New-Tech Europe | September 2016 | Digital Edition

Chip Scale Packaging Helps Portable Medical Devices Save Size and Weight

Mike Delaus and Santosh Kudtarkar, Analog Devices, Inc.

Wafer-level packages designers

chip

scale

terms of performance and reliability, designers should heed the best practices in designing the printed- circuit board (PCB) land pattern, pad finish, and board thickness. Wafer-level chip scale packaging is a variant of the flip-chip interconnection technique (Figure 1). With WLCSPs, the active side of the die is inverted and connected to the PCB using solder balls. The size of these solder balls is typically large enough (300 μm pre- reflow for 0.5-mm pitch, and 250 um pre-reflow for 0.4-mm pitch) to avoid the underfill that is required for flip-chip interconnects. This interconnection technology offers several advantages. First, considerable space savings are obtained by eliminating the first level package (mold compound, lead frame, or organic substrate). For example, an 8-ball WLCSP occupies only 8% of the board area taken up by an 8-lead

SOIC. Next, inductance is reduced and electrical performance is improved by eliminating the wire bonds and leads used in standard plastic packages. Also, designs yield a lighter weight and thinner package profile, due to the elimination of the lead frame and molding compound. No underfill is required, as standard surface-mount (SMT) assembly equipment can be used. And finally, high assembly yields result from the self-aligning characteristic of the low mass die during solder attachment. Package Construction WLCSPs can be categorized into two construction types: direct bump and redistribution layer (RDL). A direct-bump WLCSP consists of an optional organic layer (polyimide), which acts as a stress buffer on the active die surface. The polyimide

are

allowing

of portable healthcare equipment - such as invasive sensing, medical implants, and disposable monitors - to reduce size and power requirements. ne of the key trends in medical equipment design is to bring equipment closer to patients at the doctor’s office or at their own home by making these devices more portable. This involves all aspects of the design, but especially effects size and power consumption. Shrinking the electronic portion of these instruments is being aided greatly by the use of wafer-level chip-scale packages (WLCSPs). These new applications include invasive sensing, medical implants, and disposable, portable monitors. But to get the most out of WLCSPs in

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