Water diffusion in micro- and nano-particle filled encapsulants

Polymer materials mainly epoxy resins are widely used in microelectronics packaging. They are established in printed circuit board manufacturing, for adhesives as die attach glues or for encapsulants as molding compounds, glob tops or underfill materials. Low cost and mass production capabilities are the main advantages of these materials. But like all polymers they cannot provide a hermetical sealing due to their permeability properties. The susceptibility to water diffusion through the polymer and along the interfaces is a drawback for polymer materials in general, as water inside a microelectronic package might lead to softening of the material and to a decreasing adhesive strength and resulting delaminations close to solder bumps or wire bonds reducing package reliability by decreasing the package structural integrity. During package reflow, the incorporated humidity might lead to popcorning, i.e. abrupt evaporation of humidity during reflow soldering. This effect is one major problem during plastic package assembly. The introduction of high temperature lead-free soldering processes has even increased this issue. Therefore, plastic packaging materials with enhanced humidity resistance would increase package reliability during assembly and lifetime ideally without cost increase and with no changes in processing. As filler particles have an important influence on the final material properties of microelectronic encapsulants, they are well suited for material modifications. Typically micro-sized silica particles are incorporated into the polymer matrix as the thermo-mechanical properties could be well adapted to reliable packaging demands. However, there are a lot of nano- and micro-sized filler particles with potential to enhance the humidity barrier properties of encapsulants. Working principles of these particles may range from large surface impact of nano-particles, barrier functionality due to stacked layer formation (nano-clays), highly hydrophobic particle s- - urface and molecular water catcher function. Micro- and nano-sized SiO₂, bentonite, zeolites, AI₂O₃, carbon black and carbon nano tubes have been selected for a systematic study. To evaluate the potential of such additives concerning moisture resistance particles are mixed with a microelectronic grade epoxy resin. Formulations are characterized regarding their influence on humidity diffusion, absorption and desorption behavior as well as their influence on other material properties as reaction kinetics and viscosity. Different methods for characterization of the diffusion properties have been developed and applied for material analysis. The pros and cons of simple weight measuring for absorption testing, sorption analysis and encapsulated humidity sensors are presented and discussed along testing results with formulations with the different filler particles. In summary this paper describes the potential of different nano- and micro-sized particles as additives for plastic packaging materials for enhanced humidity barrier enhancement within microelectronic packages as well testing methods for material characterization concerning their humidity diffusion and absorption properties. This topic is gaining increased importance when considering the trend towards System in Package, where a multitude of components is encapsulated to form one miniaturized SiP that incorporates a large number of different material interfaces and interconnects. These SiPs need to be protected from the environment by encapsulants layers with ever decreasing thickness and thus increased moisture barrier properties.