PCB Technical - ​The quality of the PCB itself can't see the problem

For PCB, the common resistance characteristic resistance.

For example, in terms of the characteristic impedance of the PCB board, if the transmission line of the PCB board is consistent everywhere, it is usually of high quality. What kind of circuit board is called a controlled impedance circuit board? Controlled impedance circuit board refers to the characteristic impedance of all transmission lines on the PCB. Generally, the characteristic impedance value of all transmission lines is between 25 and 70.

What is the main reason for the high impedance of PCB circuit board? Or the quality of the PCB circuit board itself cannot be seen as a problem, but over time (including impedance) or performance is unstable, what is the main reason?

Please refer to the academic analysis of the following majors.

Impedance-actually refers to the parameters of resistance and resistance, because the PCB circuit (PCB circuit) circuit (PCB circuit) circuit (PCB circuit) should consider the conductivity and signal after plug-in installation of electronic components. Therefore, the lower the impedance, the more the impedance Well, the resistance is lower than 1*10 minus 6 times per square centimeter.

On the other hand, the PCB circuit board should go through copper electroplating tin (or chemical plating) during the production process. Or, the bottom of the resistivity must be guaranteed to ensure that the overall impedance of the circuit board is low and meet the quality requirements of the product. Otherwise, the circuit board will not work properly.

In addition, the electronics industry as a whole, the PCB circuit board in the tin plating link is a key factor affecting impedance. Electroless tin plating technology has been widely used in tin plating, but as a recipient of the electronics industry, the electronics or electronics processing industry has been in contact and observed for more than ten years. In the entire country, not many companies can do chemical tin plating (PCB or electronic tin plating), because tin plating technology is an emerging star in China. The technological level of enterprises is uneven.

Investigate the electronics industry. The most fatal weakness of the electroless tin layer is that it is easy to change color (easy to be oxidized or wetted), resulting in too high welding resistance, resulting in poor electrical conductivity or unstable performance of the entire board.

According to reports, tin plating was the first research in chemicals after graduating from Kunming University of Science and Technology in the early 1990s. In the past 10 years, both institutions have received the best recognition. According to our contact screening, experiments and long-term endurance tests for many companies, it is confirmed that the Tongqian electroless tin layer is a pure tin layer with low resistance. The quality of conduction and welding can be guaranteed to a higher level. No wonder they dare to ensure that the coating is not protected by any blocking and discoloring agents. It can keep its color for one year, no bubbles, no peeling, no permanent tin beard.

Later, when the entire social production industry developed to a certain level, many subsequent participants often plagiarized each other. In fact, quite a few companies did not develop or explore themselves. Leading to the poor performance of many products and their users' electronic products (the bottom of the circuit board or the entire electronic product), the main reason is impedance. Because when the unqualified electroless tin plating technology is plated on the PCB circuit board, tin is not really pure tin (or pure metal). In contrast, tin compounds are not metal units at all, but metal compound oxides or halides. More directly, it is a non-metallic material or a mixture of tin compound and tin metal, but it is difficult to find it with the naked eye.

Since the main circuit of the PCB circuit board is copper foil, the solder joints of the copper foil are the tin-plated layer, and the electronic components are soldered on the tin-plated layer. In fact, the solder paste is soldered between the electronic component and the tin coating in the molten state, that is, a single piece of metal with good conductivity, so it can be simply pointed out. The electronic components are connected to the copper foil at the bottom of the PCB through the tin coating, so the purity and resistance of the tin coating are the key, but there is no connection before the electronic components. When we directly use the instrument to detect impedance. In fact, the two ends of the instrument probe (or watch pen) also contact the copper foil surface at the bottom of the PCB board, and then connect with the copper foil at the bottom of the PCB board. Therefore, the tin coating is the key to affecting the impedance and also the key to affecting the overall performance of the PCB.

As we all know, in addition to simple metals, compounds are bad conductors of electricity, and even non-conductors of electricity (which are also the key to line distribution capacity or transmission capacity). Therefore, there are conductive but not conductive tin compounds or mixtures in the tin plating layer. Its off-the-shelf resistivity or the resistivity caused by oxidizing humidity in the future and its corresponding resistivity are relatively high (the pin affects the level or signal transmission of the digital circuit.. In addition, its characteristic impedance is also inconsistent. Therefore, it will affect the circuit board and its entire Machine performance.

Therefore, in the current PCB production, the coating material and performance of the bottom of the PCB board are the main and most direct reasons that affect the characteristic impedance of the PCB board. However, due to the aging of the coating and the change of wet electrolysis, the influence of impedance becomes more concealed and changeable, mainly because of its concealment. First of all, you cannot be seen by the naked eye (including its changes) and measured, because it changes the time and environmental humidity, so it is easy to be overlooked. Or remarry by mistake.

Therefore, after understanding the cause of high impedance, solving the coating problem is the key to impedance.