JP7632820B2 - Method for manufacturing power semiconductor substrate and heat-resistant glass substrate - Google Patents

Description

The present invention relates to a manufacturing technology for substrates suitable for power semiconductors that control and supply electric power.

In recent years, the demand for so-called power semiconductors for high power and high voltage applications has been increasing due to the spread of electric vehicles. Power semiconductors are inevitably prone to high temperatures, so the substrates on which they are mounted must be heat resistant. For this reason, ceramic substrates such as alumina are used as substrates for power semiconductors.
Photolithography is used to form electrodes on the substrate. In other words, the process involves forming a metal layer on the ceramic substrate (vacuum deposition), forming a resist layer, exposing the pattern to light, developing, etching, and removing the resist, to produce a ceramic substrate on which power semiconductors can be mounted by soldering in a later process.

However, the conventional techniques have the following problems.
Conventional circuit boards for mounting power semiconductors had the problem that the ceramic material used was not cheap.
In addition, multiple processes such as vacuum deposition and photolithography are required. In addition, ceramics require high-temperature firing, and since their hardness increases after firing, they are generally not easy to process and tend to chip or crack. This makes cutting after photolithography difficult, and generally results in low manufacturing efficiency.
Furthermore, there is a method of forming a copper thick film circuit on ceramics using a copper-based conductive paste, but it is necessary to adjust the amount of glass component to improve adhesion; a small amount of glass component results in poor adhesion, while a large amount of glass component improves adhesion but poor conductivity. In addition, the glass component is precipitated on the surface of the conductor wiring during firing treatment, which deteriorates solder wettability, making it difficult to mix the paste components.

Patent Publication No. 2008-7399 JP 5-206635 A JP 8-298359 A JP2008-243840

The present invention has been made in consideration of the above, and aims to provide a power semiconductor substrate or a heat-resistant substrate that is inexpensive and has excellent manufacturing efficiency.

The method for manufacturing a power semiconductor substrate according to claim 1 is characterized by including, in order, an adhesion layer forming step of applying an adhesion paste to a glass substrate by screen printing to ensure adhesion between the glass substrate and the electrodes, an electrode pattern layer forming step of applying a metal fine particle paste by screen printing along a pattern for forming electrodes, a wet layer forming step of applying a solder paste by screen printing to ensure wettability when soldering power semiconductor elements and other components, and a solidification step of baking at a predetermined temperature to solidify each paste portion.

In other words, the invention of claim 1 increases the freedom of material selection and mixing of metal particles and binders by using different pastes, ensures reliability and workability of soldering to electrodes and adhesion of electrodes to substrates, and makes it possible to manufacture electrode substrates that are excellent in heat resistance and suitable for large current applications by a simple and inexpensive method using glass, which is cheaper than ceramics and has excellent workability.

The glass substrate is not particularly limited, but it is preferable that it does not change in quality even in a high-temperature environment and has a small expansion coefficient. In addition, as described later, the substrate may be cut or divided, so it is preferable that it is suitable for such processability. Silica glass or borosilicate glass, which are of good quality and inexpensive, can be used. For example, glass for liquid crystal displays can be used.
The material for ensuring adhesion in the adhesion paste may be appropriately selected and mixed according to the glass and metal particles (and possibly the binder) used. For example, phenol resin may be used as the binder, and butyl carbitol may be used as the solvent.
The metal microparticle paste may also be prepared by appropriately selecting and mixing materials in combination with glass, adhesion paste, solder paste, etc. Since the substrate is for power semiconductors, the metal microparticles, i.e., the filler, are preferably microparticles mainly made of copper or silver. The particle size and particle size distribution may also be appropriately designed, but the particle size is preferably on the order of micrometers to nanometers. Examples of binders include phenolic resins, and examples of solvents include butyl carbitol. The paste may contain a small amount of dispersant, etc., as long as it does not affect the product using the substrate.
The material for improving the wettability of the solder in the solder paste may be appropriately selected and mixed according to the metal particle paste used. For example, a paste using silver-coated copper as a filler and epoxy resin as a binder may be used.

