JPH0740569A - Mounting structure of electronic parts - Google Patents

Abstract

PURPOSE:To provide a mounting structure of electronic parts with which excellent connecting reliability for thermal change can be maintained and high mounting density of electronic parts can be realized. CONSTITUTION:In an area A of a thermal head 21, an electric insulating layer 40 consisting of polyimide is formed on a ceramic substrate 22, on which a Pd metallic layer 41 is formed and Ni electrode layers 42a, 42b are formed thereon. At a connected part of a drive element 28 and the Ni electrode layers 42a, 42b, a bump-like solder layer 38 is formed on the Ni electrode layers 42a, 42b. At a pad of the drive element 28, a Cr layer and a Cu layer are formed in the mentioned order on an Al layer. The solder layer 38 and the pad of the drive element 28 are connected to each other by reflow method.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a heating element such as a thermal head, an LED (light emitting diode) head, and an EL.
The present invention relates to a mounting structure of an electronic component which is preferably used for a line head for printing such as an (electroluminescence) head.

[0002]

2. Description of the Related Art FIG. 4 is a sectional view showing an example of a conventional thermal head 1. As shown in FIG. 4, the heat storage layer 3 made of glass or the like is formed at a predetermined position on the substrate 2 made of ceramic or the like. Laminate it on the whole surface,
A heating resistor layer 4 made of tantalum nitride TaNx or the like and an electrode layer 5 made of Al (aluminum) or the like are formed. The heating resistor layer 4 and the electrode layer 5 are patterned by photolithography or the like. As a result, the first common electrode 5a, the heating resistor 6, the individual electrode 5
b, a second common electrode 5c for energizing the driving element 8 such as an IC (Integrated Circuit) and a control terminal (not shown) for transmitting a control signal to the driving element 8 are formed.

On the substrate 2 of the thermal head 1 obtained as described above, the driving element 8 is connected to each individual electrode 5b, the second common electrode 5c and the control terminal via the solder bump 9. To be implemented. Further, a protective layer 7 made of Si ₃ N ₄ or the like is formed so as to cover the surface of the heat storage layer 3. The drive element 8 is sealed with a sealing resin layer 10 made of epoxy resin or the like.

FIG. 5 is a sectional view showing a joint portion between the drive element 8 and the electrode 5. As shown in FIG. 5, at the junction between the substrate 2 and the drive element 8, the substrate 2 is composed of the electrode layer 5 made of Al, the Ni (nickel) layer 11 and the Au (gold) layer 12. The connection portion 18 with the IC is formed in this order. The Ni layer 11 is formed to a thickness of 1 to 2 μm by an electroless plating method, and the Au layer 12 is formed to a thickness of 200 to 2000 Å by an electroless plating method.

Further, an electrode 14 made of Al is formed on the driving element 8 realized by an IC bare chip or the like by laminating it on the insulating film 13. A protective film 15 is formed on the entire surface of the electrode 14 by laminating the electrode 14. In order to prevent the intermetallic compound from being generated between the bump 9 and the Al electrode 14, the pad portion of the driving element 8 is laminated on the Al layer 14 to form a Cr (chrome) layer 16 and a Cu (copper) layer. ) Layer 17 is formed in this order. Further, the bumps 9 made of eutectic solder are formed by laminating on the Cu layer 17. The drive element 8 having the bumps 9 formed as described above is realized on the substrate 2 by the reflow method.

[0006]

However, as described above, when the Ni layer 11 and the Au layer 12 are laminated on the electrode layer 5 made of Al to form the connection portion 18 with the IC, the electrode layer made of Al is formed. Since the adhesion between the No. 5 and the Ni layer 11 is weak, the thermal strain generated during the reflow is absorbed by the solder in the softened state, but the Ni layer 11 is made of Al due to the thermal strain caused by the periodic thermal change of the thermal head. There is a problem that it is easily peeled off from the electrode layer 5.

