PCB direct plating process introduced
(A) Faraday’s law
The first law – when the plating bath (electrolysis), “attached” on the cathode metal weight (or anode erosion) is proportional to the amount of electricity passed.
The second law – in the different bath plating with the same amount of electricity, the weight of each attached to its chemical yield is proportional to.
The first law of the “power”, that is, the product of the current strength and time, the theoretical unit is Coulomb, real power potential for the ampere, minutes or ampere hours to cupric sulfate in the divalent copper ions, for example, the first A Coulomb battery can be plated with 0.3294 mg of pure copper at 100% cathode efficiency and 1.186 g of pure copper per 1 A hour.
The second law is different for different plating solution, the amount of copper plating refers to the copper sulfate divalent copper ions, if the bath is replaced by copper cyanide solution of copper ions, then the same 1 Since the electricity equivalent of one cup of copper is 63.57, the chemical equivalent of 1,2-copper is 63.57 / 2, the former is the latter Twice as much.
(B) the cathode film
The closer the plating is to the surface of the target to be plated, the lower the concentration of metal ions is, and the thin film of liquid whose thickness is reduced by 1% from the surface to the surface of the substrate is called “cathode film.” Due to the metal ions gradually less and the hydrogen gas resistance increases due to poor conductivity hinders the smooth landing of metals and this film is also due to fluctuations in the shape of the plating body and the original thin film is different from the convex shape of the film at the outer membrane of the high concentrations of ions Easy to add so that the place is preferentially plated, the so-called high current density area, whereas the low valley is naturally not easy to plating now the current intensity of each local area to formula to discuss:
Ilim = nFADCb / ∮
Ilim ─ ─ the size of the local area current
n ─ ─ ─ electronic number
F─── Faraday constant (the amount of electricity required to plate 1 g of metal)
A ─ ─ ─ the size of the area
D ─ ─ ─ metal ion diffusion coefficient
Cb ─ ─ a large number of bath average concentration
∮ ─ ─ cathode film thickness
It can be seen from the above formula to reduce the thickness of the cathode film contribute to the coating evenly, so the plating for various mixing such as blowing, plating flow, the cathode swing and its purpose are to reduce the thickness of the cathode film in the surface near the substrate Department to increase the supply of metal ions.
(C) electrowetting bath
Total electricity Yin = outside the road and contacts + raw liquid + cathode film.
Circuit board in the production of large area to be plated, so the need for DC current is also very high and often thousands of amperes, in order to better the voltage of the coating control more than 5 V. However, press A = V / R formula seems that its total resistance must be extremely small to meet this Ohm’s law, it should be maintained outside the road and contacts and low external contacts, heating the bath to reduce the liquid, stirring the bath to reduce the cathode film. Otherwise If the voltage is too high, it will cause the water to be discharged. As a result, a large amount of pressure will be generated and the quality of the coating will be greatly affected.
(D) When the metal is immersed in its salt solution, the surface of the dissolved gold ion or ion landing to become a metal replacement reversible reaction until a certain potential to reach equilibrium. If under normal temperature and pressure to dilute electrolysis Xi’an liquid white gold cathode surface of the hydrogen light as an arbitrary zero value, the various metals and the “zero-pole” connected to make contrast, you can find a variety of metal hydrogen standard electrode potential to. And then the metal ions Between the oxidation or reduction potential of the NHE compared to arrange the “electrochemical order” or electric order.In terms of the reduction point of view, than the lively metal crown with a negative value to make it arranged in the upper position of hydrogen, such as zinc is -0.762, That zinc is easily oxidized into ions, not easy landing into the metal, in theory, at least plus 0.762V above can be plated.
More noble than Hydrogen crown with positive value, ranked in the lower part of the hydrogen in the next more easy to restore the plated out, which means that the metal can be more stable in the natural conditions, whereas the upper is easy to rust.
(E) hydrogen over voltage
Electroplating hydrogen ions will swim to the cathode and the formation of hydrogen escape, the hydrogen ions in the aqueous solution in the same way and the metal, so hydrogen than the active metal plating, the theory is the first hydrogen out after the turn of the metal landing But actually more metal than hydrogen, which is called “hydrogen over voltage.” Hydrogen, which appears on the surface of the plating and does not immediately expel, will prevent the metal from later Point of landing, resulting in the potholes of the coating so try to improve the hydrogen over the bath voltage and reduce the bath surface tension and stirring to drive away the hydrogen bubbles are the pursuit of electroplating technology.
Metal electrode in its salt aqueous solution can form a reversible balance of the external world in terms of positive and negative polarization, but if the other is applied to separate the positive and negative voltage for electrolysis, the additional voltage is called Overvoltane , overpotential, or polarization, but to overcome various obstacles so that the metal can be successfully landed, must exceed a variety of polarization, such as activation polarization, concentration polarization, polarization resistance, and gas polarization, the total value of the plating The minimum voltage required to make the coating perfect to add a variety of additives often to change the surface of the local resistance phenomenon to make the coating more uniform.
(Seven) quality delivery
The positively charged metal ion clusters continue to swim toward the cathode to compensate for their constant depletion.The movement of such ion clusters proceeds in three ways: migration, convection and diffusion are described later:
(1) Migration – The absolute mobility of Cu is 5.9 * 10 <-4> cm / sec in 1 mole copper sulfate solution at a 1 v / cm potential gradient at 25 [deg.] C. When the cathodes and anodes are operated at 3 V away from 10 cm, Anodic dissolution of copper ions to 93 minutes before walking to the cathode 1cm far, to 15 hours to reach the cathode surface. Therefore, the results of electroplating, the contribution of migration is not significant, only the cathode near the metal ions pushed to land only.
