Effect of Ion Migration on Insulation of Printed Circuit Boards

Due to the development of high integrated circuit technologies and microelectronic technologies, the volume of electronic products is getting smaller and smaller, which makes the requirements for the substrates carrying various lines more and more demanding. No matter whether it is the number of layers of the multilayered board and the aperture of the through hole, or the width and distance of the wiring, it tends to be miniaturized, and a higher requirement is imposed on the manufacturing process thereof. According to the data, the distance between the circuit board's wires has been as small as 30um, and the board's interlayer thickness is only 40um. And these have the tendency to further develop to more subtle and thin layers. (1) In this case, the requirements for its reliability have also been increased accordingly. In particular, it is an important issue to prevent short circuit or open circuit.

Among the causes of short-circuit faults, a factor that has been noticed by people in the past and that has not been taken seriously has attracted attention since the miniaturization of electronic products. This is the short circuit caused by metal ionization under certain conditions and migration under the action of an electric field. This article will introduce the research progress in this area for the reference of colleagues in the electroplating industry.

1, factors affecting the reliability of the circuit board

The factors affecting the reliability of the circuit board are different from the environment used. The main factor is the quality of the substrate. In particular, some potential factors, when the external conditions change and become induced factors, will accelerate the transition of potential factors to the critical state, eventually leading to failure.

From the intrinsic factors of the substrate, the substrate material is determined by high frequency characteristics, heat resistance, moisture resistance, dimensional accuracy, processing performance, mechanical strength, surface flatness, insulation, thermal conductivity, and the like. These properties are also closely related to time and temperature, that is, anti-aging properties. Some substrates that are acceptable under normal conditions will degrade under long-term and high-humid conditions. The high temperature and humidity are related to the environment in which the product is located.

Caused by the temperature of the circuit board failure are: the work of the component heat; connection is not strong or ohmic welding caused by ohmic heat; short circuit caused by abnormal discharge; component damage caused by high temperatures.

Moisture induces: damp air, wet environment, accidental moisture, etc.

There is also a physicochemical cause of dust, chemical residues, and short-circuit faults caused by metal ionization and migration under the action of an electric field. This kind of ion migration failure has become a factor that cannot be ignored when the circuit board is becoming smaller and smaller.

2, the study of ion migration failure

2.1 History of Ion Migration Studies

The so-called ion migration failure means that the metal on the circuit board, such as copper, silver, tin, etc., undergoes ionization under certain conditions and the insulating performance caused by the migration of the insulating layer to the other pole under the action of the electric field is reduced. Due to the wide spacing of early circuit board lines, the probability of such failures is very small, and it is only possible under high temperature and humidity conditions. Therefore, this phenomenon has not attracted attention.

The earliest research on this phenomenon was conducted by Kohman et al. of Bell Labs, USA. They found that some of the silver plating on the copper posts was detected in the phenolic resin substrate in the connector of the telephone exchange. They confirmed it was silver. Ion migration. This phenomenon sometimes causes a decrease in the insulating performance of the substrate. The reason for this may be caused by the ionization of the coating that is exposed to moisture at the DC voltage. The study found that, in addition to silver, aluminum and tin have similar phenomena. But for a long period of time, almost no one has paid attention to this research. Until the 1970s, research in this area began to arouse people's attention. At this time, high-density printed wiring boards and copper-clad wiring boards using ceramics as the substrate began to develop in the direction of miniaturization, and research on the material of the substrates has progressed, and it has been noticed that ion migration has an effect on the insulation performance of the substrates.

Most of these studies are only limited to the West, and the works are rare. Since the 1980s and especially the 1990s, Japan has also begun to pay attention to this topic, and there are also many reports on this aspect. (4) Since there is no uniform standard, the reproducibility of these studies is poor, and the conclusions obtained are also different.

2.2 Factors Affecting Ion Migration

The reason for the ion migration is that when there is a DC electric field between the metals at the two ends of the insulator, the two sides of the metal become two electrodes, in which one of the anodes is ionized and passes through the insulator to the other side metal under the action of an electric field ( Cathode) migration. Thus, the insulator is in an ion-conducting state. Obviously, this will cause the insulator's insulation performance to drop even become the conductor and cause the short circuit fault.

The conditions for this phenomenon are the potential factors for the formation of electrolytes on the surface or inside the insulator in a humid environment. Including the type of insulator itself, composition, additives, fiber properties, resin properties.

