Electroplating was developed as a combination of Direct Current (DC) and a chemical bath. It was understood that this simple waveform and bath composition had considerable limitations. Numerous innovations followed to optimize the plating process for the desired deposit characteristics. There were advances in cell geometries, anode materials, temperature controls, monitoring, instrumentation and numerous others.

A key advancement was the use and optimization of chemical additives for the DC electroplating bath. We recognize the need to change our bath (electrochemical process) based on the required deposit characteristic (i.e. throwing power, flatter deposit, conductivity, etc.). Additives change the process parameters and mediate the desired results.

Performing the electroplating operation as a process (sequence of steps) enables us to properly level complex parts (such as PCBs) and achieve otherwise difficult deposit characteristics. For example, at a specific point during electroplating, we need to utilize an exact quantity of a specific additive. A second example is to stop the plating process, mask the section which was just plated, unmask another section to be plated in the next step of the process, and continue on. The point is that electroplating was developed and successfully utilized by creating processes. A process allows the user to accomplish the task by performing multiple measurable steps each specifically defined to yield a desired result. The sum of the incremental steps is a completed process with a desired result.

The timeless truth I referred to in the subhead is that “electroplating needs to be viewed and executed in terms of processes, regardless if it involves DC or pulse waves. This is especially true as our work pieces are composed of multiple geometries (fine-line traces, vias, PTHs) requiring multiple deposit characteristics.”

Somewhere in our search for continuous improvement, six-sigma quality, and reduced cycle time, we discovered pulse plating and forgot that timeless truth. Thus began our search for the “magic pulse waveform.” You know the one. You set your dials on this waveform and electroplate fine-lines, plated though holes (PTHs) and blind vias with bright deposits and high conductivity using any proprietary chemistry and additives developed for DC. The only reason we cannot find the magic pulse waveform is that those who have it will not share the information with us. We keep looking and waiting for someone to demonstrate it so that we can upgrade our plating operations.

We would not attempt to drill different sized holes with a single drill bit. Nor should we attempt to electrochemically fill different sized holes with a single pulse waveform. This approach is similar to our ancestors frustrated search for the mythical fountain of youth; they returned after several centuries with increased value placed on the old knowledge. Prior to the search for the fountain of youth, that knowledge consisted of a healthy diet, genetics, hygiene and exercise, to name a few.

Today we know to value diet, genetics, hygiene and exercise in the pursuit of a long and prosperous life. The pursuit of such ideal solutions is not itself bad; it motivates us to pursue a noble goal. What we find, however, may not be what we expected. In the pursuit of the fountain of youth, through medicine, technology and other efforts, we have in fact extended our life expectancy considerably, eliminated many deadly plagues, and are now stronger and healthier than at any other time in history. Clearly, we’ve not reached the ultimate goal, nor are our methods that which Juan Ponce de León expected to find in the 15th century, but it all adds up to a longer and more prosperous life.

Similarly with electroplating, as we search for a magic pulse waveform, we advance technology and solutions. This is not as effective as if we had the end goal in mind at the start, but it is progress, and these advances benefit our manufacturing capability and bottom line.
There are many modern examples to indicate that we will not find the magic pulse waveform, but rather, the need to incorporate new pulse technology with the old wisdom of performing the job as a process or sequence of steps. The sum of these tangible steps can yield a more efficient and higher performance process while providing time and cost reductions.

Faraday Technology, during IPC 2000, demonstrated a single waveform was not optimally able to geometrically level both PTHs and blind vias. This is important if we hope to reduce process time and cost by eliminating multiple plating baths.

fast cash

First things First
Quick solutions to problems on an electroplating line can save thousands of dollars. Experience is by far the most valuable tool a troubleshooter can possess. So successful troubleshooting begins when the line is at its best! Build your experience by walking the line when everything is running smoothly. Take note of solution color, smells, gage settings, etc. Intimate knowledge of your plating line and its idiosyncrasies will expedite the solution to future problems.

Now that a problem has developed, you must walk the line looking at temperature gages, current, anode baskets, pumps, etc. You must rule out the cleaning section of the line and the post plate section of the line. These steps may take some time but they must be done. Ninety five percent of plating problems have nothing to do with the plating bath. Occasionally a problem develops, though, which persists despite experience.

