Flexible circuits can be manufactured using a wide range of material combinations, including different copper types, copper thicknesses, flex cores, coverlays, adhesives, stiffeners, pressure-sensitive adhesives (PSAs), shielding films, and flexible soldermasks. While this material flexibility supports a broad range of design requirements, selecting uncommon materials can significantly increase lead times.
Most flexible circuit manufacturers maintain inventory of commonly used materials that satisfy many customer requirements. Material suppliers similarly stock more of their highest-demand products. When a design specifies materials not in the standard inventory, additional time may be required to procure or manufacture them before production can begin.
In some cases, non-standard material may add weeks to the delivery schedule. If the material is also not stocked by the supplier, lead times can extend even further while the material is manufactured to order.
The following material selections are among the most common sources of delivery delays in flexible circuit designs.
Flex Core Thickness and Material Availability
One of the most frequent causes of lead-time increases is the specification of a thicker-than-normal flex core.
This requirement is often driven by controlled impedance needs. While a thicker core, combined with specific trace widths and spacing, may achieve target impedance values, it can also introduce several drawbacks:
- Longer lead times due to limited material availability
- Reduced flexibility
- Increased cost
Example of flex circuits with a thicker construction.
In some applications, impedance requirements can be achieved using a thinner core combined with adjusted trace geometries and copper thicknesses.
Because flexibility and tight bend radius performance typically depend on thinner materials, the most common flex core thicknesses range from 0.0005” to 0.003”, with 0.001” being the most frequently used. Although some suppliers list thicker core materials, these materials are often available only through special order because of limited industry demand.
When additional thickness is required, laminating multiple thinner layers together may provide an alternative that helps avoid material procurement delays.
Copper Weight and Copper Type Considerations
Copper selection is another important factor in both flexible circuit performance and material availability.
Flex circuits commonly use copper thicknesses ranging from 1/3 OZ to 1 OZ, with 1/2 OZ being the most widely used. PCB designers also choose between two primary copper types:
- Rolled annealed copper, commonly preferred for very tight bend requirements
- Electrodeposited copper, often selected for applications with less demanding bend requirements where cost sensitivity is a greater concern
Higher current-carrying requirements may drive the need for heavier copper weights. However, heavier copper can also affect material availability and lead times.
While 2 OZ copper is relatively common, it is typically paired with a 0.001” flex core to maintain as much flexibility as possible. 3 OZ copper and greater may be available only as a special-ordered material from some suppliers and can introduce substantial delays.
In many situations, manufacturers create these constructions by laminating bare copper to a layer of polyimide. Although this approach can make the required material available, it typically increases both cost and delivery time.
PSA Requirements and Lead-Time Impacts
Many flex circuit designs incorporate a PSA (pressure-sensitive adhesive or double-sided tape).
PSAs may be used to:
- Mount the flex circuit within an assembly
- Attach the flex circuit to a heat sink for thermal dissipation
- Electrically connect the flex circuit to another portion of the assembly
Example of a flex circuit with pressure sensitive adhesive.
One challenge with PSA selection is the large number of available options. Maintaining inventory across a broad PSA portfolio can be expensive, making comprehensive stocking impractical.
PSA availability is often less of an issue than material format. Flexible circuit manufacturers commonly process wider material rolls, and certain manufacturing operations may not be compatible with narrow roll widths.
The most commonly used PSA materials are 3M’s 467 and 9077. The 9077 adhesive is typically used for designs with automated assembly requirements because it is reflow temperature rated.
Stiffener Material Selection
Stiffener requirements can also influence delivery schedules. Commonly stocked stiffener materials include:
- FR4
- Polyimide
- Some stainless steels
- Some aluminums
A wide range of FR4 and polyimide stiffener thicknesses is generally available as standard stock. Stainless steel and aluminum stiffeners, however, are less commonly used and therefore less likely to be stocked in a broad range of options.
Flex circuit with FR4 and polyimide stiffeners.
Specific stainless steel and aluminum alloys frequently require special ordering. In addition, fabrication of metal stiffeners may need to be outsourced, introducing both material procurement and external processing lead times.
For designs requiring metal stiffeners, these additional schedule considerations should be evaluated during the material selection process.
Coverlay and Flexible Soldermask Availability
Coverlay selection can affect both circuit performance and material sourcing.
Like flex cores, coverlays are available in multiple thicknesses, although not all options are commonly stocked. The most widely used coverlay thickness is 0.001”, while 0.0005” is commonly used in applications requiring a very tight bend radius or dynamic flexing.
Example of a flex circuit with coverlay and flexible soldermask.
Some suppliers offer coverlays as thick as 0.003” for specialized applications, but such materials are often not maintained as standard inventory.
When a thicker coverlay is required, laminating multiple coverlay layers together may provide an alternative approach.
Coverlays are available in natural polyimide as well as black and white versions. However, colored coverlays are generally limited to 0.001” thickness.
Some designs use a flexible soldermask instead of a coverlay or in combination with a coverlay. In these cases, color availability can influence lead times. Green flexible soldermask is typically standard stock, and yellow may also be available. Other colors generally have more limited availability because of lower demand.
Selecting Materials with Lead Time in Mind
Flexible circuit designs often require balancing electrical performance, mechanical performance, manufacturability, cost, and schedule requirements. While a broad range of flex materials exists, not all materials are commonly stocked by manufacturers or suppliers.
Thicker flex cores, heavier copper weights, specialized PSA requirements, uncommon stiffener materials, and non-standard coverlay or soldermask options can all extend delivery schedules.
Reviewing material selections early in the design process can help identify opportunities to meet design requirements with more readily available materials. This approach can reduce sourcing delays while maintaining the intended electrical and mechanical performance of the flexible circuit design.
Summary
The wide variety of available materials can negatively impact delivery if unique materials with limited demand in the industry are selected. We encourage our customers to allow Epec to support their design process by reviewing the material requirements to ensure timely deliveries, that design requirements are met, and the design is cost-effective.
Key Takeaways
- Non-standard materials add delays: Unique or rarely used cores, adhesives, or films may not be stocked, adding weeks or even months to lead times.
- Flex core thickness impacts timing and flexibility: Thicker cores often require special orders; using thinner cores with adjusted copper and trace widths can meet impedance needs while avoiding delays.
- Copper weight affects both bend performance and availability: Common thicknesses (⅓ oz to 1 oz) are stocked, but heavier copper like 3 oz often requires custom lamination, extending delivery time.
- PSA and stiffener choices can slow production: While common 3M PSAs and FR4/polyimide stiffeners are standard, exotic PSAs or metal stiffeners like stainless steel typically require special orders and outsourcing.
- Coverlay and soldermask options are limited: Standard 0.001” polyimide coverlay and green soldermask are readily available, but thicker coverlays or non-standard colors usually introduce delays.
