Laser micro welding in electronics: Limitations and solutions for a further miniaturization

Looking:at today's production of microelectronics, the laser is a commonly used tool for many purposes. Especially laser micro welding of electronic contacts, the application discussed here, gains more and more importance in industrial production. The reasons are its high flexibility, the absence of a third joining partner like solder (lead-free), the high temperature resistance and finally its high miniaturization potential. Projecting the trend of a further miniaturization to this joining technique, even laser welding shows some limitations. For welding electronic contacts, the smallest geometry dimension (e.g. diameter of a wire) has to be larger than the spot diameter of the laser focus. If the ratio between dimension and focus diameter is nearly one or smaller, the workpiece will melt over the whole cross section and, according to the minimum of interface tension, build a ball. Producing a joint between two contacts will therefore not be possible. This fact will be discussed in the paper. The only solution to this problem is to reduce the focus diameter of the laser beam. To accomplish this, an improvement of the beam quality to nearly M-2=1 is necessary. Working with a nearly diffraction limited laser spot diameter changes some boundary process conditions like the ratio between focus diameter and average grain size. In how far a simple scaling of parameters meets the process requirements will be shown in the article from the theoretical and practical point of view. When realizing such a laser in practice, more than one option for reducing the focus diameter down to 10 mum exists. Besides the constant improvement of conventional solid state lasers, the fiber laser shows a remarkable alternative owing to its inner build-up. A theoretical and practical description shows the particularities of this laser type. Examples of welded basic geometries will be given and the advantages and disadvantages of this laser type discussed in comparison to conventional types. The benefits and limitations of a fiber laser for welding such small geometries will be discussed and summarized.