Objective: Flexible wearable electronics consisting of passive components and active devices that are configured to act as sensors and communications interfaces for a variety of applications. Such electronics faces significant design verification/validation and testing challenges. Our objective is to develop performance characterization and reliability prediction algorithms to assess electrical and mechanical reliability as well as to design flexible wearable electronics with built-in self-test capability that allows high coverage testing to be performed post-manufacture and in the field.
Figure 1. Determining changes in electrical parameters of materials using transmission lines
Figure 2. Built-in testing and tuning infrastructure for embedded RF transmitter
Twisting experiment for mechanical reliability
Interfacial fracture experiment
Design Verification and Validation of Embedded Passives and Integrated Flexible Substrates: Design verification/validation is concerned with the problem of determining the correctness and reliability of the fabricated design. This has two major objectives:
(a) To determine if the designed system will perform correctly in the presence of realistic manufacturing process variations
(b) To determine, through correlations with stress testing techniques, the expected lifetime of manufactured parts under realistic field operating conditions.
Figure 1 shows two test structures that can be used to measure the permittivity, permeability and loss tangent of dielectric material (flexible substrate) using a vector network analyzer up to 60GHz. Multiple test structures of different dimensions corresponding to different characteristic impedances are designed and tested for calibration and measurement validation. The fabricated test structures are subsequently subjected to mechanical stress and the permittivity, permeability and loss tangent of the stressed dielectric relative to that of unstressed material is determined from the measured electrical parameters.
Built-In Testing of Flexible Electronics: After the design is validated, the next step in the testing process is to ensure that defects introduced into manufactured flexible substrates are detected with high coverage (such as defective interconnect) and the effects of manufacturing process variations (such as changes in L, C values due to changes in substrate thickness and permittivity) are compensated using low cost built-in tuning mechanisms. Figure 2 shows how an RF front-end can be tested and tuned using resources available on-substrate.
- The mixed-signal sensors/electronics/communication subsystems and interconnect are stimulated by test patterns generated by a digital processor or logic. In Figure 2, the baseband digital processor is used to stimulate the RF transmitter directly using the digital-analog interfaces that are part of the transmitter. The stimulus is designed carefully to functionally exercise the RF front end. In general, the digital processor can be replaced by dedicated test stimulus generation logic that can placed in a “controller” ASIC that is incorporated onto the flexible substrate to enable post-manufacture testing and tuning (note that the same infrastructure can also be used for design validation as described earlier).
- Sensors designed into the electronics can be used to infer the performance of the underlying circuitry. For example, the envelope tracker connected to the output of the transmitter of Figure 2 is used to infer a suite of transmitter performances such as gain, nonlinearity, I-Q mismatch, etc., with limited additional processing. This information is then used to “tune” the electronics to achieve desired performance metrics using a feedback loop as shown for the transmitter of Figure 2. This allows measurement of complex performance metrics using relatively simple circuitry without the use of external test instrumentation.
We believe the above approach has the advantages of: (a) low implementation complexity and hardware overhead due to maximal use of on-substrate resources, (b) ability to test for and determine complex mixed-signal/RF/sensor specifications on-substrate without the use of external test instrumentation and (c) ability to perform self-calibration and tuning autonomously allowing for high manufacturing yield and extended in-field lifetime.
Mechanical testing involves a wide range of experiments that focus on bending, stretching, and twisting of flexible electronic components and systems under both monotonic and cyclic loading conditions. For each category of test, different test tools and methods will be used. Under such testing, various mechanical, electrical, and thermal properties will be determined, and the changes in such material properties as a function of applied load and load cycles will be determined. Measurements will be carried out at different temperature and humidity conditions. Constitutive relations as well as process-structure-property relationships will be established. Fixture-based and fixture-less test techniques will be employed to measure adhesion strength and interfacial fracture characteristics of various printed and assembled components on flexible substrates. Special test coupons will be designed for these various suites of tests. The tests will be such that they are easy to use, applicable and relevant to the flexible wearable electronics ecosystem, adaptable to industry needs, and acceptable to Testing Standards organizations.