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Correlation between Hardness and Storage Modulus of Silicone Gels for Thermal Interface Materials
Keywords: Silicone, Storage Modulus, Shore Hardness
Silicone gels are used as thermal interface materials to both remove heat and protect the fragile components against vibration. Silicone manufacturers optimize the gel cure chemistry to create a uniform interface that minimizes the stress on the encapsulated electronics. Most silicone manufacturers formulate based on the gel’s mechanical compliance as measured on the Shore Hardness scale. Fortunately, this static testing technique is a key characteristic for predicting the contact resistance between a thermal interface material and metallic heat sink. More recently, Prasher and Matayabas (1) demonstrated dynamic mechanical behavior, specifically the storage (G”) and the loss (G’) modulus, could also be used to predict contact resistance. However, they were not able to demonstrate the correlation between the Shore Hardness scale and rheological testing techniques. In this study over 20 silicone gel formulations were tested to build a correlation between hardness and rheological behavior. Firm gels, such as NuSil’s GEL1-8155, is characterized by a high storage modulus (>1000 Pa) and low Shore 00 number, meaning it takes a large amount of stress to cause deformation (e.g., strain). The storage modulus of a firm gel is independent of the frequency of the vibration making it an effective encapsulant to dampen vibration in most applications. However, a firm gel is more likely to pull away from the interface, creating air pockets and impeding thermal transport and increasing the contact resistance. In contrast, a low storage modulus (<500 Pa) and high Shore 00 number is easily deformed under a small amount of stress. The storage modulus of a soft gel, such as GEL-8100, is also highly dependent on the strain frequency. Therefore, most soft gels must be combined with springs in order to achieve the same dampening effect as a firm gel. Despite these additional requirements, soft gels are favored because the tacky surface limits the risk of developing air pockets at the interface. Both gel types were measured in both the filled and unfilled state. Therefore, in addition to providing the correlation between hardness and storage modulus, this study provides a guideline for identifying the appropriate silicone gel to choose when adding filler to formulate a thermal interface material.
Kristina Chano, Chemist
NuSil Silicone Technology
Carpinteria, CA

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