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Plated Through-holes in Flexible Circuits

There is probably no more important feature than the plated through-hole (also called via or via hole) with regard to the reliability and integrity of a flexible circuit. The through-hole provides electrical connection between insulated layers and enables electrical functionality on double-sided and multilayer flexible circuits.

There are a number of methods that a flex circuit manufacturer may use when generating a via hole.  For example, one can create through-holes on a panel with no circuit patterns defined, or one can insert the through-holes after the circuit traces have been imaged and etched. There are advantages and disadvantages for both situations, but the basics of creating a through-hole are the same.

The first step in via formation is to create a hole through the laminate. The hole can be created by mechanical drilling, laser drilling or punching. Once the hole has been drilled, an adhesive-based laminate will look similar to the diagram below:

Figure 1: Through-hole, side view.

Figure 1 depicts the case where a laminate consisting of copper foil, adhesive, dielectric, adhesive and copper foil are used. Adhesiveless laminates are also very popular and are produced with a variety of technologies that bond copper to dielectrics sans adhesive.

Electroplating is a process where metal ions are bonded to a metal surface. The process requires a voltage potential between a copper source (anode) and the plating target (copper circuit). Both the anode and copper circuit are immersed in a copper sulphate solution with an applied voltage potential. This promotes ion flow from the anode to the cathode (the circuit). Areas of a circuit panel with a voltage potential will get plated, so any part of a copper circuit that is electrically isolated will not get plated. Since copper layers are separated by dielectric materials, and there is no electrical charge going through the through-hole, electroplating between layers is not possible. In order to allow electroplating, a conductive “bridge” must be coated over the insulating layer.

The two most common methods for creating that conductive bridge are:

• Electroless copper plating
• Shadow plating

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