What Is Involved in Electroplating Metal?

Copper plating is one of the most widely used pre-plating coatings in the electroplating industry. It includes tin solders, lead-tin alloys, and zinc die-castings before being plated with nickel, gold, and silver. Copper is used to improve the adhesion of the coating.

Electroplated copper

Background to the latest challenges of electroplated copper

The BGA ball foot's soldering copper pad has a Micro Viain Pad, which not only saves board space, but also replaces the old Dog Boning interlayer interconnect with a long indirect interconnection (Interconnection). ), And become a blind hole interconnection that is straight up and down; it can reduce the wire length and hole length to suppress parasitics in high frequency, and avoid the inner copper Gnd / Vcc large copper surface. The puncture to the through hole prevents the return path from being damaged, and the overall benefit of the high-frequency signal integrity (Signal Integrity) will be better.

However, this method will inevitably attract improper inflow of solder paste in the micro-blind holes in the pads when printing solder paste and subsequent reflow ball feet downstream. This has a negative effect; one can lead to insufficient solder joint strength due to the loss of tin, and the other can cause the vaporization of solder paste flux in the blind hole to blow out nasty voids. Both make the reliability of solder joints worrying.

In addition, in order to pursue the quality of high-frequency transmission, the design of the "1 + 4 + 1" layer-adding method before 2001 has gradually changed to the current "2 + 2 + 2" layer-adding appearance. The latest rules of this "incremental style" make the traditional double-layer (core) of the inner layer only play the reference role of the copper surface of Vcc / Gnd and others; almost all the signal lines for transmitting data (Signal Line) It has been fully deployed in subsequent layers without glass fiber (lower DK, better signal quality). In this way, some second-order blind holes in the outer layer that must be grounded or pressed according to the power supply will even be located on the top or bottom ring of the buried via. These difficult process fields have appeared in many BGA ball pads. Unfortunately, in the case of such a second-order blind hole, the hollowness of the copper plating will inevitably be worse than that of the first-order one in the case of the depression and the hole diameter becoming larger, making the already difficult small solder joint problem more serious and miserable. Therefore, customers of mobile phone boards have repeatedly requested that electroplated copper can fill the blind holes as much as possible to maintain the overall function without falling.

So far, the skill of active copper plating can only say how much to fill. The micro-blind hole has a small diameter of less than 3mi and is mostly used for packaging carrier boards. The problem of filling is not serious. Some types of commercial The copper plating process is also quite satisfactory. However, the second-order blind hole in the BGA ball foot pad of the second-type mobile phone board not only has a caliber of between 6-8mil, but also has a funnel-shaped depth close to 3mil. With the latest appearance of the ultra-difficult distance (0.5mm or 20mil-Pitch), the space of the pad is squeezed and squeezed, so that the diameter of the pad is tightly reduced to only about 12-14mil. Only 3mil is left. With such a limited and dangerous terrain soldering soldering, is it so unpleasant that the eight-band pinches a lot of cold sweat? That is why hole-filling copper plating has almost become an imperative process.

Filling holes for pre- plated copper solder

Older readers may remember that in the era of Pentium (586) seven years ago, its CPU was packaged in a "tape-and-tape (TAB) package. This large chip package is completed with multi-pin components, and the board surface must be downstream. The I / O of the QFP four-sided overhang welding has a total of 320 pins, and the dense parallel ratio of 80 parallel extended legs on one side, which forces the rectangular solder pads to be received side by side, dense and dense. The square pitch forced the pitch of the adjacent foot pads to be less than 10 mils! The pad width was only 5 mils, and the pad spacing (SPacing) was even less than 5 mils. Such a close-spaced multi-pad and narrow Who can guarantee that there is no error in the difficult solder paste printing? Even if the solder paste printing is passed, its subsequent placement steps and high temperature reflow are two more difficult procedures. Completely free from short circuits in mass production?

