1. Field of the Invention
The present invention relates to a mounting structure which reduces the occurrence of a blow hole in a soldered portion due to the passage of air through a through hole when a lead-inserted component is electrically and mechanically connected to a printed wiring board by immersing into molten solder, the air being expanded in a space surrounded by the lead-inserted component and the printed wiring board.
2. Description of the Related Art
In a wiring board such as a printed wiring board, electrical and mechanical connection between an electronic component and the printed wiring board is made by inserting leads of the electronic component into through holes formed in the wiring board to attach the electronic component to the wiring board and soldering the leads to the through holes. It should be noted that a leaded electronic component is hereinafter referred to as a lead-inserted component. An example of the lead-inserted component is an electrolytic capacitor.
Generally, a lead-inserted component is mounted on a printed wiring board with a space left between a lower end portion of the lead-inserted component and the printed wiring board. However, when a lead and a through hole are joined with a space left between the lower end portion of the lead-inserted component and a mounting surface of the printed wiring board, if the lead-inserted component has a certain amount of weight, the joint is subjected to loading and vibration, and cracking may occur in the joint to reduce the reliability of solder joint.
Conversely, if the lead-inserted component is mounted on the printed wiring board such that the lower end portion of the lead-inserted component contacts the mounting surface to reduce loading and vibration on the joint, a closed space surrounded by the surface of the printed wiring board and the lower end portion of the lead-inserted component may be formed at the time of soldering performed by immersing the lead-inserted component into molten solder. When soldering is performed in that state, air in the closed space is thermally expanded, and the expanded air exits to the outside while pushing, out of the through hole, part of solder which has filled the through hole once. As a result of the solder being pushed out of the through hole, a blow hole is formed in the soldered portion to reduce the reliability of solder joint.
FIGS. 4A to 4C are diagrams for explaining the mechanism of formation of a blow hole. FIG. 4A is a diagram showing that a lead-inserted component 10 and a printed wiring board (base material 15 covered with solder resist 16) contact each other at a contacting portion 21 and that a hermetically closed space 20 is formed over a through hole 26 when soldering is performed. FIG. 4B is a diagram showing that heat generated during soldering increases the temperature of air in the above-described hermetically closed space 20 to increase the internal pressure of the hermetically closed space 20. FIG. 4C is a diagram showing that after soldering is performed, air in the hermetically closed space 20 exits through the through hole 26 to form a blow hole 22.
In the case where soldering is performed by immersing into molten solder a surface of the printed wiring board that is opposite to the surface thereof on which the lead-inserted component 10 and the printed wiring board contact each other, air in the hermetically closed space 20 is thermally expanded by the heat of the molten solder as shown in FIG. 4B. The expanded air flows into the through hole 26 and exits to the outside through the through hole 26 as shown in FIG. 4C to form the blow hole 22. Thus, the reliability of solder joint is reduced.
To cope with this, there has been a method for preventing a blow hole by providing an air vent hole 23 in the base material 15 as shown in FIG. 5 (see Japanese Patent Application Laid-Open No. 8-321668 and Japanese Utility Model Application Laid-Open No. 3-104773). However, in this method, the hole passing through all layers strongly constrains high-density pattern wiring. Moreover, there is the problem of entry of a solder wave from the air vent hole 23.
As another technique, there is a method for preventing a blow hole by raising the lead-inserted component 10 from the base material 15 with a silkscreen 24 and solder resist 16 to provide a groove serving as an air vent 25 as shown in FIGS. 6A and 6B (see Japanese Utility Model Application Laid-Open No. 1-171024, Japanese Patent Application Laid-Open No. 2009-295812, and Japanese Patent Application Laid-Open No. 8-204323). However, in this method, resin of the lead-inserted component 10 may soften at the time of soldering to block the air vent groove. Thus, an intended effect may not be produced.
Moreover, as still another technique, there is a method in which a spacer 27 is mounted under the lead-inserted component 10 so that the space 20 under the lead-inserted component 10 may be in an open state as shown in FIG. 7 (see Japanese Patent Application Laid-Open No. 2005-302929). A portion 28 without the spacer is in an open state to prevent the space 20 formed under the lead-inserted component 10 from becoming a hermetically closed space.
However, in this method, if the spacer 27 is thin, the space formed by the lower end of the lead-inserted component 10 and the surface of the printed wiring board (base material 15 covered with the solder resist 16) is hermetically closed due to the softening of resin of the lead-inserted component 10. Thus, an intended effect may not be produced. Conversely, if the spacer 27 is thick, a soldered position and the center of gravity of the lead-inserted component 10 are separated, and the lead 11 may be broken when the printed wiring board is subjected to an external force such as vibration.