Understanding Smart Sensors - Nomads.usp

Packaging, Testing, and Reliability Implications of Smarter Sensors 239Mounting or additional circuit(i.e., temperature) connectionsSolder bumpbefore reflowPlated copperPhotoresistSputtered CuSputtered TiWPassivation nitrideAl-Cu metal padElectricalconnections(a)Control ICSolder bumpbefore reflowPlated copperPhotoresistMCU(b)(c)(d)Sputtered CuSputtered TiWPassivation nitrideAl-Cu metal padFigure 10.9 Flip-chip assembly: (a) die with bumps, (b) bumped sensor on MCU, (c) bumpedcontrol die on sensor, and (d) bump structure itself.manufacturability and yields a well-protected, hermetic sensor device such as anaccelerometer. The technology has far-reaching implications and opens thedoor to new methods of low-cost, hermetic packaging over the more expensivemethods used today. Protection of the two-layer structure has been accomplishedwith a molded plastic package for current products. Future protectivepackaging may take a different form.Epoxy protection for ICs has provided a low-cost packaging solution forconsumer electronics. The glob-top package shown in Figure 10.10(a) is simplyan epoxy mound that protects the die surface and wire bonds for an ICmounted on a circuit board. For flip-chips, a protective layer around the die hasbeen proposed, as shown in Figure 10.10(b) [13]. In that case, the protection isfrom silicon to silicon with sensitive layers sealed inside the epoxy. Lower costpackaging will be essential for many new smart sensor applications. Adaptingthe approaches described here could provide the cost breakthrough.10.5 Reliability ImplicationsSensors require tests similar to those performed on ICs and unique qualificationtests to verify that they will have acceptable performance of both the