The above-mentioned Al electrode layer 5 and Ni layer 11 made of Al
The adhesion force with is mainly due to the physical adsorption force between Al and Ni, and the adsorption energy per atom is about 0.
It is 1 eV. Therefore, the area of the IC connecting portion 18 is about 1
By forming it to a relatively large size of 00 μm ² ,
The adhesion between the electrode layer 5 made of and the Ni layer 11 is enhanced. However, by forming the area of the IC connecting portion 18 to be large in this way, the wiring density of the electrode layer 5 made of Al cannot be increased, which hinders the miniaturization of the thermal head.

Further, since the surface of the substrate 2 made of ceramic is rough, it is impossible to form the pattern of the electrode layer 5 made of Al having a high wiring density by photolithography. Therefore, a glass layer is formed on the substrate 2,
The surface of which is flattened, and the electrode layer 5 having a high wiring density is formed on the surface.
May be formed. However, in this case, there is a problem that the adhesion between the glass layer and the electrode layer 5 is weak.

Furthermore, due to the periodic thermal change of the thermal head, thermal strain is concentrated on the connecting portion between the drive element 8 and the substrate 2 described above, and the thermal stress caused thereby is generated by the drive element 8 and the substrate. The smaller the distance from 2, the larger. Therefore,
The bumps 9 cannot be made small, and the wiring density of the electrode layer 5 made of Al cannot be made high.

An object of the present invention is to solve the above-mentioned problems, to maintain excellent connection reliability against heat change, and to mount a driving element on a high density wiring electrode. Is to provide.

[0011]

SUMMARY OF THE INVENTION The present invention is an electronic component mounting structure for mounting an electronic component on a ceramic substrate by joining electrodes of the electronic component to a wiring layer provided on the ceramic substrate via solder. The wiring layer has a two-layer structure of a lower layer made of palladium and an upper layer made of nickel or copper.

[0012]

According to the present invention, an electric insulating layer made of, for example, polyimide is formed on one surface of a ceramic substrate, and a lower layer made of palladium Pd and an upper layer made of Ni or Cu are laminated thereon. Layers are formed,
A plurality of bump-shaped solder layers are formed at predetermined positions on the wiring layer.

Further, an electrode layer made of Al is formed on the electronic component, and a plurality of pad portions are formed at predetermined positions on the electrode layer. On the plurality of pad portions of the electrode layer, a conductive layer made of Cr and a conductive layer made of Cu are formed in this order on the electrode surface.

The electronic component is mounted on the substrate by melting the plurality of bump-shaped solder layers and press-bonding them to the conductive layer made of Cu.

When a thermal change is applied to a mounting portion of the electronic component, the electronic component, the substrate, and each layer interposed between the electronic component and each of the layers have different shrinkage rates, so that thermal distortion occurs, and thus each of the layers. Thermal stress is generated due to this thermal strain. However, the adhesion between polyimide and Pd is strong, and the Young's modulus of polyimide is extremely low. Therefore, even when the above-mentioned thermal strain occurs repeatedly,
By the plastic deformation of the electrical insulation layer, in addition to absorbing thermal stress, since the adhesion between the electrical insulation layer and the wiring layer is strong, film peeling occurs between the electrical insulation layer and the wiring layer. It can be prevented from occurring. At the same time, it is possible to prevent the substrate from warping.

Further, this makes it possible to reduce the solder area between the electronic component and the substrate, which allows the electronic component to be mounted on the substrate at a high density.

Further, since the irregularities on the substrate surface are flattened by the electrically insulating layer made of polyimide formed under the wiring layer, it is possible to form high density wiring.
Further, since the wiring is formed on the electric insulating layer, the occurrence of open short circuit is reduced.

[0018]

1 is a sectional view showing the structure of a thermal head 21 according to an embodiment of the present invention, and FIG. 2 is a sectional view showing the structure of a connecting portion between an electrode 42 and a driving element 28 of the thermal head 21. Is. As shown in FIG. 1, a semicylindrical heat storage layer 23 made of glass or the like is formed in a thickness of about 25 μm at a predetermined position on a substrate 22 made of ceramic or the like. A heat generating resistor layer 24 made of tantalum nitride is laminated on this, and a thickness of about 500 is formed by a sputtering method.
Å (Angstrom) is formed. An electrode layer 25 made of Al is formed on the heating resistor layer 24. The electrode layer 25 is patterned by photolithography or the like, and the first common electrode 25a and the individual electrode 2 are formed.
5b is formed. Furthermore, by stacking on the electrode layer 25,
A protective layer 27 made of Si ₃ N ₄ or the like is formed so as to cover the heat storage layer 23.

