JP2003264351A - Aluminum nitride multilayer substrate - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an aluminum nitride multilayer substrate which is extremely excellent in mounting reliability to a printed wiring board as a mother board. <P>SOLUTION: In the aluminum nitride multilayer substrate 26 which is surface- mounted on the printed wiring board 14 via a plurality of pins for connection, at least a pin 28a for connection wherein thermal stress is liable to be concentrated is made more non-rigid than a pin 28 for connection wherein thermal stress is hard to be concentrated, or the diameter of the pin 28a is made smaller than that of the pin 28. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to an aluminum nitride multilayer substrate.

[0002]

2. Description of the Related Art In recent years, similarly miniaturized and high-density IC chips, LSI chips and the like are being used in electronic devices which are remarkably miniaturized and highly densified. However, since the amount of heat generated increases as the density of the chips increases, high heat dissipation is required for the package in which the chips are mounted on the ceramic multilayer substrate. For this reason, it has been strongly desired in recent years to realize a ceramic package having a further excellent heat dissipation property than the conventionally well-known alumina package.

Aluminum nitride, which belongs to non-oxide ceramics, has excellent physical characteristics that it has a higher thermal conductivity than alumina and a thermal expansion coefficient close to that of silicon. Therefore, at the present time, it is considered that a package using a multilayer substrate made of aluminum nitride will be suitable.

[0004]

This type of package is, for example, a so-called PGA having a large number of pins as connecting terminals on the lower surface side of an aluminum nitride multilayer substrate.
A (pin grid array) type one is well known. The PGA type package is designed to be surface-mounted on a pad on a printed wiring board, which is a mother board, via pins.

However, the ceramic package using the aluminum nitride multilayer substrate has the following problems. That is, since the thermal expansion coefficient of aluminum nitride is about 4.5 (× 10 ⁻⁶ / ° C.), the difference from silicon having a thermal expansion coefficient of about 3.6 (× 10 ⁻⁶ / ° C.) is relatively small. small. Therefore, regarding the combination of the aluminum nitride multilayer substrate and the silicon chip, there is no particular thermal mismatch between them. Therefore,
No increase in resistance is observed between the aluminum nitride multilayer substrate and the silicon chip, and no connection failure occurs.

On the other hand, a resin such as glass epoxy, which is a material for forming a printed wiring board, is generally characterized by having a coefficient of thermal expansion several times larger than that of ceramics or metal.
Therefore, regarding the combination of the aluminum nitride multilayer substrate and the printed wiring board, thermal mismatch is apt to occur between them.
For this reason, there is a high probability that an increase in resistance will occur between the aluminum nitride multilayer substrate and the printed wiring board, or a connection failure will occur. Further, it is predicted that the above-mentioned defects will become remarkable when, for example, the package encounters a rapid temperature change.

Based on the above predictions, the inventor of the present application actually manufactured a package using an aluminum nitride substrate and mounted the package on a printed wiring board in a surface mounting state, and the following mounting reliability was obtained. The test was conducted.

The size of the test package is 20 mm
There are four types: square, 25 mm square, 30 mm square, and 35 mm square. Further, as a printed wiring board for surface-mounting these packages, a FR-4 printed wiring board which is commonly used is selected. Short pins made of Kovar (length 2 mm, 0.2 mm) are used as pins for connection.
φ) was used. Further, eutectic solder (Pb: Sn = 63: 37) was used for joining the pins. Then, the thermal cycle test (vapor phase, -65 ° C to 150 ° C) and the thermal shock test (liquid phase, -55 ° C to 125 ° C) were each performed 1000 cycles, and the resistance value was measured every predetermined cycle. Then, the rate of resistance change (%) was obtained, and the point at which the initial resistance value increased by 10% was determined to be “NG”. The resistance value was measured separately for two groups, (a) a pin existing on the outermost periphery of the package and (b) a pin not existing. Hereinafter, for convenience of description, the former (a) will be referred to as an “outer peripheral portion pin” and the latter (b) will be referred to as a “central portion pin”.

9 and 10 are graphs showing the results of the above test. As you can see from these graphs,
For the pins on the outer circumference, 10 in any test
The result obtained was that NG was reached by the end of 00 cycles. On the contrary, with respect to the pin in the central portion, the result was obtained in which NG was not reached even after 1000 cycles. Further, when observing the connection portion of the pins on the outer peripheral portion reaching the NG, many cracks were found near the interface between the pin and the solder.

