Flexible Electronics: Thin Silicon Die on Flexible Substrates

Silicon thinned to 50 $\mu{\hbox {m}}$ and less is flexible allowing the fabrication of flexible and conformable electronics. Two techniques have been developed to achieve this goal using thinned die: die flip chip bonded onto flexible substrates [polyimide and liquid crystal polymer (LCP)] and die flip chip laminated onto LCP films. A key to achieving each of these techniques is the thinning of die to a thickness of 50 $\mu{\hbox {m}}$ or thinner. Conventional grinding and polishing can be used to thin to 50 $\mu{\hbox {m}}$. At 50 $\mu{\hbox {m}}$, the active die becomes flexible and must be handled by temporarily bonding it to a holder die for assembly. Both reflow solder and thermocompression assembly methods are used. In the case of solder assembly, underfill is used to reinforce the solder joints. With thermocompression bonding of the die to an LCP substrate, the LCP adheres to the die surface, eliminating the need for underfill.