonded by a first solder layer 18, and the chip holding 
insulation substrate 12 and a heat sink 13 are bonded by a second solder 
layer 14. 
The chip holding insulation substrate 12 comprises an insulating plate 15 
made of material such as ceramics. Metal patterns 16 which are formed of 
metal such as Cu are located on a chip mounting surface of the insulating 
plate 15. A metal layer 17 formed of a material such as Cu is located on a 
back surface of the insulating plate 15. 
The chip 11 is bonded on the chip mounting pattern 16 by a first solder 
layer 18. The terminal connecting patterns 16, which are connected to main 
terminals 101, are bonded to pad sections (not shown) of the chip 11 by 
bonding wires 19, and a metal layer 17 is bonded to the heat sink by a 
second solder layer 14. 
In this figure, 101 denotes main terminals of the power system and the 
signal processing system. The main terminals are bonded by solder to the 
terminal connecting patterns 16 for connecting the main terminals 101. 
To sum up, the power semiconductor device shown in FIG. 1 comprises: the 
semiconductor chip 11 on which the semiconductor elements (not shown) are 
formed; the chip holding insulation substrate 12 having metal patterns 16 
on the element mounting surface of the insulating plate 15 and a metal 
layer 17 formed on the back surface of the insulating plate 15; the solder 
layer 18 bonding the chip mounting pattern 16 on the chip holding 
insulation substrate 12 to the semiconductor chip 11; bonding wires 19 
connected between the metal patterns 16 and the pad sections (not shown) 
of the semiconductor chip 11; the heat sink 13; and the second solder 
layer 14 bonding the heat sink 13 to the metal layer 17 formed on the back 
surface of the insulating layer 15. 
The semiconductor device of the present invention further comprises the 
thermal resistance detection means for detecting an increase of the 
thermal resistance of the heat radiating path which is provided to radiate 
the heat generated in the semiconductor device during operation, and the 
thermal resistance result detection means for outputting a result of a 
detection by the thermal resistance detection means to an output of the 
semiconductor device. 
It has been reported by Wuchen Wu, Marcel Held, et al. in "Thermal Stress 
Related Packaging Failure in Power IGBT Modules", Proceedings of 1995 
International Symposium on Power Semiconductor Devices & ICs, Yokohama, 
1995, May-24, pp.330-334, for example, that the deterioration of the 
solder layer due to the crack in the solder layer bonding the insulation 
substrate and the heat sink occurs at the periphery of the solder layer at 
first. 
The examples of the states of the second solder layer 14 formed between the 
insulation substrate 12 and the heat sink 13 before and after the 
temperature cycles are shown in FIGS. 3A and 3B. FIG. 3B shows that the 
void region 14a (the region deteriorated due to the occurrence of the 
crack and the like) exists on the periphery of the second solder layer 14 
after the temperature cycles. 
FIG. 2 is a schematic plan view of an example of the pattern on the 
insulation substrate shown in FIG. 1. 
The thermal resistance detection means can be constituted, as follows: 
In one example as shown in FIGS. 1 and 2, in order to detect that the 
thermal resistance exceeds a predetermined value, one or a plurality of 
temperature sensing element/elements 21 such as thermistor and diode are 
arranged on at least the periphery of the insulation substrate 12. In this 
example, if a crack 14a has occurred on the periphery of the second solder 
layer 14 as shown in FIG. 3B, the thermal radiating efficiency will 
decrease and the temperature of the heat radiating path increases. The 
increase of the thermal resistance of the heat radiating path is detected 
by the temperature detection element 21 provided on the periphery of the 
insulation substrate 12. 
When one temperature detection element 21 is provided only on the periphery 
of the insulation substrate 12 as indicated by the solid line in FIG. 2, 
the output from the temperature detection element 21 is compared with a 
reference value by a comparator (not shown), and when the detected value 
exceeds the reference value, it is determined that the thermal resistance 
of the heat radiating path is higher than the reference value. 
As shown by a broken line in FIG. 2, when at least one temperature 
detection element 22 is provided to the portion other than the periphery 
of the insulation substrate 12 (e.g. the central portion of the insulation 
substrate 12) in addition to the one temperature detection element 21 
provided on the periphery of the insulation substrate 12, the output of 
the temperature detection element 22 is used as a reference value. More 
specifically, the detection element 21 arranged on the periphery of the 
insulation substrate 12 is compared with the output of the temperature 
detection element 22 arranged on the portion other than the periphery of 
the insulation substrate 12 (i.e., compared with the reference value), and 
when the output of the temperature detection element 21 exceeds the output 
of the temperature detection element 22, it is determined that the thermal 
resistance of the heat radiating path is higher than a predetermined 
value. As should be clear from this, the detection output of the 
temperature detection element 22 is used as a reference value for 
determination. 
