MPXM2102AS datasheet(30/670 Pages) MOTOROLA

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Motorola Sensor Device Data

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Mechanical Failures

The occurrence of mechanical failures include components

of fatigue, environment assisted cracking, and creep.

Packaging materials, process, and residual stresses are all

contributors to mechanical failure. A summary of acceleration

stresses is shown in Table 3. Contact with harsh media is an

accelerating stress for all of the mechanical failure mechanisms.

Table 3. Mechanical Failure Mechanisms

Failure Mechanism

Acceleration Stresses

Fatigue crack initiation

Mechanical stress/strain range

Cyclic temperature range

Frequency

Media

Fatigue crack propagation

Mechanical stress range

Cyclic temperature range

Frequency

Media

Environment assisted cracking

Mechanical stress

Temperature

Media

Creep

Mechanical stress

Temperature

Media

PRESSURE SENSOR SOLUTIONS

The range of solutions for pressure sensors to media

compatibility is very diverse. Mechanical pressure sensors still

occupy a number of applications due to this media compatibility

concern. These devices typically operate on a variable

inductance method and are typically not as linear as a

piezoresistive element. Figure 10 shows a comparison

between a mechanical pressure sensor and a piezoresistive

element for a washing machine level sensing application. The

graph shows a nonlinear response for the mechanical sensor

and a corresponding straight line for the piezoresistive element.

A common method of obtaining media compatibility is to

place a barrier coating over the die and wire interconnection.

This organic encapsulant provides a physical barrier between

the harsh environment and the circuitry. The barrier coating

can range from silicone to parylene or other dense films that

are typically applied as a very thin layer. This technique offers

limited protection to some environments due to swelling

and/or dissolution of the encapsulant material when in contact

with media with a similar solubility. When a polymeric material

has a solubility parameter of the same value as the

corresponding media, swelling or dissolution will occur.

Stainless steel diaphragms backfilled with silicone oil

provide a rugged barrier to most media environments, but

generally are very costly and limit the sensitivity of the device.

The silicone oil is used to transmit the stress from the

diaphragm to the piezoresistive element. If a polymeric

material is used as the die attach, the silicone oil will permeate

out of the package. This concern requires a die attach that is

typically of higher modulus than a silicone and may not

adequately isolate the package stress from the die.

Figure 10. Graphical comparison of the output from a mechanical pressure sensor compared to a piezoresistive

sensor during a washing machine fill cycle.

140

145

150

155

160

165

170

175

0.2

0.4

0.6

0.8

1.2

1.4

1.6

1.8

TIME (MINUTES)

WASHING

MACHINE

SENSOR

PIEZORESISTIVE

PRESSURE

SENSOR

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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