This invention relates to a chip type solid electrolytic capacitor, a method of manufacturing the capacitor, and a lead frame used for the method.
Solid electrolytic capacitor using tantalum or niobium as a valve function metal is small in size, large in capacity, and excellent in high frequency characteristic. Therefore, the solid electrolytic capacitor is widely used for a power supply circuit of a CPU (Central Processing Unit). In a small size electric equipment such as a portable electric equipment, a chip type solid electrolytic capacitor is often used. With the further downsizing of the small size electric equipment, the further downsizing and further thinning-down of the chip type solid electrolytic capacitor is also proceeding.
When the chip type solid electrolytic capacitor is surface-mounted on a printed circuit board (mounting object) with the use of solder, a fillet made of the solder is formed between the printed circuit board and an end surface (fillet surface) of a terminal of the chip type solid electrolytic capacitor. Because the fillet joins the chip type solid electrolytic capacitor to the printed circuit board, a condition how the fillet is formed is important. As a factor influencing on the forming condition of the fillet, there is solder wettability.
For example, when the solder does not sufficiently wet-up on or permeate onto the fillet surface, the solder is stopped and piled as far as a lower surface of the chip type solid electrolytic capacitor as a mounted surface contacted to the printed circuit board. In this state, the chip type solid electrolytic capacitor stands out or looses on the printed circuit board. Further, when the solder does not equally wet-up on the fillet surfaces of anode and cathode terminals, the chip type solid electrolytic capacitor tilts or leans to the printed circuit board.
In addition, there is an estimating method of the quality of soldering on the lower surface of the chip type solid electrolytic capacitor, by observing a condition of the solder in the fillet after soldering. When the solder does not sufficiently wet-up on the fillet surface, the estimating method is disturbed.
It is known that the fillet surface is plated in order to improve the solder wettability.
Regarding the plating for the fillet surface, a chip type electrolytic capacitor called as a lower surface terminal type as an example will be hereinafter described.
An art of the lower surface terminal type is suitable for downsizing and thinning-down of the chip type solid electrolytic capacitor. In the chip type solid electrolytic capacitor of the lower surface terminal type, a lower surface of a lead frame is exposed on the lower surface of the capacitor. Further, a cut surface, that is, an end surface of the lead frame is exposed on an end surface of the capacitor. The cut surface of the lead frame is used as the fillet surface.
Hereinafter, a common manufacturing process of the chip type solid electrolytic capacitor of the lower surface terminal type will be described.
First, a solid electrolytic capacitor element manufactured by a known process is mounted and bonded onto a lead frame. The bonded capacitor element and a part of the lead frame are molded and covered with a resin package. The packaged capacitor element and the packaged part of the lead frame are cut off from a main body of the lead frame. The part of the lead frame serves as a terminal of the chip type solid electrolytic capacitor. An end surface (cut surface) of the terminal exposed from an external surface of the package serves as the fillet surface. Further, the fillet surface of the terminal is plated. Thus, the chip type solid electrolytic capacitor of the lower surface terminal type has been manufactured.
In the above-mentioned plating step, a barrel plating is generally done as an afterplating. In the barrel plating, products (the packaged capacitor element and the packaged part of the lead frame) are dropped into a barrel with plating solution or liquid.
For example, such as a chip type solid electrolytic capacitor comprising a terminal with a plated fillet surface is disclosed in Japanese Patent Application Publication (JP-A) No. H9-298256.
However, in the case that the fillet surface of the terminal formed by cutting off the part of the lead frame from the main body of the lead frame is plated, there are disadvantages as follows.
Namely, the plating solution soaks into the package and therefore the performance of the chip type solid electrolytic capacitor may be deteriorated.
Furthermore, directions of the plated products become inconsistent with one another after the barrel plating. Therefore, it is necessary to draw up the products in upside and downside, in length and breadth, and in polar direction. This causes the increases in the number of manufacturing process, in the number of man-hour, and in the manufacturing cost of the chip type solid electrolytic capacitor. Moreover, because it is necessary to product and to use expensive equipment such as a product aligning apparatus, the manufacturing cost of the chip type solid electrolytic capacitor is further increased.