Olaf van der Sluis

Design and implementation of flexible and stretchable systems

This paper presents a summary of the modeling and technology developments for flexible and stretchable electronics. These technologies can achieve mechanically bendable and stretchable subsystems by incorporating the electronic circuit into a matrix made of a soft polymer. The base substrate used for the fabrication of flexible circuits is a uniform polyimide layer, while silicones or polyurethanes materials are preferred for the stretchable circuits. The method developed for chip embedding and interconnections is named Ultra Thin Chip Package (UTCP). Extensions of this technology can be achieved by stacking and embedding thin dies in polyimide, providing large benefits in electrical performance and still allowing some mechanical flexibility. These flexible circuits can be converted into stretchable circuits by replacing the polyimide by a soft and elastic silicone material. The integration of ultra thin dies at package level, with thickness in the range of 10–30 lm, into flexible and/or stretchable materials are demonstrated. Furthermore, the design and reliability test of stretchable metal interconnections at board level are analyzed by both experiments and finite element modeling. We have shown through finite element modeling and experimental validation that an appropriate thermomechanical design is necessary to achieve mechanically reliable circuits and thermally optimized packages.

Analysis of the Three-Dimensional Delamination Behavior of Stretchable Electronics Applications

Stretchable electronics offer potential application areas in biological implants interacting with human tissue, while also facilitating increased design freedom in electronics. A key requirement on these products is the ability to withstand large deformations during usage without losing their integrity. 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. Interestingly, peel tests show that the rubber is severely lifted at the delamination front caused by its high compliance. To quantify the interface in terms of cohesive zone properties, these parameters are varied such that the experimental and numerical peel-force curve and rubber-lift geometry at the delamination front match. The thus obtained interface properties are used to simulate the delamination behavior of actual three-dimensional stretchable electronics samples loaded in tension.

Molecular simulation on the material/interfacial strength of the low-dielectric materials

Abstract In this paper, the material stiffness of amorphous/porous low- k material and interfacial strength between amorphous silica and low- k have been simulated by the molecular dynamics (MD) methods. Due to the low stiffness of the low- k material, the interfaces which include this material are critical for the most delamination and reliability issues around the IC back-end structure. MD simulation technique is applied to elucidate the crack/delamination mechanism at these critical interfaces. However, due to the amorphous nature of the low- k material (e.g., SiOC:H), the atomic modeling technique of the amorphous/porous silica is first established. Through the experimental validation, the accuracy of this amorphous modeling technique is obtained, and the results show that this algorithm can represent the trend of the mechanical stiffness change due to different chemical composition of low- k material. A novel interfacial modeling technique, which model the status of chemical bonds at interface during the delamination loading, is developed. Afterward, the simulation of the mechanical strength of the amorphous silica/SiOC:H interface, is implemented. The simulation depicts that the existence of the strong Si–O covalent bond will significantly enhance the adhesive strength of the interface. Instead of the covalent bond at interface, the simulation results also reveal the multiple atomic scaled crack path within the material during the interfacial delamination. Hence, improving the material stiffness of the soft low- k material and preventing the pore at interface can increase the adhesive strength of the silica/low- k interfacial system.

On the epoxy moulding compound aging effect on package reliability

Most semi-conductor devices are encapsulated by epoxy moulding compound (EMC) material. Even after curing at the prescribed temperature and time in accordance with the suppliers curing specifications often the product is not yet 100% fully cured. As a consequence, the curing process of a product continues much longer, leading to curing effects of the EMC during the lifetime of the package. In this paper, the effect of EMC curing during lifetime on package reliability is investigated. The mechanical properties of one EMC material are measured as a function of aging time and used in FE calculations. Aging effects on critical semi-conductor failure modes such as die cracking, compound cracking, wedge break, and delamination are addressed. Die and compound crack risks are predicted by common stress analysis. The risk of wedge break occurrence is investigated by detailed 3D modeling of the actual wires in the package using a global-local approach. Conclusions on delamination risks are made based on a parameter sensitivity analysis using a 3D cohesive zones approach to predict transient delamination. The package reliability study shows that the effect of EMC aging affects relevant failure modes in different ways.

The competition between adhesive and cohesive fracture at a micro-patterned polymer-metal interface

It is Common Practice for Polymer-Metal Interfaces, Frequently Encountered in Microelec-Tronic Devices, to Improve Adhesion by Surface Roughening or Micro-Patterning. the Competitionbetween Adhesive Fracture and Cohesive Fracture in the Vicinity of a Patterned Interface, i.e., Inter-Face Crack Deflection, is One of these Key Mechanisms that Contribute Significantly to the Macroscopicadhesion. in this Paper, these Fracture Phenomena are Described Simultaneously by Cohesive Zoneelements with an Exponential Traction-Separation Law (TSL) for the Adhesive Failure and an Initiallyrigid, Exponentially Decaying, TSL for the Cohesive Failure. it is Demonstrated that the Conditions Atwhich Crack Kinking Occurs are Dominated by Fracture Strength Values as Opposed to the Commonlyused Fracture Toughness Values. Experimental Verification is Performed by Means of Four Point Bend-Ing Tests on Specifically Designed Micro-Patterned Polymer-Metal Samples.

