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
Electroless Copper Plating
Electroless copper plating involves immersing the laminate in a series of baths that include a catalyst (usually palladium) followed by an alkaline, chelated solution of copper. Copper is thereby chemically bonded to all surfaces that are immersed. This chemically bonded coating is rather thin, but it allows electrical current to flow across the dielectric, which enables electroplating. Figure 2 depicts the through-hole immediately after electroless copper plating (technically it is not a plating process since electrical current is not used).
Figure 2: Electroless plating on the hole wall.
Figure 2 shows that a very thin layer of copper is deposited over the through-hole. Electroplating is next. Figure 3 depicts the through-hole after electroplating.
Figure 3: Electrolytic plating over electroless copper.
The through-hole now has a solid coating of copper that is both electrically and mechanically robust.
The Shadow® process performs the same basic function as the electroless plating process, which is to create a conductive bridge across the insulating layers so that electroplating can be performed.
The drilled copper laminate is immersed in a solution with conductive carbon particles. The carbon will adhere to the entire surface, creating a very thin, fragile layer. A micro-etch is then performed that removes the carbon from the copper material, so that only the dielectric areas remain coated, as shown in figure 4:
Figure 4: Carbon deposited on dielectric.
The Shadow process ultimately leaves conductive carbon only where it is needed. The carbon-coated through-hole can now be electroplated as shown below:
Figure 5: Electroplated copper over carbon.
It is important to note that both the Shadow and electroless processes are only used to create the conductive bridge. Neither coating is robust enough to withstand environmental stress or conductive enough to carry significant current.
The Shadow Process vs. Electroless Copper
Some flexible circuit manufacturers are primarily using electroless copper and some are using conductive carbon. Each process has its benefits and drawbacks.
Plated through-holes can be tested/inspected in a number of ways.
- Conductivity: There are special electrical measurement devices that will precisely characterize the electrical conductivity of the through-hole. Any anomaly such as a plating void will change the electrical conductivity and be detected by the tester. This is a non-destructive test which is outlined in IPC-TM-650.
- Micro cross-sections: Cross-sections are created by encasing the through-hole in a hard epoxy and creating thin slices to inspect the through-hole sidewalls under high magnification. Micro cross-section requirements are specified in MIL-PRF-31032/1 and IPC-6013.
- Visual Inspection: Through-holes can also be inspected by using special magnification with reflected lighting. This inspection is considered a “quick” method for spotting problems and can supplement more formal inspection methods.
Good process control techniques may require a combination of several different inspection methods. The most critical aspect is controlling the inputs such as chemistries. Some applications, markets and/or customers may have exact specifications on through-hole test methods, minimum thicknesses, and may require cross-section data and/or images to accompany individual manufacturing lots.
Dave Becker is vice president of sales and marketing at All Flex Flexible Circuits LLC.