Small deflection bending response

The simplest level of approximation for small deflection of the film is geometrically linear elastic plate theory. In effect, it is presumed that the pressure is resisted exclusively by bending resistance of the film in this case, and the bending stress is not coupled to any membrane stress which may be present. In this case, the transverse deflection w x) is known from linear elastic plate 

The small deflection approximation is usually thought to be accurate for a center point deflection up to roughly wq hf. Therefore, the pressure at which Wq = hf is identified as the reference pressure 

The corresponding phase angle i) for the stress state on the interface immediately beyond the pressurized zone is given by (5.26) as 0 = —45°. Even if tm / Oj the energy release rate is given by Qh a) because bending and membrane stresses are not coupled at this level of approximation. 

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In addition to monotonic loading conditions, certain repetitive operations could also lead to mechanical failures. A typical example is a PWB used in cell-phone applications. The PWB could be subjected to multiple small deflections each time the ceU-phone keypad is pressed.This can cause an incremental deterioration in the mechanical response of the PWB and lead to solder joint interconnect failures in BG As mounted on the PWB. This sort of failure mechanism is more prevalent in relatively thin PWBs used in cell-phone applications. However, similar failures could also occur if, for example, multiple runs of ICT are performed on PCAs in high-end server/network applications. An industry standard test method has been developed by JEDEC to characterize the response of a PCA to cychc loading conditions (JESD22-B113). The layout of the test board as specified in the test method is similar to that for the monotonic four-point bend test. Some data on the cyclic bend response of Sn AgCu solder joints can be found in Reference 39. 

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