Electronic Circuit Package Including Plural Bare Chips Mounted on a Single Wiring Substrate

The present invention relates to the miniaturization of an electronic circuit package and more particularly to an extra-small computer for use in space exploration.

A number of computers are being employed for various uses, and there is an increased demand for smaller and lighter computers. Computers for use in space are required to be particularly smaller and lighter in order to decrease launching cost while increasing the payload.

As shown by a photograph 1 of "Development of LSI for Radiation Resistant 16-Bit Microprocessor", pp 10-411, Goke et al, Collection of Papers at 32nd Space Science and Technology Federation Lecture Meeting, a space computer is built of, for instance, discrete parts with reliable, resistant-to-environment single chips contained in one package.

There has been no serious consideration to decrease the size and weight in the computer as the computer comprises discrete parts.

On the other hand, a so-called multiple chip mounting technique, that is, the technique of mounting a plurality of bare chips on one wiring substrate for use on the ground, is being studied. It has heretofore been arranged that, as shown in FIG. 3 of "Nikkei Micro Device", pp 32-40, December Issue, 1989, a wiring conductor to be connected to a bonding pad is led out of the bonding pad.

It was not considered to make the wiring density uniform in this technique. The wiring density around the die bonding pad in particular is made extremely high and consequently effective wiring cannot be implemented. The wiring density in the outermost layer thus causes a bottleneck, and the package size is not sufficiently reduced. As the via hole connecting the upper and lower layers occupies most of the area on the particular multilayer wiring substrate, the via holes account for a large percentage of area on the outermost layer, particularly around the die bonding pad.

With respect to a fault tolerant system, a checking unit for detecting errors and faults and a unit under check are accommodated in one the same chip to reduce the size as described in "Trial Manufacture Evaluation of Fault Tolerant Quartz Oscillation IC", by Tsuchimura et al, Research Material, 24th FTC Study Meeting. With the diffusion of ASICs (Application Specified ICs) in particular, attempts have been made to add an MPU inspection circuit by making an ordinary MPU a core through the ASIC technology.

Faults and trouble affecting the whole chip were not taken into consideration in this technique. When the checking unit and the unit under check develop trouble simultaneously, the irregularity might not be detected.

An object of the present invention is to provide a small and light electronic apparatus, in particular an electronic circuit package, with high reliability.

Another object of the present invention is to provide a small and light electronic apparatus in which the number of pins affixed to the outside of the package is reduced to prevent an increase in package size.

Further, another object of the present invention is to provide a small and light electronic apparatus in which the concentration of the wiring density is reduced to prevent an increase of the package size.

According to the present invention, all semiconductor chips to be substantially connected by means of a bus line are bare chip packaged on a wiring substrate, and the semiconductor chips and the wiring substrate are connected by wire bonding between wire bonding pads formed on the semiconductor chips and the wiring substrate. The wiring substrate may be a multilayer element.

Preferably, there is an insulating layer formed on a portion of the surface of the multilayer wiring substrate, and a die bonding ground is formed on the surface of the insulating layer in order to use a portion of the multilayer wiring substrate under the die bonding ground as a wiring or a via hole region. At least one semiconductor chip is formed on the die bonding ground. In a preferred embodiment of the present invention, there are wiring conductors which extend from the wiring pads and via holes formed in the periphery and inside of the die bonding ground, and the wiring conductors are connected to other wiring conductors of a different wiring layer in the multilayer wiring substrate through the via hole.

Preferably, the bus line includes two data bus lines. The semiconductor chips connected with one data bus line are formed on one side of the wiring substrate and the semiconductor chips connected with the other data bus line are formed on the other side of the wiring substrate.

Preferably, the semiconductor chips include a checking unit and a unit under check.

Electronic apparatus as represented by computers typically include a plurality of semiconductor chips connected to a bus having a number of signal lines. With discrete parts constituting the apparatus conventionally, an enormous number of pins are required as the signal lines connected to the bus are used to couple the internal and external devices of a package. In the present invention, as all the semiconductor chips connected to the bus are formed into a single package, the number of signal lines connecting the inside and outside of the package is drastically reduced. In other words, the number of pins connecting the signal lines inside and outside the wiring substrate decreases. As a result, the electric apparatus become small and light.

