Page 75 - MEMS Mechanical Sensors

64 Mechanical Sensor Packaging

Table 4.2 Relative Merits of Die Attachment Methods
Process Advantages Disadvantages
Adhesive Low cost Outgases
Easily automated Contamination/bleed
Low curing temperatures Susceptible to voids
Reduced die stress Inferior thermal/electrical conductivity
Special plated surfaces not required Can require careful storage (e.g., –40°C)
Rework possible and mixing before use
Not suited to harsh environments
Solder Good electrical/thermal conductivity Requires wettable metallized surfaces
Good absorption of stresses arising from on the die and substrate
of thermal expansion coefficients Usually requires processing temperatures
mismatches greater than 200°C
“Clean” Needs flux or an inert gas atmosphere
Rework possible Porosous
Poor thermal fatigue resistance of some
alloys
Eutectic Good thermal conductivity Poor absorption of stresses arising from
Electrically conducting of thermal expansion coefficients
Good fatigue/creep resistance mismatches
Low contamination High processing temperatures
“High” process/operating temperature Die back metallization may be required
capability If bare die are used, a scrubbing action is
required to break down surface oxide
Rework difficult
Glass Low void content High processing temperature
Good thermal/electrical conductivity Glass requires an oxygen atmosphere,
Limited stress relaxation which can lead to oxidation of other
Low contamination plated systems
High process/operating temperature Not commonly used
resistance

4.3.2.3 Eutectic Bonding

A eutectic bond typically uses gold and silicon, which, when heated, diffuse together
at the interface. This diffusion continues until a suitable eutectic alloy is formed,
which melts at a more workable temperature than would be the case for the base
materials (for example, a 97Au-3Si eutectic melts at 363°C). The eutectic bond can
be produced by heating the die then scrubbing it against a gold foil/metallization or
by placing a eutectic foil preform at the interface.

4.3.2.4 Glass Die Attach
This process uses a glass layer between the die and the substrate. The glass can be
either a solid frit placed beneath the die or be made into a screen printable paste and
deposited onto the substrate. The assembly is then heated to typically between
350°C and 450°C until the glass softens to form a low viscosity liquid that will wet
the die and substrate. The glass film solidifies upon cooling, thereby attaching the
die. As with adhesive attachment, silver particles can be added to the glass to
improve the thermal and electrical conductivity of the material. This is a more spe-
cialized process not commonly employed.