Electroplating process of multilayer circuit board
With the rapid development of surface mount technology, the future trend of printed circuit board is bound to go to thin, small holes, multi-layer high-density packaging type.However, the manufacture of such high-level circuit board copper plating process will also face some technology Bottlenecks, such as: how to make the center and edge of the panel to be uniform coating, how to improve the distribution of small hole wall
Strength, how to improve the physical properties of the coating such as ductility, tensile strength and so are worth the effort in the future issue, the main purpose of this paper is to explain the basic principles of the process and seek ways to cope with the difficulties, I hope the individual can see the circuit board In recent years with the rapid development of semiconductor and computer industry, the production of printed circuit boards are increasingly complex, we can use the following empirical formula to judge the degree of difficulty of the circuit board pointer.
Circuit board complex program pointer = number of circuit boards * the number of wire between two solder joints / two solder pitch (inches) * wire width (mil)
(1) For example, a 16-layer board, the solder joint spacing of 0.1 inches, wire width of 5mils, two solder joints between the three conductors of its complexity index of 96, since the 80's surface mounted technology popular PCB industry towards the high level of the multilayer forward, so that the rapid increase in complex targets, from the traditional circuit board 20 or so to the current 100 or higher, in this update, product evolution process, of course not To some of the technical bottlenecks to copper plating process, for example, I try to macroscopic, micro and microstructure of three aspects to explore its basic principles and to seek response strategies.
In this case,
(A) the macro aspects
Refers to the board of the board, the board is usually a large board size of about 24 "* 18", to make the central and edge coating thickness is not easy, according to Faraday's law of electrolysis, the thickness of the coating and the applied current into The thickness distribution of the coating is the distribution of the cathode current, many factors that affect the current distribution, including resistance in the solution, the electrode polarization, the plating of the geometry, the distance between the anode and cathode, plus Current size, mass transfer rate, etc., we will discuss the impact in the following sections, when the current distribution on the electrode does not produce polarization or other interference factors in the case referred to as the primary current distribution depends entirely on the plating tank geometry Shape, when a certain voltage applied to the two electrodes, each point in the plating bath also has a certain voltage exists between the size of the electrode voltage between the two, because the metal electrode conductivity is very strong, we can assume that the electrode surface voltage of each point Are the same, in the bath can also find some of the potential of the hypothetical plane with the potential, in general, to near the electrode position, the equipotential plane and electrode shape is very similar but its shape with the electrode distance The potential distribution of the equipotential plane is more dense in the isopotential distribution, and vice versa.The electric field theory shows that the equipotential plane and its corresponding stress The plane is perpendicular to each other, and the electrode itself belongs to the equipotential plane, so the current flows into or out of a certain point of the electrode and the point where the plane must be perpendicular to each other.Figure 2 shows the equipotential plane and current flow distribution relationship. The potential plane is replaced by an integral conductor or the plane of the stress on the equipotential plane is replaced by an insulator without affecting its electric field.If the equipotential surface is cut by any alternative, the whole electric field will be equalized The current distribution will be changed to an example to A and BB as the electrode and A and C as the electrode will get the same current distribution, mainly because the BB plane coincides with the equipotential surface coincidence , It will not affect the electric field. Assuming that A and C in Fig. 1 are slightly shifted from the center, the distribution of the equipotential lines will be very different from the original, because the change of the electrode position affects the electric field. The current distribution has also changed.
(A) 1. The method of changing the primary current distribution
The basic theory of the electric field is that the primary current distribution depends entirely on the geometry 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, For example, increasing the distance between anode and cathode, increasing the area of ??the anode, the use of insulation of the shield to change the position of the mask, such as to increase the thickness of the anode, Plane, the use of auxiliary anode to improve the current distribution of the low current region, the use of auxiliary cathode to disperse the high current area of ??the current are feasible methods.
(A) 2. Secondary current distribution
Since the electrode is polarized to change the primary current distribution, the resulting current is referred to as the secondary current distribution, where the concept of polarization must first be explained in brief, the polarization is near the electrode The electrochemical reaction increases the resistance of the solution.To make the desired reaction can be successfully generated, it is necessary to increase the applied voltage.This will produce additional heat and power consumption due to polarization, the electrode voltage and the level The difference between the two known as the difference between the two over-voltage near the cathode, the electrode reaction due to participate in the electrode, too late to add too much time to consume, at this time caused by the concentration of over-voltage is called, to make the ion smoothly The total overvoltage is the sum of the concentration overvoltage and the activation overvoltage, and is the index used to measure the degree of polarization of the electrode. Because of the magnitude of the current, and the total overvoltage, The distance between the anode and cathode is inversely proportional to the increase in the distance between the anode and cathode, this distance is called the characteristic length because of this effect, the secondary current will be more or less to reduce the primary current non-uniformity The phenomenon.
