Electroplating Process for Multilayer Circuit Boards (PCB)

With the rapid development of surface mount technology, the future trend of printed circuit boards will inevitably move towards high-density packaging with fine lines, small holes, and multiple layers. However, the copper plating process for manufacturing such high-level circuit boards will also face some technical bottlenecks, such as how to obtain uniform plating in the center and edges of the panel, how to improve the distribution force of the small hole wall, and how to improve the physical properties of the plating such as ductility and tensile strength. These are all topics worth working on in the future. The purpose of this article is to explain the difficulties of the process and seek ways to deal with them based on basic principles. I hope that my personal opinions can be helpful to circuit board practitioners. In recent years, with the rapid development of the semiconductor and computer industries, the production of printed circuit boards has become increasingly complex. We can use the following empirical formula as a guide to judge the degree of difficulty of circuit boards.

PCB complex program pointer = PCB layer number * number of wires between two solder joints / distance between two solder joints (inch) * wire width (mil) (1)

For example, a 16-layer board with a solder joint spacing of 0.1 inches, a wire width of 5 mils, and three wires between two solder joints has a complexity index of 96. Since the 1980s, the popularity of surface mount technology has driven the circuit board industry to move towards high-level multi-layer boards, which has led to a rapid increase in complexity indicators, from about 20 for traditional circuit boards to the current 100 or higher. In the process of such updates and product evolution, of course, some technical bottlenecks are inevitable. Taking the copper plating process as an example, the author attempts to explore its basic principles and seek corresponding strategies from three aspects: macro, micro and microstructure.

Macroscopic aspect
Refers to the board surface of the circuit board. Usually, the size of a large board is about 24"*18". It is not easy to make the thickness of the coating uniform in the center and the edge. According to Faraday's law of electrolysis, the thickness of the coating is proportional to the external current. Assuming that the density of the coating is a certain value, the distribution of the coating thickness is the distribution of the cathode current. There are many factors that affect the current distribution, including the resistance in the solution, the polarization of the electrode, the geometric shape of the plated part, the distance between the cathode and the cathode, the size of the external current, the mass transfer rate, etc. We will discuss the influence in the following sections. When the current is on the electrode, The distribution of current without polarization or other interference factors is called primary current distribution. It is completely determined by the geometric shape of the plating tank. When a certain voltage is applied to the two electrodes, each point in the plating bath also has a certain voltage, which is between the voltages of the two electrodes. Because the metal electrode is highly conductive, we can assume that the voltage at each point on the electrode surface is equal. Similarly, some imaginary planes with equal potential can be found in the plating bath. Generally speaking, the equipotential plane is very similar to the electrode shape near the electrode, but its shape changes as the distance from the electrode gradually increases. Figure 1 illustrates the distribution of the equipotential plane. The current density is larger where the equipotential plane is denser, and vice versa. According to the electric field theory, the equipotential plane and the plane of its corresponding stress are perpendicular to each other, and the electrode itself belongs to the equipotential plane, so the current flowing into or out of a certain point on the electrode must be perpendicular to the plane where the point is located. Figure 2 illustrates the relationship between the equipotential plane and the distribution of the current flow. If the equipotential plane is replaced by a certain conductor or the plane of the stress corresponding to the equipotential plane is replaced by an insulator, it will not affect On the contrary, if the equipotential surface is cut by any substitute, the entire electric field will be disturbed to an equal extent, and the current distribution will also change. For example, if A and BB are used as electrodes and A and C are used as electrodes, the same current distribution will be obtained. The main reason is that the BB plane coincides with the equipotential surface, so it will not affect the electric field. If A and C in Figure 1 are slightly moved to deviate from the center position, the distribution of equipotential lines will be very different from the original one, because the change of electrode position affects the electric field and causes the current distribution to change.

a. Methods for changing the primary current distribution
From the above basic electric field theory, we know that the primary current distribution is completely determined by the geometric shape of the plating tank. That is, the distance, size, and shape of the anode and cathode will affect the current distribution. For the circuit board surface, the equipotential surface distribution is denser at the edge, so the plating layer is thicker and the central part is thinner. To improve this phenomenon, the design concept must be emphasized. For example, increasing the distance between the anode and cathode, increasing the area of ​​the anode, using insulating shielding to change the equipotential plane, using auxiliary anodes to improve the current distribution in low current areas, and using auxiliary cathodes to disperse the current in high current areas are all feasible methods.

b. Secondary current distribution
The primary current distribution is changed due to the polarization of the electrode. At this time, the current obtained is called the secondary current distribution. Here, the concept of polarization must be briefly explained. In simple terms, polarization is caused by the electrochemical reaction near the electrode, which increases the resistance in the solution. If the desired reaction is to occur smoothly, the applied voltage must be increased. In this way, additional heat and power consumption will be generated. Due to polarization, the electrode voltage will be different from the average potential. The difference between the two is called overvoltage. Near the cathode, the ions are consumed too quickly to be replenished due to participating in the electrode reaction. The overvoltage caused at this time is called concentration overvoltage. If the ions are to pass through a certain energy barrier and reach the electrode to participate in the reaction, the overvoltage required is called activation overvoltage. The total overvoltage is the sum of the concentration overvoltage and the activation overvoltage, which is an indicator used to measure the degree of electrode polarization. Since the current size is inversely proportional to the distance between the cathode and the anode, under the action of electrochemical technology, it is equivalent to increasing the distance between the cathode and the anode. This distance is also called characteristic length. Because of this effect, the secondary current will more or less reduce the phenomenon of primary current unevenness.

c. Polarization parameter
From the above basic electric field theory, we know that the distribution force of current is actually affected by the following two factors: the resistance in the solution and the resistance generated by polarization. ALKIRE once defined the polarization parameter P as follows.