Screen printing is a technique in which a plate (screen mask) is placed directly on the substrate or on the lower layer, and the paste is rubbed from above, transferring, applying, and forming a paste layer through the mesh in the screen mask. The mesh (holes) form the pattern. Since it is easy to apply multiple layers, it is possible to apply them thicker as desired to reduce the resistance of the electrodes and suppress heat generation.
The screen mask is replaced depending on the paste. The mesh part of the screen mask used in the adhesion layer forming process and the screen mask used in the electrode pattern layer forming process have approximately the same shape (same pattern) (the mesh part for the adhesion layer may be slightly larger). In addition, if the mesh part of the screen mask used in the wetting layer forming process is limited to the part or area where the power semiconductor element and other components are soldered, waste of paste can be reduced.
Depending on the specification, after the wetting layer formation step, screen printing may be added to form an insulating layer, an oxidation-resistant layer, or a protective layer.
Solidification has a broad definition and includes not only firing to remove each binder and solvent and bond the metal particles to exhibit the desired resistance value, but also drying and hardening so that the paste does not drip, if this is to be fired separately at a later time.
In the present invention, the electrodes can also be expressed as a circuit as appropriate.

The method for manufacturing a power semiconductor substrate described in claim 2 is the method for manufacturing a power semiconductor substrate described in claim 1, characterized in that the electrode pattern layer formation process is repeated multiple times to increase the thickness of the electrode.

In other words, the invention of claim 2 makes it possible to easily achieve low electrode resistance and to suitably manufacture substrates for power semiconductors.

The heat-resistant glass substrate described in claim 3 is characterized in that an adhesion paste for ensuring adhesion between the glass substrate and the electrodes is applied to the glass substrate by screen printing, then a metal fine particle paste is applied by screen printing along the pattern for forming the electrodes, then a solder paste for ensuring wettability when soldering power semiconductor elements and other components is applied by screen printing, and then the substrate is fired at a predetermined temperature to dissipate the binders in each paste, forming a layer that ensures solder wettability on part or all of the surface of the electrode pattern that is in contact with the glass.

In other words, the invention of claim 3 provides an inexpensive electrode substrate that is excellent in heat resistance and processability, while ensuring the reliability and ease of soldering to the electrodes and the adhesion of the electrodes to the substrate, due to the high degree of design freedom.

"Dispersing the binder" means dispersing unnecessary components, and includes dispersing solvents, etc.

When forming electrode pattern layers multiple times as in claim 2, depending on the viscosity and properties of the paste, a drying process for drying the previous electrode pattern layer may be inserted as appropriate to ensure stable lamination of each layer.

In addition, the electrode pattern layer forming process may be repeated several times with different patterns, and an insulating layer forming process may be inserted between the patterns to form a three-dimensional circuit. This increases the degree of freedom in electrode design.
In this case, the insulating layer may be formed by screen printing a paste mainly made of fine particles of an insulating material, or a separate (local) insulating sheet may be inserted.

In addition, a separation process of cutting or dividing the glass substrate may be further included. This allows a large number of power semiconductor substrates of the same or multiple patterns to be manufactured from a single glass substrate, improving manufacturing efficiency. In addition, compared to ceramics, there is less damage during processing, which improves yield and allows substrates to be supplied at low cost.
The separation process can be performed before or after the power semiconductor elements and other components are soldered, and can be determined taking into consideration the processes before and after and workability.

The present invention makes it possible to provide power semiconductor substrates or heat-resistant substrates at low cost with excellent manufacturing efficiency.

4 is a flowchart showing a method for manufacturing a power semiconductor substrate according to the first embodiment. 2(a) is a schematic diagram showing the state of layer formation, in which Fig. 2(a) is a plan view of the electrode pattern layer printed, Fig. 2(b) is a plan view of the wetting layer itself, and Fig. 2(c) is a plan view of the substrate immediately after the wetting layer is formed. 10 is a flowchart showing a method for manufacturing a heat-resistant substrate for power semiconductors according to a second embodiment.

