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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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.

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.