P=R/Rp =ａcFLj/RgTK (2)

Where ac is the transfer coefficient, F is the Faraday constant, L is the distance between the cathode and anode, j is the average current density, Rg is the gas constant, T is the temperature, and K is the conductivity of the solution. If P<<1, it means that the polarization effect far exceeds the electric field effect, and the current tends to be distributed in the secondary current and will be very uniform. If P>>1, the current tends to be distributed in the primary current, which depends entirely on the geometry of the plating tank. They also used copper sulfate plating bath to conduct copper plating experiments on multilayer boards. The basic data of each parameter are ac=0.5, Ma-sec/g-ep, L=30.5cm, j=26.9Ma/cm2, K=0.55 (ohm cm)-1,
RgT/F=25.6Mv/(23℃) Result P=29.13>>1 means the current tends to be distributed in a primary current. Whether it is uniform or not is completely determined by the design of the plating tank. The conductivity of the solution and the polarization reaction have little effect. In addition, the brightener or additive has little effect on the macroscopic current distribution of the board surface. If you want to get a uniform current distribution, you can use a shield or auxiliary cathode.

Micro-etching
This is for the plated through holes (PTH) of circuit boards. In the past five years, the widespread use of surface mount components has made circuit boards tend to be thinner, smaller, and more multi-layered. As a result, drilling, desmearing, and copper plating have all faced unprecedented challenges. For example, if a 0.3-inch thick multi-layer board is drilled with a 15-mil through hole, the aspect ratio is as high as 20:1. Such a small hole is similar to a capillary tube with a considerable degree of surface tension. According to theoretical calculations, at least 0.093 psi of external pressure is required to allow the liquid to pass smoothly through this thin double-depth hole. The traditional air blowing and stirring method can no longer meet this requirement. Therefore, the plating tank must be specially designed.

a.三次电流分布
于通孔及其附近而言,影响电流分布的因素甚多包括镀槽几何形状、镀浴导电性、质量传迅速率、铜离子之浓度等.电流受这许多因素错综复杂的影响其分布称为三次电流分布,在此最值得一提的是小孔内质量传迅的问题.晚们知道,在纵横比甚高的小孔,内溶液穿过不易,再加上离子褵的速率远比离子消耗来得慢,所以在靠近孔壁及远离孔壁之区域间形成了扩散层.此扩散层将影响电镀的速率,如果希望增加电镀速率则必须提高外加电流,但电流增高将使镀层品质逐渐恶化.当电流上升至某一程度时,镀层呈粗糙、松散而无法接受,此时之电流称为极限电流密度,以Jlim=nFDCb/∮ (3)

Where n is the number of electrons, F is the Faraday constant, and Cb is the thickness of the diffusion layer. Generally speaking, if the applied current density is kept within 25% of the limiting current density, a good quality coating can be obtained. If the limiting current density can be increased, the electroplating rate will also increase. From formula (3), it can be seen that the methods of increasing the limiting current density include increasing the copper ion concentration, increasing the diffusion constant, and reducing the thickness of the diffusion layer. Increasing the temperature also has the effect of increasing the diffusion constant, and the use of pulsed electroplating technology has a considerable effect on reducing the thickness of the diffusion layer. Basically, pulsed electroplating is an electroplating technology that uses current or voltage of different waveforms to attach metal to the substrate. The waveforms used can be roughly divided into three categories: square wave, sine wave and triangle wave. In addition, different shapes of waveforms can be produced from the three basic waveforms according to the special needs of various materials. If we use direct current as the applied current.

b.特殊搅拌方式之使用
若要提高小孔内之电镀速率,必须使产生电极反应的金属离子迅速得到补充,通常有两种方式,一是藉助扩散作用,一是藉助对流,前者是由不孔内之铜离子往孔壁浓度低的地方运动,后者则是由镀液的快速流动使孔外的新鲜镀液流入孔内而褵消耗的铜离子,当小孔内毛细现象十分显著时,扩散层也具有一定的厚度使得扩散作有的进行受阻碍,如果孔内外对流良好,不但可降低扩散层之厚度亦可提高电镀的速率,亦即可使用高电流之快速电镀方式,至于采用何种搅拌方式以增强对流有以下两种可行的方法

The impact spray method uses a pump to spray the plating solution directly vertically into the through hole through a high-pressure nozzle. Its advantage is to increase the mass transfer rate in the hole, but the arrangement, aperture, and spray direction of the nozzle must be specially designed, which increases the cost of equipment manufacturing and management.