[First embodiment]
Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings.
1 is a flow chart showing an embodiment of a method for manufacturing a power semiconductor substrate according to the present invention. A schematic diagram showing how an electrode pattern or the like is formed is added to the right of the flow chart.

The screen printer or screen printing technology used in this invention is general-purpose, so a detailed description of its configuration will be omitted.

In this embodiment, the paste used has the following features.
・Adhesion paste: Paste with a high resin content ・Metal fine particle paste: Highly conductive paste that mainly uses copper fine particle powder with particle diameters of 10 nm to 200 μm ・Solder paste: Paste with a low resin content that is resistant to oxidation

Here, an example is described in which 4 x 5 glass substrates 10 measuring 200 mm x 200 mm x 4 mm are arranged and printed. The size of the printing surface is 800 mm x 1000 mm. The glass substrates 10 are made of borosilicate glass that is commercially available for use in LCD televisions. Glass manufactured for use in LCD televisions is mass-produced, easily available, inexpensive, and has no distortion in the flat surface (excellent flatness), making it ideal.

First, the adhesive layer 20 is formed on the surface of the glass substrate 10 (step S1).
The adhesion layer 20 is an intervening layer for ensuring that the electrodes 35, which are essentially made of copper and originate from the electrode pattern layer 30, adhere completely to the glass substrate 10 and do not peel off.
When forming the adhesion layer 20, first, a screen mask on which a mesh having the same shape as the electrode 35 is formed is fixed above the glass substrate 10 with a clearance of 0.5 mm. Next, adhesion paste is placed on the screen mask, and the same paste is also applied to the squeegee. The squeegee is pressed against the screen mask with a predetermined pressure or sinking depth, and moved from one end of the screen mask to the other end at a predetermined speed.
By the above operations, the adhesive layer 20 is transferred and formed on the glass substrate 10 in accordance with the shape of the electrodes.

In addition, when forming each subsequent layer, a drying process is inserted depending on the specifications in order to increase the coating stability of the layer placed on top. The drying process does not necessarily mean heating, but it is sufficient to create a surface condition that allows the paste to adhere stably on top, and it also includes leaving it for a specified period of time.

Next, the electrode pattern layer 30 is formed on the adhesive layer 20 (step S2a).
The electrode pattern layer 30 refers to a layer that is to become an electrode 35 made of copper after the binder is evaporated (i.e., diffused) in the firing step (step S4).
When forming the electrode pattern layer 30, a screen mask with a mesh formed in the portion that will become the electrode 35 is first fixed above the adhesive layer 20 with a clearance of 0.5 mm plus an increase in film thickness. Next, metal fine particle paste is placed on the screen mask, and the same paste is also applied to the squeegee. The squeegee is pressed against the screen mask with a predetermined pressure or sinking depth and moved from one end of the screen mask to the other at a predetermined speed.
Here, multiple layer formations are performed based on the design value for reducing the resistance of the electrode 35. In this embodiment, a three-layer thickness is used. That is, the electrode pattern layer 30 is placed on top again (step S2b), and then another electrode pattern layer 30 is placed on top (step S2c).
By the above operation, an electrode pattern layer 30 having the same shape as the adhesive layer 20 is formed on the adhesive layer 20. The thickness of the electrode pattern layer 30 is 10 μm by one printing and 30 μm by three printings.
The clearance between the screen mask and the adhesive layer 20 during printing may be fixed at a position that does not add to the increase in film thickness since the film thickness formed by screen printing is 10 μm.

