Dynamic flexing

Protection Against Flex Cracking. Most antioxidants including waxes provide oxidation protection under static conditions. However, most of them are not effective in mbber products subjected to dynamic flexing, eg, sidewall compounds in tires. The best dynamic protection is provided by either /V-alkyl-/V-phenyl or diaryl-/)-phenylene diamines. 

Fig. 14. Mastication of polystyrene — synthetic rubbers systems. Effect of the amount and type of rubber on dynamic flex resistance. 1 styrene rubber (SKS 30) 2 nitrile rubber (SKN 18) ...

ASTM D 1565, a specification, outlines a test method for dynamic flexing of flexible vinyl cellular materials. This test uses a flexing machine which oscillates at 1 Hz. A minimum of 250,000 flexes are applied. After alternate compression and relaxation the effect on the structure and thickness of the foam is observed. The percentage loss of thickness is reported. Flexural modulus of microcellular urethane is described in ASTM D 3489. This method uses the general procedure in ASTM D 790, Method I. ASTM D 3768 outlines a procedure for determining flexural recovery of microcellular urethanes. The method is used to indicate the ability of a material to recover after a 180° bend around a 12.7-mm (0.5 in.) diameter mandrel at room temperature. 

Efficient energy conversion has been demonstrated under dynamic flexing conditions (using a nanoscale cantilever beam ). Disadvantageously, however, ceramics are fragile and difficult to stress, cannot maintain a large bend and suffer from fatigue in a dynamic environment. Furthermore, flexoelectric crystals must have a suitable shape, which makes the fabrication process for crystalline or ceramic solid materials much more difficult. 

De Mattia type dynamics flexing testing machine ... 

This family of elastomers is produced by the random chlorination of HDPE (the proprietary Tyrin is from Dow Chemical). The properties of chlorinated polyethylene (CM) include excellent ozone and weather resistance, heat resistance to 149°C (300°F) and even 177°C (350°F) in many types of oil, dynamic flex resistance, and resistance to abrasion. 

Properties of Polyimide Films. Table 61.8 shows major properties of typical poly-imide films, Kapton H and Apical AV. These have good mechanical properties and provide excellent performance as base substrate films and coverlay films in dynamic flexing appUca-tions.They have flame-retardant characteristics, and it is not difficult to achieve Underwriters Laboratories (UL) flame class 94-V-O or 94-VTM-O. The largest disadvantage of polyimide film is its higher cost compared to other common plastic films such as PET. 

As flexible circuits have various mechanical stresses, their conductors also have requirements for more flexibility and toughness than traditional ED copper foils of standard rigid circuit boards. An RA copper foil is the solution for high flexing endurance. It is necessary for dynamic flexing use. RA copper foils are more expensive than ED copper, especially at thicknesses of less than 18 micron. Therefore, high-ductility electrodeposited (HD-ED) copper foils have been developed that have greater flexibility than ED copper foils and a lower cost than RA copper foils. The differences between the copper foils are shown in Table 61.11. 

Generally, because of poor resolution, a screen-printing process does not satisfy the requirements of high-density SMT assembling on flexible circuits. Also, such as process does not provide good mechanical performance for dynamic flexing.Typical property examples are shown in Table 61.15. 

New materials of photoimageable coverlay have been developed without bromine molecules. Not many choices offer a balanced performance, especially high flexibihties for dynamic flexing. Polyimide-based photoimageable coverlays are the solution to satisfy the all of the physical requirements, but these polyimide materials are remarkably more expensive. 

FIGURE 62.15 Layer construction for dynamic flexing, (a) Standard layer construction, (b) Dynamic flexing construction with thin adhesive layers, (c) Dynamic flexing construction without adhesive layer. ... 

Basically, double-sided flexible circuits with through holes are not available for dynamic flexing. The flexibility depends on the configuration of conductors in both sides of the base substrate, as shown in Fig. 62.19 shows examples. Bending the drcuit with thin copper circuits on the outside of the flexible circuits may cause them to break, because the mechanical stresses are concentrated on the small traces. 

A dynamic flexing area of a double-sided flexible circuit should have only one conductor layer. Coverlay on the other side of conductor should also be eliminated to enable symmetrical layer constructions. Copper... 

FIGURE 62.16 Typical dynamic flexing mode of flexible circuit specified by IPC-TM-650. ... 

FIGURE 62.18 Design of dynamic flexing part of flexible circuits (a) Unacceptable technique, (b) guide board, (c) loose guide, (d) silicone rubber. 

FIGURE 62.19 Circuits configuration of double-sided flexible circhits at flexing parts, (a) Preferred, (b) Acceptable, (c) Unacceptable, (d) Meshed shield layer, (e) Crossing traces, (f) Separated shield layer for dynamic flexing, (g) Fan folded, (h) Coiled. ... 

The basic concept of quality assurance for flexible circuits is the same as that for rigid boards, in the sense that any critical defect must be eliminated before shipping. This includes failures due to opens and shorts dimensional degradation of pad arrays, serious defects on conductors, substrates, coverlay, and so on. These defects need to be inspected for each circuit, whether it is flexible or rigid. Technically, however, there are several differences in quality assmance for flexible circuits because of their additional functions, such as dynamic flexing capability. 

Under service conditions of dynamic flexing, wax alone will be inadequate to protect the rubber against ozone attack. The flexing action will rupture the surface film of the wax and thus provide an avenue of entry for the attacking ozone. Thus dynamic flexing requires the joint use of wax plus chemical antiozonant to attain proper protection against ozone attack. 

---