(2) convection ─ ─ bath must be fast flow, the rear of the high concentrations of metal ions can quickly complement the consumption of the cathode film, so the convection is the main force of mass delivery to inflatable, filtration flow, stirring, and Heating and other ways to make rapid exchange of plating is the most important plating project.
(3) Diffusion ─ The cathode film thickness is about 0.2m / m and can be thinned to 0.1m / m at a fast stirring flow rate of 25cm / sec, which greatly accelerates the slow effect of natural diffusion from high concentration to low concentration. However, How the ionic groups discarded all kinds of coordination of other things through the last electric double layer alone or with a small part of the landed, the principle is still unknown.
From the above we can see that the copper plating on the PCB, the most effective way to complete the quality of delivery is the rapid mixing bath, especially for PTH, the fast circulation of the hole in order to effectively establish the hole wall thickness specifications, But not to the phenomenon of dog bone.Because the PCB board area is very large, to the plating solution by mixing through the hole or the board so that the flow through the hole is not easy to do, the more likely approach is to strong Feng Liquid column in the bath to spray holes to go, of course, most of the hit or in the board or interfere with each other and invalid, can be changed to some or withdraw the back of the board more promising.In short, how to make the bath Quick flow through each hole is the key to the growth of the copper wall.
(H) add additives
In addition to the basic method of preparation, whether the plating can be applied in all additives, especially for difficult holes such as small holes deep hole, additives is very important .About 18% Agents, fine-grained agents, wetting agents. The theoretical basis of these auxiliaries is not yet mature, mostly from the results of continuous experiments, it is almost all commercial range, the reference source for a variety of patents, but has been published Almost all of them are outdated and are no longer top-notch products, and most are on the active market.
It appears from the data that the micro-force of copper sulfate plating bath is very good to be able to be kind of small scratches and sag on the surface to be pre-filled, and then coated a full, but PTH hole wall, in order to play The advantages of this micro-distribution, you must make a high concentration of the bath can continue to flow in to reduce the thickness of the cathode film, it is to display their strengths of the first condition.
Second, the hole or deep hole copper discussion
The assembly of circuit boards is becoming more and more compact, with the benefits of reducing the size of the final product and increasing the capacity and speed of information processing, especially since VLSI has been extensively developed and the assembly of the IC on the board has been screwed in from early through-hole, Surface mount SMT of the thin lines and holes on the board is bound to be the problem.And the holes, the impact is the largest impact of the existing copper plating technology, the hole length to width ratio Very high, it is necessary to get 1mil thick hole wall, but also can not occur dog bone phenomenon, the surface and the coating of various physical properties through the existing two kinds of norms, of which the need to be a breakthrough in many difficulties. The industry is now from the basic formula, additives, equipment, and other efforts, there are still few major breakthroughs now difficult to pinhole the following facts to discuss the physical part is much larger than the aperture part, than the kind of powerful water almost Are wasted on the board of the obstacles.One solution is to make the liquid copper concentration increased, or can reduce the number of passes, but this is also an unworkable cul-de-sac, because the 2oz / gal of copper is almost the board Face and hole wall coating even awarded the upper limit, and then raise the dog Become serious, is not the additive can help solve Approach II is to improve the physical properties of electroless copper plating to achieve the standard requirements of the current Hitachi TAFⅡ process, it has been several years of research.
Now copper sulfate bath can do about things:
(1) The choice of high-purity material additives, such as special leveling agent to inhibit the coating at high current increase, so that low current can still have a normal landing, and rigorous analysis, careful to add, careful treatment to maintain the bath The best effect
(2) change the design of plating, increase the distance between yin and yang, reduce the difference between high and low current density.
(3) Reduce the current density to below 15ASF to improve the DC output ripple of the rectifier to less than 2%, and if it does not work, reduce the current density to 5ASF again and replace it with time.
(4) to enhance the frequency of bath into and out of the hole, or known as hole mixing, this point is the most important, but also the most difficult to solve, and strengthen the filter cycle at least 2 times per hour, Sisi ultrasound mixing.
(5) Do not increase the concentration of copper But to increase the concentration of sulfuric acid and copper ratio, at least 10/1 or more.
(6) Add additives should reduce the amount of brightener, increase the amount of carrier, and the use of ampere hour management to add, regular use of CVS analysis of the cleavage of additives.
(7) Trial pulse current test plating to reduce the difference between surface copper and hole copper, and to increase the ductility of the copper layer, and can be added without additives to leveling the coating.
Pulse current, is a very worthwhile way to study, the first results are also worth studying the path, the first results are very exciting, but unfortunately, many additives suppliers in the market are not keen to protect both Interest is not much supportive of the research, as once the current can be supplied from the current coating to be improved, the long-awaited additives may be lacking in human interest, or need to start a second, all non-neglected .In short, PCB holes and deep hole plating Copper is still very much to be a breakthrough place, it is not at your fingertips.
In theory, each time into the hole in the bath of copper in the hole all the time required to stay on the wall when the number of 300. Moreover, in the actual copper plating suffered current density efficiency, the cathode film and so on the trouble, the actual 20% copper may not be plated out, and if it encounters an aspect ratio of .125 / .012 or 10/1, the required theoretical fluid change is as low as 380, with a 20% landing Results, at least have more than 2000 turn Turn
However, the difficulty is still more than that, pore size decreases, the reduction of pore wall area far less than the drastic reduction of flow rate to 8mil hole compared with 25mil hole area reduced by only 33%, but the flow rate dropped sharply 1.25%, but also increased the difficulty of replacing 2000 times.
Even if the above feasible, but also to overcome the cohesion of the bath, and the hole wall resistance, molecular gravitation, and the cathode film barrier, so the strong water flow is absolutely necessary.