From the composition of the substrate, it is divided into three aspects: First, the resin, the resin composition, functional groups, curing degree, ion concentration (impurity, hydrolysis properties, etc.), hygroscopicity; the second is the fiber: the density of glass fiber , The moisture absorption of organic fibers; Third, the processing conditions: the conditions of the through hole (with or without electroplating solution residues), the lamination conditions (adhesion between the resins), processing residues (coarsing, electroplating, etc. ).

From the aspect of the circuit board's surface, there are even more factors. In addition to the substrate factors, there are components that are installed on the substrate and metal components that remain in the plating pores and the unwashed electrolytes, solders, and adhesives. Prone to electrolytic material, dust plasma pollution, condensation and so on.

From more in-depth studies, the design of circuits and structures is also related to the occurrence of ion migration failures. Because the electric field distribution on the circuit board and the polarity of the electricity carried by the easily oxidized metal material are related to the design of the circuit and the structure. The distance between the circuit and the DC electric field existing between the circuits is directly related to the occurrence of ion migration. When a DC electric field exists between two adjacent phases, the presence of a potential difference between the in-phase currents makes it easy to operate under humid conditions. Ionization of the metal in the anode state occurs and migrates to the opposite cathode. This is also related to the physical properties of the metal materials used in the components and devices mounted in the circuit. For example, the ionization energy of metals, the hydrolyzability of ions, and the mobility of ions all have a great influence on the occurrence of ion migration.

From the environmental aspects of the circuit board, moisture and high temperature are important causes of ion migration failure. The effect of these factors on the insulation life of the circuit board can be expressed by the following relationship:

MTF = α (1+ β D n / V). H γ . exp (Ea/kT)

Or ts=A [E - γ . H -n .exp ( Ea/kT )]

In the formula, MTF and ts are the time (life) when the insulation performance drops to a certain degree, D is the insulation gap, V is the voltage between the two conductors, H is the relative humidity, E is the electric field between the insulation gaps, α, β, Both a and k are constants and coefficients. Using this evaluation method can only indirectly reflect the impact of ion migration, but in any case, using the change in insulation resistance to evaluate the impact of ion migration is still a simple and practical method.

2.3 Studying ion migration methods

In order to study the mechanism of ion migration failure, the ion migration process was simulated in the experiment. There are two commonly used methods, one is the vapor test method. This is to place the test piece into a container containing an oversaturated salt solution, subjecting the test piece to ion migration under high humidity and a certain temperature, and then insulating the test piece through an ultra-insulated resistor connected to the test piece. The changes are measured.

Another method is an aqueous solution test method. The test piece used is a metal electrode such as a silver electrode. Put the two electrodes on the glass plate, and then press a layer of transparent organic glass plate on the top. The two electrodes are respectively connected with the positive and negative poles of the power supply, and deionized water is dripped between the electrodes to simulate condensation and then shoot with a camera. Under the action of the electric field under different conditions and under different conditions of electromigration.

With this method, the physical form of ion migration can be visually observed, and the change in insulation resistance at different times can be measured until a short circuit is finally formed between the two electrodes.

3 Ion Migration Failure Effects and Countermeasures

Ion migration caused by the failure is mainly to reduce the electrical insulation properties of printed circuit boards, serious is caused by the short circuit between the lines, so that the burning of electrical appliances or even cause a fire. This kind of failure has the possibility of increasing under the condition that the distance between online lines is becoming smaller, and thus attracts attention. In Japan, there were 284 failures in the design and manufacture of home appliances during the four years from 1995 to 1998. Of these, 8 occurred due to circuit board heat and fire. Four of these 8 fire accidents were caused by ion migration. In addition, because the fire accidents often damage the original state of the circuit board, some of them cannot be determined even if the ion migration fails.

An important measure to prevent ion migration failures is of course to keep the environment dry, but the circuit board manufacturing process is also very important. Nowadays, paper-based resin boards have rarely been used, and great attention has been paid to preventing chemical contamination. Among the electronic chemicals used, flux is the most easily overlooked. Today, fluxes are generally composed of natural or synthetic resins, organic or organic sulfonic acids and their salts, organic solvents, and the like. These organics, especially organic acids, remain on the circuit board and become electrolytes in wet conditions to initiate electromigration. Therefore, it is very important to choose and use the appropriate flux. Hydrophobic organic acids and halogen-free salts should be used as active agents, and their residues on the circuit board should be avoided. This is a more important tip, especially when using a no-clean process. At present, China's research on this failure is rare, but small electronic products have been put into production in China. International electronic products are also very optimistic about the rapid growth of China's vast market. In this case, it is necessary to strengthen the research on the reliability of electronic products.

Source: HC Network Industry Channel

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