Why doesn't every application use rigid-flex technology? Good Question! Rigid-flex technology is a hybrid between rigid boards and flexible circuits, combining the most favorable aspects of both. The flexible portion helps solve space, weight, and packaging issues because it can bend, fold, and flex in either installation (flex to install) or end use (dynamic flex). The rigid portion supports dense component areas, allowing higher layer counts, complex routing, surface mounting on both sides of the board. Using this construction for all applications makes sense to me!
More realistically, cost is almost always a factor when selecting the technology to use for a particular design. Rigid-flex, with all its benefits, might not always be the best total cost solution. In a future blog, we will discuss the importance of comparing total design cost rather than just the cost of a rigid or flexible circuit to a rigid-flex design. For now, let's look at why flex and rigid-flex are more expensive to manufacture than a standard rigid board.
First, the raw materials are simply more expensive than standard FR4 laminates. The PCB market consumes less flexible materials than their rigid counterparts, driving a fairly significant delta in the cost of raw materials.
Secondly, handling flexible materials can be challenging. Designers choose flexible materials because they are thin, lightweight, and can bend and fold, but these things require special attention during fabrication. Picture an 18” x 24” or even a 12” x 18” piece of laminate, only two or three mils thick. It is like trying to hold a piece of paper. Any little flutter can potentially create a ding or crease in the copper, turning it to as the circuitry is created.
Fabricators of flex materials need special handling procedures. For example, you can only pick the material up by opposing corners to keep the laminate flat. Transporting materials through the facility typically requires special trays or racked carts. Because most PCB fabrication wet-process equipment is roller based, flexible materials often require leader boards taped and untaped throughout the process to ensure the panels do not get caught in the rollers.
Comparing the cost and handling requirements of rigid and flexible circuit materials explains why flexible is more expensive. Moving to a more complex rigid construction furthers the gap with the special processing required to laminate and fabricate dissimilar materials. I think it is also important to understand how different rigid flex constructions impact cost.
The simplest and least expensive option typically is rigid flex with rigid outer layers and flex interconnecting layers with all rigid layers having the same thickness. This is the most common rigid flex construction, but both flex and rigid-flex allow a lot of creative designs., as mentioned in previous blogs.. For example, certain design solutions require plated through holes in the flex regions. While certainly possible, it adds additional cost due to additional processing at the fabricator. In simplest terms, the flex layers need “wet-process” processing to create the through holes before they get integrated into the rest of the stack up.
Another creative way to solve packaging issues is to have certain flex layers, or tails, break off into separate sections. For example, flex layers 1 and 2 go in one direction, flex layers 3 and 4 go off in another direction, and flex layers 5 and 6 go in yet another direction. This is an excellent use of rigid-flex technology! However, this construction requires significantly more processing during fabrication than the simple version described earlier. There are several different ways to approach this complex design, and it is highly recommended to work with your fabricator early in the design process to ensure you are not adding unnecessary costs.
To answer the original question, even though rigid-flex technology offers the benefits of both rigid and flexible circuit constructions, you cannot use it for all applications, primarily due to cost. Fabrication of rigid-flex is more complex than the fabrication of either rigid PCBs or flexible circuits. When rigid-flex technology does solve a packaging issue, you must look at the total cost of the design, and not just the cost of the circuit itself. Often, you can save costs as rigid-flex eliminates wires, cables, and another bill of material items. Stay tuned as that is a topic for a future blog!
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