JP2013100199A - Translucent wiring board - Google Patents

Abstract

PROBLEM TO BE SOLVED: To maintain the adhesion of a wiring pattern to a translucent alumina substrate after applying a heat cycle, in a translucent wiring board, and to ensure translucency of the wiring board.SOLUTION: A translucent wiring board 8 comprises a translucent alumina substrate 1 and a wiring pattern 2 formed on the translucent alumina substrate 1. Translucent alumina constituting the translucent alumina substrate 1 has an SiOcontent of 150-300 mass ppm and an MgO content of 100-250 mass ppm, the SiOcontent is larger than the MgO content, the purity of alumina excluding SiOand MgO is ≥99.9 mass%, and a crystal grain diameter is ≥20 μm. The total forward transmittance of the translucent alumina substrate 1 in a part other than the wiring pattern 2 is ≥50%.

Description

  The present invention relates to a translucent wiring board having high translucency and having high adhesion strength of a wiring pattern even after thermal cycling.

  Alumina ceramics are widely used as substrates for electronic components because of their characteristics such as electrical insulation and chemical stability. In recent years, the number of cases used as a substrate for a semiconductor light emitting device is increasing by taking advantage of the characteristics of high thermal conductivity. However, general alumina ceramics are opaque, and only one side of the light emitted from the semiconductor light emitting element can be used directly, and the light emitted to the opposite side of the substrate is used after being subjected to treatment such as reflection. (Patent Document 1). If a light-transmitting substrate is used, such a structure is not necessary. However, when glass or resin, which is a general light-transmitting material, is used as the substrate, the thermal conductivity is low, so the temperature of the semiconductor light-emitting element increases. In addition, the light emission efficiency of the light emitting element itself is lowered.

  On the other hand, particularly high-purity alumina ceramics have high translucency by firing in a reducing atmosphere such as vacuum or hydrogen. (Patent Document 2).

  Translucent alumina ceramics have high thermal conductivity in addition to translucency, and if this can be used as a substrate for semiconductor light-emitting devices, the light-emitting efficiency will not decrease due to temperature rise from both sides of the light-emitting device. Can be extracted (Patent Document 3).

  In addition, a metal wiring pattern is formed on the surface of an alumina substrate. Here, in order to improve the adhesion between the wiring pattern and the alumina substrate, it is known that the entire surface of the alumina substrate is etched using alkali, acid, ion beam or the like, and then the wiring pattern is formed (Patent Document). 4, 5). Moreover, a method of providing irregularities larger than alumina particles on the surface of an alumina substrate is known (Patent Document 6). In addition, in the patent document 7, the present applicant discloses a light-emitting diode element diffusion plate made of a homogeneous translucent alumina sintered body.

JP 2003-243717 A Patent 2780941 JP 2002-289925 JP 61-251589 JP-A-5-024959 JP 2010-30280 JP2007-258228A

When an alumina ceramic substrate is used as a substrate for a semiconductor light emitting device, it is necessary to form a wiring pattern for supplying power to the light emitting device on the surface of the substrate. The wiring pattern is usually a silver or copper paste printed by a thick film process and then heat treated to combine the binder component contained in the paste with the components other than alumina (mainly SiO ₂ ) contained in the substrate. To make it in close contact. However, translucent alumina ceramics have a higher purity than ordinary alumina, and the amount of impurities is extremely small, resulting in insufficient adhesion, and problems such as peeling of the wiring pattern due to repeated temperature cycles due to lighting are likely to occur.

  Increasing the amount of impurity components in alumina can increase the adhesion to the wiring pattern, but the translucency is lowered, making it difficult to use the light emitted from the light emitting element. Further, if the crystal particle diameter is reduced, the adhesion can be improved, but the translucency is lowered and it becomes difficult to use the light emitted from the light emitting element.

  In order to solve this problem, the present inventor also tried a method as described in Patent Documents 3, 4, and 5. In Patent Document 3, an alumina substrate that is not light-transmitting is etched with an acid / alkali, whereby the surface of the alumina substrate is finely roughened over the entire surface. The surface roughness does not change before and after etching. Next, a wiring pattern is deposited on the alumina substrate.

  However, according to the study of the present inventor, even if the surface of the translucent alumina substrate is processed by acid / alkali etching, the translucent alumina has few impurities and has a high smoothness as described above. For this reason, fine etching that does not increase the surface roughness as described in Patent Document 4 has no effect of improving the adhesion of the wiring pattern, and the wiring pattern peels off after the thermal cycle. Further, when the surface roughness of the translucent alumina substrate is roughened by etching, the light transmittance is lowered.

  Moreover, etching of the surface of a translucent alumina substrate using an ion beam as described in Patent Document 5 was also examined. However, this method also causes the same problem as in Patent Document 4.

