At All Flex Solutions, we’re often asked about the cost of rigid flex circuits, namely the reasons behind their higher price point compared to standard rigid boards and flexible circuits with stiffeners. For context, a typical rigid flex PWB will cost about seven times that of the same design in a hard board, and two to three times an equivalent flexible circuit with stiffeners. So why is rigid flex so expensive, and what are the cost drivers relative to other board options? In this blog, we’ll take a look at the rationale behind rigid flex PCB pricing, and give you a quick method of estimating your rigid flex costs.
Flex vs. Rigid PCB Costs
Compared to standard rigid boards or flex boards with stiffeners, rigid flex PWBs’ exponentially higher cost can largely be attributed to the cost of the raw materials used to build them.
Rigid Flex Prepreg Costs
To that end, the number one cost driver is the no-flow prepreg. Rigid flex manufacturers have to use no-flow prepreg, or occasionally low-flow prepreg, to ensure the resin does not flow out onto the flexible areas of the board. No-flow prepregs are designed to offer just enough flow to fill the circuits in the hardboard areas, but not enough to overflow out onto the flexible areas of the boards.
No-flow and low-flow prepreg can range in costs from $ 1.50-3.00/sq. ft. Conventional rigid board high-flow prepregs run around 25¢/sq. Ft. This means that, at its highest price point, the prepregs required to build a rigid flex PWB can be up to 12 times more expensive than the standard high-flow prepreg used in rigid boards. These striking raw material costs are among the central causes of the major price differential between the finished boards.
It is wise for you — and us, as your board manufacturer — to use two (or more) plies of prepreg between layers within the construction. The no-flow resin resists flowing, as its name suggests, so we use two or more plies of prepreg between each layer to assure adequate encapsulation of your circuits. Prepreg, like all manufactured items, has some variability across the sheet, which includes resin content. If there is an area that is low (or comparatively lower) in resin content, it may not fill adequately, causing resin starvation and ultimately shorts in your circuitry. There are occasions in which we do build boards with a single sheet of prepreg (with half ounce copper or less,) but there is always a risk of inadequate flow that will allow air entrapment and, ultimately, shorted circuits.
A simple, straightforward eight-layer rigid flex will be manufactured with 10 sheets of no-flow prepreg. Eight sheets are used to bond the package, and two are used to mate up to the coverlayer to create a cut back coverlayer design for high reliability packaging. If no-flow prepreg costs $2.00/sq. ft. and each sheet is three square feet, this 10-sheet build would require a prepreg investment of $60. The equivalent materials in standard prepreg would cost $7.50 total. Clearly, the costs can add up fast, even for basic, low-layer-count designs. In complex and high-layer-count designs, the cost impact is even more profound.
Other Rigid Flex Material Costs
Of course, the cost of prepreg is not the only driver at play. Other factors, like availability of certain components and styles, also contribute to the price difference.
For example, no-flow prepreg is only available in 1080 and 106 glass cloth styles, which typically press out at .0025″ and .002″ respectively. If your design is primarily half- and/or one-ounce copper weights, we will usually use 1080 glass styles, which are lower cost. If your design has two- or three-ounce copper weights, we will use the 106 glass styles for their slightly more favorable flow characteristics — but they cost 50% more than 1080 styles Multiple plies of 106 to fill the thicker copper weights can rapidly drive up the panel price.
Traditional PCB manufacturers use 1080 and 106 as well, but they also have other low cost prepreg glass fabrics such as 2113, 2116, 7628, etc. that are cheaper and thicker than 1080 and 106 fabrics.
A smaller cost driver in rigid flex constructions is the flexible copper clad laminate. We almost exclusively use adhesiveless flex materials, which are recommended by the IPC (IPC 2223 5.2.2.2) for high-reliability rigid flex constructions. Adhesiveless flex materials run $6.00-10.00/sq. Ft. for popular thicknesses and can run up to $ 30 to $ 60 per sq. ft. for thicker constructions. Equivalent hardboard laminates are closer to $2.00/sq. ft., so there is a 3-5x cost increase for flexible adhesiveless laminates.
However, if your design has controlled impedance requirements (most of our multilayer work now is controlled impedance,) we usually use thicker flex dielectrics to meet the most popular values. Those thicker flex materials have an exponential price curve, doubling with each additional mil. of thickness.
In rigid boards, a .014″ core laminate is essentially equivalent in cost to a .003″ laminate. This is not so in flexible laminates, where thickness of the base dielectric determines price. Controlled impedance rigid flex PWB’s usually incorporate the thicker dielectrics at an elevated price.
Rigid flex manufacturers come out of the gate using relatively expensive materials, then face the additional challenge of getting materials with widely varying X and Y CTE values to line up within the design. A .002″ Kapton flex dielectric moves dramatically differently — and less consistently — than a .004″ glass reinforced core material. Adding coverlayer, multiple lamination cycles to build the internal flex layers, makes material movement even harder to predict accurately, especially in first-run designs.
These various discrepancies in cost on the material level, which are often driven by design and performance requirements, are the primary reasons behind the significantly higher cost of rigid flex PWBs compared to other board types. However, our experience in the industry has shown that the benefits of opting for a rigid flex construction typically justify the cost for most of our rigid flex clients.
Estimating Your Rigid Flex Costs
For your convenience, we thought we’d provide an easy way to estimate the cost of your rigid flex board in low-level production quantities. In general, we like to expect costs of $0.35-0.45/sq. in./layer. For example, if you have an eight-layer board that is 4”x6”, your price estimate breakdown would look like: 8 layers x 4” x 6” x $0.40 = $76.80/board.
This calculation is a useful one to help you quickly estimate what your part might cost as a rigid flex. For a more accurate and specific quote, contact All Flex Solutions to tell us more about your needs and requirements. We’ll provide a quote and guide you toward the flex circuit solution that is right for you and your project.
Contact All Flex Solutions
Interested in whether a rigid flex PCB is the right solution for your unique application? Contact All Flex Solutions to connect with a knowledgeable member of our team, or to request a quote for your project.