According to PCB Trace Technologies Inc, flexibility is a major critical element in the design of flexible circuits. Flexibility is the primary reason users look for in flexible circuits. This may be an application involving one-time bending, or the application may require the circuit to bend in infinite cycles. To conform to these requirements, the designer must keep the flexible circuit as thin as possible. This involves using the right type of copper, and the right process of copper plating.
By using the most optimum combination of the above processes, the designer can ensure the design will not exceed the physical properties of the copper traces when the circuit flexes in use. This is necessary, as surpassing the properties of copper will cause it to deform, making it brittle, just like any other metal. Bending brittle copper is very likely to develop cracks in the traces. For best flexibility, the recommendation is to use rolled annealed copper. This minimizes the finished copper thickness, and improves flexibility. This type of copper starts as a plated or electro-deposited copper, with a vertically oriented grain structure.
Copper Plating Methods
Printed circuit boards, with two or more layers, whether rigid or flex, require depositing copper with a copper plating process. The plating process deposits copper not only on the traces and pads, but also within the barrel of drilled vias and holes for creating vertical electrical interconnections between the different layers in the PCB. The PCB board manufacturing industry has two methods for copper plating.
Pattern Plating
Pattern plating is the original or traditional plating process that started with rigid printed circuits. The technique involves depositing copper both in the barrels of plated through vias and holes, and on the surfaces of all traces, pads, and lands present on the external layers of the board.
Manufacturers begin the process by applying a photo imaged mask on the copper clad on the external layers of the board. Subsequent drilling creates the holes and vias. The board then undergoes plating that not only covers the entire exposed circuit, but also forms the barrels of the holes and vias. By etching away the unwanted copper, only the required circuitry is left on the board. This is a cost-effective method, meeting the requirements of the majority of rigid PCB manufacturers and to some extent, flexible circuits also.
Pad Plating
The pad plating process is exclusively suitable to flexible circuitry. This is a variation of the pattern plating process, and is a two step process. In the first step, an image mask covers the entire board, except for a small ring around each hole. When plating, this allows only the barrels of the through holes and vias to receive the copper deposit, along with a small ring on the pad around the hole.
The operator then removes this initial mask and replaces it with an image mask defining and protecting the circuit pattern. They etch away the unwanted copper leaving only the required circuitry. This way, only the holes and vias receive the plating, and the copper traces remain at the thickness of the starting base copper. Each hole or via is now surrounded by a raised copper plated ring somewhat thicker than the copper in the barrels. As there is no extra copper layer on the traces, they maintain their original flexibility. Therefore, the pad plating process is the preferred solution for flex circuits.
Advantages of Pad Plating in Flex Circuits
In a flex circuit, copper is the most critical element and the stiffest one. Presence of copper is necessary to produce the electrical interconnections, while at the same time, it must also withstand the requirements of mechanical bending.
By minimizing the thickness of the copper circuitry, designers gain the advantage of higher flexibility, thereby allowing for more reliable and tighter bend radius. This is important, as exceeding the minimum bend radius can result in a failure and cracked circuit in a flex circuit construction.
The pad plating process is advantageous in that it does not add to the thickness of the base copper. This helps to retain the flexibility of the base material to the end of the process.
As an additional advantage, the pad plating process is best suited to the type of copper that flex circuits use. Most designers prefer to use rolled annealed copper in their flex designs. Rolled annealed copper is more flexible and has higher reliability for mechanical bending.
Initially, the copper starts as ED or electro-deposited copper with a vertical grain oriented structure. For flex circuits, manufacturers subject the copper foil to a rolling and annealing process. This transforms the vertical grain structure of the ED copper to horizontal, increases its flexibility, and improves the reliability for tight bend applications.
The pad plating process also improves the controlled impedance designs on flex circuits. During the process of copper plating, the operator applies electrical current, which forms eddy currents in the copper panel. The eddy currents cause a higher deposit of copper around the perimeter of the copper panel rather than at its center. As the settings of the plating process is set to meet the plating thickness at the center of the copper panel, according to IPC standards, the result is a thicker deposit of copper at the panel extremities. This difference in the copper thickness leads to a variation in the impedance values between parts at the center of the panel versus those at its periphery. The central part of the panel also produces more uniform line widths and spacing during the etching process.
Design Requirements for the Pad Process
The size of the via and its PTH pads must follow a specific relation to the hole size. This requirement allows a proper formation of the raised copper-plated ring surrounding each plated hole. Therefore, the Gerber layout must properly define the opening of the photo image mask that forms the top of the pad plating ring surrounding each hole. In relation to the hole size, this pad plating ring must be a minimum oversize of 0.012 inches. This provides the minimum surrounding pad for the imaging mask.
The PCB documentation must include the type of RA copper necessary along with the requirements of pad plating. This must find mention in the drawing notes or in the material stack up. The document must also mention the orientation of the grain direction of the RA copper to be along the length of the flex board with the tightest bend.
Conclusion
While both methods of pattern and pad plating serve to deposit copper in the barrels of plated through holes and vias, designers and manufacturers prefer the pad plating process for use on flex circuits. According to PCB Trace Technologies Inc, the benefits of the pad plating process are mainly in improving the reliability and flexibility of the finished flex circuits when they are bent into required configurations.
