Electronic device and semiconductor package

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ELECTRONIC DEVICE AND
SEMICONDUCTOR PACKAGE

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an electronic device having a semiconductor package mounted on a motherboard or the like, and more particularly to an electronic device having a highly reliable structure.

The invention also relates to a semiconductor package having a highly reliable structure even when it is mounted on a motherboard or the like. The invention also relates to a semiconductor package configured to have high productivity when it is mounted on a motherboard or the like.

2. Description of the Related Art

With electronic equipment being made lightweight and compact in size, it has been demanded to mount various types of electronic parts on a wiring substrate and the like in high density. To mount the various electronic parts in high density, for example, a semiconductor package is required to be made smaller and thinner in size and also connected with high reliability. Specifically, to provide a compact and high-performance electronic device which has electronic parts such as a semiconductor package mounted on a wiring substrate as the host, it is necessary that the electronic parts such as the semiconductor package are made highly functional, compact and thin; the wiring substrate, on which the electronic parts are mounted, is made to have an improved wiring density; and electrical and mechanical connection between the electronic parts and the wiring substrate is improved in reliability.

Especially, since fine connecting terminals mounted in high density such electronic parts are connected on many points, high reliability must be secured over a range from the mounting process to the actual use of the product. Otherwise, stable performance cannot be obtained over a long period.

For example, a high thermal load is applied in a process of mounting the electronic parts on the wiring substrate by soldering or the like. In addition, a cyclic thermal load is also applied when an electronic part which has the electronic parts mounted on the wiring substrate is mounted. Therefore, a stress (mechanical stress) due to such a thermal load is applied to the interior of the electronic parts and the wiring substrate and also to the connections between the electronic parts and the wiring substrate. Conventionally, such a stress is the cause of degrading the reliability of the electronic device, and the electronic device has lost its functions as a result.

Now, a conventional semiconductor package will be described briefly.

FIG. 9 is a diagram schematically showing the structure of a conventional semiconductor package. This semiconductor package 90a has a semiconductor element 91 mounted with a face down on a wiring substrate 92.

This semiconductor package 90a has electrodes 91a of the semiconductor element 91 and connecting pads 92a mounted on a face of the wiring substrate 92, which has the semiconductor element 91 mounted. Conductive bumps 93 are intervened between the electrodes 91a and the connecting pads 92a. In addition, an encapsulant (sealing resin) 94 is formed in a gap between the semiconductor element 91 and the wiring substrate 92 to protect the connection between them. Connecting pads 92b are mounted on a face of the wiring substrate 92 opposite from its face on which

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the semiconductor element 91 is mounted, as external connecting terminals to connect the semiconductor package with an external circuit. The connecting pads 92b are mounted to have, for example, a grid shape on the wiring

5 substrate 92, to configure a so-called LGA (Land Grid Array) type semiconductor package. In addition, mounting of solder balls or the like on the connecting pads 92b makes a BGA (Ball Grid Array) type semiconductor package. The connecting pads 92a and the connecting pads 92b are mutually connected with a wiring pattern formed on the wiring substrate intervened between them.

In the semiconductor package shown in FIG. 9, the wiring substrate 92 is an alumina substrate having alumina or other ceramics layer as an insulating layer and has a thickness of about 0.5 to about 0.7 mm and a square with each side of

15 about 13 to about 40 mm. Further, the semiconductor element 91 has a square with each side of about 8 mm to about 15 mm.

FIG. 10 is a diagram schematically showing another embodiment of the structure of a conventional semiconduc

20 tor package, which has the semiconductor element 91 mounted with a face up on the wiring substrate 92.

This semiconductor package has electrodes 91a of the semiconductor element 91 and connecting pads 92a mounted on a face of the wiring substrate 92 having the

25 semiconductor element 91 mounted connected by bonding wires 96. The semiconductor element 91 is fixed to the wiring substrate 92 with an adhesive agent 95 or the like. In addition, the semiconductor element 91 is sealed with a molding resin 97. On a face of the wiring substrate 92

30 opposite from the side on which the semiconductor element 91 is mounted, the connecting pads 92b are mounted as the external connecting terminals in the same way as described above to connect the semiconductor package with an external circuit.

35 In the semiconductor package shown in FIG. 10, the wiring substrate 92 is also an alumina substrate having alumina composite as an insulating layer and has a thickness of about 0.5 to about 0.7 mm and a square with each side of about 13 to about 40 mm. Further, the semiconductor

4Q element 91 has a square with each side of about 8 mm to about 15 mm.

The semiconductor packages shown in FIG. 9 and FIG. 10 were described with reference to the wiring substrate 92 having alumina composite as the insulating layer. But, the

45 insulating layer configuring the wiring substrate may also be formed of various types of insulating materials including ceramics such as aluminum nitride and polymers such as glass epoxy or BT resin.

The semiconductor package described above is then

50 mounted on a wiring substrate (motherboard) of the host to configure an electronic device.

In addition, FIG. 11 is a diagram schematically showing a structure of the electronic device having the semiconductor package on the wiring substrate. The structure of an elec

55 tronic device having the semiconductor package 90a shown in FIG. 9 mounted will be described.

For example, the connecting pads 101a mounted as a part of the wiring layer are mounted on a face of a wiring substrate 101 which has a laminated structure of an insulat

60 ing layer of a glass epoxy resin and a wiring layer of copper or the like. The connecting pads 101a are mounted to face with the connecting pads 92b, which are the external connecting terminals of the semiconductor package 90a. In addition, the connecting pads 101a of the wiring substrate

65 101 and the connecting pads 92b of the semiconductor package are connected with the solder balls 103 intervened between them.