Therefore, the inventor of the present application deduced the mechanism causing the above-mentioned phenomenon as follows.
FIG. 8 shows a view of the package as seen from the bottom surface side of the aluminum nitride multilayer substrate S. Of the pins protruding from the aluminum nitride multilayer substrate S, pins P1 and P1a
Are so-called outer peripheral pins, and pins P2 and P2a are so-called central pins. Here, focusing on the pin P2a, it can be seen that eight pins exist around the pin P2a. In comparison, 5 around the pin P1
It can be seen that there are only three pins and only three pins around the pin P1a. Therefore, when thermal stress is applied to the pins, it can be seen that the smaller the number of pins existing in the periphery, the smaller the degree of dispersion of thermal stress.

That is, it is concluded that cracks may occur frequently on the pins P1 and P1a on the outer peripheral portion where the burden of thermal stress per pin is large, especially on the pin P1a located at the corner portion C. .

Therefore, the inventor of the present application has found that the pin P on the outer peripheral portion is
It was thought that some measures should be taken so that a large thermal stress is not applied to a specific pin such as the pin P1a. Then, the present inventor has completed the present invention described in detail below by further developing the knowledge obtained so far.

An object of the present invention is to provide an aluminum nitride multi-layer substrate which is extremely excellent in mounting reliability for a printed wiring board which is a mother board.

[0014]

In order to solve the above problems, according to the invention of claim 1, in an aluminum nitride multilayer substrate surface-mounted on a printed wiring board through a plurality of connecting pins, The gist of the aluminum nitride multi-layer substrate is characterized in that at least the connecting pin in which the thermal stress is easily concentrated among the connecting pins is softer or has a smaller diameter than the connecting pin in which the thermal stress is difficult to concentrate. .

According to a second aspect of the invention, in an aluminum nitride multilayer substrate surface-mounted on a printed wiring board via a plurality of connecting pins, all the connecting pins are made of a material or material capable of absorbing thermal stress. The gist is an aluminum nitride multilayer substrate characterized by having a shape.

[0016]

According to the structure of the invention described in claim 1, since at least the connecting pin, on which thermal stress is easily concentrated, is soft or has a small diameter, the pin itself has a certain degree of thermal stress even when a temperature change is encountered. Will be absorbed. Therefore, breakage of the connecting pin where thermal stress is likely to concentrate is avoided.

According to the second aspect of the present invention, since all the connecting pins are made of a material or shape capable of absorbing thermal stress, thermal stress is applied to a specific pin even when a temperature change is encountered. There is no need to concentrate. Therefore, breakage of the connecting pin is avoided in advance.

[0018]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS [Embodiment 1] An embodiment in which the present invention is embodied in a ceramic package using an aluminum nitride multilayer substrate will be described in detail with reference to FIGS.

As shown in FIG. 1, the package 1 of this embodiment is a PGA type package, and is mainly composed of an aluminum nitride multilayer substrate 2, a sealing ring 3 and a cap 4, an LSI chip 5 and the like. Has been done.

The aluminum nitride multilayer substrate 2 used in the present embodiment is a five-layer plate obtained by hot pressing a laminate of green sheets. Inside the aluminum nitride multilayer substrate 2, through holes 7a and 7b penetrating the front and back and a wiring pattern 6 are formed by printing a tungsten paste or the like. On the upper surface of the aluminum nitride multilayer substrate 2, a thin film multilayer circuit of 5 layers (L / S = 2
A build-up layer 9 having a thickness of 0 μm / 20 μm) 8 is formed. At the center of the upper surface of the buildup layer 9, LS
The I-chip 5 is mounted. The LSI chip 5 and the thin film multilayer circuit 8 of the buildup layer 9 are electrically connected via a bonding wire 11.

The diameter (0.8 mmφ) of the through hole 7a in the lowermost layer of the aluminum nitride multilayer substrate 2 is larger than the diameter (0.2 mmφ) of the through hole 7b in the other layers. In the case of the aluminum nitride multilayer substrate 2, the surface of the through hole 7a is used as a pad for attaching the connecting terminal. The surface of the through hole 7a is previously plated with Ni-Au.