  As described in Patent Document 6, a method of providing irregularities larger than alumina particles was also examined. However, in the case of translucent alumina, the average particle diameter of the alumina particles is very large in order to develop translucency. For this reason, if unevenness sufficiently larger than the alumina particles is provided, a fine pattern or a complex pattern cannot be formed on the substrate surface, and the wiring cannot be made finer. For this reason, this technique cannot be used industrially in the field of translucent wiring boards. Moreover, if the substrate surface has irregularities sufficiently larger than the translucent alumina particles, the printing plate making is damaged when the wiring pattern is screen-printed on the substrate surface.

  An object of the present invention is to provide a light-transmitting wiring board having a light-transmitting alumina substrate and a wiring pattern formed on the surface of the light-transmitting alumina substrate. It is intended to maintain adhesion to the alumina substrate and to obtain the translucency of the wiring substrate.

The present invention is a translucent wiring board comprising a translucent alumina substrate and a wiring pattern formed on the translucent alumina substrate,
The content of SiO _{2 in} the translucent alumina constituting the translucent alumina substrate is 150 to 300 ppm by mass, the content of MgO is 100 to 250 ppm by mass, and the content of SiO _{2 is} the content of MgO. More than that, the alumina purity excluding SiO ₂ and MgO is 99.9% by mass or more, the crystal particle diameter is 20 μm or more, and the total transmittance of the translucent alumina substrate other than the wiring pattern is 50% or more. It is characterized by being.

  According to the present invention, it is possible to prevent peeling of the wiring pattern from the surface of the alumina substrate even after the thermal cycle is applied, while maintaining the translucency of the translucent alumina substrate.

It is a top view showing typically wiring board 8 concerning one embodiment of the present invention. FIG. 4 is a cross-sectional view of the wiring board 8 after the wiring pattern 7 is provided on the wiring pattern formation region 6 of the translucent alumina substrate 1. It is a schematic diagram for demonstrating the measuring method of an average particle diameter. It is a schematic diagram which shows the measuring method of the translucency of a wiring board.

(Wiring board formation procedure)
A translucent alumina substrate is produced by molding and sintering the alumina raw material powder. At this time, it is preferable that the surface of the translucent alumina substrate is a smooth surface, and thereby the total transmittance of the substrate can be increased.

  In a preferred embodiment, as shown in FIGS. 1 and 2, the surface 1a of the translucent alumina substrate 1 is a smooth surface. Although 1b is a back surface, it is preferable to make it a smooth surface. Next, as shown in FIG. 2, a wiring pattern 7 is formed on the substrate surface 1 a in the wiring pattern formation region 6. The wiring pattern 7 is not formed in the non-formation region 5. 3 is a soldering part.

(Translucent alumina substrate)
Translucent alumina can be used, for example, as a substrate for mounting a light-emitting diode element, whereby the life of the light-emitting diode element can be dramatically extended.

  The thickness of the translucent alumina substrate is preferably 0.2 mm or more and 2 mm or less. If the substrate is too thin, it will be easily cracked by impact, or the ratio of linearly transmitted light will be too high, and light diffusion will be insufficient. If the substrate is too thick, the total light transmittance is lowered and the heat dissipation is also lowered.

  The linear transmittance in the visible light region of the translucent alumina substrate is preferably 40% or less, and more preferably 15% or less, for light diffusion. The total front transmittance of the portion other than the wiring pattern of the translucent alumina substrate is preferably 50% or more from the viewpoint of luminous efficiency.

(Translucent alumina)
The average crystal grain size of the translucent alumina constituting the substrate is preferably 20 μm or more from the viewpoint of obtaining the above high translucency. The crystal grain size of alumina is preferably 100 μm or less, and more preferably 40 μm or less so that the substrate strength is maintained and the ratio of linear transmitted light is not excessively increased.

In addition, the average crystal grain diameter here is a value measured by the following method.
(1) A micrograph (100 to 200 times) of the surface of the sintered body is taken, and the number of particles crossed by a unit length straight line is counted. This is performed at three different locations. The unit length is in the range of 500 μm to 1000 μm.
(2) Take the average of the number of particles in three places.
(3) The average particle diameter is calculated by the following formula.
[Calculation formula]
D = (4 / π) × (L / n)
[D: average particle diameter, L: linear unit length, n: average number of particles at three locations]
An example of calculating the average particle diameter is shown in FIG. When the number of particles 32 traversed by the straight line 30 of unit length L (for example, 500 μm) is 22, 23, and 19 at three different positions, the average particle diameter D is calculated by the above calculation formula:
D = (4 / π) × [500 / {(22 + 23 + 19) / 3}] = 29.9 μm
It becomes.

The content of SiO _{2 in} the translucent alumina constituting the translucent alumina substrate is 150 to 300 ppm by mass. Within this range, the inventors have found that it is possible to prevent the wiring pattern from being peeled off from the surface of the alumina substrate even after the thermal cycle is applied, while maintaining the translucency of the light transmitting alumina substrate. The content of SiO ₂ is more preferably 200 ppm or more.

