Mechanical
properties

Parameters of mechanical properties

Mechanics is a very broad subject, so it is worth providing an exact description of the crucial parameters of a resin system that characterise its mechanical properties. 

a. Shore hardness

Shore hardness is the easiest way to describe an encapsulant’s mechanical characteristics. It can always be used as an initial indicator. In the pottings’ and adhesives’ area there are usually three ranges of Shore units, all of which are dimensionless with a scale of 0–100.

• Shore 00: used for very resilient, partly gelatinous materials; often used for silicone-based materials
• Shore A: used for soft to tough elastic materials; can therefore be used for many PU-based materials
• Shore D: used for hard to brittle materials

Since Shore hardness is always measured on a surface, it can only be used as an indicator to describe mechanical properties. Other parameters are needed to describe material-specific mechanical properties.

b. Modulus of elasticity

The modulus of elasticity is used to describe the elasticity or, conversely, the stiffness of a material. The higher the value, the greater the resistance of a material to elastic deformation. For example, an elastic material will have a modulus of elasticity of only a few N/mm², while a hard and brittle epoxy resin may have a modulus of elasticity of several thousand N/mm². The modulus of elasticity is an important characteristic for assessing an encapsulant or an adhesive in terms of the overall strength of the material and the stresses in the component caused by temperature changes. Here it is important to point out that the modulus of elasticity is not directly related to the stiffness of the material.

c. Tensile strength

Tensile strength describes the stress calculated from the maximum tensile force achieved in the tensile strength test of the original cross section of a test piece. It therefore describes the material’s mechanical resistance to mechanical stresses such as e.g. tension and vibration. The unit is N/mm².

d. Elongation at break

Elongation at break is, as the name describes, the elongation at which a test piece breaks during a tensile strength test. The more elastic and linear the structure of the material, the higher the value of the elongation at break. Elastic, rubber-like systems display an elongation at break of more than 100 % of the original measured length, while some brittle materials have less than 1 % elongation at break.

e. Glass transition temperature T_g

The glass transition temperature is a material-specific characteristic of polymeric materials. It describes the temperature range within which the mechanical behaviour of the plastic changes from elastic and plastic-like to glass-hard and brittle, or vice versa. The glass transition point can only be influenced through the choice of polymer structure in the formulation. The glass transition temperature also serves as an indicator of the resin’s characteristics after installation.

Since systems with a high glass transition point also display a high degree of hardness, they are often unsuitable for sensitive electronic components. In this case, materials with a low glass transition point can be selected for use in temperatures below freezing. These encapsulants have nearly constant mechanical characteristics over the entire temperature range of the electronic component. As a rough rule of thumb, the thermal coefficient of expansion (ppm/K) of a PU system below T_g is about a third of that above T_g.

The glass transition temperature is therefore viewed in relation to the sensitivity of the potted or bonded component and the temperature in the intended application. It should also be pointed out that the glass transition temperature generally extends over a range of several degrees Kelvin, without a well-defined transition point. It is also important to mention that 90 % of all potting applications use systems where the glass transition temperature is within the temperature range of the component to be potted or bonded.

f. Coefficient of thermal expansion (CTE)

The coefficient of thermal expansion (CTE) in ppm/K is used to describe the expansion characteristics of a material. The higher the value, the more elastic the material in most cases. In many cases, thermal expansion alone is thought to be responsible for the deterioration of a component, but it is usually caused by the combined effects of expansion and, in particular, the stress in the component resulting from the modulus of elasticity of the material. Geometrical factors may also play a part.