Primary Cost Drivers for Flex Circuit Designs
Someone once told me that the potential applications for flexible circuits are really only limited by our imaginations. After pondering that a bit, I had to agree. In fact, one of the things I like best about what I do is that moment during a discussion when I can see the lightbulb go off in a designer’s head. Something in our discussion, or a sample that we were looking at, triggered an idea. Flexible circuits continue to be a growing part of the printed circuit board industry. While most people are comfortable with the cost drivers of rigid PCB designs, many are not as comfortable with flex. Although the three primary cost drivers are the same – panel utilization, materials and technology – there are subtleties of each to be mindful of with flexible circuit design. Elizabeth Foradori and I sat down to discuss these cost drivers and trade-offs. A link to that discussion is included at the end of this column.
Panel Utilization: Typically, panel utilization, or the “number up”, is the biggest cost driver for flexible circuit designs. Fabricators charge for material by the panel, so the piece part price is the panel price divided by the number of parts on the panel. As with rigid PCB designs, it is critical to understand the panel sizes that the fabricator is working with. Panel sizes are most often 12” x 18” or 18” x 24”. Fabricators commonly use the outside one-inch border of the manufacturing panel for coupons and tooling holes. Effectively, when designing, optimizing the useable space of 16” x 22” and 10” x 16” either with individual pieces or arrays, will result in the lowest cost option.
There are a few unique things about flexible circuit panelization. Flex circuits are often unusually shaped, not the standard square or rectangular shape typically seen in rigid PCBs. Standard panelization programs do not necessarily take this into account. In the example shown here, the flexible circuit is “L” shaped. Standard panelization would put six pieces per panel. But, by reverse nesting the parts, this can be increased to eight. Another thing to keep in mind is that flexible circuits are intended to be folded, bent, or flexed in use. This design could be straightened for fabrication, allowing even more efficient panel utilization with 10 pieces per panel. The “L” shape could be created once the circuit is complete. The lesson here is to not rely on the standard panelization programs, but to analyze each design with material utilization in mind.
Materials: There are many different material options for flexible circuits and the number of options is even greater when looking at rigid-flex designs. For the purposes of this discussion, the focus will be on the commonly used copper/polyimide combinations. In general, there are three types of materials: copper with acrylic adhesive and polyimide; copper with flame retardant adhesive and polyimide; and adhesiveless copper with polyimide. These materials are available in many different options ranging from ¼-ounce copper to 2-oz. copper, and 0.5-mil polyimide to 6-mil polyimide.
Assuming there is no electrical or performance reason driving material selection, choosing the materials most commonly used and stocked at the fabricator will prevent adding unnecessary cost to the design. In terms of construction, the copper-acrylic adhesive – polyimide material is most common with lower layer count designs. The flame retardant adhesive option is sometimes lower cost, but outside of a UL requirement, it is not as popular and not as commonly stocked. Adhesiveless material is more expensive, but when working with higher layer count designs and rigid-flex, this would be the material of choice based on the lower CTE value of the material.
In terms of copper and polyimide thickness, 1-oz. copper with 1- or 2-mil polyimide is most common, followed by ½-oz. copper. Material price increases quickly when going below 1-mil polyimide or increasing to 3-mil and 5-mil polyimide. This pricing also increases substantially when you move to ¼-oz. or 2-oz. copper.
Another material choice to make is using polyimide coverlay or flexible liquid photoimageable coverlay. The flexible LPI is going to be less expensive and requires less processing by the fabricator, but there are trade-offs to bear in mind when looking at dynamic flex applications and reliability. The polyimide coverlay is considered the most reliable in high flex applications.
Flexible circuit stiffeners are another material to consider. Typically, stiffeners are either FR-4 or polyimide. Flexible circuits are often rigidized with a piece of FR-4 material to help support component weight, while polyimide stiffeners may be added to increase thickness in specific areas, create a bend area, or provide a barrier in a high wear area. Both types of stiffeners can be bonded with either a pressure-sensitive adhesive (PSA) or a thermal-set adhesive. The cost driver behind each adhesive option is different, dependent on the stiffener material. If the application environment allows it, pressure-sensitive adhesive will be less expensive than thermal-set adhesive for FR-4 stiffeners. This is driven by the need for the fabricator to put the panels in an additional press cycle to cure the thermal-set adhesive. Conversely, the polyimide stiffeners are commonly placed and bonded while the circuit is still in panel form and during the same press cycle that cures the polyimide coverlay. Using PSA for the polyimide stiffeners will increase cost, due to the added labor needed to hand place these after processing through the press.
Technology: Moving on to cost drivers based on technology, line width and space and hole size are the common cost drivers in standard designs. Of course, with any type of PCB manufacturing, the bigger the better in terms of ease of manufacturability. Reaching out to several flex circuit fabricators, the most common threshold that moves from a standard process to a more advanced process is 0.004” line/space and 0.010” hole size. Anything below these will increase costs.
Multiple surface finishes and selective plating requirements also drive costs. This should be avoided if at all possible. Running the flex panels through two surface finishes is an obvious cost adder, but the cost increase is compounded by the taping and de-taping process required and the subsequent yield loss associated with that process.
Button plating is another cost adder to consider. This process creates the plated-through-hole connection without adding extra copper to the rest of the circuit. While this does increase cost, certain applications require a level of flexibility that cannot be achieved with the addition of electrodeposited copper during the manufacturing process.
Recap: To summarize, the biggest cost driver in flexible circuit design is material utilization. Take time to investigate how the flex will fit on the production panel to ensure the best use of that space. Consider material selection and if at all possible, select materials that are commonly stocked; these are also typically the lower cost materials. Before adding additional layers, use smaller line width and spacing. Stiffeners, button plating, and controlled impedance would all be considered medium cost factors, while layer count, dual surface finish requirements, and line width and space below .004” would be considered higher cost adders.
Involving the fabricator early in the design process can help avoid unnecessarily adding costs to your design. They see hundreds of flex designs each year – tap into that pool of knowledge!
Recording Link: https://youtu.be/M-t9xkbTBM0
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