However, there must be a brave man under the reward. At that time, the Japanese merchant "Furukawa Electric" developed a very peculiar Super solder process (for details, see Issue 74 of the Circuit Board Advisory Magazine). The method is to open a simple gap on the steel plate (Stencil) against one side of 80 dense pads, and then fill the above "Super Solder Paste" regardless of copper pads or spacing. Ingeniously, in the subsequent high-temperature tin melting process, the molten tin layer only grew on the narrow copper pads, and there was no tin layer in the pitch, and even the residual tin or tin bead slag was never seen. Therefore, under this secret weapon of accurate soldering, as long as you carefully align all 320 pins of the PI, you can press the hot bar (hot bar) like an iron, then Huatai Electronics of Kaohsiung That is, a large number of such small precision card boards equipped with a CPU have been assembled.

Good times are rare. This kind of difficult TAB is used in the CPU and was eliminated in less than three years. The objective situation forced Intel to abandon the TAB that it has always advocated, and switched to Motorola's BGA for high-priced and difficult CPU packaging. So the SECC card with Pentium II assembled on the ball foot officially came on stage in 1999, which caused the superb solder paste's brilliant performance to immediately fade, and the expensive "Saurian" production line almost became scrap iron. The investment loss caused by technological changes is not only helpless but also unpredictable.

However, no one expected that a few years later, the first or second-order blind holes in the tiny BGA ball pads on the mobile phone board could be welded and filled in advance using the outdated "super solder", which greatly satisfied Yield and reliability of downstream assembly. With the power of this secret weapon, of course, there is no need to worry about copper plating to fill the holes. However, the residual value of this type of flower-receiving wood is just an unexpected opportunity for some specific manufacturers. Copper plating with blind hole filling is still a common and urgent need in the industry.

Basic formula and operation of electroplated copper acid copper

Before the 80s, the copper plating process of copper sulfate (acid copper for short) only appeared as a primer for decorative bright nickel. After 85 years, due to the unsatisfactory high-temperature copper pyrophosphate used in PCBs, low-temperature copper sulfate was gradually adopted, and this kind of unpopular saitama gold was finally given the opportunity to shine. However, its basic formula is also changed to a new type of acid copper (10: 1) with a higher acid copper ratio (10: 1) in order to respond to the distribution of perforations and improve the ductility (elongation).

Since 2001, due to the high current density requirements of horizontal copper plating and the latest challenges of blind hole filling, its acid-to-copper ratio has returned to the previous goal of 1: 1 for decorative copper. The biggest feature of this type of decorative acid copper is that the "microthrowing power" is very good, and it is easy to heal and smooth defects such as surface scratches and dents. Therefore, the filling of small shallow blind holes below 3mil is greatly affected. benefit. And because the aspect ratio of the hole length to the hole diameter is still low, it is not likely to be broken by thermal stress. However, once the diameter of a micro-blind hole reaches more than 6 mils or even a second-order deep blind hole, its filling probability is greatly reduced. This difficulty has not yet been overcome.

The formula of electroplated copper decorative acid copper

The following is the typical composition of high-speed copper plating bath (cathode electrode density CCD average 80-100ASF), where the weight ratio of acid to copper is 1: 1:

If this formula adopts the conventional current density (20-40ASF), the acid copper ratio should be more than 6: 1. If you want to change to low-speed copper plating (5-15ASF), the acid copper ratio can be increased to 10-15 / l. Therefore, the basic formula has a wide range of variation, which depends entirely on the operating conditions. As for the most influential organic auxiliaries, the properties of their commercial medicaments are different from each other, and they must be operated in the field to find the best situation. Generally, the thickness of this decorative copper is very thin (less than 0.5mil). The main purpose is to reduce the scratches and dents and pave the bottom of the village, so that the subsequent decorative nickel and thin chromium layers have the opportunity to exert better gloss. The tensile strength and elongation are usually not very particular about decorative applications.

Formula of hanging copper plating on electroplated copper circuit board

In order to enable the copper thickness of the hole wall to meet the requirements of the specification (average lmil), and to withstand the test of thermal stress (previously, blistering tin at 288 ° C for 10 seconds and continuously holed, currently due to the addition of the package carrier, Strictly five-time non-breakable hole in tin bleaching) The acid copper used in PCB has generally been changed to the following formula with acid-copper ratio of 10/1.

