Nano-particle enhanced encapsulants for improved humidity resistance

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 can not 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. 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, 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 without cost increase and with no changes in processing. The incorporation of nano-particles into plastic packaging materials is discussed as one potential solution for improved humidity resistance as it is a rather low effort approach to material modification opposed to chemical modification of the matrix. To evaluate the potential of such additives concerning moisture resistance the effect of nano-particles mixed with a microelectronic grade epoxy resin is studied. From the large variety of fillers available this work mainly focuses on three different types: nano-sized silica, modified bentonite and zeolites. Working principles of these particles range from large surface impact of nano-particles, barrier functionality due to stacked layer formation and molecular catcher function. Formulations with different particle concentrations and surface modifications are characterized regarding their influence on humidity diffusion, absorption and desorption behavior as well as their influence on other material properties as reaction kinetics, viscosity and thermo- mechanical properties. Additionally the combination of nano- and standard micro-particles needed for thermo-mechanical adjustment of the polymer properties is studied. Experimental work is accompanied by simulations, in order to provide further qualitative understanding on effects of particle form, size and surface properties. In summary this paper describes the potential of different nano-particles as additives for plastic packaging materials for enhanced humidity resistance/barrier enhancement within microelectronic packages. This topic is gaining increased importance when considering the trend towards system in package, where a multitude of components is encapsulated to form one SiP that incorporates a large number of different material interfaces and interconnects. All these interfaces and interconnects need to be protected from degradation caused by moisture ingress, without allowing much increased package volume or package cost. Polymers with improved moisture resistance can be one building block of future moisture resistant packages - the results of this study show their large potential for this field of application.