High viscosity paste dosing for microelectronic applications

Today’s microelectronics packaging especially for SiPs relies on the processing of a wide variety of materials. Materials for components, for substrates, for contact materials (solder & adhesives) and encapsulants. Most materials are processed as bulk material but precision dosing of pastes is key to many assembly processes. Examples are dosing of solder paste, typically done by stencil printing, Underfilling for Flip Chip encapsulation, typically done by dispensing or jetting, or Glob Top encapsulation of Chip on Board assemblies, where also dispensing is the typical process. When working with those paste materials, viscosity is one of the key parameters for processing, and viscosities too high do not allow dosing of the materials, not even to transport the material from a reservoir to the dosing head, which may be a simple needle or a jet valve. [i, ii] To overcome this obstacle, i.e. to dose materials of high viscosity precisely and homogeneously from a syringe to the dosing head, a research program has been set up, where Vermes microdispensing as a valve manufacturer and TU Berlin/IZM as a research institute are cooperating. TU Berlin is working on material rheology effects and flow models; Vermes is researching valves modifications and material flow path optimization. Core of the research is to find methods that allow a reduction of paste viscosity without leading to irreversible changes in the material, as would be the case when simply applying heat to the paste. As reference process for material dosing, FO-WLP has been chosen, materials selected for the investigations are GlobTop dam and fill material and liquid molding compound – using both rheological experiments as well as actual material dosing and processing. Apart from temperature, mechanical and ultrasonic stimulation of the material have been evaluated to achieve optimized dosing of high viscous pastes, As a result, a first description of paste behavior during processing is given, being the basis for future work towards homogeneous precision dosing of high viscous pastes for microelectronic applications