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The affects of adsorbed water on printed circuit boards, and the process of ionic migration
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Hiroko Katayanagi* Hirokazu Tanaka* Yuichi Aoki* Shigeharu Yamamoto*
This report considers the insulation resistance characteristics of printed circuit boards (PCBs) and the effects of ionic migration in environmental testing. By measuring insulation resistance in a variety of environmental test conditions, the authors have determined that the initial changes in insulation resistance values is caused by water adsorption *1 and electrolysis. While investigating the occurrence of ionic migration, the authors were able to confirm that metal ions eluted in response to changes in the pH near the electrodes. These changes in pH were determined to have been caused by the electrolysis of water, which affected both pH and applied voltage.
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In recent years, the trend toward compact and lightweight electronic devices has been accompanied by crucial problems in insulation reliability. Such reliability problems have been particularly noticeable with ionic migration (IM).
To properly evaluate the insulation reliability of PCBs during environmental testing, standards have been set for insulation resistance measurement in hightemperature, high-humidity tests. (Table 1)1) These standards consist of applying flux and solder paste on prescribed electrodes and evaluating (1) the diffusion of water absorption in the flux and the resin base material, and (2) the time elapsing in the progress of the insulation deterioration due to the reduction effect of metal ion elution caused by the electric field coming from the applied voltage.
The insulation resistance values seen in environmental tests appear as a result of IM caused by (1) electrolysis due to moisture adhering to the surface of the PCB and its adsorption, and (2) by the elution and diffusion of metal ions, and their subsequent reduction. (Photos 1 and 2)
This report points out the correlation between IM and the changes in insulation resistance due to the adsorption of water. By running actual environmental tests and analyzing the IM mechanism, the authors noticed the following relationships between IM and insulation resistance values:
(1) Insulation resistance characteristics and the adsorption of water during environmental tests,
(2) Characteristics of changes in current due to the electrolysis of water,
(3) Changes in pH in the vicinity of electrodes and the elution of metal ions, and
(4) The affects of pH on the occurrence of IM.
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| Photo 1 Dendrites occurring on a glass epoxy PCB (60X) |
Photo 2 Example of paper phenolic board and CAF*
throughout substrate material(X30)
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*CAF=Conductive Anodic Filaments
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| Table 1 Main standards for evaluating insulation resistance |
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Standard
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Test name
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Test conditions
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Applied voltage
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Measurement voltage
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(1)SAE
- J - STD - 004
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Requirements for
Soldering Fluxes
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85°C 85%RH, 168h
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50VDC
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100VDC
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(2)JIS
- Z - 3284
Appendix 14
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Solder paste
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40°C 90 to 95%RH,1000h
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45 to 50VDC
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100VDC
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85°C 85 to 90%RH,1000h
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(3)IPC*-
TM - 650 - 2.6.3
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Moisture and Insulation
Resistance, Printed
Boards
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35°C
85 to 93%RH, 4days(class1)
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100VDC
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Decided in
consultation
with purchaser.
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50°C
85 to 93%RH, 7days(class2)
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| *IPC??The institute for Interconnecting
and Packaging Electronic Circuits |
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| 2. Water adsorption and insulation resistance characteristics during environmental tests |
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| Changes in PCB insulation resistance values during environmental tests is intimately related to the amount of water adsorption. These experiments measured both PCB resistance values and changes in the amounts of moisture absorbed under high-temperature, high-humidity test conditions. Table 2 shows the test conditions.
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| Table 2 Insulation resistance test conditions |
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Item
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Details
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Test
conditions
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3 sets of conditions:
40°C 87%RH, 60°C 87%RH,
85°C 85%RH
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Applied
voltage
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50VDC
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Measurement
intervals
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Every hour
(measurement voltage = 50 V DC)
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Specimen
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Copper-clad glass epoxy (FR-4)
Conductor intervals:
0.318 mm (JIS type 2)
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Fig. 1 shows the insulation resistance characteristics during high-temperature, high-humidity tests. The specimens are glass epoxy PCBs with JIS type 2 copper electrodes. Measurements consisted of applying 50 V DC after the temperature and humidity had stabilized for 24 hours, and measurements were recorded hourly using an insulation resistance continuity tester. (Photo 3 shows an ionic migration Evaluation System.) Fig. 1 shows a sharp rise in insulation resistance values in the initial stage of the tests, changing to a roughly stable condition after the passage of a specific amount of time. During the stable period, the insulation resistance values showed a tendency to become increasing lower in response to more severe temperature and humidity conditions.
Fig. 2 shows humidity absorption characteristics. Humidity absorption was measured by exposing the specimens to 100°C for 24 hours, then taking the initial value of the absolutely dry weight, and then measuring the weight every hour and finding the change according to the following formula, which compares the absorption weight of the PCB with its initial weight.
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The humidity absorption of the PCB shows a sharp increase in the initial period, followed by a gradual increase. The results also confirmed that the humidity absorption of the PCB during the environmental tests corresponded to temperature and humidity conditions.
The adsorbed water is dispersed within a solid object due to the water molecules being dispersed in the intermolecular intervals of the solid.2) Therefore, changes in the humidity absorption rate can be hypothesized as due to such factors as the amount of water adsorbed into the interior of the solid, the diffusion weight, and the diffusion time, and can be conjectured to be determined by the PCB materials and environmental conditions.
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Fig. 1 Insulation resistance characteristics during environmental tests
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Fig. 2 Humidity absorption characteristics during environmental tests
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Photo 3 Ionic migration Evaluation System
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