Into the Lab
Once the problem has been determined to be a result of the plating solution or the material being coated, the troubleshooting should be done in the laboratory.

Before starting a laboratory investigation, the first thing to do is ship samples of the plating bath and reject work to your supplier. Suppliers often have sophisticated labs with experienced people. Follow up with a phone call to your supplier. Speak directly with a technical service representative and discuss your problem and investigation.

Once in the laboratory, define the condition of the bath with a routine analysis and a routine hull cell. Correct any chemistry problems found by the routine analysis. The routine hull cell should be one you re used to looking at. Suggested conditions for a routine panel are:

1. A two amp, five minute, unagitated panel for acid zinc.
2. A one amp, ten minute, agitated panel for alkaline zinc.
Compare the routine hull cell panel to ones from when the problem was not present. Measure thicknesses across the panel and again compare them to past hull cell panels. If the routine hull cell panel appears normal, chances are your problem lies outside of the plating bath. Re-walk the line and review your observations. Pay particular attention to the electrical portion of the plating bath, as poor electrical connections will make the plating bath appear to be at fault. Investigate the material of the parts exhibiting the problem. Again, material problems will not manifest themselves in the lab. If you are still convinced that the plating bath is the source of the problem, continue with the lab investigation.

The next step is to use a hull cell to generate the problem. Make sure the conditions and time all hull cells were run are clearly marked on the resulting panel. Vary the conditions in the hull cell to give yourself the best opportunity to produce the problem. Some variations, which may prove useful, are:
1. Panels run at low amperage
2. Panels run at high amperage
3. Bent panels to create a shelf area
4. Bent panels to create an extreme low current density area
5. Panels run at a high temperature
6. Panels run for thirty minutes then scribed with an exacto knife (to reproduce blistering Knowing how your bath appears when operating normally will make the interpretation of these hull cells easier. Once the problem has been produced, we can proceed to the next step.

Target Identified
With the ability to produce the problem, one now needs to know how to remove the problem. The problem probably will fall into one of several broad categories:
1. Organic contamination
2. Metallic contamination
3. Poor filtration
4. Imbalance of proprietary chemicals
5. Unknown

With an unlimited supply of solution, take the opportunity to begin multiple treatments. After each of the following treatments, rerun the hull cell test, which produced the problem. First, for organic contamination, treat three hundred milliliters of solution with two grams of activated carbon. Mix the solution continuously for at least thirty minutes, then filter and run the hull cell. Second, for metallic contamination, treat three hundred milliliters of solution with one-half gram of zinc dust. Again, mix the solution continuously for at least thirty minutes, then filter and run the hull cell. Third, filter the solution thoroughly. The solution must be clear after this step. Use a filter aid if necessary. Fourth, if the solution is an acid bath, metallic or organic contamination may be affected by adding one tenth of a gram of potassium permanganate to three hundred milliliters and mix the solution for five minutes. Filter thoroughly and run the hull cell. For an alkaline solution, freeze out carbonates by putting three hundred milliliters in a lab refrigerator. Cool the solution to 30….F for fifteen minutes. Decant the solution, raise the temperature, and run the hull cell test. If one of these treatments affects the problem, you may be well on your way to solving the problem. Give yourself a pat on the back! Not too fast though. You now must translate the lab results to the production line. You must also locate and eliminate the source of the contamination. The quick results in the lab may take a couple of days to accomplish in production. But don t give up. Plug away until the job is finished.