The principle of the one-way pressure difference method is to divide the plating tank into two areas with a circuit board and align them tightly. Then use a pump to create a pressure difference between the two areas, so that the plating liquid will have no choice but to pass through the small hole. The advantage of this method is that it eliminates the trouble of high-speed nozzle design, but the disadvantage is that it cannot achieve the goal of mass production.

c. Principles of plating tank design
Plating of small holes involves too many factors, making plating tank design quite difficult. However, Kessler and Alkire proposed some basic rules as the basis for design, which are worth our reference. They first defined two basic parameters N and E, N represents the average current parameter, and E represents the current distribution force parameter. In physical terms,

N = resistance in solution / resistance caused by mass transfer (4)

E = resistance in solution / resistance generated by polarization (5) If the N value is large, it means that the current tends to the primary current distribution, which is relatively uneven. If the N value is small, it indicates that it tends to the mass transfer limit, that is, the quality of the coating deteriorates. When E<<1, the influence of polarization is greater than the effect of resistance in the solution, making the current tend to the secondary current distribution, which will be very uniform. If E>>1, the current distribution uniformity deteriorates. If N and E are considered at the same time, the following two criteria can be summarized: E<1 will simultaneously achieve uniform current distribution on the panel and in the hole; N>=64E can achieve a balance between the uniformity of current distribution in the hole and the quality of the coating.

Microstructure
High-level multi-layer printed circuit boards are often used for military purposes, and their reliability must be particularly particular. They need to pass US military standards such as tinning or temperature cycling tests. Therefore, the physical properties of the coating, such as ductility and tensile strength, often determine the success or failure of the test. Gloss and additives play a very important role in improving the physical properties of the coating. Here we briefly explain their basic reaction mechanism.

a. Basic reaction mechanism of electroplating
The plating of metal ions on the cathode substrate is usually carried out in two steps. The first step is that the metal ions in the solution move to the cathode through the electric double layer, which is the charge transfer reaction. The second step is that the ions that reach the electrode combine with each other or with the original crystal grains. This step is called crystallization. Since metal ions are often bound by several water molecules in the solution to form dissociated ions, the water molecules must be slowly removed to allow the metal ions and electrons to combine. Because the lattice planes on the surface of the substrate have different shapes, the curved shapes include plane, curved, edge gaps and holes, so the charge transfer reaction of dissociated ions to water first goes from plane to step and then curved, which saves energy compared to directly placing them in the hole-shaped position. As for the formation of crystals.

b. The role of additives
In order to reduce the surface roughness or increase the ductility of the coating during electroplating, additives or glossing agents are often added. It has been proposed that the convex parts of the reaction mechanism with a flat surface can absorb more glossing agents, thus increasing the resistance. Since the current always flows toward the part with lower resistance, the current flows smoothly to the concave part of the coating to reduce the roughness. Because most of the glossing agents are organic, Bockris also proposed a theory to explain its adsorption on the electrode surface. When the electrode itself carries too much negative or positive charge, the electric dipole of the water molecule will show a falling or rising shape, that is, the adsorption force between the electrode and the electrode is increased, so that the adsorption force of the organic molecules is reduced. On the contrary, if the electrode itself does not carry a charge, the upward and downward trends of the water molecules offset each other, the adsorption force is reduced, and thus the adsorption force of the organic molecules is enhanced. Based on this basic theory, the place of additives can be studied for coatings of different properties.

c.脉波电镀对微结构的影响
脉波电镀最显著的功能是其以物性的方式来改变镀层的微结构,根据许多数据显示,在脉波电流下得到的镀层延展性、附着力均比传统直流电的方式来得大,且粗糙度降低,于于经由何种机构产生此种效应呢?至今仍不十分明了.

结论
本文就多层印刷电路板的镀铜制程以巨观、微观及微结构三种观点来分析其基本理论,所得结论归纳如下:

a. As for the PCB panel, its current distribution is mainly determined by the geometric shape of the plating tank, such as the distance, arrangement, size, etc. between the cathode and anode. The effect of brightener or additives on current distribution is very small. If you want to change the phenomenon of uneven current distribution, you can use auxiliary devices such as shielding or auxiliary cathode.

b. As for the through-hole of the circuit board, the current distribution and the properties of the coating are mainly affected by complex factors such as the resistance of the solution, electrode polarization and mass transfer. If you want to obtain a coating with good quality and uniform distribution, you must emphasize the concept of design, such as applying a special stirring method and using pulse plating technology.

c. Additives or glossing agents can change the physical properties of the coating, such as ductility and tensile strength, but excessive use may also cause organic pollution to the quality of the coating, and also increase the inconvenience of management. Therefore, the method of using pulse electroplating to improve the physical properties of the coating is worth studying. my country's circuit board industry is currently booming, with output ranking third in the world, but there are still many bottlenecks to be broken through in the production of high-level circuit boards. The future trend of copper plating process is to use specially designed plating tanks and strive for improvement in chemical matching, in order to improve the technical level and create a bright future.