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Figure 2. Direct Bump WLCSP
Figure 1. WLCSP Package
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
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
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