MEASUREMENTS AND VERIFICATION

The thermal and air flow behaviour of an electronic device is investigated in experimental verification. This produces the measurement results called for by the test plan, as well as images of air flow obtained with smoke or tests based on other visual methods and observer notes of the flow behaviour of the device. Parameters to be measured are surface temperatures, air temperature, air humidity, air volumetric flow, velocity distributions, pressures and pressure differences. In the case of materials, components, and structural parts one can also measure fundamental quantities such as conductance, convection, and emission properties. In principle these data should be available in device development, but this often includes design and implementation of new components (modules), which requires one to delve further into these fundamental measurements.

The stages of experimental research include planning of measurements, execution of measurements, processing of measurement results, analysis of

results, and reporting and documenting the data set. There is considerable literature published on the execution of experimental measurements, different measurement methods (such as /6.1/, /6.2/, /6.12...17/) and standards of separate measurement methods.

When experimental research is planned the measurement objects and measured quantities in the device are defined and the measuring apparatus is chosen. Selection of the apparatus is governed by economic resources and the physical and measurement technical properties, and usefulness of the apparatus. Properties to be considered are performance, accuracy of the method, measurement frequency, ability to observe measured values in order to monitor and control the test, and the storage of measurement results. In the selection of measurement sensors appropriate to the measurement conditions, the sensor accuracy, size, and reaction speed must be considered. Other considerations affecting selection of the measurement system include the training of operations personnel, maintainability of the equipment, and ease of calibration.

When assessing the reliability of measurements and mapping the errors, one must consider among others things the measurement error, systematic error, random error, and inaccuracy and drift of the measuring device /6.17/. When presenting the measurement results, errors different sources are estimated with error limits. The presentation format of the measuremement results is y ± dy, where y is the value of the measured quantity and dy is the

error in the measurement results caused by various factors.

0 10 20 30 40 50 60 70 80 0 30 60 90 120 150 180 210 240 270 300 330 Environment Component PWB 1 PWB 2 Temperature ( °C) Time (min) Figure 10. Warm-up of the device during measurements.

Figure 10 is an example of normal warming up of a device. When planning the measurement the time constants of different parts of the device must be

considered in order to perform the measurements at the correct times. Figure 10 shows the temperatures measured from a television set The voltages are switched on at time zero. The warm-up of a fairly hot, large component continues when measured from its surface even past 120 min after voltage switch-on, although the circuit board temperature near the component stabilised already at 90 min. The same rate of change is seen after voltage switch-off at 300 min. In this case one must wait at least 120 min before an accurate impression of the static performance of the device is obtained. Correspondingly in thermal cycling tests sufficient stabilisation times must be reserved to ensure that all parts have reached their final temperature.

In verification at least the following items are covered:

- Existence of a measurement program - Existence of a test program

- Processing and documentation of results (assessment of usefulness of results, comparison with calculated results)

- Do the measurements and tests cover all structural models and field conditions?

- Definition of the contents of measurements (temperatures, surfaces, air, flow, heat power, etc.). What is worth measuring and in what phase? - Definition of physical measurement locations. Do they also correspond

to the thermal design and the model?

- Definition of measurement timing. Have sufficient times for thermal equilibrium been ascertained, have the different time constants of different parts and thermal capacities been considered when studying transients (for example the reaction of the last components at the end of a long path)?

- Definition of the measurement (test) conditions. Have external

disturbances been considered, and are the measurements performed in operating conditions or representative test conditions?

- The measurement methods and their applicability to the measurements, inaccuracy, measurement equipment, and relation to the stated design margins

- Decision procedure in interpretation of observations - Field measurements under operating conditions

- Measurements of materials, boundary surfaces and components, possible needs

- Utilisation of feedback information, pruning of measurements based on design experience and/or change of measurement content when making new versions of the product.

Based on the preceding discussion, measurements related to design verification should extend from the early structural models (even cardboard models) to the final product and even to the field where feedback information of real operational environments can be obtained. The structural/thermal models, which are simplified models of the functional device, are verified to the accuracy level appropriate for the current phase. These measurements performed in different phases must be included as part of the thermal design to gain the fullest possible information on the accuracy of the design criteria.

Documentation of the measurement plans and results is a mandatory part of the utilisation of the results. For example, the existence of result sheets prepared in advance certifies that the desired properties have been measured and reduces the need for supplementary measurements or reprocessing of the results.