(A) 3. Polarization parameters
From the basic theory of the electric field that the current distribution of the actual force by the following two factors, respectively, for the resistance and polarization in the solution of resistance and ALKIRE once the polarization parameter P is defined as follows: P = R / Rp = acFLj / RgTK where ac is the transfer coefficient, F is the Faraday constant, L is the cathode-anode distance, j is the average current density, Rg is the gas constant, T is the temperature, and K is the conductivity in the solution. If P >> 1, the current tends to the primary current distribution, depending entirely on the geometry of the plating bath, if P >> 1, the current tends to the secondary current distribution, even if the polarization is far more than the electric field effect, , They are copper plating bath for multilayer copper plating experiments, the basic parameters of each parameter ac = 0.5, Ma-sec / g-ep, L = 30.5cm, j = 26.9Ma / cm2, K = 0.55 (Cm) -1, RgT / F = 25.6Mv / (23 ℃) Results P = 29.13 >> 1 indicates that the current tends to the primary current distribution, the uniform or not entirely depends on the design of the plating tank, The effect of polarization reaction is not large, in addition, the gloss agent or additives on the plate surface of the giant current distribution of the force are not affected, to get a uniform distribution of current shielding or auxiliary cathode can be used.
This is for the circuit board plated through-hole (PTH), the past five years, a large number of surface mount components used, making the trend of thin circuit boards, small holes, multi-layer of difficult levels, and thus drilling, For example, a 0.3-inch thick multi-layer board if the drilling 15mils of the through-hole, voltage aspect ratio up to 20: 1, so the hole has been similar to a capillary With a considerable degree of surface tension, according to the theoretical calculation of at least 0.093psi plus pressure in order to make the liquid smoothly through the small double-deep hole, the traditional blowing gas mixing method has been unable to meet this requirement. For special design.
In this case,
(B) 1. Three current distribution
The factors influencing the current distribution in the through-hole and its vicinity are many factors including the bath geometry, bath conductivity, mass transfer rate, concentration of copper ions, etc. The current is affected by the complexity of these many factors, Three times the current distribution, the most noteworthy is the quality of the hole within the mass transmission problem.We know that in the aspect ratio is very high holes, the solution is not easy to penetrate, together with the ion rate than ion A diffusion layer is formed between the cell wall and the region remote from the cell wall.The diffusion layer will affect the rate of plating and if the plating rate is desired to increase the applied current, the current quality will deteriorate gradually as the current increases (3) where n is the number of electrons, F is the Faraday constant, and Cb (C) is the maximum current density, If the limiting current density is maintained within 25% of the limit, a good quality coating will be obtained, and the plating rate will be increased if the limiting current density can be increased by the addition of the current density. (3) can be seen to increase the limit current density method, including increasing the concentration of copper ions to improve the diffusion constant, reducing the diffusion layer thickness, etc. Elevated temperature also has the effect of increasing the diffusion constant, while the pulse wave plating technology is used to reduce The thickness of the diffusion layer has a considerable effect. Basically, pulse wave plating is a different waveform by the current or voltage metal attached to the substrate plating technology, the waveform can be divided into three categories are square wave, Sine wave and triangular wave. In addition, the need for a variety of special needs can also be performed by the three basic waveform shapes of different waveforms, if we use DC for external current.
In this case,
(B) 3. The use of special mixing method to improve the plating rate of small holes, must produce the electrode reaction of metal ions quickly replenished, usually in two ways, one with the diffusion, one by means of convection, the former Is by the hole of the copper ions to the hole wall where the concentration of low movement, which is by the rapid flow of the bath so that the fresh hole outside the plating solution into the hole and consumption of copper ions, when the capillary capillary phenomenon The diffusion layer has a certain thickness so that diffusion can be impeded. If the inside and outside of the well conveys well, not only the thickness of the diffusion layer can be lowered but also the plating rate can be increased, that is, a high-current fast plating method , As to what kind of agitation to enhance convection has the following two possible methods:
(1) the impact of injection method is to use the pump to fight the bath when the high pressure nozzle directly to the vertical through-hole, the advantage is to increase the quality of the hole speed transmission rate, but the nozzle arrangement, aperture, jet direction and so on To make a special design, thereby increasing the equipment production and management costs.
(2) one-way pressure difference method, the principle is to the circuit board is divided into two areas of the plating tank, and to be tight. Then use the pump to make a pressure difference between the two regions, so the bath will have no alternative Forced from the hole through the advantages of this method is to avoid the trouble of high-speed nozzle design, but the disadvantage is that can not achieve the goal of mass production.
