| Field Report : Top : Evaluation validity in systems with continuous, automatic measurement |
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| Evaluation validity in systems with continuous, automatic measurement |
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| 4. Validity of continuous, automatic measurement |
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| Now, let's look at the validity of continuous measurements in test environments using examples from evaluating ion migration, solder connection reliability, and short interruptions, and let's consider the possibility of using automatic measurement systems.
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4-1 Evaluating ion migration
4-1-1 Necessity of evaluating ion migration
Ion migration is the phenomenon of precipitate growth in the gap between facing or adjacent electrodes on a printed circuit board. This growth results from metal ions from one electrode being reduced as metal at the base of the other electrode.
Ion migration can only occur when there is moisture in the presence of an electrical field between the electrodes. In reality, occurrence is usually linked to the effect of impurities on the circuit board that precipitate out on the positive side. Photo 1 shows a picture of a ion migration occurrence in a test using the water drop method. Because it is extremely fragile, the ion migration substance fuses and disappears in a very short instant due to current occurring in the instant of a short circuit.
The ion migration phenomenon has been well known for years. Recently, though, ion migration has been occurring more frequently due to such factors as finer pitches being incorporated in the printed patterns in wiring in IC packages such as BGA*5 in build-up circuit boards. Other factors include stronger electrical fields between the patterns, smaller insulation gaps, impurities in flux residue and new materials, and the impact of humidity absorption due to the portability of the electronic equipment. The fact that ion migration leads quickly to failure places a high priority on evaluation of the phenomenon.
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4-1-2 Validity of continuous, automatic measurement in evaluating ion migration
Ion migration is generally evaluated using comb-patterned printed circuit boards as specimens, and applying voltage between the comb-patterned electrodes in a high-temperature, high-humidity environment. The insulation resistance between the electrodes is measured by removing the specimens from the temperature and humidity chamber at specified intervals and measuring them at room temperature. However, using this evaluation method that I have just described, it is possible for the specimen to recover high insulation characteristics at the time of measurement. Therefore, there is a danger that the specimen will be mistakenly determined to have no malfunction, even though a malfunction has occurred. In addition, manually measuring the insulation resistance of each specimen and processing that data is timeconsuming and inefficient. To resolve these issues, it would be possible to continuously and automatically measure insulation deterioration and changes in insulation resistance occurring in an extremely short time due to ion migration while applying stress voltage between the electrodes under conditions of high temperature and high humidity. Fig. 4 shows an example of deterioration characteristics obtained by continuously measuring flux insulation resistance in a high-temperature, high-humidity environment.
As you can see from these measurement examples, by measuring insulation resistance under conditions of high temperature and high humidity, we can accurately capture the insulation deterioration characteristics caused by ion migration, as well as the time at which the failure level was attained. From fluctuations in the resistance values, we were able to confirm that a gap exists between specimens at the initial stage when the test started. We were also able to obtain information confirming the major fluctuation in resistance values leading up to failure.
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Fig. 4 Insulation deterioration characteristics under high-temperature, high-humidity conditions
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