## 1 Introduction  
The space traveling-wave tube amplifier (STWTA) is a key component of a satellite system for satellite communication and the spaceborne transponder. It is mainly responsible for the amplification, forwarding, and transmission of microwave signals [1]. As a critical component of STWTA [2], the electronic power conditioner (EPC) is required to undergo several months of environmental simulation tests during the development process [3]. It often happens that the sampling value of EPC increases in thermal and moisture stress environments, which leads to telemetry abnormity of the TWTA. The main reason for the increase in sampling value is leakage currents on the high-voltage printed circuit board (HV-PCB) surface due to a diminution in surface insulation resistance (SIR). Researchers believe that the presence of contamination of the HV-PCB surface acts as a factor in surface insulation resistance deterioration [4]. Contamination of the PCB mainly originates from processes of PCB manufacture, handling, operating environment, storage, and transportation [4; 5]. Commonly found contaminations are residual salts from electroplating, etching, and soldering, flux residues [6; 7; 8],dust particles [9; 10; 11], and even fingerprints of sweat. Residues and contamination in the manufacturing process, and improper operation cannot only affect SIR but also lead to electrochemical migration (ECM) [4; 5], which is generally considered to be the failure mechanism most relevant to insulation failure behavior [12]. Research institutions such as Bell Labs (where ECM was first discovered), the Technical University of Denmark, the KTH Royal Institute of Technology, and the Beijing University of Posts and Telecommunications have conducted extensive research on the decrease in surface insulation resistance and aggravation in ECM caused by PCB surface contamination, as well as protection methods. Through the temperature-humidity-bias test [13; 14; 15], water drop test [16], thin electrolyte layer test [17], and electrochemical test on different types of PCB samples, different surface contaminants, and different protection methods, researchers have shown that: (1) under a certain humidity condition, the presence of contaminations will affect SIR and ECM [4; 18]; (2) compared with non-cleaning flux residue containing weak organic acid active agents [7; 19], ionic contaminations have a greater impact on the decrease of SIR [20], while the residue of moderately active rosin flux has little contribution to the degradation of SIR [21; 22]; (3) soluble inorganic salts contained in natural dust in the air become one of the main reasons for the failure of the circuit board [9; 10; 11; 23]; (4) the deterioration of SIR and ECM are closely related to humidity, temperature and applied voltage [24; 25]; (5) PCBs are generally coated for protection [21; 26], while integrated circuits are usually encapsulated to counter the effects of ECM [27]. It is worth noting that previous studies were all based on low-voltage circuits with small spacing between adjacent electrodes, operating voltage of only a few volts, and electric field strengths of only 100-1000 V/cm [28]. In addition, researchers usually used standard Y-type and comb circuits as experimental test circuits, test components are surface mount devices (SMD), while high-voltage applications were rarely reported. Compared with low-voltage circuits, the typical operating voltage of an EPC used in space is 4 kV-10 kV, and the electric field intensity is much higher than that of the low-voltage circuit [3]. HV-PCB welding of the EPC uses medium active rosin flux, and the pollution source that affects the insulation performance of the EPC is mainly ionic contamination. In high electric field strength, temperature, and humidity environments, surface contamination may cause two forms of insulation failure: surface insulation resistance (SIR) decline, and surface discharge [29].