INKtelligent connection

European researchers have developed a printing process that can be used to print conductor paths onto microelectronic components to replace obtrusive connecting wires.

The Fraunhofer IFAM’s INKtelligent printing platform uses maskless, CAD-driven direct write technology to print specially engineered inks in the form of functional structures onto surfaces ranging from silicon wafers to ceramics.

‘The formulation of printable inks can be done by different processes,’ said Christian Werner, IFAM project manager. ‘The simplest is to disperse a powder [of a functional material, such as a metal] into a liquid medium. More sophisticated is a process we developed known as the VERL [vacuum evaporation on running liquids] process, where we sputter the desired material directly into the liquid medium.’

The nanoscale particles would usually be suspended in a solvent, such as alcohol or glycol. ‘Alcohols are used because they are volatile,’ said Werner. ‘When you print, you have to get the solvent out of the suspension very quickly because you just want to have the nanoparticles on the substrate.’

The functional inks are then delivered to the required substrates using inkjet or aerosol printing. The main difference between the two is that the more established inkjet printing produces single droplets of ink, measuring ten picolitres, which are pressed out from a cartridge by a piezo or thermal element. The latter, newer technique uses an ultrasonic transducer or pneumatic system to transform the ink into an aerosol, which is then guided to the substrate using a focusing gas.

Functional structures are then activated by heat, for example in a furnace or with a laser, which has the benefit of providing localised heat treatment.

In electronic applications, the technology could be used to print a functional structure onto a circuit board. ‘Usually in electronics, when you want to have conductive tracks on a circuit board, they are etched, but we print on rather than take out,’ said Werner. ‘So what we can do with the direct write technologies is additive manufacturing, which means we have a reduction in waste.’

According to the researchers, any material that can be sputtered can be sputtered into a liquid medium. This means metals such as silver, gold and platinum can be used to create a variety of nanoscale functional inks. The inks can also be printed on many different types of substrate.

‘There are virtually no substrate limits. In principle, you can use flexible [such as textiles] as well as rigid substrates,’ said Werner. ‘For example, from microelectronics we often have silicon wafers and printed circuit boards; ceramics and glass are possible too. They can be printed onto metals, but then you have to prevent short-circuiting when printing metallic structures onto these substrates.’

The substrates do not need to be flat, because the CAD-driven aerosol technique allows printing of precise structures of ten to 100 microns on surfaces with complex geometries, such as right angles.

As well as conductive paths, the technology could be used to make more sophisticated components, such as antennae or temperature sensors.

Anh Nguyen