Moreover, as each semiconductor chip is subjected to bare chip packaging, a fault of one chip does not influence the other chips.

As lead-out lines can be formed from wiring bonding pads and via holes even under the die bonding ground, according to the present invention, the wiring density in the neighborhood of the wire bonding pads is prevented from centering therearound. Therefore, the wiring density in each wiring layer is uniform to the extent that they are effectively utilizable. As a result, the package becomes small.

The data line connected to a microprocessing unit (MPU) ranges from 4, 8, 16, 32 up to 64 bits in width, for instance, depending on the kind of MPU. On the other hand, the data line connected to ROM (Read Only Memory) and RAM (Random Access Memory) ranges from 1, 4 up to 8 bits in width, for instance, depending on the kind of data line to be connected to the storage element. In other words, the data width of the storage element in this case is narrower than that the MPU in many cases. Therefore, according to a preferred embodiment of the present invention, the data lines are grouped according to the width to be connected to the storage element and the storage element to be connected to the data line belonging to the same group is packaged on the same surface of the wiring substrate. As a result, the number of wiring layer-to-layer connections (via holes) is reduced and the wiring substrate is minimized in size as the area occupied by the via hole is decreased.

Although a semiconductor element often causes transistor (gate)-based trouble in many cases, the trouble may involve the whole element (chip). In the case of trouble involving the whole chip, an electronic apparatus comprising a checking unit and a unit under check is unable to detect the trouble, thus making the addition of the checking unit meaningless.

Even when the checking unit and the unit under check are constructed in different packages, it will be essential for checking signal lines to be provided between the units. This will increase the size of the electronic apparatus.

Consequently, the checking unit and the unit under check are separately provided in respective chips on the same wiring substrate on a bare chip basis. The wiring substrate and the bare chips are connected by wire bonding into a single package. The omission of a fault involving the whole chip is thus prevented and furthermore, a small, light-weight electronic apparatus can be made available.

These and other objects, features and advantages of the present invention will be understood more clearly from the following detailed description with reference to the accompanying drawings, wherein

FIG. 1 is an overall diagram of an embodiment of the present invention;

FIG. 2 is a sectional view of a die bonding portion of a wiring substrate according to the present invention;

FIG. 3 is a diagram showing an arrangement of via holes in an embodiment of the present invention;

FIG. 4 is a diagram showing a division of a data bus in an embodiment of the present invention;

FIG. 5 is a diagram showing a division of a data bus having 32 bits width in an embodiment of the present invention;

FIG. 6 is a diagram of an MPU with checking circuit and a RAM with an error correction code on a wiring substrate in an embodiment of the present invention;

FIG. 7 is a block diagram of an MPU with a external ROM on a wiring substrate in an embodiment of the present invention;

FIG. 8 is a circuit diagram of an electronic apparatus according to the present invention;

FIG. 9 is a diagram showing packaging of semiconductor chips as shown in FIG. 8 on one side of a wiring substrate;

FIG. 10 is a diagram showing packaging of semiconductor chips as shown in FIG. 8 on the other side of the wiring substrate; and

FIG. 11 is a sectional view of a package in an embodiment of the present invention.

FIG. 1 illustrates the inner construction of an electronic apparatus embodying the present invention by way of example. In the embodiment shown, MPU 101, RAM 102, ROM 103, FPU (Floating-point Processing Unit) 104, DMAC (Direct Memory Access Controller) 105, and an interface circuit 106 are connected via a bus 100 formed in a wiring substrate 10. What is particularly noticeable according to this embodiment is that the bus 100 is not led out of the wiring substrate 10, but only an interface line 107 for external devices is led out of the wiring substrate 10.

All semiconductor chips to be connected to the bus 100 are totally packaged on the wiring substrate 10 according to this embodiment. As the bus 100 is not led out of the wiring substrate 10, the number of signal lines connecting between internal and external devices is reducible by a large margin. Accordingly, the number of pins connecting between the signal lines inside and outside the wiring substrate 10 decreases and this overcomes an obstacle to rendering the wiring substrate smaller and lighter.

