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PCB board face HASL process characteristics and process description

With the vigorous development of the surface stick pack technology, printed circuit board, is destined to fine lines, pores, the trend of the future of the multilayer high density packaging type. However, such a high-level circuit the copper plating process will also face some technical bottlenecks, such as: how to make the panel of the central and edge of uniform coating, how to improve the distribution of small hole hole wall.

Force, how to improve the physical properties such as ductility, tensile strength of coating etc. Topic worth trying in the future, this theme is that the basic principle to illustrate the process difficulties and seek the way of coping, hope personal humble opinions can help workers who working for circuit boards. In recent years, with the rapid development of semiconductor and computer industries, the production of printed circuit board is also increasingly complex, we use the following empirical formula as the circuit board difficulty of pointer.

PCB complex program pointer = PCB layer * number of wires between the two solder/second spot pitch (inches) * wire width (mil)

for example, a 16 layer board, the solder joint spacing is 0.1 inches, wire width is 5 mils, there are three wires between two solder joints is its complexity index of 96, since the 80 s, popular in surface mount technology drive the sandwich plate of the circuit board industry towards high level, so that the complex index rising fast, from the traditional PCB rise 20 to 100 or higher, in the process of evolution of this update, product, of course not encountered some technical bottlenecks, copper plating process, for example, the author try to macroscopic, microscopic and microstructure three aspects to discuss the basic principle and coping strategies.

1. (A) change A current distribution method

By the basic theory of the electric field, a current distribution completely depend on the geometry of plating tank. That is, Yin and Yang of polar distance, size, shape, will affect the current distribution. For PCB board face, the edge parts of equipotential surface distribution is intensive, so the coating thicker and the central part of the thinner. If you want to improve this phenomenon must be an emphasis on design. The concept of Yin and Yang, such as increasing extremely distance, increasing the area of the anode, using insulation shield to change the equipotential plane, auxiliary anode is used to improve the current distribution of low current area, use the auxiliary cathode to dispersed current of high current region is feasible method.

(A) 2. The secondary current distribution 

Due to electrode polarization and changed a current distribution, at this point, the resulting current is called secondary current distribution, in this, you must first to the idea of polarization slightly. Simply put, because near the electrodes in electrochemical reaction and increase the solution resistance. To make the desired reaction to produce smoothly, must increase the applied voltage. In this way, will generate additional heat and power consumption. Due to the polarization effect, with flat thin electrode voltage potential differences, the difference of the two called overvoltage near the cathode, ion of an electrode reaction, the consumption of too fast too late, at this time cause the overvoltage is called concentration of overvoltage, if you want to make ions pass some energy barriers and electrode in response to the need of the overvoltage is called the activation over-voltage, and concentration is the total overvoltage overvoltage and activate the sum total of overvoltage is used to measure the electrode polarization degree of indicators. Because the current is inversely proportional to the size and Yin and Yang of the interelectrode distance, in under the action of electric operation technique, is equivalent to increase the Yin and Yang are extremely distance. This distance double called characteristic length for this effect, the secondary current more or general can reduce a current uneven phenomenon. 

(A) 3. The polarization parameters

By the basic theory of the electric field to understand the distribution of current force actually dominated by the following two factors, respectively, in the solution of polarization resistance and resistance to generate and ALKIRE once polarization P parameters are described in the following definition. P = acFLj = R/Rp/RgTK (2) the ac is transfer coefficient, F is Faraday constant, L is the distance, Yin and Yang j is the average current density, Rg is the gas constant, T is temperature, K is the electric conductivity of a solution. If P < < 1, on behalf of the polarization effect is far more than the effect of electric field, the current tends to secondary current distribution, will be very even. If P > > 1, the current tends to a current distribution, completely depend on the geometry of plating tank, they are with copper sulfate plating bath as a multilayer copper plating experiments, the parameters of the basic data for ac = 0.5, Ma - SEC/g - ep, L = 30.5 cm, j = 26.9 Ma/cm2, K = 0.55 (om cm) - 1, RgT/F = 25.6 Mv/results 23 ℃) (P = 29.13 > > 1 represents the current tends to a current distribution, the uniform or not completely depends on the plating tank design, and the electric conductivity of solution and the effects of polarization reaction is not big, in addition, the luster agent or additive macroscopic current distribution of the plate were not what effect, if you want to get the uniform current distribution can use the shield or auxiliary cathode.

(B) micro erosion 

Here is for the circuit boards of plated through hole (PTH), over the past five years, a large number of adoption of glue surface mounted components, makes the circuit board to fine lines, holes, and the difficulty of multiple stratification level, thus in the hole, in addition to the glue residue, such as copper plating are facing unprecedented challenges, for example, if a 0.3 inch thick laminated drill hole 15 mils, Zha aspect ratio as high as those days, so the holes is similar to a capillary tube with a fair degree of surface tension, according to the theoretical calculation of at least 0.093 psi plus some pressure can make the liquid smoothly through the thin double deep hole, traditional way of blowing air mixing have been unable to meet this requirement. So the plating tank is bound to make a special design.

