Using Flex in High-Speed Applications

Posted on Sept. 1, 2017

The printed circuit board industry makes copper-clad circuits in four different classes: traditional rigid circuits, flexible circuits, circuits meant for high speed and high frequency, and High Density Interconnect (HDI). Flexible circuits are available in many variations. Traditional flex circuits are popular, as they are primarily of low layer-count, are highly flexible, and are used in static or dynamic motion applications. Static cases are usually applications of a one-time bend — assembly forms the circuit into a specific shape that does not need to flex again. However, circuits in dynamic applications must flex continuously during the life cycle of the product: for instance, the read-write flex circuit within a hard disk drive.

There are also the rigid-flex circuits with the ability to join flexible circuits with multilayer rigid PCBs. Conceptually, this is like multilayer PCBs with built-in flex circuit layers. Typically, FR-4 type materials form the rigid areas, and polyimide-based materials make up the flex layers. Although consumers find the rigid-flex circuits highly useful, manufacturers find them problematic because of reliability and manufacturing issues. However, manufacturers have now fine-tuned their rigid-flex technology, and are better equipped to handle the reliability and manufacturing issues. As a result, they can now make rigid-flex effectively, and with excellent reliability.

Difference in Material and Construction of Rigid and Flex Circuits

The properties of materials used for flexible circuits are great for typical end-use applications. However, as all materials may do, those for flex circuits also have potentially limiting properties, such as high coefficient of thermal expansion (CTE), poor thermal conductivity, relatively high moisture absorption, and poor dissipation factor. Manufacturers are addressing these issues by using specific combinations of a wider range of adhesives and polyimide films for constructing flex circuits.

Customary flexible circuit materials suffer from one major issue, that being performance at high frequencies. There are two basic reasons for this: first, unlike rigid circuits, flexible circuits do not have glass reinforcement in their base dielectric. Rather, they contain different grades of polyimide as the dielectric, providing both flexibility and mechanical integrity.

The second reason is flex circuits do not use soldermask to cover the outer layers. Soldermask is a brittle substance that cracks when flexed. Instead, the sides of a flex circuit are covered with an adhesive as a conformal coating and referred to as coverlay. Additionally, copper clad flex cores are bonded together on both sides of a polyimide layer, referred to as bondply.

All the above not only make the dielectrics of flex circuits different, but the processes involved in building up the base dielectrics are also different from those used for rigid PCBs. While manufacturing, the dielectrics are made in large rolls of coated films and the lamination to copper takes place as a separate step. The process allows very consistent thicknesses of these cast films in the range of 100 µm.

As opposed to electrodeposited (ED) copper in rigid circuits, flex circuits use rolled-annealed (RA) copper, which has a better and smoother surface finish, and is less likely to crack due to flexing or bending. At high frequencies, the smooth RA copper offers optimum performance.

Performance of Flex Circuits at High Frequencies

At operational frequencies above 1 GHz, a complex quantity, called the relative permittivity, assumes greater significance for polymers, rather than the customary dielectric constant, as the latter now varies with frequency. Materials with higher losses are problematic as they absorb more energy at higher frequencies. Flexible copper clad laminates using Advanced Kapton and Teflon have lower dispersion, which allows use of these materials for high frequency applications.

With reducing thickness of layers, the copper traces have significant impact on signal integrity at high frequencies. This is related to skin-effect at high frequencies, as it increases the fundamental resistivity of thin copper structures. This is the opposite of the performance of thick copper structure, where the effect of thin dielectric is more prominent. That means designers of flex circuits have to be careful with calculating line widths to meet controlled impedance requirements.

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