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.

This program consists of an Operator Interface (OI) for parameter entry and display and a Control Server (CS) to control and monitor the rectifiers. These two applications use a Windows socket connection to interchange data using the Internet protocol. This two-part design allows DynaComm II to be installed on a single computer for a stand alone application or to be installed on separate computers where the operator’s and rectifier’s positions can be separated by feet or miles.

This dual application design also permits any application program that will support the Internet protocol to be connected to the Control Server. The company can create custom additions to the Operator Interface to control and monitor other plating line functions, such as temperature, pH, and so on, whether servers exist or need to be written.

DynaComm II allows multiple monitoring and controlling options. More than one Operator Interface can be connected to a CS for multiple monitoring/control locations for a single line. Or, multiple Control Servers can be connected to a single OI for monitoring of multiple stations from a single supervisory office workstation.

Data logging is an optional feature, and when enabled, this function automatically stores data accumulated during the plating operation. This data can be downloaded to an electronic spreadsheet for further analysis, providing users with a written record to verify rectifier performance for all parts and batch runs against specifications and tolerances required.

To prevent loss or corruption of data, DynaComm II also features UPS power monitoring and control. If AC power is interrupted, it senses the line loss or brownout, closes the open files, closes itself, shuts down Windows and disables the UPS output. The minimum system requirements include a 233 MHz Pentium PC, 32 MB RAM, 4 MB Video RAM, 6 MB free hard drive space, two RS232 serial ports and a 10/100 Base T Ethernet port. Additional hard drive space will be required for plating recipes and data logging storage.

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.