Next, the wetting layer 40 is formed on the electrode pattern layer 30 (step S3).
The wetting layer 40 is a layer that is placed to ensure wettability when soldering power semiconductor elements and other components that will be mounted in a later process.
When forming the wetting layer 40, first, a screen mask with a mesh formed only around the element legs is fixed above the electrode pattern layer 30 with a clearance of 0.5 mm plus the film thickness increase. Next, solder paste is placed on the screen mask, and the same paste is also applied to the squeegee. The squeegee is pressed against the screen mask with a predetermined pressure or sinking depth and moved from one end of the screen mask to the other at a predetermined speed.
By the above operation, a wetting layer 40 is partially formed on the electrode pattern layer 30 .

The mesh of the screen mask in each process has a thickness and hole size appropriate for each paste.

Next, the glass substrate 10 on which each layer has been formed is baked (step S4). The maximum temperature and baking time, as well as the temperature increasing and decreasing steps, can be designed as appropriate.
This causes the binder in each paste to evaporate, forming a low-resistance copper electrode 35 that is fully bonded to the glass substrate 10 and has good wettability in the spot where it is soldered. The substrate in this state is referred to as substrate 11.

Next, the power semiconductor elements and other components are soldered to the substrate 11 (step S5).
Finally, the substrate 11 is cut out and separated into products 13 on which the power semiconductor elements and other components are attached (step S6). The substrate 11 can be cut out by using a suitable glass cutting technique. Alternatively, grooves for cutting out may be formed in the glass substrate 10 from the beginning.

By going through the above steps, a product 13 on which a power semiconductor is mounted can be obtained. If the substrate 11 is cut out before the power semiconductor is mounted, a substrate for a power semiconductor can be obtained.
FIG. 2 shows the state of layer formation.

[Embodiment 2]
Next, a method for manufacturing a heat-resistant substrate for a power semiconductor having three-dimensionally crossed portions will be described.
3 is a flow chart for manufacturing a heat-resistant substrate for a power semiconductor, which is three-dimensionally crossed. An enlarged schematic diagram showing how an electrode pattern or the like is formed is added to the right side of the flow chart.

It should be noted that the description of the same steps as those in the first embodiment will be omitted as appropriate.
In this embodiment, in addition to the paste of the first embodiment, the following paste is also used.
・Insulating paste: Paste containing a large amount of insulating material

First, the adhesive layer 20 is formed on the surface of the glass substrate 10 (step S1).
The adhesion layer 20 is an intervening layer for ensuring that the electrodes 35 and 36 adhere completely to the glass substrate 10 and do not peel off.

Next, the electrode pattern layer 30 is formed on the adhesive layer 20 (step S2a).
Furthermore, the electrode pattern layer 30 is placed on top (step S2b).

Next, the insulating layer 50 is formed (step S3).
The insulating layer 50 is a layer that is interposed at the intersection between the electrode pattern layer 30 and the electrode pattern layer 31 that is placed directly on the insulating layer 50. It is provided to prevent the electrodes from shorting out.
When forming the insulating layer 50, a screen mask with a mesh formed at the intersections of the electrodes is fixed above the electrode pattern layer 30 with a clearance of 0.5 mm plus the film thickness increase. Next, insulating paste is placed on the screen mask, and the same paste is also applied to the squeegee. The squeegee is pressed against the screen mask with a predetermined pressure or sinking depth and moved from one end of the screen mask to the other at a predetermined speed.
By the above operations, the insulating layer 50 is formed on the electrode pattern layer 30.

