**4. Heated and humidified sample charge accumulation behavior**

In this chapter, we discuss the relationship between space charge distribution and breakdown phenomena of polyimide under heating and humidified condition.

Recently, various and many electronic devices have been used the flexible Print-Circuit Board (PCB) to reduce the size of them. At the moment, polyimide film is usually used for PCB [4]. Polyimide film has an appropriate flexibility and it shows a good insulating performance even at high temperature. On another front, since the devices are improved for downsizing and lightweight every year, it is necessary to reduce the thickness of PCB. When the thickness of PCB becomes thinner, the applied electric field becomes higher. It means that the possibility of electrical breakdown in PCB becomes higher. It is necessary that inverter can control at high frequency and drive at high voltage for high efficiency of inverter control. But inverter serge becomes high voltage in these conditions. The partial discharge happens and insulating covering material becomes depleted. And these insulating materials using electric devices need to become high insulating capacity at high temperature and high humidity. Therefore, it is necessary to understand the mechanism of breakdown in polyimide film to prevent the breakdown. Thus, we tried to measure the space charge distribution in polyimide under heating and humidified condition for understanding the phenomena.

### **4.1 Samples and measurement procedure**

In this chapter, a commercially available polyimide film, Kapton® 500H (nominal thickness 125 μm) supplied from Dupont-Toray Co.Ltd. is used.

For simulating the real usage condition, Heat with humidifying treatment and heat treatment are applied before measuring space charge as pre heat and humidifying treatment. The samples are set in a thermostatic chamber IH 400 manufactured by Yamato Scientific Co., Ltd., (80 °C and 80 or 60%) for one hour.

Furthermore, we prepared the heated sample whose were set in silicone oil boiled at 100 °C in beaker to dry off moisture of sample.

### **4.2 Results and discussion**

**Figure 5** shows space charge [2] behaviors in non-treated (1) and heated and humidified (2) sample under DC stress of 60(a), 100(b), 120(c) kV/mm at 80 °C. Measurement interval is 5 s. Amount of accumulated space charges are displayed using color chart. As shown in **Figure 5(a)**, positive and negative homo charges are observed near anode and cathode from the start of measurement in non treated sample. The amounts of homo charges are increasing until about 10 mins later,

**Figure 5.**

*Space charge behaviors in non treated (1) and heated with humidified (2) sample under DC stress of (a)60, (b)100, (c)120 kV/mm at 80 o C.*

### *Space Charge Accumulation Phenomena in PI under Various Practicable Environment DOI: http://dx.doi.org/10.5772/intechopen.96786*

but notable changes are not observed after 10 mins. On the other hand, positive and negative homo charges are observed near anode and cathode from the start of measurement in heated and humidified sample. But the amounts of homo charges are increasing until about 5 mins later. In 60 kV/mm, there is no difference point except time to increasing of homo charge and space charge behavior of end of measurement is similar to each other. The time to saturating space charge accumulation in heated and humidified sample is shorter than one in non treated sample. It is appeared that moisture of heated and humidified sample is larger than one of non treated sample. So, moisture of sample is large, it is easily occurs to ingress space charge and migration speed of space charge is more fast.

As shown in **Figure 5(b)**, positive and negative homo charges are observed near anode and cathode from the start of measurement in non treated sample along with space charge behavior of 60 kV/mm(a). But migration speed of space charge is faster and migration position of space charge is larger than one of 60 kV/mm in non treated sample. On the other hands, positive and negative homo charges are observed near anode and cathode from the start of measurement in heated and humidified sample. And positive hetero charge becomes to be observed near cathode. The amount of positive hetero charge is increasing until breakdown at 43 mins later. In our previous research(1), it is observed that positive and negative homo charges are observed near anode and cathode and positive hetero charge becomes to be observed near cathode and breakdown occurs. It is similar process of space charge behavior to heated and humidified sample. And moisture of sample is large, it is easily occurs ingress of space charge and migration speed of space charge is more fast. So, in heated and humidified sample, space charge behavior process until breakdown occurs faster than non treated sample.

As shown in **Figure 5(c)**, homo charges are observed near anode and cathode and positive hetero charge becomes to be observed near cathode and breakdown occurs at 25 mins later in non treated sample. On the other hands, breakdown in heated and humidified sample occurs at 5 mins in same space charge behavior process at non treated sample. However, negative hetero charges are observed near anode from the start of measurement.

In these results, space charge behavior process of non treated sample and heated and humidified sample until breakdown is same. It is said that the breakdown process is promoted by increasing moisture of sample and rising applied electric field. Therefore time to breakdown is shorter than ingress of space charges and migration speed of space charges are faster.

From the above results, we describe process of chare accumulation behavior to breakdown under DC stress. **Figure 6** shows typical process of space charge accumulation behavior in Kapton® under high DC stress (130 kV/mm).

As shown in the figure, a few positive and negative hetero charges are observed near anode and cathode from the start of measurement (charge behavior annotation number 1 in the figure). These hetero charges are same hetero charge in heated sample. Therefore, these space charge accumulations are looked when moisture of sample is low and sample is applied at high electric field. After hetero charges are observed, positive and negative homo charges are observed near anode and cathode and hetero charges decrease (charge behavior annotation number 2 in figure). These homo charges are injected from anode and cathode. When moisture of sample increase, amount of accumulation of homo charges is large. In fact, injection of charge easily happens from electrodes when moisture of sample is large. And charges can easily move in the bulk. And positive and negative hetero charges may disappear by accumulating homo charges. These homo charges decrease and large positive hetero charge accumulated at next period (charge behavior annotation number 3 in figure). And accumulation of positive hetero

**Figure 6.**

charge is larger than that of negative hetero charge. However large hetero charge is observed in **Figure 6**, a few hetero charges can be observed at some sample. Amount of hetero charge is influence at amount of homo charges mentioned at the charge behavior annotation number 2 in figure. Because hetero charge is observed before breakdown happens, this hetero charge promote breakdown. But accentuation of electric field by accumulating these space charges is a little. And so, it is not say that accumulation of space charge is cause which breakdown happens by. But this phenomenon is distinct pre-breakdown phenomenon. After this hetero charge was observed, positive hetero charge decreases a little in this measurement result (charge behavior annotation number 4 in figure). This behavior is not always observed in all samples. In fact, breakdown occurs when accumulation of space charge was saturated or decreased. Therefore, the processes of accumulating of the hetero charge and decreasing ones are equilibrium condition. And electric field around electrode is large by accumulating hetero charge. And given the accumulation of homo charge before hetero charge is observed, homo charge injected from electrode and hetero charge generated from bulk is saturated. In this research, source of hetero charge is not clear up. It is considered that hetero charge is generated by ion breaking the molecular chain or ionization of impure substance of the sample.
