BioSciences - Polysciences, Inc.

Electronic Adhesives & Encapsulants
High Performance / Electronic Materials Glossary of Terms
ALPHA-1 & ALPHA-2 (a1 & a2): Alpha 1 is the Coefficient
of Thermal Expansion (see also CTE) below the glass transition
temperature (Tg). Alpha 2 is the CTE above the Tg.
ALPHA EMISSIONS: Are radioactive, high energy particles
that can damage semiconductor devices. Memory devices are
especially sensitive to alpha emission because the passing of the
alpha particle through the device can cause the memory to change
its state from a 1 to a 0 or a 0 to a 1. The device does not fail,
but because the memory now has the wrong value stored, it is
called a soft failure. To avoid such failures, memory devices use
only “low alpha” solder. Low alpha solder contains Lead which has
less radioactivity than normal Lead. Because this low alpha Lead is
scarce, it is more expensive.
ANISOTROPIC: material that has different values when measured
in different directions. Some methods of die connections use
anisotropic conductors, which is a suspension of metal particles in
an adhesive tape or paste. The metal conducts through the tape/
paste (vertically), but not in the plane of the tape (horizontally).
The products are commonly called ACF for film and ACP for paste.
The ACF or ACP is placed on the substrate and the die to be
connected is flipped upside down and pushed into the paste to
make the connections between the chip pads and the substrate
conductors. Sometimes the pads have a gold bump on them to
improve the connection.
BLEEDING: Die Attach adhesives will leak out from under the die
before it is cured. This leakage is called “bleed”. Since wire bonds
will not stick in areas where bleed has occurred it is important
that bleed not exceed a few mils. Bleeding can be controlled
by formulation (filler particle size, resin/hardener selection) and
process (cure temp, out time). In addition, material will bleed
differently on different surfaces (Silver, Nickel, Palladium, solder
mask, polyimide).
CAPILLARY ACTION: The action by which the surface of a liquid
in contact with the walls of a capillary tube or between two closely
spaced plates is drawn into the tube or between the plates by
a wicking action. An example of capillary action is in Flip Chip
assembly, the underfill is dispensed next to the die and the material
is pulled into the gap by the force of capillary action.
CATALYST: A chemical that when added to other materials starts
or speeds up a chemical reaction. The catalyst is not consumed in
the reaction, it only acts to make the reaction take place.
CONDUCTIVITY, ELECTRICAL: How well a material allows
electricity to flow through it. The inverse of conductivity is
resistivity. Most polymers do not conduct electricity, so they have
very low conductivity. To make them conduct electricity we add
metal particles, usually flakes of Silver. In order for the combined
polymer and metal to conduct electricity, the particles must be
close enough to touch.
Table of Electrical Conductivity
Material Resistivity
(Ohm-cm)
10 E-6
Copper (Cu) 1.5
Silver (Ag) 1.5
Gold (Au) 2.0
Aluminum (Al) 2.4
Solder (60/40) 1.5
Die Attach Epoxy 100 to 400
(.0001 to .0004 ohm-cm)
CONDUCTIVITY, THERMAL: How well a material allows heat
to flow through it. Most metals have good thermal conductivity
but ceramic and polymers have poor thermal conductivity. In order
to increase the thermal conductivity of polymers, they need to
be filled with a material that conducts heat well. This can be a
metal (cheapest and best way) but adding a metal will also make
the polymer electrically conductive. To achieve improved thermal
conductivity while allowing the polymer to remain an insulator,
ceramic powders such as Alumina or Aluminum Nitride are added.
The unit of measure of thermal conductivity is Watt/meter-Kelvin.
Table of Thermal Conductivity
Material Thermal Conductivity
(Watts/meter-Kelvin)
Diamond (C) 600
Silver (Ag) 410
Copper (Cu) 390
Gold (Au) 300
Aluminum (Al) 240
Alumina 40
Solder (60/40) 50
Silica 4
Typical Die Attach 1.5 to 2.0
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