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.

The process of electroplating (also referred to as electrodeposition) is fairly simple. To start, a negative charge is placed on the object that will be coated. The object is then immersed in a salt solution of the metal that will be used to plate the object. From there, it’s simply a matter of attraction; the metallic ions of the salt are positively charged and are thus attracted to the negatively charged object. Once they connect, the positively charged ions revert back to their metallic form again and you have a newly electroplated object.

Controlling the thickness of the electroplated object is generally achieved by altering the time the object spends in the salt solution. The longer it remains inside the bath, the thicker the electroplated shell becomes. Of course there must also be an adequate amount of metallic ions in the bath to continue coating the object. The shape of the object will also have an effect on the thickness. Sharp corners will be plated thicker than recessed areas. This is due to the electric current in the bath and how it flows more densely around corners.

Before electroplating an object, it must be cleaned thoroughly and all blemishes and scratches should be polished. As mentioned, recessed areas will plate less than sharp corners, so a scratch will become more prominent, rather than being smoothed over by the plated material.

The process of electroplating began at the beginning of the 20th century and continues to evolve today. Many common objects such as tin cans are actually electroplated steel with a protective layer of tin. Medical science has experimented with electroplating to create synthetic joints with electroplated coatings, and new advances in electronics have been made with electroplated materials.

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.

Taking Electroplate Orders
2. Commercial electroplating companies vary in their specialties. Some focus on the automotive industry, some the computer industry, and some are large enough to take on many industries and use a full range of plating technologies. In the process of writing an order, directions or specifications for the process are outlined. The electroplating company and its employees must understand the specs and fulfill the order accurately. Required delivery times, transportation methods and payment dates are agreed upon and signed for. Delayed orders or orders delivered on time or before the agreed date may be further defined by payment penalties or additional incentives.

Managing Electroplating
3. Commercial electroplating companies have the option of working with tool manufacturers and suppliers to enhance their production capabilities. From the management of the waste from electroplate processes—metals must be worked with and disposed of properly by law—to the need for hoists, material handling and purchasing, commercial electroplate businesses have a need for strong management. The scheduling and timely completion of several large orders simultaneously require close attention.

Conductive Electroplating Process
4. Traditional electroplating methods require the substrate (object) to be conductive or able to take a negative electrical charge. Once charged, the object is placed into a bath solution of salt and the chosen metal. The positive ions of the salt solution are attracted to the negatively charged object. When the two connect, the ions return to a metallic state and are now adhered to the substrate (object). Before being electroplated the object must be cleaned of any debris, oils or waxes, or the areas of dirt will not receive the process. Dimples or rough spots must be polished smooth or the plating will take on the irregularities.

Non-conductive Electroplating Process
5. Plastics have presented an interesting aspect to the plating process. Plastics are insulators, not conductors, which means charging them with a negative charge is not possible–at least not in the context of electroplating. This has given rise to all manner of processes designed to adhere metals to various forms of plastic. From using friction to developing special paints, this portion of the commercial electroplating industry is in constant discovery and development.

Application of FPC
FPCs are employed in a wide variety of applications due to the nature of their characteristics. Examples of applications for FPCs include cell-phone liquid crystal display enclosure, hinge parts, keypad, battery enclosure and interface components. FPCs are also used in optical pickup and device interfaces inside hard disk drives, digital still cameras and digital camcorders. Desired performance characteristics are: 1) wiring within small spaces; 2) wiring connection accompanied by mechanical functions within working part/device and motherboard; and 3) high density interconnect resulting from denser and narrower features.

FPCs fall into three broad categories: single-sided flexible printed wiring boards, double-sided flexible printed wiring boards and multilayer flexible printed boards. Single sided and double sided FPCs are widely employed for personal computers, optical pickup (OPU), HDD and cell phones. When calculated based on substrate area, half of these are single sided and the remainder are double sided. Multilayer FPCs are mainly used in cell phones, OPU, portable music players and DSC/DVC. However, multilayer FPCs only represent approximately 3 to 4% of the total FPC production by finished board area base. This is because there are relatively few large volume applications for multilayer FPCs.

FPC Materials
Polyimide is a crucial material which provides key features to FPCs and is used in almost all FPCs. In general, FPC is manufactured with a flexible copper clad laminate (FCCL) or one of many combinations. FCCL may be broadly grouped into the following four types: 1) material made from single polyimide and copper clad sheets connected with epoxy adhesive; 2) material laminated using polyimide adhesive (laminate); 3) material made using polyimide film and a sputtering/plating method; and 4) material made by coating polyimide varnish on copper foil (casting) followed by a curing step.

Today, the dominant films for FPC applications are 12.5 to 25 microns thick, with the industry trend being toward ever thinner materials. Two major types of copper foil are used for FCCL–electrolytic copper foil and rolled copper foil. Electrolytic copper foil is typically 18 or 12 microns in thickness, and rolled copper foil 18 microns thick. Both types of copper foils are moving to thinner dimensions. Generally, rolled copper foils demonstrate flexural properties superior to those of electrolytic copper foil. HDD applications, in particular, require high flexibility and reliability, so rolled copper foils dominate this segment. In recent years, flexural properties of electrolytic copper foil have been much improved and these foils are being increasingly used for optical pickup applications.

FPC Manufacturing Process
As mentioned previously, FPCs fall into three broad categories; single sided, double sided and multilayer. Each type of FPC has a series of required manufacturing process steps, examples of which are provided below. In particular, multilayer FPCs have a wide variety manufacturing processes based on the specifics of desired structures and performance characteristics.