Transcript coefficient of thermal expansion (CTE)

Chapter 18
Fundamentals of Packaging
Materials and Processes
Jason Mucilli
Vincent Wu
18.1 Role of Materials in
Microsystems Packaging
 Materials provide several functions in
microelectronic packaging.
 It transmit signals from IC to IC, supply power
to ICs, provide interconnections to form the
system-level hierarchy, mechanically and
environmentally protect Ics, and dissipate heat.
18.1 Role of Materials in
Microsystems Packaging cont.
18.1 Role of Materials in
Microsystems Packaging cont.
• Integrated Circuit Packaging
 Packaging of an integrated circuit (IC) provides electrical
connections to the rest of the components by means of a
systems-level board.
 Ceramics provides thermo-mechanical reliability
 Polymers perform better electrically than ceramics because of
the low dielectric constant, except for applications where ultralow loss is required.
18.1 Role of Materials in
Microsystems Packaging cont.
 IC Assembly
• The electrical interconnections between the chip
and package are provided by metal wirebonding
techniques.
• The conducting wire should have a high
electrical conductivity, oxidation resistance, and
good wetting to the bonding pads and
mechanical properties to withstand creep and
fatigue.
• Wirebonding needs any two of the three
conditions that assist joining: heat, compression
or ultrasonic vibration.
18.1 Role of Materials in
Microsystems Packaging cont.
System – Level Packaging
• System-level packaging provides wiring
that forms an electrical interconnection for
all components within the system.
• The organic substrate that provides these
functions is called a printed wiring board
(PWB).
18.1 Role of Materials in
Microsystems Packaging cont.
 System – Level Packaging cont.
• Surface mount technology (SMT) interconnections
are achieved by soldering, with the most common
soldering compound being an eutectic Pb-Sn alloy
with a melting point of 183C.
• A huge coefficient of thermal expansion (CTE)
mismatch between the PWB and IC induces
significant stresses that cause failure at the solder
joints.
18.2 Packaging materials and
properties
 The properties relevant to packaging are electrical and
thermal conductivity, coefficient of thermal expansion,
electrical permittivity, polymer glass transition
temperature and Young’s modulus.
 These properties are determined by the lattice or
molecular structure, the atoms that constitute the
lattice and their interactions, and the extrinsic effects
such as impurities. No single material has the required
combination of properties.
18.2 Packaging materials and
properties
 Conductivity
• Electric field is applied onto a conductor, the
electrons drift towards the positive potential,
resulting in a current.
• Electrical conductivity is the ratio of current
density and the applied electric field
• Most covalent and ionic solids are insulators,
whereas metals are good conductors.
Semiconductors form an intermediate group
between these two.
18.2 Packaging materials and
properties cont.
 Electrical conductivity is limited by the collisions
between ‘‘electrons’’ and ‘‘imperfections’’ in the lattice
of the conductor. These collisions will cause the
electrons to lose their energy and momentum.
 Joule heating manifests as an electrical resistance
 The resistance in almost all metals increases with
temperature.
18.2 Packaging materials and
properties cont.
Thermal Conductivity
• The amount of heat transferred through a
material per unit of time, denoted as heat
flux Q, is proportional to the temperature
gradient (dT/dx).
• The Ratio of heat flux and temperature
gradient is called thermal conductivity.
18.2 Packaging materials and
properties cont.
Coefficient of Thermal Expansion
• Dimensional change that occurs during
heating or cooling of a material is
characterized by its coefficient of thermal
expansion (CTE).
18.2 Packaging materials and
properties cont.
 Glass Transition Temperature
• It is characterizes the transition of an amorphous
material from a brittle state to a rubbery state.
• Glass transition is manifested by drastic changes
in many of material’s physical properties such as
volume and modulus.
• Glass transition temp. is characterized from
thermochemical analysis (TMA) and dynamic
mechanical analysis (DMA).
18.2 Packaging materials and
properties cont.
Glass transition temp.
phenomena in polymers.
18.2 Packaging materials and
properties cont.
Mechanical Properties
 Materials in electronic system packages
are always subjected to large forces
 Forces may be caused by flexure and
impact during fabrication or actual use, or
from the internal thermal gradients and
differential expansion properties at the
interface with other materials.
18.2 Packaging materials and
properties cont.
 Young’s Modulus
 Materials deform in response to an applied force.
