New-Tech Europe | September 2016 | Digital Edition

Figure 2. Direct Bump WLCSP

Figure 1. WLCSP Package

reduce die chipouts induced during sawing and to ease the handling of the package. Best PCB Design Practices The critical board design parameters are pad opening, pad type, pad finish, and board thickness. Based on the IPC standard, the pad opening equals the UBM opening. The typical pad openings are 250 μm for a 0.5-mm pitch WLCSP and 200 μm for a 0.4- mm pitch WLCSP (Figure 4). The solder mask opening is 100 μm plus the pad opening. The trace width should be less than two-thirds of the pad opening. Increasing the tracewidth can reduce the stand-off height of the solder bump. Hence, maintaining the proper trace width ratio is important to ensure the reliability of the solder connections. For board fabrication, two types of pads/land patterns are used for surface-mount assembly (Figure 5): Non-solder-mask defined (NSMD): The metal pad on the PCB (to which the I/O is attached) is smaller than the solder-mask opening. Solder-mask defined (SMD): The solder-mask opening is smaller than

the metal pad. Because the copper etching process has tighter control than the solder- mask opening process, NSMD is preferred over SMD. The solder-mask opening on NSMD pads is larger than the copper pads, allowing the solder to attach to the sides of the copper pad and improving the reliability of the solder joints. The finish layer on the metal pads has a significant effect on assembly yield and reliability. The typical metal pad finishes used are organic surface preservative (OSP) and electroless nickel immersion gold (ENIG). The thickness of the OSP finish on a metal pad is 0.2 μm to 0.5 μm. This finish evaporates during the reflow soldering process, and interfacial reactions occur between the solder and metal pad. The ENIG finish consists of 5 μm of electroless nickel and 0.02 μm to 0.05 μmof gold. During reflow soldering, the gold layer dissolves rapidly, followed by reaction between the nickel and solder. It is extremely important to keep the thickness of gold below 0.05 μm to prevent the formation of brittle intermetallic compounds. Standard board thicknesses range from 0.4 mm to 2.3 mm. The thickness selected

covers the entire die area except for openings around the bond pads. An under-bump metallurgy (UBM) layer is sputtered or plated over this opening. The UBM is a stack of different metal layers serving as diffusion layer, barrier layer, wetting layer, and antioxidation layer. The solder ball is dropped (which is why it's called ball-drop) over the UBM and reflowed to form a solder bump (Figure. 2). RDL technology allows a die designed for wire bonding (with bond pads arranged along the periphery) to be converted into a WLCSP. In contrast to a direct bump, this type of WLCSP uses two polyimide layers. The first polyimide layer is deposited over the die, keeping the bond pads open. An RDL layer is sputtered or plated to convert the peripheral array to an area array. The construction then follows the direct bump, with a second polyimide layer, UBM, and ball drop (Figure 3). Post ball-drop are wafer backgrind, laser-marking, testing, singulation, and tape and reel. There is also an option of applying a backside laminate after the backgrinding process to

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