MPXM2102AS datasheet(27/670 Pages) MOTOROLA

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Figure 3. Examples of uniform corrosion of a gold leadframe in nitric acid at 5 Vdc and galvanic corrosion on an

unbiased device at the gold wire/aluminum bondpad interface in commercial detergent.

PIEZORESISTIVE

TRANSDUCER

DIAPHRAGM

SILICON DIE

UNIBODY PACKAGE

DIE ATTACH

LEAD FRAME

WIREBOND

NITRIC

Part of figure 3 shows an example of what we have

described as electrolytic corrosion (i.e., corrosion of similar

metallic surfaces in an electrolytic solution caused by a

sufficient difference in potential between the two surfaces).

This appears to be uniform corrosion of the gold leadframe

surface. It should be noted that this type of failure is observed

even on ‘noble’ metals like gold. Applied potential is the driving

force for the reaction. All metals can corrode in this fashion

depending on the solution concentration (pH) and the applied

potential. Pourbaix diagrams describe these thermodynamic

relationships [18].

Figure 3 shows an example of galvanic corrosion. The

figure illustrates that corrosion can also occur because of

dissimilar metals that are connected electrically and are

immersed in an electrolytic solutions. A difference in the

corrosion potential between the two metals is the driving force

for the reaction. Localized corrosion examples are prevalent

as well. Often they may be the precursor to what appears on

a macro scale to be uniform or galvanic corrosion.

In situ

monitoring of devices in electrolytic media will allow better

diagnosis of this failure mechanism. Typical

ex situ or interval

reliability testing may not allow diagnosis of the root cause to

the failure, thus limiting the predictive power of any resulting

reliability models.

Silicon Etching

Figure 4 shows the result of an accelerated test of a

pressure sensor die to a high temperature detergent solution.

The detergent used was a major consumer brand and resulted

in dramatic etching of the silicon. Alkaline solutions that

undergo a hydrolysis reaction may result in etching of the

silicon similar to a bulk micromaching operation. This failure

mechanism can cause a permanent change in the sensitivity

of the device because the sensitivity is proportional to the

inverse square of the silicon thickness. Moreover, it can lead

to loss in bond integrity between wafers (Fig. 4). Silicon

etching [19–20], like corrosion reactions, is a chemical

reaction, so the contributing factors include the silicon

material, its crystal orientation and its doping level, the

solution type, concentration and pH, and the applied potential.

Temperature, concentration (i.e., pH), and voltage all act to

accelerate this process. Figure 5 shows an example of

modeling results that illustrates two of these variables.

Figure 4. Photograph of silicon etching after

exposure to an aqueous detergent solution at

elevated temperature for an extended time. A frit layer,

horizontally in the middle, adheres to silicon on

either side. The amount of etching is evident by

referencing the glass frit edge on the far left.

These two silicon edges were aligned to the frit

edge when the die was sawn.

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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