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  MSE / Research / Projects / Silsesquioxane Materials for Flip/Chip No Flow Underfill

Silsesquioxane Materials for Flip/Chip No Flow Underfill
Materials: Organic Nanomaterials Composites
Application: Nanotechnology Structural
Technique: Processing Characterization Synthesis

The objective of this project is to develop new silsesquioxane-based epoxy resins for flip-chip underfill. Flip-chip underfills are used to transfer heat from Si chips (coefficient of thermal expansion, CTE ≈ 2-4 ppm/°C) to substrate printed circuit boards (CTEs ≈ 40 ppm/°C). Thus, the CTEs of these materials must be a compromise between the two components, about 25 ppm/°C to minimize shear stress. Typical epoxy resins have CTEs near 100-150 ppm/°C and thus must be filled with silica fillers to reduce the CTEs to those needed for the flip-chip applica-tions. Thus, such fillers raise the viscosity of the underfill resin to near 50,000 MPa-sec making it very difficult to apply. The goal here has been to design silsesquioxanes that use the nanosilica particle at the center of cube materials as a nanofiller and thereby escape the high processing vis-cosities of current flip chip underfills.

The most common underfill technology used in industry forms solder joints between sub-strate and IC first, and then underfills with resin (capillary-flow underfill). A variation of this method adds the underfill directly to the substrate surface and places the IC chip over the sub-strate; the solder joints are formed simultaneously with curing of the underfills (no-flow under-fill). Commercial materials used for capillary-flow underfill are typically silica-filled epoxy res-ins with very high viscosities (up to 50,000 MPa-sec) because of the silica filler, making them difficult to flow under capillary action. Most underfill material also have Tgs below 150oC, which means that there will be a significant change in CTE at temperatures approaching Tg, and the solder joints could suffer from cyclic fatigue.

Cubic silsesquioxanes offer considerable potential for this application. The rigid silica core should restrict expansion of the materials and increase the Tgs, while the functionalized organic corners provide potential for crosslinking, hydrophobicity (to prevent moisture uptake), and ad-hesion, as illustrated below. Studies of the thermal expansion behavior of silsesquioxane-based epoxy resins show that systems with long, flexible organic tethers between the silsesquioxane cubes give high values of CTE (> 100 ppm/oC), while those with short and rigid organic tethers give low values of CTE (20-60 ppm/oC). These systems are stable in air up to 200oC and have Tgs higher than 200oC. Physical doping of these systems with nanoalumina was found to de-crease the thermal expansion of the resins and improve their thermal stabilities. We have now developed simple epoxy resins/cube systems that cure to materials with CTEs of 25 ppm/°C.
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