Unknown
If the above treatments did nothing to affect the problem, things just got a lot tougher. Get on the phone to your supplier and ask for their assistance on-sight. Review their analysis of your bath. Is your bath low on carrier, high in brightener, out of balance, etc? Many suppliers have doctor solutions. By comparing notes with your supplier, they will be able to send in a new arsenal of weapons along with technical assistance. Meanwhile, there are still a few hull cells to run:

For an alkaline bath try:
1. Adding 1% sodium
hypochlorite to the hull cell
2. Adding one-half ounce per gallon of EDTA or Rochelle salts
3. Diluting the bath by 25% with virgin solution
For an acid bath try:
1. Heating the bath above the cloud point, then carbon treat
2. Reduce the pH of the solution with 50% hydrochloric acid to kick-out most organics.
Usually a pH of 2.5 is sufficient. Filter the bath, raise the pH, and add carrier.
3. Diluting the bath by 25% with virgin solution
Out of all the tests you have now run, hopefully something you can build on has emerged. If not, you are into the rare problem, which falls into the unknown classification. This type of problem will take time and effort to resolve. Calling in electricians, sending samples to outside laboratories, etc. are examples of the steps that may be necessary to solve the problem. In this case, the economics of dumping the bath and making a new one must also be considered.

Conclusion
Troubleshooting a zinc-electroplating bath will be much easier if one takes the time to observe the line when things are running well. When a problem develops, split the line into a pre-cleaning section, the plating bath, and a post-plate section. Run tests to isolate the problem to one of the three sections. When the plating bath is identified, follow these steps:
1. Send samples and reject parts to your supplier.
2. Use the hull cell tests outlined above to treat the problem. Remember, even if you can treat the problem, you will also have to eliminate the source of the problem.
3. Demand prompt service from your supplier.
4. Label or identify all tests run during the troubleshooting process.
Once the problem is solved, go back and review the troubleshooting process and learn from it. This will build your troubleshooting skills and shorten the duration of future problems

This article describes a number of key factors affecting copper electroplating for microvia filling and the levels of performance that are currently available to meet the needs of this important market.

Bath Chemistry Parameters Affecting Via Fill
The vast majority of via fill electroplating baths are based on electrolytes consisting of copper sulfate and sulfuric acid. Combining low cost and convenient operation, these sulfate based systems are a well established technology, having now been used in the PCB industry for over 50 years and for via fill applications for over 10 years.

A typical acid sulfate system contains copper sulfate (the primary source of cupric ions), sulfuric acid (for solution conductivity) and chloride ion (as a co-suppressor). Of these components, copper sulfate, typically at concentrations above 200 g/L, has the most significant affect on via filling ability.

Acid copper sulfate system operated without additives typically yield deposits of poor physical properties. Organic additives, typically consisting of materials described as brighteners, suppressors and levelers, are therefore used to further refine deposit characteristics.
Carriers are typically large molecular weight polymers that work in conjunction with small amounts of chloride to form a surface film on the plating surface, which retards the plating reaction. This limits the lifetime of individual growing grains, causing the deposit grain size to become smaller than that obtained without carrier. Carriers are present in relatively high concentration (500 to 3,000 g/L) and show relatively low sensitivity to variations in the rate of mass transfer to the surface. However, in the absence of additional additives, deposits from such formulations do not have smooth, bright surfaces.

Brighteners are typically small, molecular weight sulfur-containing compounds that locally increase the plating reaction by displacing adsorbed carrier. The impacts of brightener additions occur preferentially at points of lower field density, typically in surface recesses or at the bottoms of vias or trenches. The function of the brightener is to locally accelerate the rate of the copper plating reaction and further refine the grain size of the deposit.

Levelers, a further class of additives, act as selective suppressors and typically operate at low concentration (< 10 ppm). At these low concentrations, the activity of levelers is much more mass transfer dependent then that of carriers, with the consequence that less isolated locations (such as the panel surface) are more suppressed than more isolated locations, such as the interior surfaces of vias and recesses within via hole walls.

Bottom-Up Fill Mechanism
For blind vias to be filled with a high quality continuous copper deposit, the plating rate within an individual via must vary. The plating rate at the base of the via must be substantially faster than that that of the remaining areas to avoid premature closure of the mouth of the via opening and the consequent formation of voids or seams.

Accelerated bottom-up filling has been attributed to the mode of action of the organic additive system (1). The suppressor or carrier forms a current inhibiting film on the Cu surface. This film forms uniformly at all locations, assisted by the high solution concentration of suppressor. The accelerated bottom-up filling (i.e. “superfilling”) is believed to be driven by brightener concentration enhancement at the base of the feature (via or trench) during the plating process. Progressive reductions in surface area of via bottoms during deposition “squeeze” the brightener into ever decreasing areas. This localized concentration of brightener further accelerates the plating rate relative to the surface. The leveler acts to suppress the plating at the corners of vias, and aid in reducing the formation of a void. To maintain bottom-up filling behavior, brightener concentration must be controlled within specified limits.

