Phm Peter Timmermans

Stretching-induced interconnect delamination in stretchable electronic circuits

Stretchable electronics offer increased design freedom of electronic products. Typically, small rigid semiconductor islands are interconnected with thin metal conductor lines on top of, or encapsulated in, a highly compliant substrate, such as a rubber material. A key requirement is large stretchability, i.e. the ability to withstand large deformations during usage without any loss of functionality. Stretching-induced delamination is one of the major failure modes that determines the amount of stretchability that can be achieved for a given interconnect design. During peel testing, performed to characterize the interface behaviour, the rubber is severely lifted at the delamination front while at the same time fibrillation of the rubber at the peel front is observed by ESEM analyses. The interface properties are established by combining the results of numerical simulations and peeling experiments at two distinct scales: the global force–displacement curves and local rubber lift geometries. The thus quantified parameters are used to predict the delamination behaviour of zigzag- and horseshoe-patterned interconnect structures. The accuracy of these finite element simulations is assessed by a comparison of the calculated evolution of the shape of the interconnect structures and the fibrillation areas during stretching with experimental results obtained by detailed in situ analyses.

Correlation between electrical and mechanical properties of polymer composite

The common features in dielectric and mechanical characteristics of the Epoxy Molding Compound (EMC) were found in temperature dependencies of the dielectric permittivity and the dielectric loss factor obtained in experiments by Electro-Dynamic Analysis (EDA) in comparison with the temperature dependence of storage modulus and mechanical loss factor in Dynamical Mechanical Analysis (DMA). Experimental results obtained give a basis for establishing a correlation between electrical and mechanical characteristics of the material. The relaxation model of the dynamic dielectric and mechanic response is suggested and verified by the experimental results. The same relaxation model has been used for a description of the frequency dependent storage modulus and inverse dielectric permittivity of the EMC sample. The temperature dependence of these parameters was introduced by the temperature dependent relaxation time model. The fitting parameters were found from the DMA and EDA experimental characteristics. A correct description of the mechanical and electrical characteristics of EMC samples in the glass transition temperature area has been obtained.

On the effect of microscopic surface roughness on macroscopic polymer-metal adhesion

Surface roughening is a generally accepted way to enhance adhesion between two dissimilar materials. One of the key mechanisms, besides the obvious increase in surface area, is the transition from adhesive to cohesive failure, i.e., crack kinking. This chapter presents several analysis methods to study this phenomenon. First, a semi-analytical approach is discussed in which the competition between adhesive and cohesive cracking is analyzed by means of the theoretical relation between interface and kinking stress intensity factors. Accordingly, the crack kinking location and kinking angle are readily calculated. Second, transient crack propagation simulations are performed to calculate crack paths at a roughened surface by means of cohesive zone elements. Third, delamination experiments are performed on samples containing well-controlled surface roughness profiles.

Quantification of the leadframe roughness effect on adhesion properties

Numerical analysis of delamination at polymer-metal interfaces depends on phenomenological descriptions of interface adhesion. Therefore, extensive experimental characterization is needed. A physics-based multi-scale framework enables the derivation of interface properties by numerical analysis. Such a framework depends on both molecular and continuum analysis giving information on interfacial behavior at different scales. This work relates to the micro-scale analysis of leadframe/epoxy moulding compound (EMC) interfaces. At the micro-scale, intrinsic adhesion properties resulting from chemical and physical interactions at the molecular scale are used to analyse the geometric effect of leadframe roughness on the resulting macro-scale adhesion properties. It is well-known that a roughened surface enhances interfacial adhesion. Two main reasons for this effect are the increase of effective interfacial area and mechanical interlocking of the EMC within the roughness valleys of the leadframe. This enhancement is investigated by developing a micro-scale FE model consisting of two materials bonded solely by chemical and physical interactions. The topology of the interface follows from the conversion of statistical data of a measured roughness profile into a representative geometrical description. The intended FE model contains cohesive zone elements for both interface and bulk fracture to simulate the competition between adhesive and cohesive failure. The macroscopic traction-separation law is then determined from the micro-scale simulations by means of a homogenisation scheme. Preliminary results for mode I and mode II loadings are presented. These results indicate an enhancement of interface properties from 20% for mode I loading to over 2200% for mode II loading due to interfacial area increase and mechanical interlocking.

Analysis of the three-dimensional delamination behavior of stretchable electronics applications

Stretchable electronics offer potential application areas in biological implants interacting with human tissue. Furthermore, they facilitate increased design freedom of electronic products. Typical applications can be found in healthcare, wellness and functional clothes. A key requirement on these products is the ability to withstand large deformations during usage without losing their integrity (i.e., large stretchability). One of the possible basic designs for stretchable electronics is to interconnect small rigid semiconductor islands with thin metal conductor lines on top of a highly deformable substrate, such as a rubber material. In this case, large stretchability must also be provided by these thin metal conductor lines. The adhesion of the conductor lines to the rubber substrate is of major importance from a reliability point of view. Experimental observations show that delamination between the metal conductor lines and the stretchable substrate may eventually lead to short circuits while also the delaminated area could result in cohesive failure of the metal lines. To understand and quantify the behavior of the copper-rubber interface, peel tests are performed and analyzed by means of experiments and numerical simulations. Interestingly, experimental observations show that the rubber is severely lifted at the delamination front caused by its high compliance. To quantify the interface properties, numerical simulations of the peel test have been performed by developing a finite element model comprising of cohesive zone elements by which the transient delamination process during the peel test is described in detail. By means of an extensive model parameter sensitivity study combined with the measured peel-force curves and the rubber-lift geometry at the delamination front, the final set of model parameters has been determined. Finally, the thus obtained model parameters are used to simulate the delamination behavior of actual three-dimensional stretchable electronics samples loaded in tension.