Page 248 - An Introduction to Microelectromechanical Systems Engineering

Wiring and Interconnects 227

deposition of very thin gold improves surface wetting. Immersing the part in flux
(an organic acid) removes metal oxides and furnishes clean surfaces. In a manufac-
turing environment, the solder paste is either dispensed through a nozzle or screen
printed on the package substrate, and the die is positioned over the solder. Heating
in an oven or by direct infrared radiation melts the solder, dissolving in the process
a small portion of the exposed thin metal surfaces. When the solder cools, it forms
a joint bonding the die to the package. Melting in nitrogen or in forming gas pre-
vents oxidation of the solder.
Organic adhesives are attractive alternatives to solder because they are inexpen-
sive, easy to automate, and they cure at lower temperatures. The most widely used
are epoxies and silicones, including room-temperature vulcanizing (RTV) rubbers.
Epoxies are thermosetting (i.e., cross linking when heated) plastics with cure tem-
peratures varying between room temperature and 175ºC. Filled with silver or gold,
they become thermally and electrically conductive, but not as conductive as solder.
Electrically nonconductive epoxies may incorporate particles of aluminum oxides,
beryllium oxides, or magnesium oxides for improved thermal conductivity. RTV
silicones come in a variety of specifications for a wide range of applications from
®
construction to electronics. For example, the Dow Corning 732 is a multipurpose
silicone that adheres well to glass, silicon, and metal, with a temperature rating of
–65ºC to 232ºC [9]. Most RTV silicones are one part condensation-curing com-
pounds, curing at room temperature in air while outgassing a volatile reaction prod-
uct, such as acetic acid. Another class of RTVs, however, is addition-cure RTVs,
which do not outgas, making them suitable for many optical applications. Unlike
epoxies, they are soft and are excellent choices for stress relief between the package
and the die. The operating temperature for most organic adhesives is limited to less
than 200ºC; otherwise, they suffer from structural breakdown and outgassing.
Epoxies and RTV silicones are suitable for automated manufacturing. As vis-
cous pastes, they are dispensed by means of nozzles at high rates or screen printed.
The placement of the die over the adhesive may also be automated by using pick-
and-place robotic stations employing pattern recognition algorithms for accurate
positioning of the die.

Wiring and Interconnects

With the advent of microfluidic components and systems, the concept of inter-
connects is now more global, simultaneously incorporating electrical and fluid
connectivity. Electrical connectivity addresses the task of providing electrical wiring
between the die and electrical components external to it. The objective of fluid
connectivity is to ensure the reliable transport of liquids and gases between the die
and external fluid control units.

Electrical Interconnects
Wire Bonding
Wire bonding is unquestionably the most popular technique to electrically connect
the die to the package. The free ends of a gold or aluminum wire form low-resistance