Screen Printing

When applying a specific amount of material onto a defined area screen printing provides a possible alternative to ink jet printing (Fig. 1). The material usually comes in form of a paste. The paste is transferred onto a substrate by squeezing it though a steel or polymer mesh. The mesh is previously masked using photosensitive resist and a stencil (foil mask). After printing the structured paste is usually being tempered to burn out organic binders to be bonded subsequently. This is how glass frit bonds or ceramic compounds are usually formed. The CTEs (coefficient of thermal expansion) of the paste and the substrates need to matched and the temperature profile of the bonding process needs to be optimised.

Other applications include the areal adhesive bonding of various kinds of components. For good adhesion properties and high structural accuracy the wetting characteristics of the paste as well as its viscosity need to be adjusted. This is of particular interest when dealing with adhesives since the process is driven by surface tension.

Screen printing offers a variety of applications. It is used to print user defined patterns of  

  • ceramic slurries
  • glass frits
  • functional layers (catalytic coatings, electrically or thermally conducting layers,  layers with defined wetting properties, etc.)
  • adhesives like epoxy resins
  • solder pastes
  • dielectric coatings and passivations

There is a in-house developed screen printing machine available at MNT (Fig. 2). It allows to print feature sizes ranging from about 50 μm to several centimetres. A camera with a beam splitter allows the exact alignment of the mesh and the substrate or of two substrates (e.g. align structures on one substrate/mesh to those on the other substrate).

Fig. 1: Screen printed adhesive layer (0.03mm thickness) on a thermally oxidised Si substrate.
Fig. 2: Screen printing machine with integrated optics to align the mesh to the substrate.