Testing Microcomponents
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Testing microcomponents
General
A comprehensive and accurate understanding of material properties is of great importance for ensuring the full functionality of microcomponents and microsystems made from plastics and for selecting suitable materials. In this context, the damage behaviour (see, for example: fracture and crack formation) under thermo-mechanical loading is becoming increasingly important (see: failure analysis plastics products, VDI Guideline 3822). Furthermore, it is the case that finite element methods (FEM), utilising appropriate material constitutive equations or theoretical material models, are frequently employed to assess the mechanical and thermal reliability of microelectronic components. The material properties required for this (see: material value) must be determined through appropriate experimental investigations. A further objective of applying micro testing technology is to characterise typical defects and evaluate them locally, in order to derive effective material properties from a given microscopic structure.
The influence of test specimen geometry
The influence of test specimen dimensions on material properties is a key issue, as established concepts such as continuum mechanics and fracture mechanics reach their limits of validity at the micro-scale. However, even when dimensioning microcomponents, the characteristic values used should be geometry-independent (see: geometry criterion) in order to ensure optimum reliability and operating safety for these applications. However, the standard test specimens used in conventional polymer testing or fracture mechanics are often unsuitable for describing material properties, as they can only inaccurately reflect the actual strength and deformation behaviour of microcomponents. Furthermore, standard test specimens are very material-intensive and are therefore often unavailable for the development of new, optimised material systems. Moreover, when assessing damage cases (see: failure analysis – basics), only very small quantities of material are usually available for the extraction of test specimens, making it necessary to determine the properties using miniaturised test specimens (Fig. on the right).
| Fig.: | Anwendungsbeispiel Mikrotechnik (links), schematische Darstellung eines Mikrobauteils (Mitte) und miniaturisierter Prüfkörper (rechts) |
The transferability of characteristic values to microcomponents
When moving from the macro to the microscale, material properties take on particular significance. As miniaturisation increases, the ratio of the test specimen’s surface area to its volume rises very sharply. The material properties determined, such as strength, stiffness and deformability, are determined to a considerable extent by the quality of the surface. Similarly, variations in the test specimen’s cross-section can have an impact on the level of mechanical properties. For example, as the test specimen size decreases, an increase in strength and tensile strain at break (see: tensile strength) is observed; this is attributable to the fact that, as the test specimen size increases, the probability of inhomogeneities with low strengths (microcracks, defects) occurring also increases. Another aspect to be taken into account in connection with miniaturisation is the significance of residual stresses. The following test methods are frequently used in micro testing:
- uniaxial micro-tensile testing,
- micro-bending beam testing,
- micro-fracture mechanics,
- nanoindentation testing,
- and biaxial bulge testing.
It should be noted, however, that micro testing technology needs to be further developed through modified test methods and measuring equipment so that the characteristic values determined are representative of the respective material behaviour, thereby enabling the results to be extrapolated to microcomponents and allowing an assessment of their functional capability.
See also
References
• Michel, B., Walter, H.: Testing of microcomponents. In: Grellmann, W., Seidler, S. (Eds.): Polymer Testing. Carl Hanser, Munich (2022) 3rd Edition pp. 679–682 (ISBN 978-1-56990-806-8; E-Book: ISBN 978-1-56990-807-5; see AMK-Library under A 22)