FIG. 2 is a sectional view of a die bonding portion of the wiring substrate according to the present invention. Wire bonding pads 11 are formed on the wiring substrate 10 and an insulating layer 16 is formed on a wiring conductor 14 for use in leading out the wire bonding pad. A die bonding ground 15 is formed on the insulating layer 16 and a semiconductor chip 20 is bonded thereto by die bonding. A bonding wire 30 is then used for connecting between a wire bonding pad 21 on the semiconductor chip 20 and the wire bonding pad 11 on the wiring substrate 10. According to this embodiment, as shown in FIG. 3, via holes 13, 13' may be formed in the periphery and inside of the die bonding ground 15, respectively. The via hole 13 formed in the periphery of said die bonding ground 15 and the wire hole 13' formed inside of said die bonding ground 15 are preferably arranged alternately. As a result, a portion beneath the die bonding ground 15 as the outermost layer that has heretofore not been utilized may be put to practical use as a wiring and a via hole region. An area to be occupied by wiring and via hole regions can thus be made drastically smaller than what is occupied by the semiconductor chip of the wiring substrate.

FIG. 4 illustrates an embodiment wherein signal lines of the data bus 100 connected to MPU 101 are divided into two groups 100-1 and 100-2. RAMs 102-1-102-k and ROMs 103-1-103-k connected to the data bus 100-1 are packaged on one side (B side) of the substrate, whereas RAMS 102-(k+1)-102-N, ROMs 103-(k+1)-103-N connected to the data bus 100-2 are packaged on the other side (A side) of the substrate, wherein k and N is an integer. According to this embodiment, it is unnecessary to connect the data bus on the A side to what is on the B side, and hence the number of via holes in a wide area is reducible. As a result, an area to be occupied by wiring and via hole regions can thus be made drastically smaller than what is occupied by the semiconductor chip of the wiring substrate, so that the electronic apparatus can be made smaller and lighter.

FIG. 5 refers to an embodiment, where the data bus 100 connected to the MPU 101 is 32 bits wide and the data bus connected to ROM and RAM is 8 bits wide. Among data line D0-D31 constituting the data bus 100, D0-D15 are formed into a group of data bus lines 100-1, and D16-D31 are formed into a group of data bus lines 100-2. D0-D7 in the group of data bus lines 100-1 are connected to RAM 102-1 and ROM 103-1, and lines D8-D15 are connected to RAM 102-2 and ROM 103-2. Moreover, D15-D23 in the group of data bus lines 100-2 are connected to RAM 102-3 and ROM 103-3, and lines D24-D31 are connected to RAM 102-4 and ROM 103-4. According to this embodiment, the electronic apparatus can be made smaller and lighter as in the case of the embodiment shown in FIG. 4.

According to the embodiment shown in FIGS. 2 to 5, it is also possible to package as many bare chips as possible on a single wiring substrate which is limited in size.

FIG. 6 refers to an embodiment wherein MPU 101, a checking circuit 111 of the MPU 101, and RAM 102, an error correction code encoding/decoding circuit 112 are packaged in the form of bare chips on the wiring substrate 10. In this case, a bonding wire has been omitted for simplification.

MPU 101 and the checking circuit 111 are different bare chips and are connected by wire bonding on the wiring substrate 10. Heretofore, various systems have been proposed as the checking circuit 111. There are the following, for instance:

(1) A watch dog timer for resetting MPU 101 after sensing its operation on impulse when it is unaccessible within a fixed period of time.

(2) A system having a reference MPU (not shown) within the checking circuit 111 and while comparing the output signal of the reference MPU with that of MPU 101, determines that the reference MPU or MPU 101 is irregular once a nonconformity is found, for example.

In the conventional method of packaging MPU 101 and the checking circuit 111 separately, the number of packages, the number of wires and the dimensions of the electronic apparatus tend to increase. In the method recently followed for forming the MPU 101 and the checking circuit 111 on the same chip, moreover a fault involving the whole chip is not completely detectable even if the checking circuit 111 ceases to function.

