JPH0225246B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to solid electrolytic capacitors, and in particular to improvement of characteristic deterioration caused by migration of metal members constituting electrode extraction layers into capacitor elements. In general, this type of solid electrolytic capacitor is made of a capacitor element made by press-molding and sintering metal powder that has a valve function into a cylinder, and a metal wire that has a valve function is installed in advance as an anode lead. Welding a first external lead member to the lead-out portion, and soldering a second external lead member to an electrode lead-out layer formed on the circumferential surface of the capacitor element via an oxide layer, a semiconductor layer, and a graphite layer, The entire circumferential surface of the capacitor element is covered with a resin material. By the way, in view of the fact that the graphite layer of the electrode lead layer in the capacitor element shows almost no wettability to the solder member, and it is almost impossible to solder the second external lead member to the graphite layer, It is made of a conductive material that has excellent electrical and mechanical connectivity to the graphite layer and excellent wettability to solder members. For example, the conductive member has an average particle size of 2 to 2.
One that contains 3μ silver powder and resin and has a silver powder ratio of 70% by weight to the total is widely used. The conductive member is usually constructed as a conductive suspension consisting of silver powder, an inorganic material, a resin, and a solvent, and the electrode extraction layer is formed by immersing the capacitor element in this conductive suspension and pulling it up.
Formed by heating to about 150°C.
The silver powder is fixed to the circumferential surface of the capacitor element by thermosetting the resin, and good electrical connection between the silver powder and the graphite layer is maintained. However, when such a solid electrolytic capacitor is used in a humid atmosphere, the silver constituting the electrode lead layer is ionized by the presence of moisture, and a migration phenomenon occurs.
For this reason, leakage current characteristics are impaired due to silver migration to the graphite layer, semiconductor layer, and oxide layer. This migration phenomenon is affected by ambient conditions, operating conditions, etc., but it becomes more pronounced when no direct current is applied to the first and second external lead members and when the humidity is high. The leakage current characteristics also tend to be significantly impaired. For this reason, there is a problem in that its use is severely restricted in highly reliable equipment that requires a stable and small leakage current value over a long period of time, such as precision equipment, audio equipment, and the like. Therefore, if it is possible to suppress the migration of silver into the capacitor element under no-load conditions and in a high-humidity atmosphere, it is possible to improve the leakage current characteristics and significantly increase the reliability of the capacitor. This makes it possible to apply it to highly reliable equipment, which is desirable. In view of these points, the present inventors believe that characteristic deterioration caused by the migration phenomenon can be suppressed by lengthening the migration path of silver constituting the electrode extraction layer to reach the oxidized layer in the capacitor element. Considering this, we investigated the relationship between the thickness of the graphite layer and the failure rate of leakage current, and obtained the results shown in the figure. In addition, tantalum powder is used in the capacitor element.
The material used was pressure-formed into a cylindrical shape of 3.5 x 4 mm and sintered. In addition, the failure rate of leakage current is determined by leaving the capacitor unloaded in an atmosphere with a temperature of 65℃ and relative humidity of 95% for 1000 hours, charging it with a DC voltage of 46V for 3 minutes, and measuring the leakage current. Calculated based on the results. According to the figure, as the thickness of the graphite layer becomes thicker, the incidence of defects due to leakage current decreases. For example, when the film thickness is in the range of 2 to 7μ, the defect rate is low.
While it is 60 to 66%, it drastically decreases to 30% at 10μ, and it gradually decreases even at higher film thicknesses. This is considered to indicate that increasing the thickness of the graphite layer makes it difficult for silver to reach the oxide layer. The present invention has been realized based on these facts, and includes a conductive layer containing silver powder interposed between an oxide layer, a semiconductor layer, and a graphite layer on the circumferential surface of a capacitor element made of a metal member having a valve action. The electrode drawing layer is formed from a member, and the graphite layer is characterized in that the thickness of the graphite layer is set to 10 μm or more and less than 50 μm. According to this invention, the thickness of the graphite layer can be
By setting the value to 10 μ or more and less than 50 μ, the path for silver to reach the oxidized layer constituting the electrode extraction layer can be lengthened. However, the incidence of defects due to leakage current can be significantly reduced compared to conventional products. However, when the film thickness becomes less than 10μ,
The incidence of defects due to leakage current increases rapidly, and conversely, when the film thickness exceeds 50μ, cracks tend to occur in the graphite layer, contact resistance increases, and it becomes difficult to expect highly reliable capacitors. Become. Next, examples will be specifically described. An oxide layer and a semiconductor layer are sequentially formed on a capacitor element made by press-molding tantalum powder into a 3.5 x 4 mm cylinder and sintering it. Then, this capacitor element was immersed in a graphite liquid prepared to have a viscosity of 300 CPS, and after being pulled up and heat treated, the surface layer (mainly the circumferential surface) of the capacitor element was coated.
A graphite layer of 13.5μ was formed. On top of this
An electrode lead layer is formed using a conductive material containing silver powder with a particle size distribution of 0.1 to 20 μm and an average particle size of 3 μm, and a tantalum solid electrolytic capacitor is manufactured using a conventional method. The initial characteristics of this capacitor and the characteristics after being left unloaded for 1000 hours in a high temperature and high humidity atmosphere were measured, and the results shown in the table below were obtained.

[Table] The above table shows that the product of the present invention has a significantly improved failure rate after 1000 hours compared to the conventional product, even though the electrode extraction layer contains minute silver powder that is easily ionized in the presence of moisture. Although it is clear that it is possible,
This is thought to be because the thicker graphite layer lengthened the route for silver to reach the oxide layer. As described above, according to the present invention, migration of silver constituting the electrode lead layer into the capacitor element can be suppressed with a simple configuration, and
The incidence of defects due to leakage current can be effectively reduced, and a high quality capacitor can be obtained.

[Brief explanation of drawings]

The figure is a diagram showing the relationship between the thickness of the graphite layer and the failure rate of leakage current.

Claims (1)

[Claims] 1. An electrode lead layer formed of a conductive material containing silver powder via an oxide layer, a semiconductor layer, and a graphite layer on the circumferential surface of a capacitor element made of a metal member having a valve action, in which the graphite layer is A solid electrolytic capacitor characterized by having a film thickness of 10μ or more and less than 50μ.