In the area A where the drive element 28 is mounted, as shown in FIG. 1, an electrically insulating layer 40 made of polyimide (hereinafter abbreviated as "polyimide layer 40") is formed on the substrate 22. , Laminated on the polyimide layer 40,
A metal layer 41 made of palladium Pd is formed to have a thickness of several atomic layers by immersing it in an alkaline organic palladium complexing solution, and an electrode layer 42 made of Ni is formed on the metal layer 41 by electroless plating. Depending on the thickness 2-3 μm
It is formed. A wiring layer is formed with the Pd metal layer 41 as a lower layer and the Ni electrode layer 42 as an upper layer. The wiring layer is patterned by photolithography or the like, and electrodes 42a, 42 for energizing the drive element 28 are provided.
b and a control terminal (not shown) for transmitting a control signal to the driving element 28 are formed. Individual electrode 25b
Are electrically connected to the electrode 42a.

In addition, the drive element 28 realized by an IC bare chip or the like is provided on the electrodes 42a and 42b and the control terminal (not shown), and the bump-shaped solder layer 38 is provided.
Connected via. The drive element 28 is sealed on the substrate 22 by a sealing resin layer 30 made of epoxy resin or the like.

As shown in FIG. 2, an electrically insulating layer 40 made of polyimide is formed on the substrate 22, and a metal layer 41 made of Pd is laminated on the polyimide layer 40.
Is formed, and the electrode layer 42 made of Ni is formed on the metal layer 41. In the connection portion of the electrode layer 25 with the driving element 28 as described above, the solder layer 38 is formed in the opening portion provided with photoresist or the like by the electrolytic plating method.
Is formed to a thickness of 50 to 70 μm. After that, the photoresist is removed and a bump-shaped solder layer 38 is formed.

An electrode 34 made of Al is formed on the driving element 28 so as to be laminated on the insulating film 33. A protective film 35 is formed on the entire surface of the electrode 34 by laminating the electrode 34. A bump-shaped solder layer 3 is formed on the pad portion of the driving element 28.
8 and the Al electrode 34 to prevent the formation of an intermetallic compound, UBM (under bump metal)
As a result, the Cr layer 36 and the Cu layer 37 are laminated on the Al layer 34 in this order.

In the drive element 28 and the substrate 22 having the connection portions formed as described above, a flux (oxide removing agent) is applied on the bump-shaped solder layer 38 formed on the substrate 22 side. After that, the drive element 28 is mounted on a predetermined position on the substrate 22. The substrate 22 on which the drive element 28 is mounted is melted with solder in a reflow furnace,
The drive element 28 and the substrate 22 are electrically and mechanically connected.

The above-mentioned Pd metal layer 41 and the polyimide layer 4
The chemical adhesion with 0 is 200 kg / cm ² , which is very strong as compared with the conventional adhesion between the Al layer 5 and the Ni layer 11. This is because the adhesion density of Pd particles to the polyimide layer is high, and therefore the adhesion is improved. Therefore, the Pd metal layer 41 can prevent the Pd metal layer 41 from undergoing thermal strain between the drive element 28 and the substrate 22 having different contraction rates due to the periodic thermal change during the operation of the thermal head 21.
It can be prevented from peeling off. Adhesion between Pd metal layer 41 and Ni electrode layer 42 and Cu metal layer and P
The adhesion with the d metal layer 41 is about the same. Therefore, a Cu metal layer may be used on the Pd metal layer 41 instead of the Ni electrode layer 42.