According to this embodiment, MPU 101 with the checking circuit 111, which is capable of detecting a fault involving the whole chip, is provided without causing the number of packages and the number of wires to increase. Therefore, a small, lightweight, reliable electronic apparatus can be provided. RAM 102 and the error correction code encoding/decoding circuit 112 are different bare chips and are connected by wire bonding on the wiring substrate 10.

The error correction code adds an error detection/correction redundant bit to the data stored in the memory, thus causing an error to be detected and corrected by making the code-to-code Hamming distance 4 or greater. When the code-to-code Hamming distance is set to 4, 1-bit error correction is possible, but a 2-bit error remains only detectable. Consequently, it is called SECDED (Single-Error-Correction, Double-Error Detection). For instance, a 6-bit detection/correction redundant bit needs adding when SECDED is meant to be realized concerning 16-bit data. A detailed description of an error correction code will be omitted since the present invention is not directly concerned with an error correction code.

In the conventional method of packaging RAM 102 and the error correction code encoding/decoding circuit 112 separately, the number of packages, the number of wires and dimensions of the electronic apparatus tend to increase. In the method recently followed for forming the RAM 102 and the error correction code encoding/decoding circuit 112 on the same chip, moreover, a fault involving the whole chip is not completely detectable since the error correction code encoding/decoding circuit 112 ceases to function.

According to this embodiment, RAM 102 with error correction code encoding/decoding circuit 112 capable of detecting a fault involving the whole chip is provided without causing the number of packages and the number of wires to increase. Therefore, a small, lightweight, reliable electronic apparatus can be provided.

Like other semiconductor elements, the storage element (ROM) storing the program involved is packaged on the same wiring substrate in the form of a bare chip, and if it is incorporated into the same package, the electronic apparatus may be made drastically smaller and lighter. If a ROM is incorporated into the package, it will be necessary to devise progamming and its erasing methods. Use of an EEPROM (Electrically Erasable Programmable ROM) will make programming readily possible and make the program erasable. Even when an UVEPROM (Ultra-Violet Erasable Progammable ROM) is used, the program may be executed or erased by providing the apparatus with a window which allows erasing ultra-violet rays to pass therethrough.

When an EPROM is used as a space electronic apparatus to be exposed to cosmic rays, the data written may be erased by means of a cosmic ray. Moreover, an EPROM is not desirable for use as an electronic apparatus to be used over several hundred thousand years due to the electronic thermal movement. Therefore, mask- or fuse-ROMs will have to be used for the purpose described above.

For program development, the program involved has to be modified and rewritten. For this reason, a mask- or fuse-ROM may not be used efficiently for such program development.

According to the following embodiment of the present invention, the electronic apparatus leads the line connected to a ROM out of the package and makes it possible to operate the ROM outside the package. Consequently, no wire bonding is provided for the ROM in the developing package. By connecting a program externally, that is, its easily erasable EPROM to an external device, any program may be developed by means of a wiring substrate having the same pattern as that proposed in the present invention.

FIG. 7 refers to an embodiment wherein either the ROM inside the wiring substrate 10 or an external ROM may be used to operate the MPU. RAM 102 and ROM 103 are connected to MPU 101 via the bus 100 in the wiring substrate 10. Moreover, RAM 102 and ROM 103 selection signals CS# are formed by an address decoder 107. Although a signal name with a line thereon is provided for each active low signal in FIG. 7, the signal name followed by a `#` mark is employed in this specification for convenience of description. The address decoder 107 decodes higher significant bits in an address signal suplied to the bus 100 and when the address signal indicates the address of RAM 102 or ROM 103, the address decoder 107 applies the corresponding selection signal CS# to RAM 102 or ROM 103. While the selection signal CS# is active, RAM 102 or ROM 103 reads or writes desired address data in accordance with the lower significant bits.

According to this embodiment, a ROM 103 selection signal CS# 108 is also sent out of the wiring substrate 10. Consequently, ROM 103' outside the wiring substrate 10 in place of ROM 103 inside the wiring substrate 10 may be used for operation. Moreover, since part of the lower significant bits in the address bus signal are sufficient for an address line to be connected to ROM 103', the number of leader lines from the wiring substrate 10 is also prevented from increasing. In order to develop a program, it is only necessary to write the program to ROM 103' outside the wiring substrate 10 without packaging ROM 103 inside the wiring substrate 10. Hence, efficient program development can be made as a program is readily written to and erased from ROM. If a mask- and a fuse-ROM are used as ROM 103 inside the wiring substrate 10 for an actual apparatus after program development, the fear of the risk of erasure of data in ROM 103 is eliminated, and an electronic apparatus will likely remain in good condition after long use.

