A rundown of electrical and physical considerations.
In my previous columns, I’ve stressed the importance of the partnership between OEMs (or ODMs) and flex manufacturers throughout the product lifecycle, to properly apply flex technology. A good flex design has the right blend of performance, manufacturability, and cost: three factors that ultimately impact the success of a product launch.
This month, I focus on how and when OEMs and flex manufacturers can use material selection to optimize design for performance. Material selection plays a critical part in improving manufacturability and offering performance gains. Material selection begins with a thorough understanding of the application requirements, including the components to be used and the physical constraints for the flex assembly.
Flex design engineers, mechanical engineers and electrical engineers determine the required outline, stackup and layer construction to achieve the necessary connectivity of the flex circuit. To illustrate the different stackup options, Figures 1 to 3 show available choices for a two-layer flex circuit, depending on bending requirements. After engineers determine the stackup and layer construction, the flex manufacturer can start selecting materials to meet the requirements for bending, flexibility, dimensional stability, components, circuit density, impedance characteristics, electrical properties, thermal resistance, tear resistance, low moisture absorption and chemical resistance.
Simulation analysis and empirical knowledge enable correct material selection to achieve optimum mechanical and electrical performance. Simulations also offer fast, thorough insight into the potential conflicts during component assembly. OEM engineering teams should engage with their flex manufacturer/assembler to review simulation data, to make better decisions on the tradeoffs between cost, performance and manufacturability. Different materials available for flex circuits are:
Dielectrics. Dielectrics for flex circuits (compared to glass-reinforced dielectrics for rigid PCBs) are characterized by their thinness, flexibility, mechanical strength, thermal properties, and dielectric properties. The suitable dielectric film depends on operating environment, electrical and mechanical properties, assembly requirements and cost. Polyimide films are typically used for applications requiring high tensile strength and excellent flexibility. Polyester films are used for lower-cost applications. High-performance fluoropolymer films offer superior electrical properties and are considered for low insertion-loss requirements for high-speed applications. The thickness of dielectric materials commonly ranges from 12 to 50µm. Dielectrics can be used as a base laminate and a coverlayer when combined with adhesive.
Conductors. Choosing a conductor material depends largely on the material’s performance during specific applications. In particular, dynamic applications require the right choice of copper type and thickness. Commonly used copper thicknesses are 12µm, 18µm and 35µm. Electrodeposited copper, rolled annealed (RA) copper and high tensile elongation (HTE) copper are commonly used. While the most common solution for dynamic applications is HTE, RA copper is still in wide use. In addition to copper, other conductive materials include copper-nickel alloys and conductive inks.
Solder masks. Liquid photoimageable (LPI) solder masks are solder resist inks applied onto a flex circuit, then cured with a thermal or ultraviolet process. The result is a permanent, durable insulation. The preferred insulation material is coverlayer. In case of small component areas, solder mask may be required for insulation.
EMI shielding. EMI shielding protects against electromagnetic inference. Copper layers provide the best shielding effect, while flexible silver epoxy paste is the next best option. Silver or aluminum film materials 9 and 22µm thick are commonly available shielding solutions for most applications.
Stiffeners. Stiffeners are used to reinforce flex circuits under areas where components are assembled. When selecting a stiffener, consider overall thickness, thermal properties, shielding, adhesion method and cost. Another consideration is the process to singulate the stiffener to the required shape, and the process needed to attach it. Common materials include polyimide, polyester, FR-4, metal and plastic.
Adhesives. Adhesives fall into three groups: thermal set adhesives create permanently bonded laminates; pressure-sensitive adhesives bond stiffeners or mounting hardware to flex, and conductive adhesives connect shielding stiffeners and flex to various structural components inside a device.
For instance, the outlined approach can be used to select the right materials for a flex circuit to meet dynamic bending requirements to exceed 100,000 cycles. Simulation analysis shows HTE copper on 12µm polyimide dielectric adhesiveless laminate with epoxy adhesive coverlayer provides best results. Copper thickness depends on the fabricator’s ability to meet the characteristic impedance and insertion loss requirements. Simulation analysis also shows the impact of solid ground versus hatch ground to meet the impedance requirements. To meet the dynamic bending requirements, shielding film is the preferred option. Shielding film thickness depends on the flex manufacturer’s capabilities. Even though this example is for a consumer electronics product, it can be easily adapted to other products in any other market.
Designing flex circuits is a collaborative process. The above methodology is one approach for flex manufacturers and assemblers to effectively guide designers toward structures and circuit layout using the right materials to optimize design for performance and improve new product time to market by decreasing ESI and NPI times.
Jay Desai is director of marketing at MFLEX (mflex.com); This email address is being protected from spambots. You need JavaScript enabled to view it..