As shown in FIGS. 1 and 2, on the surface of the through hole 7a in the lowermost layer, a short pin (length 2 mm, 0.2 mmφ) 12,1 made of Kovar as a connecting terminal is provided.
2a is joined. In addition, in the present embodiment, Au- is used for joining the short pin 12 and the through hole 7a in the lowermost layer.
Cu brazing material is used. Therefore, these short pins 12 and 12a are connected to the LSI through the through holes 7a and 7b, the thin film multilayer circuit 8 and the bonding wire 11.
It is in a state of being electrically connected to the chip 5. In this embodiment, the pitch of the short pins 12 and 12a is 1.2.
It is set to 7 mm.

As shown in FIG. 2, the short pin 1
A dummy short pin 13 is provided so as to surround the short pins 12 and 12a on the outer periphery of the region where the second and 12a are provided. Unlike the short pins 12 and 12a, the dummy short pin 13 here does not particularly participate in electrical connection. Further, in this embodiment, as the dummy short pin 13, one having the same shape and the same material as the short pin 12 involved in the electrical connection is used.

As shown in FIG. 1, on the upper surface of the aluminum nitride multilayer substrate 2 on which the LSI chip 5 is mounted, L
A Kovar-made sealing ring 3 and a cap 4 are joined to protect the SI chip 5 from moisture and the like. As a result, a so-called face-up type substantially square-shaped package 1 (35 mm square in this embodiment) is formed.

As shown in FIG. 3, this package 1
Is a printed wiring board (made of FR-4) that is a motherboard 1
4 is implemented. Connection pads 15 are formed at predetermined positions on the printed wiring board 14 corresponding to the protruding positions of the short pins 12 and 12a on the package 1 side and the dummy short pins 13. And eutectic solder (P
b: Sn = 63: 37) 10, the short pin 1
2, 12a and the dummy short pin 13 and the connection pad 15 are joined.

Now, according to the package 1 of this embodiment,
The dummy short pin 13 surrounds the short pin 12a located on the outer peripheral portion (particularly the corner portion C) where thermal stress easily concentrates. Therefore, even when a sudden temperature change is encountered, the thermal stress can be dispersed in the dummy short pin 13. Therefore, breakage of the short pin 12a located on the outer peripheral portion can be avoided in advance. Also,
Even if a part of the dummy short pin 13 is broken, it does not cause any serious problem because it is not involved in the electrical connection by itself. Therefore, the package 1 using the aluminum nitride multilayer substrate 2 of the present invention has extremely excellent mounting reliability for the printed wiring board 14.

By the way, when the above-mentioned mounting reliability test was carried out also in this embodiment, the short pin 12a located on the outer peripheral portion was N even after 1000 cycles.
The extremely favorable result of not reaching G was obtained.

As is clear from the above test results, according to this example, it is understood that no problems such as an increase in resistance or a connection failure between the aluminum nitride multilayer substrate 2 and the printed wiring board 14 occur.

The present invention is not limited to the above embodiment, but can be modified as follows. For example, (a) a dummy short pin 13 may be provided only at the corner C, as in a package 21 using the aluminum nitride multilayer substrate 20 of the first modification shown in FIG. Even with such a configuration, there is an advantage that at least the short pin 12a in the corner portion C where thermal stress is most likely to be concentrated is protected. Further, when this configuration is adopted, the number of dummy short pins 13 used can be small, so that the cost can be reduced.

(B) The dummy short pin 13 shown in the embodiment may be provided inside the region R where the short pins 12 and 12a are projected, for example. That is,
If a dummy short pin is provided in a region R indicated by a chain double-dashed line in FIG.
This is because the thermal stress applied to 2b can be dispersed.

(C) Like the package 23 used for the aluminum nitride multilayer substrate 22 of the second example shown in FIG. 5, the connection terminals involved in the electrical connection are the bumps 24, and the surroundings are electrically connected. It may be a dummy bump 25 that does not participate in the. In this case, it is preferable to use, for example, high melting point solder, silver solder, or tungsten as the material for forming the bumps.

Even with the structure of the second example, the same operational effect as that of the above-described example can be obtained, and the mounting reliability of the package 23 on the printed wiring board can be improved. (D) A structure such as the package 27 used for the aluminum nitride multilayer substrate 26 of the third example shown in FIG. 6 may be adopted.

In the case of this aluminum nitride multilayer substrate 26, among the connecting pins 28, 28a involved in electrical connection, the outermost connecting pin 28a has a smaller diameter (0.15 mmφ) than the other connecting pins 28. ) Has become. With this configuration, even when a temperature change is encountered,
The outermost connecting pin 28a itself, on which thermal stress is easily concentrated,
It absorbs some thermal stress. Therefore, breakage of the connecting pin 28a is avoided in advance.