FIG. 8 is a circuit diagram embodying the present invention. MPU 101, RAM 102, ROM 103, FPU 104, DMAC 105, and a gate array 110 in the form of bare chips are mounted on the wiring substrate 10. Although RAM 102 and ROM 103 consist of a plurality of chips, depending on the memory capacity and bit width, each of them is indicated as being one in FIG. 8 for simplification. In the gate array 110 the checking circuit 111 is formed with a watch dog timer, for example for detecting the operation of MPU on impulse, the error correction code encoding/decoding circuit 112 for correcting the inversion of data in RAM 102, the address decoder 107, the interface circuit 106 with external devices and the like as built-in elements. (These circuits in the gate array are not shown in FIG. 8.) The number of chips can thus be reduced significantly as the peripheral circuits of MPU 101 are made into such a gate array form.

As the checking circuit 111 and the error correction code encoding/decoding circuit 112 are accommodated on chips which different from those for the MPU 101, RAM 102 with respect to gate array 110, the omission of detection of a fault involving the whole chip is prevented.

Although use can be made of various kinds of respective MPU 101, FPU 104, DMAC 105, the illustration of FIG. 8 is based on the assumption that a GMICRO/200 (H32/200) series is employed. Consequently, names of various control signal lines are indicated in accordance with the specification of the GMICRO/200 (H32/200) series. Since the present invention is not implemented solely by a particular product series, the description of signal names irrelevant to the present invention particularly will be omitted; the detailed description of them has been given in a document (`H32/200 Hardware Manual`, Hitachi Ltd.). Incidentally, the bit positions of address and data lines are provided in the form of a bigendian display and lower significant bits are therefore expressed by small numbers. For instance, A0 of the address line represents the highest significant bit, whereas A29 represents the lowest significant bit.

The bus signal lines led out of the wire substrate 10 according to this embodiment are only as follows: address lines A13-A29, data lines D0-D31, address strobes AS1#, AS2#, byte control signals BC0#-BC2#, a read/write switching signal R/W# and a data transfer termination signal DC#. In other words, as only part of the bus signal lines are led out of the wiring substrate 10, the number of pins affixed to the outside of the package is reduced, whereby the package size can be made smaller. If it is decided not to use ROMs outside the wiring substrate 10, all of these bus signal lines will not necessarily be led out. Thus, the number of pins is reduced.

The address decoder 107 (not shown) in the gate array generates the ROM selection signal ROCS#108, a RAM selection signal RACE0#-RACE3#, and an external element selection signal XCS# by means of the address lines A0-A12.

The ROM selection signal ROCS#108 on one of these signal lines is connected to ROM 103 in the wiring substrate 10 and simultaneously led out of the wiring substrate 10. According to this embodiment, ROM 103' (not shown) in place of ROM 103 inside of the wiring substrate 10 may be connected to the outside of the wiring substrate 10 and used for operation. Moreover, since part of the lower significant bits A13-A29 in the address bus signal is enough for an address line to be connected to ROM 103', the number of leader lines from the wiring substrate 10 is also prevented from increasing. In order to develop a program, it is only necessary to write the program to ROM 103' outside the wiring substrate 10 without packaging ROM 103 inside the wiring substrate 10. Hence, efficient program development can be made as a program being readily written to and erased from ROM. If a mask- and a fuse-ROM is used as ROM 103 inside the wiring substrate 10 for an actual apparatus after the program development, fear of risking erasure of data in ROM 103 is eliminated, and an electronic apparatus stands to remain in good condition after long use.

