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PCB | July 20, 2009

Nanotechnology Lotus Effect with Soldering

Siemens researchers have developed a non-stick coating based on chemical nanotechnology that improves the production process for PCBs. The lotus effect is achieved on the stencils through which the solder paste is printed onto boards, and enables the production of extremely fine soldering structures.

The new process also improves the quality of the circuit boards — which are at the heart of all electronic devices. Because the coated stencils do not have to be cleaned as often as conventional stencils, their use also reduces production time. The coating is already being used in manufacturing processes for various electronic devices. Non-stick coatings prevent fluids or viscous substances from wetting surfaces. Such wetting prevention can be achieved with either a special surface structure — as in the well-known lotus effect — or by lowering the material’s surface energy through the use of a Teflon coating, for example. Non-stick coatings that repel fluids have been around for a long time, but no solution had previously been discovered for very viscous materials like solder paste. The researchers at Siemens Corporate Technology in Erlangen, Germany, produce their coating using a sol-gel process. It all begins with a dispersion containing precursor molecules (the sol), from which a gel is created via several chemical processes. This gel is then hardened into a solid coating on the stencil. Liquid organometallic compounds (alcoholates) are used as the starting materials. In the course of the process, their functional groups form an inorganic-organic network. The structure of the alcoholates is selected to result in a coating with the lowest possible surface energy. The functional groups of the molecules and the structure of the network determine the properties of the coating, which is why different starting materials can be employed to adapt the resulting coating to the type of application it will be used for. A scratch-resistant coating with a long life despite being subjected to mechanical stress is used for solder pastes. Printing solder paste through a stencil whose internal sides are coated in this way leaves no residue on the stencil when the board is removed. This is especially important when working with structures whose edge length is just a few hundred micrometers, because otherwise insufficient solder paste would have been applied to the circuit board. The new process makes it possible to precisely print very high resolution structures. As for the stencil, it remains clean for a long time — roughly 50 soldering cycles. This saves time in production, which only rarely has to be interrupted to clean the stencils.
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