Portable healthcare equipment and services are becoming more and more popular. In general, these devices must be efficient and "invisible", thus bringing new challenges to designers in terms of low power consumption and small size. Now wafer level chip scale packaging (WLCSP) enables medical treatments that were not possible in the past. These new applications include wound detection, medical implantation, and disposable portable monitors. This article first introduces WLCSP technology, and then discusses the best practical techniques for PCB land pattern, pad finish, and circuit board thickness design in order to maximize WLCSP efficacy.
As shown in Figure 1, WLCSP is a variant of flip chip interconnect technology. With WLCSP technology, the active side of the die is inverted and connected to the PCB using solder balls. The size of these solder balls is usually large enough (300μm at 0.5mm pitch, pre-reflow soldering), which can eliminate the underfill process required for flip chip interconnection.
Figure 1: WLCSP encapsulation.
This interconnection technology has the following advantages:
By omitting the first layer of packaging (molded composite material, lead frame or organic substrate) can save considerable space. For example, 8-ball WLCSP occupies only 8% of the 8-lead SOIC.
By eliminating the connecting wires and leads used in standard plastic packages, inductance can be reduced and electrical performance improved.
Since the lead frame and the molded composite material are omitted, the package shape is thinner and lighter.
No underfill process is required; standard SMT assembly equipment can be used.
Due to the self-calibration characteristics of the lightweight die during the welding process, the assembly yield is high.
Package structure
WLCSP can be divided into two structure types: direct bump and redistribution layer (RDL)
Direct bump
The direct bump WLCSP contains an optional organic layer (polyimide), which acts as a stress buffer on the surface of the active die. Polyimide covers the entire die area except the window area around the connection pads. Sputter or plate an under bump metal layer (UBM) over this windowing area. UBM is a stack of different metal layers, including diffusion layer, barrier layer, wetting layer and anti-oxidation layer. The solder ball falls on the UBM (hence the name falling ball), and then the solder bump is formed by reflow soldering. The structure of the direct bump WLCSP is shown in Figure 2.
Figure 2: Direct bump WLCSP.
Redistribution layer (RDL)
Figure 3 is a redistribution layer (RDL) WLCSP. This technology can convert bare chips designed for bonding wires (bonding pads are arranged around) into WLCSP. Unlike direct bumps, this WLCSP uses two polyimide layers. The first polyimide layer is deposited on the die, and the bond pad is kept in a windowed state. The RDL layer converts the peripheral array into a regional array by sputtering or electroplating. The subsequent structure resembles direct bumps-including the second polyimide layer, UBM and falling balls.
Figure 3: Redistribution layer (RDL) WLCSP.
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