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.

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With the study of zinc electroplating process by Electrochemical Noise (EN), it was found that the EN generated during the electroplating of large conglomerate zinc deposit has large potential oscillation amplitude and positive potential drift. However, small noise amplitude and little potential drift was seen in the case of compact zinc deposit. Any metal can be plated through zinc electroplating process, but the most common are steel and iron, on which the process offers sacrificial protection.

Steps of Commercial Zinc Electroplating Process
On the commercial scale, zinc electroplating is done by the following steps.
1. Surface of the metal is cleaned in alkaline detergent type solutions, and it is treated with acid, in order to remove any rust or surface scales. Cleanliness is essential for successful zinc electroplating, as the molecular layers of oil or rust can prevent adhesion of the coating.
2. Next, the zinc is deposited on the metal by immersing it in a chemical bath containing dissolved zinc. A DC current is applied, which results in zinc being deposited on the cathode. Alkaline zinc baths are used by the finished products, to produce a more consistent zinc thickness, especially in recesses.
3. Hence an increased protection from corrosion is provided, as the corrosion of the deposited zinc is reduced. The zinc coating can increase the time required for the formation of white rust, by ten times. Finished Products also apply sealers, which are now commonly being specified by the automotive industry, further increasing corrosion protection.

It is very difficult to obtain a uniform thickness of coating, with electroplating technique. The thickness of the coating is very much dependent on the geometry of the object being plated, and it is preferentially on the external corners and protrusions of the metal body, hence not much of it is deposited on internal corners and recesses. Zinc electroplating process is used to make a clean, smooth and corrosion resistant surface. It also makes an excellent undercoat for powder coating or paint and can leave recesses on complex shaped components without sufficient zinc coating, in order to provide corrosion protection.

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.