Sensitivity evaluation circuits

When molded or premolded packages cannot be used for whatever reason, for example, a need for hermetic sealing, modules using ceramic substrates (or PCB-printed circuit board) offer almost unlimited flexibility. They can be used for simple multichip modules containing two or more elements (e.g., a sensing element and an evaluation circuit), may include external components, or may be used to construct complete systems. Stress-optimized package configurations for sensitive microsystems and sensors are no problem (Fig. 5.8.4). 

Most capacitive evaluation circuits do not achieve the maximum possible resolution but are limited by the electromechanical interface, shortcomings in the electronic circuits, or stray signals coupling into the detector and corrupting the output. Section 6.1.2 below illustrates approaches to maximize the sensitivity of capacitive sensor interfaces, potential error sources, and approaches to minimize them. Electronic circuit options are discussed in Section 6.1.3. 

Electrochemical impedance spectroscopy leads to information on surface states and representative circuits of electrode/electrolyte interfaces. Here, the measurement technique involves potential modulation and the detection of phase shifts with respect to the generated current. The driving force in a microwave measurement is the microwave power, which is proportional to E2 (E = electrical microwave field). Therefore, for a microwave impedance measurement, the microwave power P has to be modulated to observe a phase shift with respect to the flux, the transmitted or reflected microwave power APIP. Phase-sensitive microwave conductivity (impedance) measurements, again provided that a reliable theory is available for combining them with an electrochemical impedance measurement, should lead to information on the kinetics of surface states and defects and the polarizability of surface states, and may lead to more reliable information on real representative circuits of electrodes. We suspect that representative electrical circuits for electrode/electrolyte interfaces may become directly determinable by combining phase-sensitive electrical and microwave conductivity measurements. However, up to now, in this early stage of development of microwave electrochemistry, only comparatively simple measurements can be evaluated. 

The impedance can be measured in two ways. Figure 5.23 shows an impedance bridge adapted for measuring the electrode impedance in a potentiostatic circuit. This device yields results that can be evaluated up to a frequency of 30 kHz. It is also useful for measuring the differential capacity of the electrode (Section 4.4). A phase-sensitive detector provides better results and yields (mostly automatically) the current amplitude and the phase angle directly without compensation. 

The previous section gave an overview of the transport and junction properties of conjugated materials regarding their importance for photovoltaic devices. In this chapter, the bulk heterojunction device itself will be in the spotlight. Device properties will be discussed and evaluated as for classical inorganic solar cells, concentrating on the short-circuit current /sc, the open-circuit voltage Foc, the fill factor FF, and the spectral sensitivity. 

This economical test exposes die climatically unstable points of electronic components. Due to the nature of the test, the entire board is evaluated. This test accelerates the mechanisms of electrochemical migration. Consequently, faults that previously would appear after months or even years can be detected during the development process. To identify potential weak points, the assembly is operated in standby mode and immersed in deionized water. Testing while the assembly is in full operation is even more effective. The sensitivity of the circuit to moisture exposure is assessed on the basis of flie recorded test current, combined with a subsequent examination of the assembly. Through weak point analysis, a Yes/No decision can be determined concerning the expected service life, of the assembly. 

Whether these corrosive gas tests are reaUstic for materials other than those used for connectors or for operating electronic equipment is not clear. The test should be carried out, but the observation of no failures should not be taken to mean there will be no field failures in typical urban environments. Similarly, any failures that are observed should be carefully evaluated to ensure that the same mechanism would he operative in field situations. Connectors tire a somewhat unique part of an electronic assembly in that the active part is frequently a noble mettil and the sensitivity of the mated surfaces to failure may be lower thtin many other parts of electronic assemblies. Most failures in electronic assemblies attributable to the environment are due to ionic particle contamination in conjunction with atmospheric moisture. In 20 years of evaluating field failures in the United States, the author has never seen a failure that could be attributed to the effects of SOj, has seen a few caused by H2S or HCl, has heard of a few caused by NOx, and has seen several hundred that were caused by ionic contamination. Clearly, valid accelerated testing of electronic components, circuit boards, and assemblies must include ionic contamination. Emerging methods are discussed in the Fine Particle Testing section in this chapter. 

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