(B) 1. The three current distribution 

(B) (3) the use of a special mixing way Holes in to improve the plating rate, it is necessary to make produce electrode reaction of metal ions quickly replenished, there are usually two ways, one is with the help of the diffusion is a by convection, the former is not hole by copper ions into a region of low concentration of hole wall movement, the latter by the rapid flow of plating solution is to make holes outside the fresh solution into hole and copper ions Li expenditure when holes of capillarity is significant, the thickness of the diffusion layer also has a certain diffusion makes for some hampered, if hole inside and outside the convection is good, it not only can reduce the thickness of diffusion layer can also improve the rate of plating, which can use the fast way of electroplating at high current, as to what mixing way to enhance convection has the following two kinds of feasible methods:

(1) shock injection method, is to use pump will play when the high pressure solution vertical spray nozzle directly into the hole, its advantage is that increase the rate of hole quality spread rapidly, but the arrangement of nozzle, pore diameter and injection direction is to be a special design, thus increasing the cost of equipment production and management.

(2) the unidirectional pressure difference method, the principle is based on circuit board the plating tank is divided into two areas, and to be on tight. And then use to make this two pump area to produce a pressure difference, such a solution will have no choice but forced from a small hole through, this method has the advantage of obviate the necessity of high-speed nozzle design but defect is unable to achieve the objectives of the mass production. 

(B) 4. The plating tank design criteria Plating holes due to too many factors involved, the plating tank is very difficult to design, puts forward some basic laws but Kessler and alkire as the basis of design worthy of our reference. They first define two basic parameters N and E, N represents the average current parameters, E represents the current distribution force parameters, in the physical sense, N = in the solution of resistance/quality produced by rapid transfer resistance 

(4) = E in the solution of polarization resistance/produce resistance

(5) if the value of N is large, on behalf of the current tends to a current distribution, less uniform, if the value of N is small, have shown a tendency to spread fast quality limit, i.e. coating quality deterioration, when E < < 1, the role of the polarization effect is greater than the effect of solution resistance makes the current tendency of secondary current distribution, will be very even, if E > > 1 current distribution uniformity, such as N and E consider induces the following two principles. 

(1) E < 1 will make the panel and at the same time inside the hole to get uniform current distribution.

(2) N > = 64 e in the hole current distribution uniformity and coating quality to reach a balance between the two relations.

C) microstructure. A high level of multilayer printed circuit board is often treated as for military purposes, its reliability must also don't pay attention to, need to pass the specification such as tin or temperature cycle test, so the coating properties such as ductility, tensile strength, often decides the result of the test, gloss and additives on the properties of the coating play a very important role, in this will be a quick show the basic reaction mechanism. 

(C) 1. The basic reaction mechanism of plating metal ion plating on the cathode substrate is usually divided into two steps, the first step is in the solution of the metal ion to the cathode movement through the electric double layer when the charge transfer reaction, the second step is to an electrode ion combined with each other or with the original grain knot, this step is called crystallization, because the metal ions in solution by a number of water molecules? Form wrong ion, water molecules to gently remove to make metal ions and electrons, and because of the lattice plane shape on the surface of substrate, several kinds of including planar curve type, bent, edge gaps and holes, so the wrong ionic to water charge transfer reaction and ladder and bent by the plane first, than the direct placement in the position of the hole shape columns can save energy, as for the formation of ice crystals.

(C) (2) the effect of additives In plating for surface roughness to reduce or increase ductility often need to add additives or gloss of coating agent proposed reaction mechanism of smooth convex part can absorb more luster agent, thus increased the resistance, due to the low current always towards resistance parts, so the current will flow smoothly reduce coarse plating hollow part. And because most luster agent in organic matter, Bockris also put forward a theory of adsorbed on the electrode surface to explain the situation. When the electrode itself bring too much negative or positive, water molecules electrical bipolar will present falling or rising form, i.e. increased and the adsorption of the electrode force, so the adsorption of organic molecules is reduced, on the other hand, if the electrode itself without charge, water molecules to rise and falling trend offset each other, reduce adsorption force and thus enhance the adsorption of organic molecules, which basic theory will be according to different properties of the coating to study the additive.

(c) (3) pulse plating on the influence of the microstructure Pulse electroplating the most striking feature is its in a way that is physical property change of coating microstructure, according to many data show that the pulse electric flow coating of ductility, adhesion are larger than the traditional way of direct current (dc) come, and the roughness is reduced, to produce this kind of effect through what organization? Still not very clear.

conclusion

In this paper, the multilayer printed circuit board copper plating process with three views of macroscopic, microscopic and microstructure analysis to the basic theory, the conclusion is summarized as follows: (1) in terms of PCB panel, and its current distribution mainly depends on the geometry of plating bath, such as Yin and Yang pole distance, arrangement, size, etc., agent, or the influence of additives on the current distribution is very small. If you want to change the phenomenon of current distribution can make a auxiliary devices such as the shield or auxiliary cathode.

(2) in terms of PCB hole current distribution and the coating properties, mainly by the solution resistance, electrode polarization and the quality of complex factors, such as pass the newsletter. If you want to get good quality and uniform distribution of coating, certainly will emphasize the idea of design, for example the application of special mixing mode, the pulse electroplating technology, etc.

(3) additives or luster agent can change the coating properties such as ductility, tensile strength, etc., but if use too much may cause the pollution of organic matter on coating quality, moreover also increase management inconvenience. Therefore, to improve the coating property of pulse electroplating method is worth to study. The circuit board industry in China a vigorous, yield high on a third of the world,

But there are still a lot of neck to make a high level of circuit board is yet to be. The future tendency of plating copper is the use of special design of the plating tank and strive to improve on chemical distribution, in order to improve the technical level to a bright future.

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