Page 346 - A Practical Guide from Design Planning to Manufacturing

316 Chapter Ten

powder into continuous wires. This process of fusing powdered material
together without melting is called sintering. The very high temperatures
required (as much as 1500°C) would melt most metals, so metals with
high melting points like tungsten or molybdenum must be used for the
traces. These metals have unfortunately much higher resistance than the
copper or nickel-iron interconnects used in leadframe packages.
Pin brazing (step #5) attaches the pins to the substrate using a hard high
temperature solder. The die is then glued into the cavity of the package
and wire bonded to the package traces (step #6). A protective lid is fixed
in place over the die cavity once wire bonding is complete. The package
shown in Fig. 10-6 is a “cavity down” package where the die cavity faces
the package pins. This has the advantage of placing the backside of the
die in direct contact with the top of the package. A heat sink placed on top
of the package can then readily remove heat generated by the die. A“cavity
up” package places the die cavity on the top of the package. This allows
pins to be placed across the entire bottom face of the package but means
there will be an air gap between the die and any heat sink placed on the
top of the package.
Ceramic packages transmit heat much more readily than plastic pack-
ages and tolerate much higher temperatures. In addition, the ceramic
CTE is very close to that of silicon so that high temperatures will cause
little stress between the die and the package. Multiple levels of traces
allow high current levels to be delivered to the die with little electrical
noise. All of these reasons make ceramic packages (especially cavity down
packages) ideal for high-power chips. However, there are disadvantages.
Ceramic packages require the use of high-resistance metal interconnects
and have high dielectric permittivity leading to high capacitance. Most
importantly ceramic packages are much more costly to make than plastic
packages. Organic laminate packages provide better thermal and elec-
trical performance than plastic packages at lower cost than ceramic pack-
ages. The steps for creating an organic package are shown in Fig. 10-7.
Organic laminate packages begin with the same epoxy fiberglass sheets
as PCBs (step #1). These are typically about 1 mm in thickness and plated
with copper. The sheets are individually drilled and the holes plated to
form vias. The copper sheets are then etched to leave copper only where
interconnect traces are needed (step #2). Sheets of prepreg are then inter-
leaved with these package core layers and laminated together. Afterward,
there may be another drilling step to create vias through all the layers
(step #3).
For some packages, all the metal and dielectric layers are formed this
way just as they are for a PCB. However, to achieve tighter wiring
pitches some packages add buildup layers. The use of these thin layers
and tight pitches is known as high-density interconnect (HDI) technology.
Spin-on dielectrics and sputtering of metals may be used, or resin coated