Direct Measurement of Cure‐Induced Stress in Thermosetting Materials by Means of a Dynamic Mechanical Analyzer

Abstract

Summary: Significant stresses develop during cure in functional and structural applications of polymeric materials ranging from glass fiber composites to advanced functional polymers used in microelectronics, optoelectronics, and biomaterials applications. These stresses arise from a combination of chemical shrinkage and stiffness buildup in a confined geometry. In this paper, a new method for direct measurement of cure‐induced stresses during curing of thermosetting materials by using the iso‐strain mode of a dynamic mechanical analyzer (DMA) has been developed. A thermal tape was used to facilitate maintaining a constant strain and initiate the iso‐strain measurement. Two quartz rods with a small gap were used to contain the material. The top of the quartz rod and one side of the thermal tape were secured by the fixed clamp, while the bottom quartz rod and the other side of the thermal tape were clamped with the moveable force probe. The cure force was thereby directly measured by the probe during the curing process. The cure stress buildup was observed to occur after a certain duration that corresponds to the gel point. Experimental results clearly show that curing at lower temperature could lead to higher cure stress due to the earlier onset of vitrification. An investigation of the stress buildup as a function of degree of cure indicates that a majority of the cure stress was generated in the vitrification regime. The methodology proposed herein provides an accurate experimental approach to investigate the cure‐induced stress generated in a thermosetting material in applications ranging from microelectronics and optoelectronics packaging to biomaterials amongst others.

Evolution of Contact Resistance during the Bonding Process of NCA Flip-Chip Interconnections

“…The detailed experimental procedures are reported elsewhere. 10 Figure 4a shows the evolution of the contact resistance at a cure temperature of 100°C with an applied load of 10 N. The temperature and external load profile are also shown. It is evident from the graph that the resistance change can be divided into seven distinct regions.…”

“…Given this situation, comprehensive research on the formation of NCA flip-chip interconnects, which involves a complex polymerization process consisting of cure shrinkage evolution and the development of mechanical properties, is needed. In our previous study, 9,10 the cure shrinkage and cure stress of adhesives were directly measured by means of a thermomechanical analyzer (TMA) and a dynamic mechanical analyzer (DMA), respectively. In order to establish the relationship between the cure force obtained during the cure process and the electrical contact resistance, the mechanical compressive stress formed during the cure process has to be translated to the electrical contact resistance for practical interpretation and to investigate the reliability of flip-chip interconnects with NCAs.…”

Thiol‐terminated hyperbranched polymer for DLP 3D printing: Performance evaluation of a low shrinkage photosensitive resin

“…Although various photosensitive systems have been extensively studied, 8 acrylates are still the most commonly used printing materials for DLP, due to their high reaction rate and their excellent mechanical properties. However, despite the broad applicability of photopolymerizations and their outstanding characteristics, the traditional acrylate‐based free radical photopolymerization process is limited by a series of shortcomings, including oxygen inhibition, uneven cross‐linked network, and the development of shrinkage and shrinkage stress caused by polymerization urgently to be solved 9 …”

Thermal and volumetric property analysis of polymer networks and composites using elevated temperature digital image correlation

“…[10] Conversely, techniques which measure laminate dimension changes or developing mechanical strength of uncured, impregnated fibers, such as dynamic mechanical or thermomechanical analysis, do not represent the environment present in the manufacturing of high performance composite laminates. [4,11] Because of the implications of in-situ techniques such as these, deformation of laminates from initial, pre-cured to final cured states continues to be a common method for researchers. In this, process-induced strains are examined through either curvature reduction of an angled part or by the introduction of curvature upon curing a flat laminate of a non-symmetric laminate sequence.…”