1. Field of the Invention
The present invention relates to a circuit board arrangement on which at least one component is mounted and, more particularly, to a circuit board arrangement having at least one component assembled or mounted thereon by means of a method using solder to align the at least one component as well as electrical connect it.
2. Prior Art
Circuit boards are known on which a component is soldered with the help of a method which causes an automatic, passive alignment of the component on the circuit board with the help of surface tension of the melted or liquified solder that simultaneously makes electrical contact with a pad of the component and a pad on the circuit board. An appropriate spacing is maintained between the circuit board and the component by stand-offs extending between the component and the circuit board.
The so-called flip-chip solder engineering method is known and used for high precision assembly of electronic and electro-optic components on circuit boards. In this method pads are produced on appropriate surfaces on the circuit board and the component that are wettable with solder. These pads permit mechanical attachment as well as electrical contact. The pads are surrounded by a peripheral layer that is not wettable by the solder. A so-called solder bump is placed on the pad either on the circuit board or on the component. The bumps are usually deposited with a thickness of around 20 to 30 .mu.m either in a vacuum or a galvanic process. They are cylindrically structured with the help of a lacquer layer that has a thickness approximately equal to the bump height. A so-called under-plating metallization is required in galvanic deposition in order to guarantee electrical contact during the galvanic process. The thick lacquer layer and the under-plating metallization are removed in the region beyond the bumps. The cylindrical bumps are melted and take the form of a conical section as a result of the surface tension of the liquified solder and maintain that form on solidification. The base surface of a melted bump is defined by its footprint pad, which may be structured with substantially smaller tolerances than the solder bump structured with a thick lacquer layer prior to melting.
The component is pre-positioned on its mounted surface on the circuit board during assembly so that the bumps are in contact with the appropriate pads of the component. The bumps are exactly aligned on the pad by surface tension forces in the melted solder and lateral positioning errors which occur during mounting are thus automatically corrected. Because of that the lateral position of the component is close to the high precision position of the pad made during the masking process on the component or the circuit board. Simultaneously the electrical connections are made by means of the bumps. This known alignment process known according to the state of the art as "self-alignment" is, for example, shown in FIGS. 3A to 3G of European Patent Application EP 0 699 931 A1.
For electro-optical components, especially for laser diodes, an exact vertical positioning is also required as well as an exact later positioning in order to achieve a stable coupling to an optical fiber or image forming optics. The vertical position that the component mounted with the bumps takes is a balance between the surface tension forces of the liquid solder and the weight of the solder and component, wherein the volume of the individual bump, the pad surface area and the surface tension properties of the liquid solder and the pad surfaces enter as parameters. These variables are altogether difficult to control so that substantially greater errors enter into the vertical positioning than the lateral positioning.
An attempt to solve these problems has been made in the prior art by providing non-melting stand-offs of a predetermined height besides the bump on one of the supporting surfaces on the circuit board or the component or the bumps project into a cavity or depression with a predetermined depth. After melting and solidification of the bump, its height is reduced because of the change in its form from a spherical segment to a spherical zone so that the component is mounted on the stand-off or on the edge of the cavity and permits an exact predetermined height adjustment (see European Patent EP 0 312 217 A1, IBM Technical Disclosure Bulletin, Vol. 16, No. 3, August 1973, p. 767, IBM Technical Disclosure Bulletin, Vol. 23, No. 5, October 1980, pp. 2156 to 2158, EP 0 248 566 A2).
During this height adjustment it is however disadvantageous that the lateral position correction by the peripheral surface of the pad and the surface tension of the solder and the vertical positioning by lowering of the component and its placing on the stand-off or on the edge of the depression occurs simultaneously. The placing or mounting of the component is completed before lateral justification or correction, so that no further lateral motion can occur because of the static friction on the stand-off or on the edge of the depression, and the component does not achieve its best or optimum position. The height adjustment is of course improved but the lateral positioning is made poorer in contrast to the above-described method without the height stop or motion limit. Thus according to the state of the art additional mechanical guides are provided in the above-cited paper, which define the lateral position during the vertical motion. These lateral guides may however be made only with comparatively great effort or expense to the required accuracy specifications. The lateral positioning is achieved by the lateral guide elements and not by the simple and precise pad to be structured.