  The content of MgO in the translucent alumina constituting the translucent alumina substrate is 100 to 250 ppm by mass. Within this range, the inventors have found that it is possible to prevent the wiring pattern from being peeled off from the surface of the alumina substrate even after the thermal cycle is applied, while maintaining the translucency of the translucent alumina substrate. The content of MgO is more preferably 150 ppm or more, and further preferably 200 ppm or less.

Further, it is necessary that the content of SiO ₂ is larger than the content of MgO. From the viewpoint of the present invention, the difference between the content of SiO _{2 and} the content of MgO is preferably 30 mass ppm or more, and more preferably 50 mass ppm or more. The difference between the content of SiO _{2 and} the content of MgO is 200 ppm by mass or less, and more preferably 150 ppm by mass or less.

The content of SiO _{2 and} the content of MgO in translucent alumina are measured by the amount of each element by inductively coupled plasma atomic emission spectrometry and converted to the amount of oxide.

In the translucent alumina, the purity of alumina excluding SiO ₂ and MgO is 99.9% by mass or more, thereby improving the total transmittance. Since the amount of impurities with respect to alumina is small, the adhesiveness after applying a heat cycle to the wiring pattern is lowered, and the present invention is particularly effective in improving this. The purity of alumina in the translucent alumina substrate is more preferably 99.95% or more. The alumina purity of the substrate is converted to an oxide amount by measuring the amount of aluminum element in the substrate by inductively coupled plasma atomic emission spectrometry.
Examples of components other than SiO _2, MgO, and alumina contained in the translucent alumina substrate include the following. The content of these components in the ceramic constituting the substrate is the balance of SiO ₂ , MgO and alumina, and is 0.1% by mass or less.
(1) Oxides of elements such as Zr, Y, Sc added as sintering aids (preferably 300 to 1000 ppm by mass)
(2) Metal components such as oxides such as Fe, Na, Ca, and K mixed with alumina raw materials and substrates, and W and Mo

(Manufacture of translucent alumina substrate)
The method for forming the translucent alumina substrate is not particularly limited, and may be any method such as a doctor blade method, an extrusion method, or a gel cast method. Particularly preferably, the substrate is manufactured using a gel cast method. In a preferred embodiment, a molded article is obtained by casting a slurry containing ceramic powder, a dispersion medium and a gelling agent and gelling the slurry, and the molded article is sintered.

Particularly preferably, a raw material obtained by adding MgO and SiO ₂ to a high-purity alumina powder having a purity of 99.95% or more (preferably 99.99% or more) is used. Examples of such high-purity alumina powder include high-purity alumina powder manufactured by Daimei Chemical Co., Ltd.

The average particle diameter of the raw material powder of the translucent alumina substrate is not particularly limited, but is preferably 0.5 μm or less, and more preferably 0.4 μm or less, from the viewpoint of densification during low temperature sintering and improvement of translucency. More preferably, the average particle diameter of the raw material powder is 0.3 μm or less (primary particle diameter). The lower limit of the average particle size is not particularly limited. The average particle diameter of the raw material powder can be determined by direct observation of the raw material powder by SEM (scanning electron microscope).
The average particle diameter here is n = the value of (longest axis length + shortest axis length) / 2 of primary particles excluding secondary agglomerated particles on an SEM photograph (magnification: X30000, arbitrary two fields of view). It is an average value of 500.

Examples of the gel casting method include the following methods.
(1) Along with inorganic powder, a prepolymer such as polyvinyl alcohol, epoxy resin, phenol resin, or the like, which becomes a gelling agent, is dispersed in a dispersion medium together with a dispersing agent to prepare a slurry. The slurry is solidified by crosslinking and gelation.
(2) The slurry is solidified by chemically bonding an organic dispersion medium having a reactive functional group and a gelling agent.

  Next, the compact is sintered with the powder in a reducing atmosphere. An example of this reducing atmosphere is H2. The sintering temperature is preferably 1700 ° C to 1900 ° C, more preferably 1750 ° C to 1850 ° C.

(Smooth surface)
In a preferred embodiment, a smooth surface is formed on the surface of the translucent alumina substrate and exposed to a region not covered with the wiring pattern. Thereby, the transmittance of the entire wiring board can be improved and secured. The surface roughness Ra of the smooth surface is more preferably 0.3 μm or less from the viewpoint of improving the total transmittance. Further, although there is no particular lower limit of the surface roughness of the smooth surface, 0.1 μm or more is practical for processing.

  The smooth surface is exposed in a region not covered with the wiring pattern, but preferably the smooth surface is exposed over the entire surface including the wiring pattern region. The surface roughness Ra of the smooth surface is a centerline average surface roughness defined by JIS B 0601, and is measured by a stylus type surface roughness measuring instrument as defined by JIS B 0651.

