AD8235 datasheet(15/20 Pages) AD

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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 +VS or applying a voltage

within +VS − 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 −VS, or any voltage within −VS +

0.5 V, the AD8235 operates in shutdown mode and, therefore,

draws less than 500 nA of supply current, offering considerable

power savings.

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

−VS at the output of the part, some additional current draw is

noticeable. In this mode, a path from the REF pin to −VS 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 μm 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 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 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 μm to prevent the

formation of brittle intermetallic compounds.

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