This type of typical tank fluid has been used in the industry for a long time, and when the aspect ratio of the through hole is increased, the acid-copper ratio must also be increased to ensure the uniform and uniform copper thickness.

Electroplated copper blowing and filtering

The operation of acid copper must be blown. Its function is to assist the stirring of the bath to achieve a uniform concentration, reduce the occurrence of cuprous ions (Cu +), play the role of additives, and help drive away the hydrogen on the plate or orifice. Bad gathering, etc.

The air should be blown in a clean and dry manner, and the volume of the air can be set according to the liquid level of the bath (ft3 / min; CMF). Generally, the air blowing of the circuit board should not be too strong. Its air flow rate is about 1.5-2.5CFM / ft2, and the board type with high aspect ratio (above 5/1) through holes should also reduce the air blowing rate to 1.0-1.5CFM / ft2. Too strong eddy current (Turbulence) will cause Fish Eye or Dish Down on the Annular Ring at both ends of the deep hole.

The blow pipe is best placed directly at the bottom of the tank directly below the cathode plate surface, and must not be placed under the anode to avoid rough coating. It can be raised about 2-3 inches high. The blowpipes are in two rows on the left and right sides facing down, and a staggered blow port is opened every other inch. The angle between the two rows of holes is about 35-400. Such stirring will reduce the accumulation of dirt at the bottom of the tank. As for the cathode rod reciprocating mechanical stirring, the direction of the plate surface 450 is suitable. Some operators even use vertical bouncing vibration to drive away hydrogen.

The ripple of the rectifier should be controlled below 5% (note that this data should be measured in the case of dynamic continuous power supply in actual mass production, rather than simple measurement without static no load), and continuous overage is also a must to operate the equipment. The porosity of the filter core is about 3-5um. The normal turn over volume (Turn over) should be about 2-3 times per hour. It should be noted that the spit spout of the tank must not be mixed with fine air bubbles to reduce the ball pit or bullet pit on the surface to be plated.

Copper plated copper circuit board

In order to automate the compliance with deep-hole copper thickness and the possibility of mass production of large-area thin plates;

Some PCB manufacturers have changed from traditional DC rack plating to self-propelled horizontal copper plating; the power supply method also uses the original DC direct current power supply or RP reverse pulse (Reverse Pulse) changing current. And because the distance between the cathode and anode has been greatly shortened (close to 5mm), the drastic drop in the resistance of this bath, its available current density has also increased significantly to more than 80ASF, making the copper plating production rate doubled. The common recipe is as follows:

This kind of high-speed horizontal copper plating, which emerged in 1997, still uses soluble steel ball anodes in the early stage. However, in order to supplement the rapid consumption of high-speed copper plating, the press machine needs to be stopped on average every three days in order to increase its upper and lower titanium baskets. Copper ball. This painful experience of mass production in the early days forced almost all the later horizontal copper-plated wires to be converted into titanium mesh-type "insoluble" anodes.

In the latter, due to its anode reaction, there is no reaction process of "dissolving copper". All the effects of the current on the bath are almost used for the electrolysis of H2O, so that too much oxygen is accumulated near the anode, causing the additive to be attacked and cracked. Times before. This not only causes a lot of waste, but also the physical properties of the copper plating layer are inferior to the traditional slow hanging plating. The negative factors such as the expensive horizontal equipment (especially the RP reverse pulse rectifier) and the inadequate life of the insoluble titanium anode (which will be described later), and the difficult maintenance of the unit, have gradually made the surge of horizontal self-propelled copper plating large. Not as good as before. The vertical self-propelled copper plating, which is currently emerging, has restored the titanium baskets and steel balls hanging on both sides. What is the effect of this semi-retro flat deep bath and self-propelled hanging plating? It still takes long-term mass production field experience to reach the final judgment and affirmation.

