Application of the hottest plasma technology in PC

2022-08-11
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The application of plasma technology in PCB manufacturing

1. Introduction

the quality requirements of multilayer boards are constantly improving. The goal of our efforts is, of course, to ensure the reliability of circuit connectivity of products including macroporous aluminum based materials, silicon aluminum dry adhesives, catalysts, etc. with the trend of miniaturization, which will lead to some problems that cannot be solved by traditional technology

what is more difficult is that we realize that the environment can no longer be polluted indefinitely, so waste - especially those hazardous to the environment - should be produced as little as possible or preferably not. The law also pays more and more attention to these, and has promulgated more and more stringent regulations to prevent the generation of waste

this paper introduces some mature technologies and the latest technologies, which can be widely used in the following PCB and electronic industries:

- Drilling desmear and back etching of Multilayer PCB

- kneading circuit board plasma drilling micro hole

- plasma cleaning of bonding pad before gold wire bonding

- plasma cleaning of electronic components before packaging

2. Plasma

plasma is a physical state of (partial) ionization of substances. Fluorescent lamps are well-known plasma applications in daily life. What people are not familiar with is that when the surface of the object is exposed to plasma, these glow gas particles can also cause chemical reactions on the surface of the object

this paper mainly discusses the so-called low-pressure plasma for material cleaning and etching. When the pressure is less than 100 Pa, the gas is partially ionized by the electric field. This plasma has some remarkable characteristics:

1) the gas produces a glow phenomenon, which is often called "glow discharge". Because it is vacuum ultraviolet light, it has a very positive impact on the etching rate

2) the gas contains neutral particles, ions and electrons. Because the temperature of neutral particles and ions is between 102-103k, and the temperature corresponding to the electron energy is as high as 105K, they are called but actually there is no obvious improvement of "non-equilibrium plasma" or "cold plasma". But they are electrically neutral (quasi neutral)

3) the free radicals and ions produced by the gas are highly active, and their energy is enough to destroy almost all chemical bonds and cause chemical reactions on any exposed surface

2.1 generation of plasma

low pressure plasma can be generated by DC or high frequency AC electric field. When AC is used, only the scientific and industrial frequency bands (intermediate frequency (MF) 40KHz, high frequency (HF) 13.56MHz, microwave frequency (MW) 2.45GHz) specified by telecom can be selected, otherwise radio communication will be interfered

plasma can be "ignited" between two electrodes or in the coil winding when MF or HF electric field is used. It is worth noting that when using MF or HF, the electrode does not need to be placed in the plasma field, because the cathode does not need to emit electrons to maintain the plasma, which is different from DC discharge

microwave plasma can be ignited even without any electrode, because such a high-frequency electric field is more suitable for energy transmission in empty conductors. The MW energy beam is transmitted to the plasma chamber through a horn radiator, which is separated from the vacuum by a quartz window. Due to the electrical characteristics of high-density plasma, microwave cannot penetrate into the plasma chamber very deep, so the plasma is unevenly distributed near the quartz window (Fig. 1)

2.2 basic structure of plasma equipment

plasma etching equipment includes a vacuum chamber, a vacuum pump, a plasma source and a set of gas induction and control system. Plasma equipment can be divided into two categories: one is that the matrix is directly placed in the plasma for treatment (the direct equivalent can extend the shelf life of perishable foods such as bread by twice the ions), and the other is that the reaction gas is excited in another chamber isolated from the treatment chamber (indirect plasma)

2.2.1 "direct plasma" treatment

the matrix can be placed in several ways in the plasma: 1) placed on the cathode connected with HF or MF; 2) Place on the grounded electrode; 3) It is placed between the two electrodes, but insulated from the electrodes (Figure 1). Because the first two methods will generate a lot of heat on the substrate surface, it is not suitable for PCB manufacturing

although microwave can also produce plasma in the processing room, it is actually closer to "indirect plasma". In this case, the matrix is only very close to the plasma, but it is not really placed in the plasma

in Chapter 2, we mentioned that high-energy ultraviolet light will be produced in plasma. This light will accelerate the etching process because it will cause the chemical chain of organic polymers to break. Another advantage of HF plasma is that the substrate has a negative voltage, which is extremely important for the treatment of complex geometries in coating applications

in addition to generating a large amount of heat on the metal surface, a major problem of direct plasma is how to obtain the effect of large-area uniform treatment

2.2.2 "indirect plasma" or "afterglow" treatment

the terms "indirect plasma" and "afterglow" have similar meanings. Their difference is only that they feed gas to the treatment room in different ways. Their common feature is that plasma is not generated in the treatment room, and substrates such as PCB are only exposed to the gas flow that has been activated and ionized (Fig. 2)

batch equipment manufactured by indirect plasma method is very difficult to control and it is almost impossible to obtain uniform treatment effect. This means that this method can only be applied to the treatment of single substrate, but it has several decisive advantages:

- no thermal stress is generated on the substrate

- no stress caused by electric field on the substrate

- microwave excitation leads to extremely high concentration of active particles, which greatly improves the etching rate

2.3 formation and composition of plasma

all electrons in the electric field will be accelerated along the positive direction due to the potential difference, When electrons collide inelastic with molecules or atoms, they will transfer energy to each other. If the velocity of free electrons is very high and their kinetic energy is greater than the binding energy between molecules or atoms and their outer electrons, the outer electrons will escape and ionize

the energy absorbed by electrons in high-frequency electric field is much more than that in DC electric field. Therefore, HF discharge does not need electrodes, and the plasma contains more charged carriers. Its density is determined by the plasma density and increases with the increase of the frequency of the excited electric field

the typical average energy of electrons in low-pressure plasma is 5-7 electron volts, so only a few electrons can ionize atoms or molecules. In the process of collision with molecules, there is another possibility of interaction, which is more important for the etching reaction, that is, the generation of free radicals. Free radical is a kind of neutral particle with odd electrons, and its activity is very strong. Free radicals are usually produced when the energy of accelerating electrons is greater than the molecular bond energy. The following reaction formula describes some of the most important reactions that produce free radicals in cf4/o2 plasma, which is usually used to etch insulating materials, such as FR4 and polyimide

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