The last installment of this review of design issues related to bending and flexing addressed the I-Beam effect and looked at some of the many ways that a flexible circuit can be flexed, folded, and shaped. This installment will provide some simple “rules of thumb” for flexing, both statically and dynamically.

A first recommendation is to design flex dynamic areas with the copper grain direction. The orientation of the grain of the copper foil has a definite effect on flexural life of a design and it has been proven many times in testing. However, grain direction is of greatest importance for flex circuit designs that are fabricated using rolled and annealed (RA) or traditional electrodeposited (ED) copper foil. Both of these types of foil have historically shown a marked difference in flexural endurance between machine and transverse or cross directions. In contrast, grain refined electroplated copper on sputtered film does not appear to have any particular or specific grain direction, thus laminate orientation for processing is not as critical.

Fig 2 Very Small Bends in the Flex Circuit are Possible as Demonstrated by This Disc Drive Application

When it comes to bending, it is best to keep any flexural arc as small as practical for maximum flex life. In disc drive flex circuit design, it has been demonstrated that a smaller flexural arc or total angle of flexure of the circuit in dynamic designs will provide the best performance as small as possible (that is, flex the circuit over the smallest possible distance). This is a key technique used in later model disk drive applications to allow them to achieve the high flex life cycling they presently obtain that are more than one order of magnitude greater than earlier designs.

The next concern is the bend radius, which should be kept as large as possible. The designer has always been advised to always provide the largest practical radius through bend areas. This design approach or attribute is especially important, even critical, for dynamic flex. And as pointed out in the previous installment, it can also be important in flex applications that are designed for static applications, but which can potentially be subjected and must therefore endure millions, or even billions, of low amplitude, high frequency flexing cycles.

When it comes to bending design, finite element modeling can be extremely useful and the method is recommended as it can provide excellent predictive data for suggesting limits for bending. On the other hand, there are some long standing and commonly accepted guidelines that have served the industry over the years to keep the design inside the limits. For a rough, first order approximation of where the limits are, the industry has a long standing practice of looking at the application and predetermining how much strain will be induced on the circuit during bending.

This is determined by the radius and the distance to the outer surface of the copper foil. Figure 1 and its simple equation will help to determine the need. As, can be concluded by calculation, the elongation requirements for the copper foil rise significantly as bend radii decrease. Beyond this simplistic but instructional analysis, there are a few commonly used guidelines that have served well for many years.

For normal bending of different flexible circuit constructions, those guidelines are as follows: for single metal layer, the minimum bend radius is 3 to 6 times circuit thickness; for double-sided flex, the minimum bend radius is 6 to10 times circuit thickness; for multilayer flex, the general rule is a radius greater than 10 to15 times circuit thickness or more. For dynamic applications, only a single metal layer is recommended, especially for high cycle applications and the minimum radius should be 20 to 40 times circuit thickness or more. Two metal layer circuits can be flexed dynamically, but there is a need to look closely at the application. For very high flex life dynamic flex circuit designs, fabrication and testing of prototype circuits remains the preferred method of design verification for a great many applications.

In spite of these guidelines, rules are often bent (both literally and figurative). For example, creasing and hard folding of flex, while not a preferred practice, can be successfully accomplished with some attention to certain details. When required or desired, the circuit should be permanently bonded to itself to prevent it from bending back at the crease or fold line. A small dowel pin might be advisable to hold a small radius, or a separating base, such as shown in Figure 2, can be used. The ideal copper for such high strain bending applications will be a low strength, high elongation copper. Fully annealed soft copper is normally a good choice for applications requiring a small radius bend.

In summary, the flexing and bending of flexible circuits is fundamental to the technology and there is a need to understand some of the basic rules and practices to assure design quality. This topic will continue in the next installment.