Page 246 - An Introduction to Microelectromechanical Systems Engineering

Die-Attach Processes 225

The need to calibrate and compensate extends beyond conventional sensors.
For example, the infrared imaging array from Honeywell must calibrate each
individual pixel in the array and compensate for any manufacturing variations
across the die. The circuits perform this function using a shutter: The blank scene,
that is the collected image while the shutter is closed, incorporates the variation in
sensitivity across the array; while the shutter is open, the electronic circuits subtract
the blank-scene image from the active image to yield a calibrated and compensated
picture.

Die-Attach Processes

Subsequent to dicing of the substrate, each individual die is mounted inside a pack-
age and attached (bonded) onto a platform made of metal or ceramic, though plastic
is also possible under limited circumstances. Careful consideration must be given to
die attaching because it strongly influences thermal management and stress isola-
tion. Naturally, the bond must not crack over time nor suffer from creep—its reli-
ability must be established over very long periods of time. The following section
describes die-attach processes common in the packaging of silicon micromachined
sensors and actuators. These processes were largely borrowed from the electronics
industry.
Generally, die-attach processes employ either metal alloys or organic or inor-
ganic adhesives as intermediate bonding layers [7, 8]. Metal alloys comprise of all
forms of solders, including eutectic and noneutectic (see Table 8.2). Organic
adhesives consist of epoxies, silicones, and polyimides. Solders, silicones, and epox-
ies are vastly common in MEMS packaging. Inorganic adhesives are glass matrices

Table 8.2 Properties of Some Eutectic and Noneutectic Solders
Alloy Liquidus Solidus Ultimate Uniform Creep
(ºC) (ºC) Tensile Elongation Resistance
Strength (MPa) (%)

Noneutectic 60%In 40%Pb 185 174 29.58 10.7 Moderate
60%In 40%Sn 122 113 7.59 5.5 Low—soft
alloy
80%In 15%Pb 5%Ag 154 149 17.57 — Low
80%Sn 20%Pb 199 183 43.24 0.82 Moderate
25%Sn 75%Pb 266 183 23.10 8.4 Poor
5%Sn 95%Pb 312 308 23.24 26 Moderate to
high
95%Sn 5%Sb 240 235 56.20 1.06 High
Eutectic 97%In 3%Ag 143 143 5.50 — Low—soft
alloy
96.5%Sn 3.5%Ag 221 221 57.65 0.69 High
42%Sn 58%Bi 138 138 66.96 1.3 Moderate
—brittle alloy
63%Sn 37%Pb 183 183 35.38 1.38 Moderate
1%Sn 97.5%Pb 1.5%Ag 309 309 38.48 1.15 Moderate
88%Au 12%Ge 356 356 — — Moderate
96.4%Au 3.6%Si 370 370 — — Moderate
(Source: [7].)