Page 344 - A Practical Guide from Design Planning to Manufacturing

314 Chapter Ten

The assembly process begins with creation of the leadframe (step #1).
The leadframe is punched out of metal sheets, usually an alloy of copper
or nickel and iron. Alloy 42 is the most common material used. It is an
alloy of 42 percent nickel and 58 percent iron. Nickel-iron alloys are
stronger than copper and have a CTE that more closely matches silicon.
However, copper has lower electrical and thermal resistance.
The leadframe consists of a die paddle (also called a flag), which will sup-
port the die during assembly, and internal and external leads. Internal leads
will ultimately be encased inside the plastic package. External leads will
remain outside and form the pins of the package. Leadframes for multiple
packages are connected into strips by strip rails. Tie bars, which also
connect it to the strip rails, support the die paddle.
Die attach and wire bonding (step #2) make the electrical connections
between the leadframe and the die. The die is glued face up to the die
paddle, and bonding pads on the die are connected to the internal leads.
The wire-bond connections are made with gold or aluminum wires. A
machine touches the wiring to a bonding pad on the die and uses heat and
ultrasonic vibration to attach the wire. The tool then draws out a length
of wire to the corresponding internal lead and bonds the wire again. The
wire is then cut and the process repeated for the next bonding pad.
During plastic molding (step #3), the leadframe strip is placed inside
a mold with cavities around the die and internal leads of each individ-
ual leadframe. Liquid plastic is forced into the molds and allowed to set.
The strip rails and external leads are left protruding from the plastic.
Finally the strip rails and excess metal are trimmed away (step #4), and
the individual packages are separated from the strip rails. The pins are
then bent to form either straight vertical pins for THT or gull-wing or
J-bend pins for SMT.
Plastic packages are the least expensive but have a number of dis-
advantages. The plastic material cannot tolerate very high temperatures
and does not pass heat readily. The only easy path for heat to escape the
die is through the package leads themselves. This is fairly inefficient.
To prevent overheating, die in plastic packages are limited to low power
levels. The leadframe also offers only a single level of interconnects,
which must be fairly large to have sufficient mechanical strength to be
reliably punched out of sheet metal. Better thermal properties and
interconnects are possible with ceramic laminate packages.
Ceramic substrates are made of multiple layers of ceramic tape
printed with traces and vias and fused together. The steps for creating
a ceramic package are shown in Fig. 10-6.
Constructing a ceramic substrate begins with an uncured (or green)
flexible tape composed of ceramic powder, usually alumina (Al O ),
3
2
mixed with a binding chemical. At this point, the tape has the feel of a
piece of leather. Blanking (step #1) is where the strip of tape is cut into