Ecole Polytechnique Fe&rale de Lausanne (l3PFZ) CH-1015 Lausanne, SwitzerlandThe relative sequence of shrinkage and evolution of modulus of a thermoset resin duririg cure leads to the build-up of internal stresses, especially if the resin is constrained by the presence of other materials in the form of a substrate or reinforcing fibers. To enable prediction of the levels of internal stress generated during processing and to determine appropriate processing windows, the evolution of the modulus of an epoxy-amine system during cure has been characterized and described with a phenomenological model. A combined reaction kinetics model is used to determine the degree of conversion of the epoxy over any complete range of cure. The chemorheological properties of the resin are measured as a function of curing temperature with a torsional parallel plate rheometer. A new phenomenological approach for time-cure-temperature superposition is proposed for predicting the relaxation modulus at any moment during cure and at any cure temperature. The combination of these two models provides a full description of the instantaneous viscoelastic properties during cure. This approach, which can be adapted to any curing resin, provides suitable tools for the analysis of viscoelastic stress build-up following any industrially relevant cure cycle.

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A novel, maskless, low‐volume bumping material, called solder bump maker, which is composed of a resin and low‐melting‐point solder powder, has been developed. The resin features no distinct chemical reactions preventing the rheological coalescence of the solder, a deoxidation of the oxide layer on the solder powder for wetting on the pad at the solder melting point, and no major weight loss caused by out‐gassing. With these characteristics, the solder was successfully wetted onto a metal pad and formed a uniform solder bump array with pitches of 120 µm and 150 µm.

During cure of epoxy resins, polymerization induces an increase in mechanical properties, which is accompanied by a volumetric shrinkage. When the resin is cured in a constrained mold to which it adheres, tensile stresses will hence develop, which may exceed the strength of the resin at a given curing stage. Voids will then form. The origin and governing parameters of void formation are studied using an epoxy resin cured in a three-dimensionally constrained glass mold following isothermal cure cycles. Two types of voids are shown to appear during cure, one early in the process and a second around the gelation point. A viscoelastic analysis of the material stress state over the whole range of cure is performed. Both the viscoelastic modulus obtained from a time-cure-temperature superposition and the volumetric shrinkage, which was continuously measured by density change, are taken into account. A value for the critical internal stress at void initiation is thus proposed. This criterion can be used to provide guidelines for tailoring the material properties toward an increase of the critical stress for void initiation. Also, since during processing of composite materials, cases may arise where the resin cures within the interstices left between consolidated fibers that do not move, this critical stress failure criterion can be of use in the establishment of a process window providing guidelines for the production of void free composites. lo%, depending on the type of monomer and hardener (1-3), and is generally around 5 O h or 6Yo. At the same time, the mechanical properties of the resin, in terms of shear modulus (and Young's modulus when the resin becomes a solid). increase dramaticallv as the intrinsic strength of the resin at a given time, which depends on its degree of conversion. Failure will then occur in the form of voids or cracks, which will remain in the final part.This phenomenon has extensively been reported in the literature, and was related to the occurrence of internal stresses in the curing cycle of epoxy systems. Defects appearing during cure have been experimentally observed in a three-dimensionally constrained epoxy resin by Plepys and Farris (4) who also deter-_ .the material evolves from a liquid state to a solid state. When the resin cures in a geometrically constrained environment, such as between plates separated by a fixed spacer, or within the interstices present between consolidated fibers in the case of composite material processing, tensile stresses thus mined the level of internal stresses arising from the curing process using a strain gauge method. These results were analyzed following the theory of incremental elasticity for an aging linear thermoelastic material subjected to dimensional constraints. The bulk behavior of the epoxy resin under volumetric constraint could then be calculated at temperatures above and below the glass transition temperature of the resin. The calculated internal stress levels were however expected to be overestimated, since the viscoelastic *Comsponding a...

Ecok Polytechnique Fed&& de Lausanne (EPFL) CH-1015 Lausanne, SWKZE'RLAND The development of internal stress during cure of epoxy and hyperbranched polymer-modified epoxy resins was characterized, taking into account the evolving viscoelastic properties, the volumetric shrinkage due to the chemical reaction, and the thermal expansion. A criterion for void formation during cure in a constrained mold was proposed, providing guidelines for the construction of a process window for manufacturing of void-free composites. It was shown that the internal stress development in epoxy resins during cure is strongly influenced by the presence of hyperbranched polymer modifiers. The role of these modifiers was illustrated for the case of autoclave processing of glass fiber/epoxy composites. This study showed that higher fiber volume fractions could be used with hyperbranched polymer-modified resins than with unmodified resins, for producing void-free laminates. It also appeared that by suitable tailoring of the process cycle, a fully stress-free laminate could be obtained after cure, using the modified resin.