Prediction of the epoxy moulding compound aging effect on package reliability

Abstract Most semi-conductor devices are encapsulated by epoxy moulding compound (EMC) material. Even after curing at the prescribed temperature and time in accordance with the supplier’s curing specifications often the product is not yet 100% fully cured. As a consequence, the curing process of a product continues much longer, leading to curing effects of the EMC during the lifetime of the package. In this paper, the effect of EMC curing during lifetime on package reliability is investigated. The visco-elastic mechanical properties of two commercial EMC materials are measured as a function of aging time. The resulting data is used to construct material models that are used in FE calculations. Aging effects on critical semi-conductor failure modes die cracking, compound cracking, wedge break, and delamination are addressed. Die and compound crack risks are predicted by common stress analysis. The risk of wedge break occurrence is investigated by detailed 3D modeling of the actual wires in the package using a global–local approach. Conclusions on delamination risks are made based on a parameter sensitivity analysis using a 3D cohesive zones approach to predict transient delamination. The package reliability study shows that the effect of EMC aging affects relevant failure modes in different ways.

Analysis of Combined Adhesive and Cohesive Cracking at Roughened Surfaces

Macroscopic delamination of polymer-metal interfaces is one of the main failure modesobserved in micro-electronic components. Due to the irregularly shaped metal roughness profile, thisdelamination not only consists of interface separation but also bulk cracking at the micro-scale ofthe roughness. In fact, one of the key mechanisms that results in increased adhesion toughness atroughened interfaces is the transition from adhesive to cohesive failure. A semi-analytical approachis discussed in which the competition between adhesive and cohesive cracking is analyzed by meansof the theoretical relation between interface and kinking stress intensity factors. The parameters thatdefine this relation, the solution coefficients, are quantified by finite element (FE) simulations. Accordingly, the crack kinking location and kinking angle into the softer polymer is readily calculated.Furthermore, the geometrical effect of roughness is evaluated by means of FE simulations in whichthe interface topology follows from measured roughness profiles while also including interface delamination using cohesive zone elements.

Thermo-mechanical analysis of flexible and stretchable systems

This paper presents a summary of the modeling and technology developed for flexible and stretchable electronics. The integration of ultra thin dies at package level, with thickness in the range of 20 to 30 ¿m, into flexible and/or stretchable materials are demonstrated as well as the design and reliability test of stretchable metal interconnections at board level are analyzed by both experiments and finite element modeling. These technologies can achieve mechanically bendable and stretchable subsystems. The base substrate used for the fabrication of flexible circuits is a uniform polyimide layer, while silicones materials are preferred for the stretchable circuits. The method developed for chip embedding and interconnections is named Ultra Thin Chip Package (UTCP). Extensions of this technology can be achieved by stacking and embedding thin dies in polyimide, providing large benefits in electrical performance and still allowing some mechanical flexibility. These flexible circuits can be converted into stretchable circuits by replacing the relatively rigid polyimide by a soft and elastic silicone material. We have shown through finite element modeling and experimental validation that an appropriate thermo mechanical design is necessary to achieve mechanically reliable circuits and thermally optimized packages.

The need for multi-scale approaches in Cu/low-k reliability issues

Abstract Since recent years, micro-electronic industry changed the basic materials from Al/SiO 2 to Cu/low- k in IC interconnect structure. As a consequence, new reliability issues at device/product level has been discovered, and most of the failure modes have the characteristics of multi-scale: the failure of the μm or nm induces the malfunction of the device/product. Under the pressure of the time-to-market, the industries, universities and research institutes developed numerous multi-scale simulation technologies/tools to analyze the failure mechanism and to achieve the high reliability design with the capability of high volume production and low cost. This paper reviews the multi-scale modeling techniques for reliability and processing issues in Cu/low- k IC back-end structure, from the continuum level to the atomic scale.

Integrating advanced interconnect technologies in a high power lighting application: First steps

This paper reports on first results towards the development of a high power lighting demonstrator of the FP7 Nanotherm project. The demonstrator aims to show an optimized ROHS compliant interconnect solution. Hereto sintered materials are considered as alternative interconnect materials. Additionally, a heat spreader concept is evaluated as alternative for state-of-the-art IMS boards. This paper shows preliminary results for: - Transient thermal measurements of the reference system. - First trials of sintered past and adhesive used to mount LEDs on DCB substrates. - Thermal finite element simulations of the heatspreader concept compared to the IMS solution.