Functions of various basic components of electroplated copper

Copper sulfate --- A chemical purity grade (CP Grade) or higher must be used. Blue fine-grained crystals containing five crystal waters (CuSo4 · 5H2O) are mixed with pure water to obtain a bivalent blue copper ion (or Copper swimmer) is the raw material that directly supplies the coating. When the copper ion concentration is high, a larger current density can be used and the plating speed can be increased considerably. Sulfuric acid provides the conductivity of the bath and prevents the loss of copper salt crystals caused by high concentrations of copper ions under the same ion effect. Generally speaking, when the "acid copper ratio" is low, the microthrowing power of the plating solution will be good, and the scratches and depressions on the plating surface will be missing. It has the special effect of giving priority to rapid filling. Is the best performance in the current metal plating process.

When the acid-copper ratio is increased to 10/1 or more, it will help the hole copper thickness of the PCB. In particular, deep holes with high aspect ratios (above 4/1) have almost become an indispensable means. In contrast, this method is not as effective in filling the surface defects as the former. Chloride --- Common acid copper plating and acid nickel plating must be added with chloride ions. The original purpose is to help the anode maintain its soluble activity in the increase of current density. In other words, when the anode reaction is overexcited, and too much oxygen occurs or the oxidation state is too strong, chloride ions can have strong negative electricity and reducing properties at this time, helping the anode dissolve to reduce its adverse effects.

Many recent studies of copper plating on PCBs have found that chloride ions can also assist organic additives (especially carriers) to perform their various functions. In addition, the chloride ion concentration has a significant influence on the ductility and tensile strength of the copper plating layer.

Management of copper plating bath

The main components in the bath can be chemically analyzed 2-3 times a week, and necessary supplementary operations are taken to maintain the proper control scope of Cu ++, SO4-, and CL-. As for the analysis of organic additives, the Hull Cell test plate, which has always been guided by experience, has been used as a tool for management and tracing. This unscientific approach was gradually replaced by the CVS method (Cyclic Voltametric Stripping) in the 1980s. In the active CVS automatic analysis instrument, not only the hardware is very precise, but also it is more scientific with the results of simulations of many experimental items. These pre-set accurate software programs for the three organic additives Ions can be accurately analyzed and recorded, so that the management of acid copper plating will be on track.

There are many basic theories of electrochemistry, but when it is applied to the actual copper plating site, it seems that it is not enough to make it feel powerless. Some common electrochemical books mostly involve only theories and explanations in the laboratory, and rarely explain the phenomena that occur in actual electroplating. The following is the author's little experience based on years of reading and practice, only bold interpretation of some electroplating behavior, and I hope that I can correct it.

(Reversible Reaction) Electroplated copper reversible reaction

Assume that a copper rod is placed in an acidic blue copper sulfate solution. Theoretically, the solid and liquid phases are not absolutely stationary. On its micro interface, two reactions of copper dissolution and copper ion deposition will occur simultaneously. The dissolution of metallic copper (such as Cu Cu +++ Ze-) is called electrochemically free or dissociated, and is a generalized oxidation that loses electrons. The plating of copper ions (such as Cu +++ 2e Cu0) is a deposition reaction that accepts electrons and is a reduction. Because there are incoming, outgoing, and coming and going, it is scientifically called a Reversible Reaction, but the net current to the outside during this time remains at zero. From a kinetic point of view, the Gibbs free energy (G1 and upper G2) required for the reversible reaction of this electrode can be seen from the diagram below.

