In a semiconductor device, an electronic component such as a capacitor is used together with an integrated circuit device. An electrolytic capacitor is a large-capacitance capacitor which is generally connected as a power supply source to a surrounding of a central processing unit (CPU) or a large-scale integrated circuit (LSIC). A typical structure for providing an electric connection to the electrodes of an electrolytic capacitor is to connect conductive tabs or conductive leads to the anodic metal (such as aluminum) foil and the cathodic metal foil that serve as internal electrodes of the electrolytic capacitor. The conductive tab or the conductive lead is connected to a lead wire which is a tin-plated copper wire or a copper-clad steel wire plated with tin. The conductive tab and the lead wire are joined to each other generally by electrical welding.
During the electrical welding, tin plating flakes fly and scatter from the lead wire and adhere on the welded part. The tin flakes solidified on the welded part constitute an uneven tin layer with non-uniform thickness and area size. Because a mechanical stress exists inherently in this uneven tin layer, tin whiskers grow from the surface of the tin layer. Whiskers are metal crystals of tin that have grown in needle-like shapes with a length of about 0.2 mm, and accordingly whiskers are conductive substances. When adequately grown whiskers drop off and scatter from the surface, such whiskers cause short-circuits between electrical terminals of other electronic components placed near the capacitor. This may further cause circuit defects.
The above-stated problem occurs in an arbitrary electronic component that involves a welding process of tin-plated lead terminals or lead wires. Lead wires may be treated by alkaline cleaning or high-temperature heating; however, such treatment is insufficient to effectively prevent whiskers. An easy solution for preventing scattering of whiskers is to physically cover the welded part. However, this solution may affect the reliability of the electronic components such as capacitors. Besides, the manufacturing cost and the price of the electronic component will increase due to the increased number of work processes and an extra part.
To prevent scattering of whiskers, a structure for providing a swelling to a part of the lead is known. See, for example, PCT International Publication No. WO 2007/043181 A1. When the lead is inserted in a through-bore for connection to a capacitor element, the swelling blocks the through-bore to prevent whiskers from scattering to the external space. This technique requires an additional step for forming the swelling during the fabrication of the lead wire.
Another known technique for preventing scattering of whiskers is to insert a sealing sheet between an electrolytic capacitor and a printed circuit board when mounting the electrolytic capacitor onto the printed circuit board in order to shut a through-hole formed to provide electrical connection to the capacitor element. See, for example, Japanese Patent Application Laid-open Publication No. 2010-3811. This method is only applicable to a vertical mounting structure for an electrolytic capacitor stuck onto a substrate.
Meanwhile, to prevent leakage of an electrolyte of an electrolytic capacitor, a fluoroplastic foam resin is provided to a caulked part of the seal packing of an electrolytic capacitor. See, for example, Japanese Utility Model Examined Publication No. H07-20911. To let gas produced in a capacitor cell out, an air-permeable sheet made of a porous plastic resin is used. See, for example, Japanese Patent Application Laid-open Publication No. H08-138982.
There is a demand for a simple structure and a simple process that can prevent scattering of whiskers produced in a welded part between a lead (or a tab) and a lead wire of an electronic component.