RACE0#-RACE1# of the RAM selection signals RACE0#-RACE3# are connected to RAM 102 inside of the wiring substrate 10, whereas RACE2#-RACE3# are led out of the wiring substrate 10. If RACE2#-RACE3# is led out of the wiring substrate 10, the byte control signal BC0#-BC2#, the read/write switching signal R/W#, the address lines A13-A29 and the data lines D0-D31 are connected to RAM 102' (not shown) outside the wiring substrate 10, an increase in storage capacity may be attained with the combination of RAM 102 and RAM 102'.

The external element selection signal XCS# is led out of the wiring substrate 10 and if the external element selection signal XCS#, the byte control signals BC0#-BC2#, the read/write switching signal R/W#, the address strobes AS1#, AS2#, the data transfer termination signal DC#, the address lines A13-A29 and the data lines D0-D31 are connected to an external element (not shown), system expendability will be improved as the external element becomes usable.

The number of pins may drastically be reduced when the external element is not connected as the RAM selection signals RACE2#-RACE3#, the selection signal XCS#, the byte control signal BC0#-BC2#, the read/write switching signal R/W#, the address strobes AS1#, AS2#, the data transfer termination signal DC#, the address lines A13-A29 and the data lines D0-D31 are unnecessary to lead out of the wiring substrate 10 to RAM 102' outside the wiring substrate 10.

In addition, the gate array 110 may incorporate the interface circuit 106 with external devices. A signal line MIL-1553B is employed for use in the so-called MIL-1553B communication standard. Moreover, a communication line CELLCOMCNTR is a communication line for coupling a plurality of computer units, each having the wiring substrate 10. If the number of wiring substrates 10 required is prepared for the communication lines CELLCOMCNTR to be connected together, it will facilitate the construction of a multiprocessor system or a multiplex computer system for fault tolerance.

FIGS. 9 and 10 illustrate methods of packaging the wiring substrate 10 embodying the present invention as shown in FIG. 8.

MPU 101, FPU 104, ROMs 103-1, 103-2 and RAM 102-1, 102-2 are mounted on the surface (B side) shown in FIG. 9. The storage element, ROMs 103-1, 103-2 and RAMs 102-1, 102-2 connected to the data lines which belong to the bus 100-1 are mounted on this surface as shown in FIG. 5.

DMAC 105, the gate array 110, ROMs 103-3, 103-4 and RAMs 102-3, 102-4 are mounted on the surface (A side) shown in FIG. 10. The storage element, ROMs 103-3, 103-4 and RAMs 102-3, 102-3 connected to the data lines which belong to the bus 100-1 are mounted on this surface as shown in FIG. 5.

As the number of wiring layer-to-layer via holes is reduced according to this embodiment, the wiring substrate 10 can be made smaller. Moreover, the concentration of heat and wiring to one side is avoidable by splitting LSI, MPU 101, FPU 104, DMAC 105 and the gate array 110 into two groups, each having a large chip size and many input-output signal lines, and alloting them to the respective sides. In view of thermal resistance, chemical stability and the like, a ceramic substrate is fit for use as the wiring substrate 10 when it is employed in space where reliability is required.

FIG. 11 illustrates a package embodying the present invention. Ceramic caps 50 are attached to the respective sides of the ceramic wiring substrate 10 to hermetically seal bare chips such as MPU 101 mounted thereon. The inside thus hermetically sealed by the caps 50 is kept under vacuum or filled with an inactive gas of nitrogen, helium or the like. If the inside is filled up with helium it will conveniently be used for leakage checking when the airtightness of the seal is checked. When a plurality of chips are accommodated into a single package, the package tends to become large, and the volume of the air inside the caps 50 also tends to increase. When the ceramic caps 50 are attached to the respective sides of the ceramic wiring substrate 10 before being used to hermetically seal the inside by soldering, the molten solder, as it cools, may be drawn into or jutted out of the caps 50 due to the difference in pressure between the inside and the outside. One of the measures to be taken to prevent the molten solder from being drawn into or jutted out of the caps 50 even though a package is large is to bore ventilating holes 51 into the caps 50 and to cover the holes 51 with lids 52 or the like after the contents are hermetically sealed with the inactive gas encapsulated.

According to the present invention, a plurality of semiconductor elements can be accommodated in a single package and the number of signal lines to be led out of the package is reduced, so that the package size is decreased. Therefore, a small lightweight electronic apparatus is made available.