A particular challenge in medical-device design is creating components so small they can adhere to the continually shrinking requirements for devices used inside the body. Some of these devices, such as those that monitor blood chemicals or correct vision, require electrical components with features that are 3 to 75µm. It is not possible to manufacture such small features using conventional subtractive flex technology. However, an additive method called Extreme Resolution Micro Flex (ERMF) has had a significant impact reducing feature size and overall circuit area. ERMF lets designers make smaller circuits that are more easily implanted and removed as needed.

ERMF circuits can handle trace and space dimensions from 75µm down to 3µm. The minimum feature sizes on traditional flex circuits are rarely less than 75µm. On the other hand, ERMF circuits are rarely larger than 50-mm square but can have minimum trace and space dimensions as small as 3µm.

When considering ERMF circuits, the primary question to ask is, “To what extent does this device or application require extreme resolution?” A few guidelines show what can and can't be done with ERMF circuits.

Maximum and minimum circuit area sizes: When considering the size of micro flex circuits, smaller circuits allow a greater yield at lower cost because more fit on a panel. For example, a 50 × 2 mm flex circuit might be quite reasonable to build but a 50 × 50 mm circuit, although possible, would be considerably more expensive. This seems counter-intuitive. The larger the circuit with extremely small details means a greater potential for one defect which scraps the circuit, hence increasing the cost.

Circuit substrate material: Polyimide is commercially available in sheet and liquid forms and serves as a base for ERMF circuits. Cast polyimide is preferred because it results in a cleaner, more uniform surface on which to form the micro traces. Cast polyimide can be deposited in thinner, more uniform, and more flexible layers than sheet-stock polyamide.

Conductive via holes: Via holes are typically laser drilled but can be photo imaged if 100µm diameter or greater. Lasers can drill via holes down to 25µm diameter.

Space and trace dimensions: Traces as narrow as 3µm have been successfully formed and circuits with 5µm traces are common. Of course, it makes sense to maximize trace and space dimensions whenever possible. For example, although 5µm features may be a necessity in some areas of a circuit, in other areas 25µm or greater is tolerable.

Trace thickness: First of all, trace thickness requirements are driven by the circuit function and are usually specified by the designer. For example, a microwave or low-current signal circuit could function well with thin film (less than 1µm) sputtered traces. On the other hand, thicker traces, to maximize the cross sectional area, would be required for RF induction coil. In this case, the limiting thickness factor is the trace thickness to width ratio.

Conductor materials: For thin film traces (thickness less than 1µm) any material that can be sputtered is possible. For traces 1µm and thicker the options are electroplated pure gold, hard gold, and copper. The copper can be overplated with pure gold or hard gold.