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High viscosity paste dosing for microelectronic applications
Keywords: Material Application, Rheology, Precision Dosing
Todays 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. 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 materials for the investigations printable solder paste and liquid molding compound as used for fan-out wafer level packaging processes have been investigated using both rheological experiments as well as actual material dosing. 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 under different load conditions has been developed and also a test bench to verify the use of different stimulation techniques on the paste flowability, paving the way for homogeneous precision dosing of high viscous pastes for microelectronic applications.
Tina Thomas, Research Scientist
TU Berlin
Berlin, Berlin
Germany


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