Process Parameters Affecting Via Fill
In addition to process chemistry formulation and bath composition, the key process factors affecting via filling are substrate condition, solution flow, current density and the pretreatment process.
Via profile, thickness and uniformity of the initial conductive layer, degree of surface oxidation and type of dielectric material have a significant impact on via filling ability. A ‘V’-shaped via, with uniform sidewalls free of overhang or protruding glass fibers, promotes consistent seed layer formation and enhances subsequent via fill. Accordingly, non-reinforced dielectric materials are generally easier to fill. A thin or discontinuous seed layer will significantly degrade via fill performance.

While lower levels of solution flow will generally improve via filling performance, particularly of large (100 µm or above) vias, this improvement comes at the price of increased risk of improperly filled small (75 µm or less) diameter vias. Improper fill may manifest itself as defects ranging from seams within the plated deposit, to completely voided vias. The consequence of this behavior is that equipment parameters must be optimized to achieve acceptable levels of fill and plating quality for the specific applications being run.
The effects of current density are somewhat less confounded, as lower current density will both enhance via filling performance and also produce product with lower levels of improperly filled vias. However, the impact of current density is strongest at the very early stages of via filling. Once vias have partially filled, higher current densities can be applied without adverse effects.

At the beginning of the line, the end of one coil is welded to the start of the next coil. Then there are two basic methods for continuously galvanizing sheet which differ in the way that the strip is cleaned before galvanizing-chemically or by thermal treatments. Coils of annealed cold reduced sheet may be fed directly to the galvanizing line, or alternatively, coiled sheet is continuously heat treated in the pretreatment line. After leaving the galvanizing bath, in which strip only stays for a few seconds, the surface is wiped to remove excess zinc and may be further treated to after the surface appearance, composition, smoothness or mechanical properties.

The steel sheet electroplating process utilizes the same basic principle as that for conventional decorative finish electroplating. However, the steel sheet process differs in that the electroplated coating is applied by passing the strip at high speeds through a series of plating cells, building the coating thickness by a small amount each time the strip passes through an individual cell. This continuous process for electroplating steel strip requires necessary equipment to transport the strip at high speeds (150 to 200 meters per minute and higher) through a series of individual plating cells, and is not as simple as it sounds.

An Electroplating Cell

The simplest electroplating cell is shown in the sketch where the plating solution bath is zinc sulfate.

The common schema of the electroplating cell.
This simple plating cell illustrates the actions during the plating process. At cathode (steel, for example), zinc ions dissolved in the zinc sulfate solution combine with two electrons and form elemental zinc, which deposits onto the cathode surface. At anode, water is converted to oxygen and hydrogen ions to maintain electrical balance. The oxygen forms a gas and nothing is deposited on the anode surface. The plating solution carries the current between the cathode and anode.

Plating of Steel Sheet in a Continuous Process

There are many types of anode arrangements. Some are horizontal, others are vertical, and one process utilizes a radial cell wherein the strip passes around large diameter rolls inside each plating cell, and the anodes have a radial design to match the diameter of the large rolls submerged into the plating solution. Each type of anode arrangement and design has advantages and disadvantages; thus, it is easy to see why different manufacturers use different methods. Each requires very close control of the anode-to-strip spacing to achieve efficient plating, avoiding arc spots and other defects in the coating.
Modern Continuous Electroplating Line.
Maintenance of the large volume of plating solution that is contained in all the cells is a science unto itself. Whether the plating solution for electrogalvanizing is based on zinc sulfate or zinc chloride chemistry, maintenance of the proper ranges of zinc ion concentration and solution pH are important control features. Besides plating zinc, some manufacturers have the ability to deposit alloy coatings. This requires, at a minimum, at least one more level of control of the plating solution. For example, producing a zinc/nickel alloy coating requires close control of the concentrations of both the dissolved zinc and nickel in the solution. Solution control has to be accomplished on a dynamic basis since these lines operate continuously.

