The Anatomy of a Flex Circuit Cutline

In a previous article we discussed the various methods of creating the cutline of a flexible circuit.  With the various tooling options the methods, process steps, tooling and technology are different. These differences affect the actual “physics” of cutting, and create slight variations on the circuit material.

In most cases the material being cut is polyimide laminate consisting of multiple layers of monolithic polyimide film, adhesive, and copper. In cases where edge insulation (distance between traces and circuit outline) is required, no copper remains in the cutline area leaving only plastic film and adhesive on the edge of the circuit.

The following are the common methods of creating a circuit outline i.e. cutline:

  • Male/female hard tool die
  • Steel rule die
  • NC routing
  • Laser ablation
  • Combination methods

Each of the above has different capabilities with respect to tolerances and through-put, and each method has a different level of tooling investment.   The mechanics or physics of the cutting action is different among these methods.  A cut edge examined under high level magnification reveals the physical differences. These differences usually do not affect the form, fit and function of the part although tolerances and specialty performance applications may encourage one method over another.

The following are the differences in the cutting action.

Male/female hard tool:   This cutting action is created by two metal surfaces meshing together to create a shearing action.  These tools are often called “zero clearance” as the punch is constructed with a harder steel and actually slightly “shears” the cavity metal during the initial punching operation. A properly sharpened hard tool will provide an excellent cut surface over hundreds of thousands of die hits. The side view of the cutline would look very straight and uniform. Circuits requiring dynamic flexing over hundreds of thousands of cycles often adopt hard tools for cutline formation. Tolerances of < .002” are possible with these tools.

Rule die:  Rule dies are analogous to a “cookie cutter” with one edge of the steel blade beveled. The cutting action is created by a high level of force concentrated in a very narrow area and actually causing a “bursting” action.  The steel rule does not need to fully penetrate the material to effectively cut it.   Under high magnification the side surface does not look as uniform as a hard tool cut, the side wall is not completely vertical and the exit surface may have a lip. Rule dies are also more likely to create polyimide “slivers” as a consequence of the bursting cutting action. Tolerances with rule dies are generally limited to about .005” or more.

NC routing:  The best way to describe the cutting action of routing is grinding.  Routing bits are rapidly spinning and shredding the material in its path.  An NC routed cutline will look ragged when viewed under magnification.  The degree of “raggedness” depends on the material stack up, drill speed and sharpness, and overall material thickness. Tolerances at .005” are possible with NC routing.

Laser Ablation: The laser focuses a high level of energy on a very small area causing rapid heating which partially burns and vaporizes the material.  Under high magnification the side wall looks smooth as if it was slightly melted. Lasers have been increasingly adopted as they can cut sizes and shapes beyond the capability of the other methods. Lasers too may have limits in some applications as it is quite frequently desirable to simultaneously cut the circuit and an applied pressure sensitive adhesive. While the laser will cut through the PSA, over time adhesive flow will cause re-attachment across the cut region. Lasers have uncanny accuracy with .001” tolerances possible.

In some cases the cutting action of rule dies and hard tool dies can create slight micro tears in the circuit material. These may propagate as the material is flexed and stressed.   The best way to mitigate this possibility is through some design features.  See figure 1:

  1. Replace sharp inside corners with rounded corners
  2. Insert tear stops (a drilled hole)at the end of all slits
cutline images
Figure 1

The top of figure 1 shows a circuit that has a right angle. The junction of the polyimide cutline should be defined by a rounded corner.  If the corner is sharp, then a micro tear could readily propagate when stress is exerted on the angled region.

The bottom of figure 1 shows a slit that allows the fold or bend line to be inside the body of the part.  In this case a hole is used to create a tear stop.  Without the hole the slit could propagate further when the tail is stressed. Either of these ‘defensive” design features significantly reduce the likelihood of tear propagation.

In most cases the differences in the cutting action will have little impact on the performance of a part and physical differences are unlikely to be noticed under low magnification. In some specialty applications, the differences among the various cuts could impact the function of the part.  An experienced application engineer can help determine the best cutting method to avoid any functional or performance problems.