(Electrode Potential) Electroplated copper electrode potential (Electrode Potential)

Because of the electrode reaction of a single electrode, it is impossible to find out how much the potential is inverted. Therefore, it is necessary to find a common reference standard and use it as a basis for comparison with each other to compare the electrodes of various metal electrodes in a certain solution. Potential. In the field of electrochemistry, "Standard Hydrogen Electrode" (SHE) is used as a reference. That is, the reaction between H2 and hydrogen ions emitted by the platinum electrode (Pt) in the figure below is zero potential when the adult is: but the prerequisite is that the reaction condition must be set to a hydrogen pressure of one atmosphere (1 atm) The state of hydrogen ion activity (Activity) is 1, and the reaction temperature is 25 ° C, so the SHE obtained in the acidic solution is as follows:

H2-> 2H + ten 2e- EO = 0.000V

If the SHE is 0 as the reference potential, then the salt bridge and the potentiometer are connected to obtain the standard electrode potential of the copper ion against the copper rod in the acid solution (Standard ElectrodePotential) can be measured as:

Cu ++ 10 2e--> Cu E0 = 0.337V

From the concept of electroplating, copper ions receive electrons and deposit into copper metal, which is a reduction reaction that occurs on the cathode. However, if the direction of the "electron flow" is opposite to the direction of the direct current in the general circuit (the scientific community mistakenly defined the current), when the current flows from the positive electrode to the negative electrode, However, the object to be plated becomes the positive electrode, and the copper anode becomes such a logic "negative electrode". Since the wrong habit can not be changed for a long time, readers should only use the concept of cathode and anode when studying or discussing electroplating. Do not quote the positive and negative thinking in general circuits, so as not to cause each other to talk with each other. baffling!

Electroplated copper electric sequence table

Therefore, various metals are measured in the acidic solution or the alkaline solution against the reference electrode of hydrogen, and the "standard electrode potentials" of various metals are obtained, and a table is created in numerical order. This table is called It is the Galvanic Series or "The Electromotive Force series". Each element in this table is marked with a negative sign according to the potential ranking lower than the hydrogen potential (referring to the upper position of the list). The more negative the negative value, the higher the activity, and the easier it is to lose electrons and oxidize in natural environment; The more positive reaction to the right is said to occur. Those who are higher than the hydrogen potential (lower in the list) are marked with a positive sign. The larger the positive value, the lower the activity; or the better the stability or corrosion resistance, the easier it is of course in nature. Oxidation. In other words: a negative value can occur naturally, while a positive value requires external assistance to occur.

If the electrode potentials of the above-mentioned various metals are compared with each other, the essence of close contact with the Giovanni effect between metals can be seen. Although the above-mentioned electromotive sequences are all "reversible reactions" areas that are not interfered by external forces, the probability of their left-right reaction to proceed (that is, between reversible) in a dilute acid solution is not completely the same. :

Zn ----> Zn ++ -0.762V ......

(Indicates the electrode potential that can naturally occur "reactive")

Cu <---- Cu ++ + 0.34V ......

(Indicates the electrode potential that can naturally occur "reversible")

Therefore, if the zinc metal is placed in a copper salt solution, that is, the reverse reaction of the formula (2) and the positive reaction of the line (1), the net potential is [-0.34V ten (-0.762)] or 1.102V, The -type reaction power will be great:

Cu +++ Zn ---> CuO + Zn ++ -1.102V ......

On the other hand, if copper metal is placed in a zinc salt solution (40%), its net potential should be: [(+ 0.34V + (+ 0.762)] or ten 1.102V, so the following formula shows the probability of success to the right It will be extremely small, and only when the voltage exceeds + 1.02V is applied, will there be a chance to react:

Cu + Zn ++ ---> Cu ++ + Zn + 1.102V ......

Electroplated copper irreversible reaction

If two copper rods are placed in dilute sulfuric acid (40%), it is assumed that the external power supply of 2V DC is turned on to force the cathode and anode to form a cathode. At this time, the anode will undergo an oxidation reaction of copper dissolution. Of course, the reduction reaction of sinking copper will also occur at the same time, that is:

However, once the external power is cut off, the "irreversible reaction" between the two steel rods will immediately stop, and the reversible reactions of the respective inherent electrode potentials will be restored. It is known that such a deliberately added external voltage of 2V. It is used to force the separation into cathode and anode. This kind of "differentiation" can be referred to as "polarization" for short. And this kind of irreversible reaction potential minus the potential of the reversible reaction, the resulting potential difference or voltage difference is the "overpotential" (overpotential) or "overvoltage" (overvoltage) that exceeds the inherent electrode potential, which is deliberately different Polarization potential or voltage.