Of the connecting pins 28, 28a, at least the outermost connecting pin 28a in which thermal stress is likely to concentrate.
May be softer than the connecting pin 28 in which thermal stress is hard to concentrate. That is, when the connecting pin 28 in which thermal stress is hard to concentrate is made of Kovar, a metal softer than Kovar such as copper or gold may be selected as the material for the outermost connecting pin 28a. Further, not only these metals but also soft alloys can be selected.

Further, the connecting pin 28 in which thermal stress is hard to concentrate
Even if a metal other than Kovar is selected as the metal for use as the metal, it is sufficient to select another metal that is softer than the metal as the outermost connecting pin 28a.

(E) It is also preferable that all of the connecting pins 31 have a shape capable of absorbing thermal stress, such as a package 30 using the aluminum nitride multilayer substrate 29 of Modification 4 shown in FIG. That is, in the case of the package 30 of the other example 4, the connecting pin 31 has a small diameter (0.15 mmφ) so that the connecting pin 31 has a shape capable of absorbing thermal stress. Further, not only the shape of the connecting pin 31 may be changed, but also copper, which is a soft metal, may be used as the material of the connecting pin 31, for example.

(F) The buildup layer 9 on the upper surface of the aluminum nitride multilayer substrate 2 is not particularly essential, and can be omitted from the configuration if it is unnecessary. The number of laminated aluminum nitride multilayer substrates 2 can be changed arbitrarily.

(G) Instead of the joining method of soldering the connecting pin to the flat pad, for example, a connecting method of inserting the connecting pin into the concave portion on the aluminum nitride multilayer substrate 2 side may be adopted.

[0039]

As described above in detail, according to the aluminum nitride multilayer substrate of the present invention, a large thermal stress is not concentrated on a specific connecting terminal, so that the mounting reliability for a printed wiring board which is a mother board is improved. It has the effect of being extremely excellent in

[Brief description of drawings]

FIG. 1 is a schematic vertical sectional view showing a package of an example using an aluminum nitride multilayer substrate.

FIG. 2 is a bottom view showing the aluminum nitride multilayer substrate of the example.

FIG. 3 is an enlarged sectional view of an essential part showing a state where the package of the embodiment is mounted on a printed wiring board which is a mother board.

FIG. 4 is a bottom view showing an aluminum nitride multilayer substrate (package) of Modified Example 1.

FIG. 5 is a schematic vertical sectional view showing an aluminum nitride multilayer substrate (package) of Modification 2.

FIG. 6 is a schematic vertical sectional view showing an aluminum nitride multilayer substrate (package) of Modification 3.

FIG. 7 is a schematic vertical sectional view showing an aluminum nitride multilayer substrate (package) of Modification 4.

FIG. 8 is a partially cutaway bottom view showing a conventional aluminum nitride multilayer substrate which constitutes a package.

FIG. 9 is a graph showing the result of a thermal cycle test for a conventional package.

FIG. 10 is a graph showing the result of a thermal shock test for a conventional package.

[Explanation of symbols]

1,2,23,27,30 ... Package, 2,20,
22, 26, 29 ... Aluminum nitride multilayer substrate, 12,
12a, 12b ... Short pins as connecting terminals, 1
3 ... Dummy short pins as dummy connection terminals, 14 ... Printed wiring board, 24 ... Bumps as connection terminals, 25 ... Dummy bumps as dummy connection terminals, 28 ... Hard to concentrate thermal stress Connection pin, 28a ...
Outermost peripheral connection pins where thermal stress is easily concentrated, 31 ... Connection pins.

Claims (2)

[Claims] 1. In an aluminum nitride multilayer substrate which is surface-mounted on a printed wiring board via a plurality of connecting pins, at least the connecting pins where the thermal stress is likely to be concentrated among the connecting pins are concentrated. An aluminum nitride multilayer substrate characterized in that it is softer or has a smaller diameter than the connection pins that are difficult to make. 2. An aluminum nitride multilayer substrate which is surface-mounted on a printed wiring board via a plurality of connecting pins, wherein all the connecting pins are made of a material or a shape capable of absorbing thermal stress. Aluminum nitride multilayer substrate.