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A combined solder pad design is of interest for all applications in which a second ceramic LED product is used in addition to an Oslon LED.
The recommendations for pc board layouts for various ceramic LED components often differ essentially in the dimensions of the solder surfaces and in the spacings between them. As a result, they are not mutually compatible. This means that the first LED cannot be used with the solder pad design of the other LED. With combined solder pad designs however, both LEDs can be used without having to adapt the board layout again.
To ensure that the component is positioned on the board as accurately as possible and that this positioning can be reproduced time after time, an effect known as self-centering is used during the reflow soldering process. Self-centering is achieved by the surface tension of the molten solder. But this only succeeds if the solder pads on the board and the solder contacts of the component are matched to one another. The crucial factor here is the matching of the dimensions and spacing of the contacts on the component with those of the solder pads on the pc board.
In addition to the LED, other optical components such as lenses, reflectors and diffusers are used in many applications. To reduce costs and development time it is always best if the same optical components can be used for both LEDs. What is needed here is for the emission characteristics of the two LEDs to be as similar as possible and for the luminous area for both LED components to be in the same lateral position on the board.
Fig. 1 shows the underside of an Oslon SSL LED and that of a second LED with appropriate component-specific solder pad designs (footprint). Both components and their solder pad designs differ in the number of solder surfaces (three or two pads), in their dimensions, in the spacings between the solder surfaces and in the lateral dimensions of the component – in other words, use of both LEDs would need two separate board designs.
How is a combination board design produced?
The combination solder pad design is produced in three stages from the component-specific solder pad designs as shown in Fig. 2. In the first stage the component-specific designs are positioned so that the central points of the luminous surfaces (optical center, OC) match and the second design is rotated through 90° with respect to the first.
In the second stage the solder surfaces from the specific design of the first LED are divided at precisely the points where the recesses of the second solder pad design are located. The result in this case is six electrically independent solder surfaces.
To improve the self-centering of the second LED component it may be beneficial to adjust some of the six solder surfaces in the third stage. In this case four solder surfaces have been enlarged to suit the contour of the second solder pad design.
This combined solder pad can now be used for both LEDs. Fig. 3 shows the Oslon LED and the sample LED on the combined solder pad design. The adjustment of the solder pads means that self-centering is successful in both cases and in both directions. Since the optical center of the luminous surface is in the same location the same optical components can be used, given the same emission characteristics.
The concept gives designers enormous freedom. As an option the solder pads can be supplemented with structures for lateral heat spreading. There are also no problems in connecting the components, in particular for narrow LED clusters, in which case the minimum component spacing is determined by the larger LED component.
Reference designs for combination solder surfaces for Oslon LEDs are available on request at