Electroplating copper practice plating and cognitive polarization

In fact, many organic additives have been added to the practical copper plating bath, so that around the simple copper ions (Cu ++), many temporarily coordinated organic substances will be automatically adsorbed, so the positively charged large copper swims away from the cathode. The external voltage required for the reaction on the polar surface will inevitably be higher than that of simple ions. Then its overvoltage or polarization situation will increase a little more. The concept of "polarization" of general electroplating industry is mostly focused on the potential that the original formula exceeds in the reaction, or the increased polarization, after adding organic additives. There are usually two cases of additives:

Increasing the polarization (Polarized or over-voltage) of the reaction process, the phenomenon of stepping on the car will slow down the plating rate.

Depolarized, which has the effect of increasing the throttle, speeds up the plating process.

Limited Current Density

In the field electroplating operation, the current will increase as the voltage is increased. From the perspective of practical current density, it can be divided into three stages (refer to the figure below):

During the start-up phase, the current increases very slowly, which is not conducive to mass production.

1. The current will increase rapidly only when it reaches a certain voltage stage. The area of steep curve in this section is the operating range of general electroplating mass production.

2. After the curve reaches the plateau, even if the voltage is gradually increased, the increase in current is extremely insignificant. At this point, the current density limit (1lim) of normal plating has been reached.

At this time, if the voltage is continuously increased and the current is forced to exceed its limit, the plating crystals will become coarse or even become nodules or powders, and a large amount of hydrogen will be generated. The inferior plating layer formed at this stage is of course unacceptable, but the copper knobs on the ridges of the copper foil wool surface are intentionally made beyond the limit to enhance the unexpected use of grip.

The following are the formulas and descriptions of the cathode current to be plated at their extreme current (Ilim) and current density (Jlim). The latter is particularly common in various articles about electroplating.

The limit current intensity of the plated part is (the unit is Ampere A):

Ilim =

The limiting electrical density of the plated part is (the unit is ASF; A / fi2 or ASD; A * / dm2)

J lim =

Electroplated coatings exceeding the limit current will cause rough crystals due to the deposition and accumulation too fast, forming poor-quality coatings with nodular or powdery appearance, often showing a grayish or dark appearance, so it is called Burning. The copper knobs on the rough surface of the ED copper skin are produced for intentionally exceeding the limit current.

The purpose of all kinds of mixing (blowing, continuous cycle, cathode swing, etc.) is to thin the cathode film (make smaller) to reduce the concentration polarization and increase its available current limit. In addition, the increase of the concentration of the main bath solution (Cb) and the diffusion coefficient (D) also contributes to the improvement of the limiting current, thereby increasing the reaction range of the plating.

Electroplated copper cathode film and electric double layer

The abnormal liquid film with decreasing solution concentration on the cathode surface is called the cathode film or diffusion layer. It refers to the thinning of the original copper concentration from Cu ++ by 1% (99%) to 0% of the cathode. In terms of liquid layer. The thickness of the cathode film originally composed of water molecules and copper ions is not uniform. After adding a carrier, the film will soon become a liquid film with increased thickness and better uniformity, making the thickness of the copper plating layer gradually uniform. . This agent can be analyzed by CVS or HPLC.

Electric double layer (Double Layer) refers to the closest to the cathode surface in the plating bath, because the charged body in the bath is induced by the strong negative charge on the cathode surface, a positively charged micro-ionic layer appears, which The thin interlayer formed on the surface of the negative electrode is called "electric double layer". The thickness of this layer is about 10A. It is the last checkpoint where metal cations deposit metal atoms on the cathode. At this time, the charged metal ion group will lose all kinds of "ligands" (Ligand, such as water molecules and CN- and NH3, or organic matter, etc.) in the swim, and then absorb the polar electrons to become atoms and then press After the energy atoms are arranged, the required metal coating can be obtained.