AN824

Vishay Siliconix

www.vishay.com

2

Document Number: 71990

06-Jan-03

TABLE 1

Main Parameters of Solder Bumps in MICRO FOOT Designs

MICRO FOOT CSP

Bump Material

Bump Pitch*

Bump Diameter*

Bump Height*

MICRO FOOT CSP MOSFET

Eutectic Solder:

0.8

0.37-0.41

0.26-0.29

MICRO FOOT CSP Analog Switch

Eutectic Solder:

63Sm/37Pb

0.5

0.18-0.25

0.14-0.19

MICRO FOOT UCSP Analog Switch

63Sm/37Pb

0.5

0.32-0.34

0.21-0.24

* All measurements in millimeters

MICRO FOOT’S DESIGN AND RELIABILITY

As a mechanical, electrical, and thermal connection between

the device and PCB, the solder bumps of MICRO FOOT

products are mounted on the top active surface of the die.

Table 1 shows the main parameters for solder bumps used in

MICRO FOOT products. A silicon nitride passivation layer is

applied to the active area as the last masking process in

fabrication,ensuring that the device passes the pressure pot

test. A green laser is used to mark the backside of the die

without damaging it. Reliability results for MICRO FOOT

products mounted on a FR-4 board without underfill are shown

in Table 2.

TABLE 2

MICRO FOOT Reliability Results

Test Condition C: −65_ to 150_C

>500 Cycles

Test condition B: −40_ to 125_C

>1000 Cycles

121_C @ 15PSI 100% Humidity Test

96 Hours

The main failure mechanism associated with wafer-level

chip-scale packaging is fatigue of the solder joint. The results

shown in Table 2 demonstrate that a high level of reliability can

be achieved with proper board design and assembly

techniques.

BOARD LAYOUT GUIDELINES

Board materials. Vishay Siliconix MICRO FOOT products are

designed to be reliable on most board types, including organic

boards such as FR-4 or polyamide boards. The package

qualification information is based on the test on 0.5-oz. FR-4

and polyamide boards with NSMD pad design.

Land patterns. Two types of land patterns are used for

surface-mount packages. Solder mask defined (SMD) pads

have a solder mask opening smaller than the metal pad

(Figure 3), whereas on-solder mask defined (NSMD) pads

have a metal pad smaller than the solder-mask opening

(Figure 4).

NSMD is recommended for copper etch processes, since it

provides a higher level of control compared to SMD etch

processes. A small-size NSMD pad definition provides more

area (both lateral and vertical) for soldering and more room for

escape routing on the PCB. By contrast, SMD pad definition

introduces a stress concentration point near the solder mask

on the PCB side that may result in solder joint cracking under

extreme fatigue conditions.

Copper pads should be finished with an organic solderability

preservative

(OSP)

coating.

For

electroplated

nickel-immersion gold finish pads, the gold thickness must be

less than 0.5 mm to avoid solder joint embrittlement.

FIGURE 3. SMD

FIGURE 4. NSMD

Copper

Solder Mask

Copper

Solder Mask