(Smooth surface forming method)
In order to form a smooth surface on the translucent alumina substrate, a smoothing portion corresponding to the smooth surface can be provided in the molding die. Further, a smooth surface can be formed by polishing the surface of the translucent alumina substrate using diamond paste or the like.

(Wiring pattern)
As a method for forming a wiring pattern, there are a screen printing method, an ink jet printing method, and a pad printing method.

  The heat treatment temperature when forming the wiring pattern is preferably 500 to 1000 ° C. The wiring pattern material is preferably Au, Ag, W, Mo, Pt, or a compound containing these. Furthermore, the thickness of the wiring pattern is preferably 0.5 to 20 μm from the viewpoint of the present invention.

(Experiment 1)
A translucent wiring board 8 having a configuration as shown in FIGS. 1 and 2 was produced.
Specifically, the translucent alumina ceramic substrate 1 was produced as follows. That is, 300 mass ppm of magnesium oxide, 350 mass ppm of silica powder, 2 mass percent of polyvinyl alcohol, 0.5 mass percent of polyethylene glycol, and 50 mass percent of water are added to high-purity alumina powder having an alumina purity of 99.99 mass%. The mixture was pulverized by a ball mill for 1 hour, mixed, and dried by a spray dryer at around 200 ° C. to obtain a granulated powder having an average particle size of about 70 μm. This granulated powder was press-molded under a pressure of 2000 kg / cm ² to obtain a substrate-like powder compact. This was calcined at 1200 ° C. in the atmosphere to obtain a calcined body.

Next, the calcined body was fired at 1800 ° C. in an atmosphere of hydrogen: nitrogen = 3: 1, and densified and translucent. As a result, a translucent alumina substrate having MgO and SiO ₂ contents of 250 ppm by mass and 300 ppm by mass, respectively, could be obtained. The contents of MgO and SiO ₂ were measured by inductively coupled plasma mass spectrometry. When the composition of this substrate was measured using inductively coupled plasma atomic emission spectrometry, the purity of alumina excluding the contents of MgO and SiO ₂ was 99.90% by mass.

  A silver paste was printed on the surface of the obtained translucent alumina substrate using a screen plate having a desired wiring pattern. Then, it was dried for 15 minutes with a 95 ° C. drier, and heat treatment was performed at a temperature of 850 ° C. to adhere the wiring pattern to the substrate surface.

When soldering was performed on this wiring pattern and the adhesive strength was measured, it was found that the wiring pattern had a sufficient strength of 18 N / mm ² . Furthermore, when the adhesive strength was measured after 1000 times of a temperature cycle in which −40 ° C. and 200 ° C. were alternately repeated every 30 minutes, 12 N / mm ² was inferior to the initial value, but still sufficient strength. I found out.

  Further, the total front transmittance of the portion without the wiring pattern (thickness: 1 mm) of the substrate was measured and found to be 55%. However, the front total transmittance was measured with an apparatus schematically shown in FIG. That is, monochromatic light having a wavelength of 555 nm is incident on the surface 1a of the wiring board 8 from the light source 10 as indicated by an arrow A, and radiated light emitted from the back surface 1b toward each point of the integrating sphere 12 as indicated by an arrow B. Detection is performed by the detector 11.

(Experiments 2-20)
A light-transmitting wiring board was produced in the same manner as in Experiment 1, and various characteristics were measured. However, the composition of the obtained sintered body was changed as shown in Table 1, Table 2, and Table 3 by variously changing the addition amounts of magnesium oxide and silica added to the high-purity alumina powder. The results are shown in each table.

  As can be seen from the table, according to the present invention, it is possible to remarkably improve the adhesion after applying the heat cycle while ensuring the translucency of the translucent wiring board.

Claims (3)

A translucent wiring board comprising a translucent alumina substrate and a wiring pattern formed on the translucent alumina substrate,
The translucent alumina constituting the translucent alumina substrate has a SiO ₂ content of 150 to 300 ppm by mass, a MgO content of 100 to 250 ppm by mass, and a SiO ₂ content of MgO. More than the amount, the alumina purity excluding SiO ₂ and MgO is 99.9% by mass or more, the crystal particle diameter is 20 μm or more, and the front total transmittance of the translucent alumina substrate other than the wiring pattern is 50 % Of translucent wiring board, characterized in that it is at least%.   The translucent wiring board according to claim 1, wherein the translucent alumina is a sintered body fired at 1700 ° C. or higher and 1900 ° C. or lower in a reducing atmosphere.   3. The translucent wiring board according to claim 1, wherein a surface roughness Ra of a surface of the translucent alumina substrate on which the wiring pattern is printed is 0.3 μm or less.

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