Michal Jablonski

Reliability of a stretchable interconnect utilizing terminated, in-plane meandered copper conductor

The pursuit for reliable, deformable electronic systems took two major paths, utilizing either conductive elastomers or metal conductors. In the case of the latter, a mechanical robustness trade-off is made in return for metallic conductor native low resistivity allowing for realization of power demanding and large area applications as well (e.g. conformable lighting and signage). The mechanical trade-off stems from the metal conductor intrinsic inability for significant elongation without failure. One of many present attempts at enabling a metal conductor to perform in an elastomeric medium without failure is the SMI (Stretchable Molded Interconnect), a PCB compatible technology developed at the CMST. Its concept relies on an in-plane, meandered, metal track embedded into a soft, elastomeric material. This work focuses at cyclic, uniaxial elongation endurance and reliability assessment (Weibull analysis) of such interconnect in its most simple form - utilizing unsupported, meandered copper tracks embedded in PDMS (Polydimethylsiloxane). The tracks are evaluated as short interconnects (a few meander wavelengths long) terminating between flexible (non-stretchable) regions to incorporate the effect of flex-stretch transition mechanics on reliability. This is an important assessment for optimizing the interconnect geometry for practical applications where flex-stretch transitions will be inevitable, and reliability under repeated deformation is of interest (e.g. stretchable circuits for integration in textile). An attempt is made to reinforce the meander geometry by tapering the transitions, but a negative impact on reliability is observed. It is clearly demonstrated that the wearout of the interconnect is strongly related to the amount of copper present in the interconnect.

SCB and SMI: two stretchable circuit technologies, based on standard printed circuit board processes

Purpose – In the past 15 years stretchable electronic circuits have emerged as a new technology in the domain of assembly, interconnections and sensor circuits and assembly technologies. In the meantime a wide variety of processes with the use of many different materials have been explored in this new field. The purpose of the current contribution is for the authors to present an approach for stretchable circuits which is inspired by conventional rigid and flexible printed circuit board (PCB) technology. Two variants of this technology are presented: stretchable circuit board (SCB) and stretchable mould interconnect (SMI).Design/methodology/approach – Similarly as in PCB 17 or 35 μm thick sheets of electrodeposited or rolled‐annealed Cu are structured to form the conductive tracks, and off‐the‐shelf, standard packaged, rigid components are assembled on the Cu contact pads using lead‐free solder materials and reflow processes. Stretchability is obtained by shaping the Cu tracks not as straight lines, like ...

Impact of geometry on stretchable meandered interconnect uniaxial tensile extension fatigue reliability

This work investigates the impact of geometry on the reliability of a high conductivity, meandered, stretchable interconnect. Meandered copper conductor interconnects of varying geometries that have been encapsulated into a PDMS matrix, are evaluated for reliability under tensile stretching conditions to 10% elongation. We present results that support our earlier findings by experiment and FEM simulation. Following, we vary interconnect parameters related to the encapsulation geometry, such as encapsulation hardness, thickness and stretchable zone perimeter, to assess impact on fatigue life of the embedded meandered copper lines. Results confirm and refine the prior simulation findings. Combinations of interconnect geometry parameters critical for stretching reliability are identified. Among others, we find that the meander radius (R) and encapsulation thickness are strongly coupled, causing very large meanders with thick encapsulation to fail very early. We show that, depending on the design of the meander transition, the characteristic life of an interconnect can differ 50 times under moderate, 10% cyclic elongation. Finally, we indicate the significance of our findings for the design of reliable, stretchable electronic systems.

Conformable, Low Level Light Therapy platform

Well-being applications demand unobtrusive treatment methods in order to reach user acceptance. In the field of light therapy this needs to be carefully addressed because, in most cases, light treatment system size has to be significant with respect to human body scale. At the same time we observe the push to make wearable devices that deliver the treatment on the go. Once scaled up, standard flexible electronics (FPC) fail to conform to body curvatures leading to decrease in comfort. A solution to this problem demands new or modified methods for fabrication of the electronic circuits that fulfill the conformability demand (flexing, but also stretching). Application of Stretchable Molded Interconnect (SMI) technology, that attempts to address these demands, will be discussed. The unique property of SMI is that its manufacturing draws mainly from standard PCB and FCB technologies to inherit the reliability and conductivity. At the same time, however, it allows soft, flexible and stretchable circuits with biomimetic haptics and high optical efficiency. In this work a demonstrator device for blue light therapy of RSI is presented that illustrates the strengths as well as challenges ahead of conformable light circuits. We report system electro-optical efficiency, possible irradiance levels within skin thermal comfort and efficiency under cyclic, tensile stretching deformation.

Reverse replication of circular micro grating structures with soft lithography

In this work, the reverse replication of circular micro grating structures on glass substrates is implemented using an ultra-violet curable resin and a polydimethylsiloxane (PDMS) mold which has the same structure as the original circular grating master. Two different techniques (“double PDMS replication” and “polymer- PDMS replication”) are employed to fabricate those reversed circular micro grating structures. Surface profiling measurements show that in case of the polymer-PDMS replication the dimensions of the resulting circular grating structures closely approximate those of the master, while the grating height is slightly decreased in case of the double PDMS replication technique, mainly due to the use of the releasing agent. For both methods, the grating slopes of the circular gratings are almost unchanged, leading to the desired optical performance. The two techniques are quite useful for more accurate reverse replications of micro optical and photonic structures.

Short, Stretchable Molded Interconnect reliability under 10% cyclic elongation

SMI (Stretchable Molded Interconnect) technology allows for realization of PCB-like manufactured electronic systems with intrinsic ability to be bent and locally stretched multiple times. The elasticity is obtained by introduction of meandered, electrical tracks that have the ability to follow the deformation of its PDMS encapsulation without electrical failure. This work investigates the endurance of 4 different meander-based interconnect types in a cyclic stretching test until 10% elongation. By modeling the failures of the unsupported copper interconnect (e.g. no polyimide support) with Weibull distribution we show that it can sustain up to 5000, 10% elongation cycles below 1% failure probability. Parameters of the distributions are compared between different interconnect geometries. SMI processing parameters critical to reliability are indicated based on failure analysis of poorly performing interconnects.