Polyurethane-Modified Epoxy Resin: Solventless Preparation and Properties S. Ong, 1 J. Ismail, 1 M. Abu Bakar, 1 I. A. Rahman, 1 C. S. Sipaut, 1 C. K. Chee 2 1 School of Chemical Sciences, Universiti of Science Malaysia, 11800 Penang, Malaysia 2 Intel Technology (M) Sendirian Berhad, Bayan Lepas Free Trade Zone Phase III, 11900 Penang, Malaysia Received 28 December 2007; accepted 25 August 2008 DOI 10.1002/app.29373 Published online 11 November 2008 in Wiley InterScience (www.interscience.wiley.com). ABSTRACT: A polyurethane-modified epoxy resin sys- tem with potential as an underfill material in electronic packaging and its preparation procedure were studied. The procedure enabled the practical incorporation of an aliphatic polyurethane precursor, synthesized from poly(ethylene glycol) and hexamethylene diisocyanate without a solvent, as a precrosslinking agent into a con- ventional epoxy resin. With a stoichiometric quantity of the polyurethane precursor added to the epoxy (ca. 5 phr), the polyurethane-modified epoxy resin, mixed with methylene dianiline, exhibited a 36% reduction in the contact angle with the epoxy–amine surface, a 31% reduc- tion in the cure onset temperature versus the control ep- oxy system, and a viscosity within the processable range. The resultant amine-cured thermosets, meanwhile, exhib- ited enhanced thermal stability, flexural strength, storage modulus, and adhesion strength at the expense of a 5% increase in the coefficient of thermal expansion. Exceed- ing the stoichiometric quantity of the polyurethane pre- cursor, however, reduced the thermal stability and modulus but further increased the coefficient of thermal expansion. V V C 2008 Wiley Periodicals, Inc. J Appl Polym Sci 111: 3094–3103, 2009 Key words: crosslinking; polyurethanes; resins; thermal properties; thermosets INTRODUCTION Underfill (UF) materials are vital in electronic pack- aging to keep solder joints reliable and the whole microchip assembly durable and functional. The materials currently in use are based solely on epoxy polymers because of their ease of processing, low cost, and moisture, heat, and chemical resistance. 1–4 Nevertheless, the coefficient of thermal expansion (CTE) mismatch between a UF material and a die of- ten leads to failure and remains a daunting chal- lenge. The ultimate goal is to reduce the CTE of the UF material to a value as close as possible to that of the solder material. The most common approach is the incorporation of inorganic fillers into the UF ma- trix, with microsized silica particles being the com- mon choice. However, this practice has its limit: even with the maximum allowable loading, it can hardly reduce the CTE mismatch to the desired level. The availability of nanosized silica primary particles in the range of 10–20 nm has created new prospects and interest in achieving the goal. 5,6 If the particles can be dispersed well with a size of 10–50 nm in the epoxy matrix, they have the potential to improve the dimensional stability and reduce the CTE mismatch, in addition to increasing the modu- lus, heat deflection temperature, and barrier to diffu- sion of solvents. 7 However, the agglomeration of particles, either in a physical fashion as agglomer- ates or by stronger sintering bonds as aggregates, poses difficulty for the dispersion process. 8 Although attempts to apply high shear stress to the constituent particles to redisperse them in the epoxy matrix remains a challenging approach, studies on improving the silica–epoxy interaction, which is fun- damentally important to the dispersion process, via chemical modification of the polymer have not been reported. Chemically modified epoxy may be tai- lored to produce molecules with chain dynamics that give the right viscoelasticity and other structur- ally related properties, such as modulus, wetting, adhesion, and thermal stability. Motivated by this prospect, we undertook this study to chemically incorporate polyurethane (PU) into a commercial epoxy. PU–epoxy systems have been reported previously and are commonly used in the coating industries and structural applications to mitigate the brittle nature of neat epoxy systems. 9–12 To achieve the intended elastomeric characteristics, PUs are often derived from flexible toluene diiso- cyanate and poly(propylene glycol). However, the Journal of Applied Polymer Science, Vol. 111, 3094–3103 (2009) V V C 2008 Wiley Periodicals, Inc. Correspondence to: J. Ismail (jamis@usm.my). Contract grant sponsor: Intel Technology (M) Sdn. Bhd.; contract grant number: 304.PKIMIA.605336.I104 (as well as a fellowship to S. O.).