(B) 4. Guidelines for design of plating baths There are too many factors involved in the design of plating baths, making the design of plating baths difficult, but kessler and alkire proposed some basic rules as a basis for design worthy of our reference, they first define two basic parameters N and E, N represents the average current parameter, E represents the current distribution of the force parameters, in physical sense, N = solution in the resistance / mass transmission speed produced by the resistance
(4) E = resistance produced by resistance / polarization in the solution
(5) If the N value is large, the current tends to a current distribution, more uneven, if the N value is small, there is a trend toward the quality of communication speed limit, that is, coating quality deterioration, when E << 1, The effect of polarization is greater than the effect of resistance in the solution so that the current tends to the secondary current distribution will be very uniform, if the E >> 1 current distribution uniformity worse, such as N and E at the same time summed up the following two criteria:
(1) E <1 will be the same time the panel and the hole to get a uniform current distribution.
(2) N> = 64E in the hole uniformity of current distribution and coating quality to achieve a balance between the two relations.
In this case,
C) high-level multi-layer printed circuit board is often used as a military use, its reliability must also pay attention to, the need to pass the US military standards such as bleaching tin or temperature cycling test, so the coating properties such as ductility , Tensile strength can often determine the success or failure of testing, gloss and additives to improve the physical properties of the coating play a very important role in this special reaction mechanism will be a little description of the basic.
(C) 1. Electroplating of the basic reaction mechanism Metal ion plating on the cathode substrate is usually carried out in two steps, the first step is the solution of metal ions to the cathode movement through the electric double layer that is charge transfer reaction, the first The second step is to reach the electrode of the ions with each other or with the original crystal grain knot, this step is called crystallization, because the metal ions in solution is often a number of water molecules form the wrong ion, water molecules must be slowly removed to make Metal ions and electronic junctions, because the surface of the substrate surface lattice shape of different shapes, including several curved planar, curved, edge voids and holes, so the wrong ion water charge transfer reaction first by the plane While the stairs and curved, than its direct position into the hole in the column can save energy, as the formation of crystals.
In this case,
(C) 2. The role of additives in the electroplating surface roughness to reduce or the strenght of coating ductility often need to add additives or gloss agent has raised the surface of the reaction mechanism of the convex part of the Department can absorb more of the gloss, So the increase in resistance, because the current is always toward the lower part of the resistance, so the smooth flow of the current will flow to the rough part of the coating was reduced, and because most of the organic luster, Bockris also proposed a theory that the adsorption of the electrode surface When the electrode itself with too much negative charge or positive charge, the water molecules will show a dip or rise in the state, that is, increase and the adsorption force between the electrodes, so that the adsorption of organic molecules decreased , The other hand, if the electrode itself is not charged, water molecules rise and fall trend offset each other, reducing the adsorption force thus enhancing the adsorption of organic molecules, which will be the basic theory of coatings for different properties of the place of their additives.
In this case,
(C) 3. pulse wave plating on the microstructure of the most significant feature is its physical properties of the way to change the coating microstructure, according to many data show that the pulse current obtained by the coating ductility, attached Focus on the way than the traditional DC is greater, and the roughness reduced, in which institutions through which such effects? It is still not very clear.
In this case,
In this paper, multi-layer printed circuit board copper plating process to macroscopic, microscopic and microstructure three viewpoints to analyze the basic theory, the conclusions are summarized as follows: (1) on the circuit board panel, the current distribution is mainly determined by the plating The geometry of the groove, such as the distance between the anode and cathode,
Arrangement, size, gloss or additives on the current distribution of the impact is very small.To change the phenomenon of uneven current distribution can have auxiliary devices such as shielding or auxiliary cathode and so on.
(2) on the circuit board through the hole in terms of current distribution and coating properties, mainly by the resistance of the solution, electrode polarization and mass transmission and other complex factors.To get good quality and uniform distribution of the coating, it is necessary to emphasize the design Concept, for example, using a special mixing method, using pulse wave plating technology.
In this case,
(3) additives or gloss agent can change the physical properties of the coating such as ductility, tensile strength, etc., but if the excessive use of organic matter may also cause pollution of the coating quality, in addition to increase the management of inconvenience.Therefore, the pulse wave plating Improve the physical properties of the coating is worth trying to study.China's current circuit board industry is booming, the output ranked third in the world,
However, in the production of high-level circuit boards still have a lot of neck yet to be a breakthrough.Powder process aspects of the future trend is the use of specially designed plating tank and chemical matching to improve, with a view to improving the technical level of a bright future.