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The semiconductor package 90a is mounted on the wiring substrate 101 by the following process, for example. First, the solder balls 103 are mounted on the connecting pads 92b of the semiconductor package 90a to make a BGA type semiconductor package. A solder paste is previously printed 5 on the connecting pads 101a of the wiring substrate 101. In addition, the solder balls 103 and the connecting pads 101a of the wiring substrate 101 are aligned, and the solder balls 103 and the solder pastes are reflowed to electrically and mechanically connect the semiconductor package 90a with 10 the wiring substrate 101. The amount of solder (or height of the solder balls 103) of the solder balls 103 is determined based on the size of the connecting terminals 101a and the weight of the semiconductor package 90a.

But, the electronic device described above has the fol- 15 lowing disadvantages.

For example, in the electronic device shown in FIG. 10, the wiring substrate configuring the semiconductor package 90a has the alumina insulating layer, and the wiring substrate 101 on the host has the insulating layer of the polymer 20 of glass epoxy resin. If this electronic device has its components made of a different material, the physical property values such as a thermal expansion coefficient are also different.

For example, the wiring substrate having the alumina 25 (A1203) insulating layer has a linear expansion coefficient of about 7xl0"6 to about 8xlO"6/K.

On the other hand, the wiring substrate, which has the insulating layer made of a cured prepreg having a glass cloth ^ impregnated with glass epoxy resin, has a linear expansion coefficient of about 13xl0"6 to about 18xlO"6/K.

In addition, due to a thermal load or the like applied during mounting or in the actual application environment, a stress is applied to the connection between the semiconduc- 3J tor package 90a and the wiring substrate 101 to cause a fatigue failure (breakage) in the connection between the semiconductor package 90a and the wiring substrate 101. The inventors made it certain that due to a cyclic temperature change which is applied in the environment that the 4Q electronic device is used, a stress is repeatedly applied to the solder balls 103 which electrically and mechanically connect the connecting pads 92b with the connecting pads 101a, indicating a fatigue failure.

The electronic device shown in FIG. 10 was undergone a 45 temperature cycle test (0° to 125° C), and its service life was estimated to be about 300 to 600 cycles though variable depending on the size and shape of the semiconductor element 91, the wiring substrate 92, the wiring substrate 101 and the solder balls 103. Durability against the test is 50 generally required to be about 1000 cycles in view of practical use.

Meanwhile, the semiconductor package, which had the wiring substrate 92 configured to have the glass epoxy resin as the insulating layer in the same manner as the wiring 55 substrate 101, was undergone the temperature cycle test (0° to 125° C). Its service life was about 1500 to 2000 cycles.

If the electronic parts can be produced by selecting the respective parts so to have conforming physical property values to configure the electronic parts, durability against 60 the thermal load cyclically applied can be improved. But, in practice, a plurality of materials having a different physical property are generally used in combination to deal with the specifications required for respective parts, product costs and productivity. 65

The tendency of a fatigue failure occurring on the connection of the above-described solder balls or the like may

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increase as the semiconductor package is made to have high performance and high capacity, namely the semiconductor element 91 or the semiconductor package is made large (density is increased, and the size is made large). This is because displacement by deformation due to a stress increases as the size as the whole is made large. Therefore, the conventional semiconductor package and electronic device hardly secure reliability for a long period.

The connection described above has a tendency to suffer from a fatigue failure in a semiconductor package configured without sealing by a resin and also in the electronic device having various types of semiconductor packages.

To deal with a fatigue failure in the connection, it is tried to relieve the stress caused due to a difference of values of physical properties by enlarging a loft of the connection, such as increasing a height of the solder balls 103. But, when the solder balls 103 have an increased height, a degree of relative displacement between the semiconductor package 90a and the wiring substrate 101 increases, tending in degrading durability because a warp on the periphery of the semiconductor package increases. Therefore, it is still hard to secure the reliability of the connection of the electronic device for a long period.

In addition, the solder balls are in a molten state in the re flow process, but there are drawbacks that it is hard to keep the semiconductor package flat with respect to the motherboard in this process, and productivity in the mounting process is lowered. When the solder balls are made higher, it becomes further difficult to keep the semiconductor package flat with respect to the motherboard.

The invention was achieved to remedy the disadvantages described above. Specifically, the invention aims to provide a highly reliable electronic device, and more particularly an electronic device having a highly reliable connection between a semiconductor package and a motherboard. Further, the invention also aims to provide a semiconductor package having a structure capable of improving reliability of the electronic device. The invention also aims to provide a semiconductor package having a highly producible structure.

SUMMARY OF THE INVENTION

To remedy the drawbacks described above, the present invention configures an electronic device and a semiconductor package as described below.

A first aspect of the invention relates to an electronic device which comprises: a first wiring substrate having a first face, the first face of the first wiring substrate having a first region and a second region, connecting terminals formed on the first region, and the first wiring substrate having a first thermal expansion coefficient; a semiconductor package, comprising a second wiring substrate having a first face and a second face, the first face of the second wiring substrate having a first region and a second region, the first connecting terminals in the first region which are faced with the connecting terminals of the first substrate, second connecting terminals formed on the second face which are connected with the first connecting terminals, and the second wiring substrate having a second thermal expansion coefficient, and at least one semiconductor chip mounted on the second face of the second wiring substrate, and the semiconductor chip having electrodes connected with the second connecting terminals; connecting means for connecting the connecting terminals of the first wiring substrate and the first connecting terminals of the second wiring substrate; and a buffer layer interposed between the second region of