Datasheet

AD8235
Rev. 0 | Page 15 of 20
TRACE
WIDTH
PAD OPENING
MASK OPENING
SHUTDOWN FEATURE
The AD8235 includes a shutdown pin (SDN) that further
enhances the flexibility and ease of use in portable applications
where power consumption is critical. A logic level signal can be
applied to this pin to switch to shutdown mode, even when the
supply is still on.
When connecting the SDN pin to +V
S
or applying a voltage
within +V
S
− 0.5 V, the AD8235 operates in its normal condition
and, therefore, draws approximately 40 µA of supply current.
When connecting the SDN pin to −V
S
, or any voltage within −V
S
+
0.5 V, the AD8235 operates in shutdown mode and, therefore,
draws less than 500 nA of supply current, offering considerable
power savings.
0 8211-044
In cases where the AD8235 is operating in shutdown mode, if
a voltage potential exists at the REF pin, and there is a load to
−V
S
at the output of the part, some additional current draw is
noticeable. In this mode, a path from the REF pin to −V
S
exists,
leading to some additional current draw from the reference.
Typically, this current is negligible because the output of the
AD8235 is driving a high impedance node, such as the input of
an ADC.
LAYOUT RECOMMENDATIONS
The critical board design parameters, as it pertains to a WLCSP
package, are pad opening, pad type, pad finish, and board
thickness.
Pad Opening
Based on the IPC (Institute for Printed Circuits) standard, the
pad opening equals the UBM (Under Bump Metallurgy)
opening. The typical pad openings for the AD8235 shown in
Figure 37 are:
• 250 µm (0.5 mm pitch WLCSP)
The solder mask opening is 100 m plus the pad opening (or
350 µm in the case of the AD8235). The trace width should be
less than two-thirds of the pad opening. Increasing the trace
width can cause reduction in the stand-off height of the solder
bump. Therefore, maintaining the proper trace width ratio is
important to ensure the reliability of the solder connections.
Figure 37. Pad Opening
Pad Type
For the actual board fabrication, the following types of
pads/land patterns are used for surface mount assembly:
• Nonsolder 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.
Pad Finish
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 m to 0.5 m. This finish evaporates during
the reflow soldering process and interfacial reactions occur
between the solder and metal pad. The ENIG finish consists
of 5 m of electroless nickel and 0.02 m to 0.05 µm of 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 m to prevent the
formation of brittle intermetallic compounds.