Since the polyimide layer 40 has a very low Young's modulus, the polyimide layer 40 elastically deforms to absorb the thermal stress caused by the thermal strain as described above. Therefore, by forming the polyimide layer 40 on the substrate 22, the substrate 2 due to thermal strain is formed.
It is possible to prevent the occurrence of the warp of 2. Therefore, the area of the connecting portion between the drive element 28 and the electrode 42 can be reduced, and the drive element 28 can be densely wired in the electrode 4.
2. For example, it can be mounted on a thermal head of 16 dots / mm or more.

Further, the Ni electrode layer 42 is the Pd metal layer 4
Since it is formed on the polyimide layer 40 through the polyimide layer 1, the unevenness of the surface of the substrate 22 made of ceramic has the polyimide layer 4
Flattened by 0 and N by photolithography
A high-density wiring pattern of the i electrode 42 can be formed. Further, in the Ni electrode layer 42, since wiring is formed on the polyimide layer 40 via the Pd metal layer 41, the occurrence of open shorts is reduced.

FIG. 3 is a sectional view showing the structure of a thermal head 21a according to another embodiment of the present invention. This thermal head is similar to the thermal head shown in FIG.
Corresponding parts are designated by the same reference numerals.

In this thermal head 21a, since the heat storage layer 40a and the electric insulating layer 40b can be formed of polyimide in one step, the thermal head 21a can be efficiently formed in a small number of steps. The other parts are similar to those of the thermal head shown in FIG.

As described above, in this embodiment, the thermal head 2
Although the mounting structure of the driving elements 28 of 1 and 21a has been described, the mounting structure of the electronic component of the present invention is, for example, an LED.
It may be used for a device such as a line head for printing such as a (light emitting diode) head and an EL (electroluminescence) head.

[0030]

As described above, according to the present invention, when heat is applied to the mounting portion of the electronic component, the electronic component, the substrate, and the layers interposed between the electronic component and the respective layers have different shrinkage rates. This causes thermal strain. This causes thermal stress in each of the layers. However, the electrical insulation layer made of polyimide and the lower layer made of Pd have strong adhesion, and polyimide has a very low Young's modulus.
Even with respect to the above-mentioned thermal strain, the electrical insulating layer elastically deforms to absorb the thermal stress. In addition, since the adhesion between the electric insulating layer and the lower layer is strong, it is possible to prevent film peeling between the electric insulating layer and the lower layer. At the same time, the warp of the substrate can be prevented.

Further, since the adhesive force between the electric insulating layer and the lower layer is strong as described above, the solder area between the electronic component and the substrate can be reduced, whereby the electronic component can be placed on the substrate. It can be mounted at high density.

Further, in the wiring layer having a two-layer structure of the lower layer made of palladium and the upper layer made of nickel or copper, the electrical insulation layer formed below flattens the unevenness on the substrate surface. Thus, high-density wiring can be formed. Moreover, since the wiring is formed on the electric insulating layer by the wiring layer, the occurrence of open short circuit is reduced.

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing a configuration of a thermal head 21 which is an embodiment of the present invention.

FIG. 2 is an electrode 31 of the thermal head 21 and a driving element 28.
It is sectional drawing which shows the structure of the connection part with.

FIG. 3 is a thermal head 21 according to another embodiment of the present invention.
It is sectional drawing which shows the structure of a.

FIG. 4 is a sectional view showing an example of a conventional thermal head 1.

FIG. 5 is a cross-sectional view showing a joint between a drive element 8 and an electrode 5.

[Explanation of symbols]

 21 Thermal Head 22 Substrate 23 Heat Storage Material 24 Polyimide Layer 25 Ni Electrode Layer 27, 35 Protective Film 28 Drive Element 30 Sealing Resin Layer 33 Insulating Film 34 Al Electrode 36 Cr Layer 37 Cu Layer 38 Solder Layer

Claims (1)

[Claims] 1. A mounting structure of an electronic component for mounting an electronic component on a ceramic substrate by joining electrodes of the electronic component to a wiring layer provided on the ceramic substrate via solder, wherein the wiring layer is palladium. An electronic component mounting structure having a two-layer structure of a lower layer made of and an upper layer made of nickel or copper.