Device makers have a choice of whether to specify an additive or subtractive process for fabricating the micron-scale circuits used in their products. Knowing the basic differences between the two is important. Choosing the right one can make a big difference in whether a circuit will perform as required.
In general, an additive thin film process is much more likely to achieve:
- Higher circuit resolution (more conductive traces packed into a smaller area)
- More consistent trace definition
- Thinner and therefore more flexible circuits (where needed)
- More uniform circuit lines (i.e., same width top and bottom)
- More consistent circuit electrical and mechanical performance
- More control across a wider range of circuit resolution, flexibility/rigidness, and trace thickness
Additive Versus Subtractive — the Basics
In order to understand why these advantages exist, it helps to have a basic understanding of how the two technologies work. Both methods are used to create circuit lines on a surface. Like its name implies, a subtractive technology creates circuit lines by subtracting or removing material — in this case, metal from a surface completely sputtered . The metallic material is removed using chemical etching, so what’s left are the circuit lines with spaces between them.
An additive process creates circuit lines by sputtering a positively charged metal to a negatively charged base, then, if needed, plating to build up conductive traces of precise height and width. The base may be either a conductive material or non-conductive polyimide to which a metallic layer was applied. Using photolithography, a photoresist “stencil” is applied on top of this conductive layer. The plated metal is attracted and bonds with the conductive layer where the nonconductive photoresist has left it exposed.
In both types of process, what manufacturers use as their base is the one key in how well the finished circuit will perform. If the base is an “off the shelf” product (e.g., a metal-bonded-to-polyimide stock), these stocks only come in certain thicknesses, which obviously limits circuit size and flexibility. Rather than buy stock, an alternative approach is to spin coat the base layer — which, in Metrigraphics’ case, can be as thin as 4 microns. (When we say we have a “proprietary” additive process, part of what makes it proprietary is our unique ability to sputter a very thin metallic base layer.)
An additive process may also employ laminate stocks, an approach that again limits the ability to control thickness and flexibility precisely. Another drawback of laminates is how they are used to create circuits with multiple layers — with an adhesive layer that bonds the laminate layers together. That adhesive layer is typically 50 microns thick, further limiting how small and flexible the circuit can be, as well as the ability to control size and flexibility in precise increments.
Additive’s Secret Sauce
A large part of what gives additive processing all the advantages listed above is photolithography, which is inherently more precise than chemical etching. Lines will be more precisely defined and will be the same width at the top and at the base, versus what is typically achieved with etching (lines thicker at the base.) Greater resolution and line uniformity means greater component reliability, especially at a much smaller scale.