Flexible circuits are normally fabricated in sheet or roll form. One of the last steps in the fabrication process is to excise the part from the substrate. There are a number of ways to define the circuits. As one might expect, the tradeoffs involve cost, accuracy or efficiency. The following are some typical methods used in the flex circuit industry.
Rule die cutting
A rule die consists of steel strips with a sharpened edge embedded in plywood. A steel rule die can also consist of round or square punches. A steel rule die can cut the outline of the circuit out as well as cut internal features such as holes. Steel rule dies are typically referred to as “soft tooling”.
Steel rule dies are relatively inexpensive and can be fabricated in a short period of time. A steel rule is less accurate than a hard tool die and does not last as long. Steel rule dies are good for lower volume flex circuits. A steel rule die will cut polyimide and polyester base substrates cleanly if the die is well maintained and the equipment is set up properly.
There are a multitude of presses that will handle steel rule die cutting, some of them with uniquely different ways of activation. There are hydraulic presses, air presses and mechanical presses. Substrates lay flat on a hardened steel plate. The rule die is pushed against the material with the pin point pressure of the rules ‘bursting” the material. Given enough precision with the press and die, the blade does not have to contact the metal plate that is under the circuit substrate. A successful cut can be made with the die rule only penetrating 90% of the material thickness. When set up with this precision, a rule die can last thousands of hits without degrading. On the other hand, with less precise equipment and set up the blade will push through the material and contact the supporting surface. This type of cutting will work but will dramatically shorten the life of the die.
For less precise equipment, a softer material is often used to provide a cushion between the support surface and the substrate. In this case the rule fully penetrates the circuit substrate and partially penetrates the cushioning material. A cushioning material or wear plate can increase the die life, but the cushioning material needs to be frequently replaced.
Hard tooling is a hardened steel tool that has male/female plates that mesh together to punch or cut the material. The two cutting actions possible with a hard tool are 1. Blanking: the outline of the circuit is cut, but the material is essentially left in place. The top plate creates a shearing action with the spring loaded bottom plate that depresses just enough for the material to be cut. 2. Piecing: Some sort of shape is punched through the die where the top punch fits into the bottom opening of the tool.
A Die that only performs the first action is called a blanking die, and a die that only does the second action is called a piercing die. There are dies that do both actions simultaneously, these are called compound pierce and blank dies. A compound pierce and blank die is more expensive but the process is more efficient and often more accurate.
There are a variety of hard tool die cutting machines that are available on the market, with various degrees of automation. Hard tool cutting is more precise than rule die cutting and the tool lasts significantly longer.
In both rule die cutting and hard tool cutting, alignment of the cutline to image can be achieved with mechanical alignment or optical alignment using vision systems.
A circuit cutline can be created by a computer controlled, highly focused laser beam. The laser beam uses a high level of energy to burn away the polyimide. By using a laser for excising, there are no tools involved so tool cost and lead times can be avoided. However, laser skiving is considerably slower than die cutting and may not be practical for higher volumes. Using a laser for excising can be a more expensive process as the capital costs for equipment are considerable. Vision systems are used to achieve a high level of alignment accuracy and this technology offers the highest degree of precision and ability to create complex shapes of any excise method.
It is possible to combine different processes to achieve desired accuracy and efficiency goals while minimizing tooling costs. For example, it is possible to use a high precision laser beam to cut some of the critical features in the substrate, and use a rule die to cut the rest of the less critical features. This way the laser beam time is minimized to only a small selected area. The viability of combining the different methods to achieve the final cutline would be analyzed by the circuit application engineer.