Power Requirements

The electroplating process requires a large amount of electric power to deposit a metallic coating. The total power requirement is a direct function of the coating thickness that is needed to meet the customer’s specification. For example, the power required to deposit a zinc coating mass of 80 g/m2 is approximately twice that required to deposit a coating of 40 g/m2. A typical line that has the capability to process 70 to 120 tons/hour with a coating mass of 50 g/m2 will consume hundreds of thousands of amperes during this one hour of processing time. It is easy to see why power costs are major cost component for a facility that processes large quantities of electroplated sheet product.

Product Types

The most common electroplated coating for steel sheet products is zinc. Electrogalvanized zinc coatings are used by a number of automotive companies for exposed car-body panels, where the typical coating mass ranges from about 50 to 80 g/m2 per side. These coatings are considerably thicker than the electrogalvanized coatings typically used for non-automotive applications, so the lines built to make products for automotive applications usually have a large number of plating cells. Also, each automotive customer has their own specific coated-product specification.

Another attribute associated with the use of electrogalvanized coatings for automotive applications is excellent surface finish that is attainable with the electroplating process. Twenty-five years ago, when automotive companies began using large amounts of galvanized sheet for exposed body panels to improve corrosion protection, one of the few coated sheet products that could meet the demanding surface quality requirements was electrogalvanized. Hot-dip galvanized was, and still is, used for unexposed body parts. As the surface of hot-dip products improves, they continue to replace electrogalvanized sheet for exposed automotive body panels.

Other zinc electroplating lines have been built through the years to make thinner coatings. Typically, the sheet that is made on these lines has a coating mass of less than 25 g/m2. The applications for this product are often indoors; applications where the environment is not very corrosive. Many applications involve painted products. These coating lines often have the ability to apply paint pre-treatment so that the customer can paint directly without additional in-house treating.

A second type of electroplated coated-steel sheet being manufactured today has a coating composed of a zinc/nickel alloy. Typically, the nickel content is 10 to 16 percent with the balance being zinc. The unique feature of this process is that the zinc and nickel ions are co-deposited to make a true alloy coating. It is not composed of alternating layers.

The application for this product has been limited primarily to a few automotive companies. These companies have developed in-house product design and manufacturing processes to take advantage of the unique characteristics of the zinc/nickel coating. For these automotive applications, the metallic coating is often coated with a special corrosion-resistant thin organic coating on top of the zinc/nickel. The zinc/nickel alloy coating is covered by ASTM Specification A 918.

A third type of electroplated coating is zinc/iron alloy coating. The attributes of this specialized coating are somewhat like those of hot-dip galvannealed product. Like zinc/nickel alloy, zinc/iron coating is co-deposited as an alloy coating. Iron is uniformly deposited throughout the coating thickness. Also, like zinc/nickel coating, zinc/iron coating is used predominantly by the automotive industry. The attributes of electroplated zinc/iron is that it is relatively easy to weld and paint if the proper electro-priming equipment is available to the automotive manufacturer. Also, the coating is very hard, making it is less susceptible to scratching during stamping and handling. This is the important feature since the zinc/iron alloy coated-sheet product is being used almost exclusively for exposed car-body panels.

Corrosion Resistance of Electroplated Coatings

Concerning the corrosion behavior of electrogalvanized versus hot-dip galvanized coating, it is important to note that it is essentially equivalent for identical coating masses. A coating mass of 100 g/m2 will provide essentially the same amount of corrosion protection whether it is a hot-dip galvanized or electrogalvanized coating.

The reason that the automotive companies can successfully use a coating mass in the 50 to 80 g/m2 range is because they apply additional treatments on top of the metallic coating, including a zinc phosphate coating, an electro-deposited organic-based coating, a primer, and multiple-layer finishing paint coatings. Clearly, the corrosion resistance needed to protect a car body panel for over 10 years is more than that afforded by the metallic coating alone. Application of the above coatings over the electroplated metallic layer results in a synergistic system, whose corrosion resistance is more than the sum of its individual components.