As shown in FIG. 4, one or a plurality of thermal resistance detection 
element/elements 31 may be provided as the thermal resistance detection 
means to at least the periphery of the insulation substrate 12. Then, it 
is determined on the basis of the electric characteristics output of the 
thermal resistance detection element/elements 31, which depends on the 
thermal resistance thereof, that the thermal resistance of the heat 
radiating path is higher than a predetermined value. In this case, a 
temperature detection element such as a diode, which has the 
temperature-depending characteristics, is used as the thermal resistance 
detection element/elements 31. In addition, a thermal resistance measuring 
IC 30 is used, which circuit applies to the temperature detection circuit 
a plurality of pulses until the temperature detection circuit becomes 
stable in temperature. The measuring circuit 30 compares the forward bias 
voltage drop of the diode after applying the first pulse with the forward 
bias voltage drop of the diode after applying the last pulse, to detect 
the increase of the thermal resistance. 
As indicated by a solid line in FIG. 4, when one thermal resistance 
measuring IC 30 and one thermal resistance detection element 31 are 
arranged on the periphery of the insulation substrate 12, the increase of 
the heat radiating path is detected by comparing the thermal resistance 
obtained by the thermal resistance measuring IC 30 on the basis of the 
detection output of the thermal resistance detection element 31 with a 
reference value. 
Further, as indicated by a broken line in FIG. 4, when at least one thermal 
resistance detection element 32 and at least one thermal resistance 
measuring IC 33 are arranged in the portion other than the periphery of 
the insulation substrate 12 (the central portion of the insulation 
substrate 12, for example) in addition to the thermal resistance detection 
element 31 and the thermal resistance measuring IC 30 which are arranged 
on the periphery of the insulation substrate 12, the thermal resistance 
detected on the basis of the output of the thermal resistance detection 
element 32 is used as a reference value for determination. More 
specifically, by comparing the thermal resistance obtained on the basis of 
the output of one thermal resistance detection element 31 arranged on the 
periphery of the insulation substrate 12 with the reference value (i.e., 
the thermal resistance obtained on the basis of the output of one thermal 
resistance detection element 32 arranged on the portion other than the 
periphery of the insulation substrate 12), the increase of the thermal 
resistance of the heat radiating path is detected. 
As still another example of the thermal resistance detection means, FIG. 5 
shows that at least one temperature detection/thermal resistance detection 
element 51 is arranged on the periphery of the semiconductor chip 11. In 
addition, a thermal resistance measuring IC (not shown) may be provided on 
the semiconductor chip. The element 51 or the measuring circuit determines 
that the thermal resistance of the heat radiating path is higher than a 
predetermined value on the basis of the output of the element 51. 
If in this case crack has occurred on the periphery of the first solder 
layer 18, the heat radiating efficiency of the heat radiating path will 
decrease, and the temperature of the heat radiating path will increase. As 
a result, the increase of the thermal resistance of the heat radiating 
path can be detected. 
While, as an example of the thermal resistance detection result output 
means of the above-mentioned embodiment, a combination of a bonding wire 
19 and a detecting terminal 102 is shown in FIG. 1. The bonding wire 19 
connects the detection element 21 (or the detection element 31/32 or the 
measuring IC 30/33, as shown in FIG. 4) to the metal pattern 16 for 
connecting the detecting terminal 102. The detection result is outputted 
from the detection element (or the measuring IC) to the outside of the 
semiconductor device through the metal pattern 16 connected with the 
terminal 102. 
FIG. 6 shows a cross-section of a semiconductor device according to a 
second embodiment of the present invention. In this embodiment, the 
detection of the thermal resistance may be attained in such a manner that 
the alarm signal (such as audio signal and visual signal) is generated by 
properly processing the output of the detection element 21 or 31, and 
outputted to the alarm apparatus 103 (such as a light emitting diode) 
located on the outside of the semiconductor device through the detecting 
terminal 102. 
Consequently, with the above-mentioned semiconductor device, the increase 
of the thermal resistance of the heat radiating path which is provided to 
radiate the heat generated in a power semiconductor device, and the 
deterioration of the semiconductor device due to the crack in the solder 
layer which bonds the chip mounting insulation substrate and the heat sink 
during the operation of the device, can be detected and outputted to the 
outside of the device. Therefore, according to the present invention, the 
user can take a suitable action such as exchange before the thermal 
resistance of the entire power semiconductor device decreases. 
The semiconductor device having various types of protection functions has 
been eagerly required. In recent years, this demand of the users increases 
more and more. The present invention can satisfy this demand of the users, 
and realizes the power semiconductor device with high reliability. 
As described above, the semiconductor device of the present invention can 
detect at the early stage the increase of the thermal resistance of the 
heat radiating path which is provided to radiate the heat generated in a 
power semiconductor device, and the deterioration of the semiconductor 
device due to the crack in the solder layer bonding the chip mounting 
insulation substrate and the heat sink during the operation of the device. 
Additional advantages and modifications will readily occur to those skilled 
in the art. Therefore, the invention in its broader aspects is not limited 
to the specific details and representative embodiments shown and described 
herein. Accordingly, various modifications may be made without departing 
from the spirit or scope of the general inventive concept as defined by 
the appended claims and their equivalents.