Power module, also known as IPM (Intelligent Power Module), is a power driver device that combines power electronics and integrated circuit technology (Driver IC). Due to its advantages of high integration and reliability, power modules are winning an increasingly large market, especially suitable for variable frequency drives and various inverter power supplies that drive motors. They are commonly used power electronic devices in variable frequency speed regulation, metallurgical machinery, electric traction, servo drive, and variable frequency household appliances.

Among them, due to the harsh working environment of power modules, such as high temperature, humidity, high ion concentration and other harsh conditions, as well as their high heat generation, good reliability design is needed to solve the problem of long-term service life.

In the manufacturing process of power modules, the substrates commonly used include copper-clad ceramic substrates (DBC), lead frame architectures (CIS), and insulated metal substrates (IMS). The manufacturing process is generally similar, including adhesive printing, power device and wafer mounting, reflow soldering, chemical cleaning or non cleaning, plasma cleaning, metal wire bonding, and epoxy resin packaging.

Metal substrates (CIS) have become the mainstream substrate for power modules due to their excellent heat dissipation performance, including adhesive printing, power devices, reflow soldering, solution cleaning, plasma cleaning, metal wire bonding, lead frame welding, and epoxy resin packaging. CIS cleaning is set before bonding metal wires to solve problems such as wire detachment. However, in order to solder the power devices and metal connections to the metal substrate in this process, it is necessary to apply flux in advance. Although the soldering process in this process uses flux assisted soldering, without cleaning treatment to remove the flux, such as directly encapsulating with epoxy resin, the bonding force between the substrate and the epoxy resin will deteriorate, and the packaging body is prone to delamination, thereby reducing the heat and humidity resistance and reliability of the power module.

Therefore, plasma cleaning can effectively remove organic substances such as flux from metal substrates that are soldered with flux before packaging, improve the bonding between the substrate and packaging materials, and thereby enhance the reliability of packaging processes and power devices.

Plasma cleaning technology

The term plasma was first proposed by Langmuir in 1928 to describe the form of matter in gas discharge tubes. Plasma is composed of positive ions, electrons or negative ions, and neutral particles, and is considered the fourth state of matter. Matter is composed of molecules, molecules are composed of atoms, and atoms are composed of positively charged atomic nuclei and negatively charged electrons surrounding them. Under high temperature or other specific conditions, outer electrons break free from the constraints of the atomic nucleus and become free electrons, a process called "ionization". As shown in Figure 1.1, when a solid (the first state of matter) is heated, the internal particles gain enough energy to break the original structure, thereby melting and transforming into a liquid (the second state of matter). As the energy further increases, particles in the liquid will break free from each other's constraints and evaporate into gas (the third state of matter). Subsequently, when a large amount of energy is injected into the gas through mechanisms such as discharge, it can cause electrons in atoms or molecules to escape, producing free electrons and ions. These electrons and ions accelerate and collide with each other under the action of an electric field, further increasing the number of charged particles in the gas. These processes alter the electrical properties of the gas, transforming it into an ionized gas with conductivity, known as plasma (the fourth state of matter).

Figure 1 Conversion process between substances

Plasma cleaning technology is based on the principle of plasma generation and utilization of active particles. By applying high voltage or radio frequency field, gas is ionized into plasma, and the generated active ions, electrons, and excited atoms and molecules can undergo chemical reactions with surface pollutants or target substances, achieving surface cleaning and modification.Compared with existing cleaning techniques for substrates, plasma cleaning technology has replaced liquid chemical cleaning methods to avoid corrosion of power devices and circuit wiring on the substrate. Similarly, it has replaced laser cleaning methods to reduce dust generation and substrate contamination.