Next, the electrode pattern layer 31 is formed (step S4a).
Furthermore, the electrode pattern layer 31 is placed on top (step S4b).
The electrode pattern layer 31, like the electrode pattern layer 30, is a layer that is to become the copper electrode 36 when the binder is removed in the firing step (step S6). Note that the electrodes 35 and 36 have crossing portions and are not the same.
When forming the electrode pattern layer 31, first, a screen mask with a mesh formed in the portion that will become the electrode 36 is fixed above the insulating layer 50 with a clearance of 0.5 mm plus an increase in film thickness. Next, metal fine particle paste is placed on the screen mask, and the same paste is also applied to the squeegee. The squeegee is pressed against the screen mask with a predetermined pressure or sinking depth, and moved from one end of the screen mask to the other at a predetermined speed.
Here, layers are formed multiple times based on a design value for reducing the resistance of the electrode 36. In this embodiment, two layers are formed.
The electrode 36 is formed across the adhesive layer 20 and the insulating layer 50, and has a height difference. This can be dealt with by partially changing the mesh holes or thickness, or, depending on the specification, for example, in step S4a, the electrode pattern layer 31 may be formed by applying different layers of paint multiple times using different screen masks. Also, since the film thickness of the electrode 36 is 140 μm as described later, it is not necessary to deal with the height difference.
By the above operations, an electrode pattern layer 31 having three-dimensional intersections is formed.

Next, the wetting layer 40 is formed on the electrode pattern layer 31 (step S5).
Next, the glass substrate 10 on which each layer has been formed is baked (step S6). The substrate in this state is referred to as substrate 12.

Finally, the substrate 12 is cut out (step S7). This results in a heat-resistant substrate 14 that can be used in high-temperature environments for power semiconductors and other applications.

As described above, according to the present invention, a glass substrate is used instead of a ceramic substrate which is difficult to process and requires vacuum deposition, so that the substrate can be easily cut and electrodes can be formed under normal pressure. The substrate also has excellent dimensional stability.
Furthermore, for use with large currents, in addition to thick coating, it is necessary to increase the proportion of metal particles in the paste, which can potentially impair adhesion to the glass. However, by using screen printing technology, a separate adhesion paste can be applied, making it easy to ensure or improve adhesion reliability.
Similarly, the wettability of the solder may be impaired depending on the type, particle size, ratio, and type of binder of the metal particles; however, by using screen printing technology, a separate solder paste can be interposed, making it possible to easily ensure or improve the wettability of the solder.

It should be noted that the present invention is not limited to the above-described embodiment.
For example, the adhesive layer may be 20 μm thick, the electrode pattern layer may be 10 μm thick by one printing, or 100 μm thick by ten printings, the wetting layer may be 20 μm thick, and the insulating layer may be 20 μm thick.
When the thickness is important, the film thickness can be set to 20 μm, and when forming fine lines, the film thickness can be 10 μm.

The present invention makes it possible to provide power semiconductor substrates or heat-resistant substrates at low cost with excellent manufacturing efficiency.

REFERENCE SIGNS LIST 10 Glass substrate 11 Substrate 12 Substrate 13 Product 14 Heat-resistant substrate 20 Adhesion layer 30 Electrode pattern layer 31 Electrode pattern layer 35 Electrode 36 Electrode 40 Wetting layer 50 Insulation layer

Claims (3)

an adhesion layer forming step of applying an adhesion paste to the glass substrate by screen printing in order to ensure adhesion between the glass substrate and the electrodes;
an electrode pattern layer forming step of applying a metal fine particle paste by screen printing along a pattern for forming electrodes;
A wetting layer forming process for applying a solder paste by screen printing to ensure wettability when soldering power semiconductor elements and other components;
a solidification step of baking at a predetermined temperature to solidify each paste portion;
A method for manufacturing a power semiconductor substrate, comprising the steps of: The method for manufacturing a power semiconductor substrate according to claim 1, characterized in that the electrode pattern layer formation process is repeated multiple times to increase the thickness of the electrode. A bonding paste is applied to the glass substrate by screen printing to ensure the bonding between the glass substrate and the electrodes.
Next, a metal fine particle paste is applied by screen printing along a pattern for forming electrodes,
Next, a solder paste is applied by screen printing to ensure wettability when soldering power semiconductor elements and other components.
Then, the paste is fired at a predetermined temperature to dissipate the binders in the pastes.
A heat-resistant glass substrate, characterized in that a layer that ensures solder wettability is formed on a part or all of the surface of an electrode pattern that is in close contact with glass.