 Deformation may be permanent or temporary, time
dependent or time independent, and is classified
accordingly.
 Force deformation relationships are expressed in
terms of stresses and strains.
18.2 Packaging materials and
properties cont.
18.2 Packaging materials and
properties cont.
 Surface Tension and Wetting
All materials in the solid or liquid state have
energy associated with their surfaces.
Energy arises from the unsaturated bonds on
the surface.
Energy depends on the surface characteristics
or the material
Degree of wetting by the molten solder will
depend on the relative magnitudes of the
surface energies for the solder and the
substrate metallization.
18.2 Packaging materials and
properties cont.
 Adhesion
Adhesion between dissimilar surfaces such as
metals/polymers or ceramic/polymers is
generally caused by weak chemical forces
Metals and polymers are typically roughened in
order to increase their adhesion
Interaction has two contributions:
Increased thermodynamic work of adhesion, resulting
from large exothermic reactions at the interface
Increased tensile strength, resulting from electrical
charge injection into the polymer from the substrate.
18.3 Materials Processing
 Main Processes used to make the single-chip
packages or multichip or multilayered
substrates.
 Thin-film, processes are used to build the
subsequent dielectric layers, conductor and
passive patterns.
18.3 Materials Processing cont.
18.3 Materials Processing cont.
Ceramic
 Ceramic are generally regarded as highperformance materials because of their
hermiticity, high reliability, low CTE and low
losses
 Single-chip ceramic packaging exists in
various forms
 dual-in-line packages (DIPS), chips carriers, flat
packs and pin grid arrays.
18.3 Materials Processing cont.
Thick Film Screen Printing
 A widely used thick-film process for applying
films of pastes on a substrate
 Alumina is used for high temperature thick film
hybrid technology
 Thick-film pastes can be ceramic or polymerbased
 Ceramic pastes are made up of active particles in
a matrix of glass particles, organic filler materials
and solvents.
 Polymer pastes are cured at a lower temperature
and aren’t stable at higher temperatures
Thick Film Screen Printing cont.
 Key components to the screen printing
process:
 The Screen: a mask with openings at locations
where paste is to be dispensed
 Solder paste: applied to the top surface of the
screen
 The Squeegee: a rubber blade that travels along
the screen pushing paste through the openings
 The Board is held in place by a suitable fixture
Organic Thick Film
Organic materials make for excellent
insulators
 Widespread use in electronics because of
their low cost, good dielectric properties,
reasonable mechanical properties and
ease of processing
Organic Thick Film Cont.
Common organic materials
Organic Thick Film Cont.
 PWB-used for system-level and multichip
packages.
 Starting material consists of laminated layers of
binder and reinforcement
 A common binder is epoxy
 Common reinforcements are woven glass fibers and
paper
 FR-4 is a glass/epoxy laminate and is the most
common PWB today
 Low stiffness, and high coefficient of thermal expansion
 Not suitable for future applications involving multilayered
thin-film structures and direct-chip attach
PWB Processes
 Simplest has only one layer of copper metal
foil for conductors on one side of the board
 Conductor patterns are formed by lithography,
using screen-printed resist or UV exposure
 Referred to as “print and etch”
Woven Glass fiber for
PWB reinforcement
PWB Processes Cont.
2-sided boards have copper conductor
patterns on both sides
 Surface mounted components are mounted
on one side and hole-mounted components
are mounted on the other with leads
passing through the vias.
PWB Processes Cont.
 Multi-layered boards are most complex
version of PWB packaging
 Conductor patterns are defined on each laminated
layer and the interconnections are obtained with
vias
 Epoxy of one board has to adhere well to the
copper of the other board. In order for this to
occur, the copper is roughened using a micro-etch
process
 Drilling often causes the epoxy to soften due to
frictional heating and creates an insulating layer on
the walls of the holes
 The smeared insulating layer is etched with plasma or
strong oxidizers to combat this
Thin-Film Processes
 Increased integration demands more layers on thickfilm technologies
 Thick film offers limited wiring density
 Thus their ability to package highly integrated, high speed
chips is limited
 Led to the development of thin-film packages where lines
are made of conductive metals
 A combination of the two technologies has provided
more design flexibility
Thin-Film Processes Cont.
Physical Vapor Deposition (PVD)
 Vacuum Evaporation-deposition takes
place in a vacuum because
 Increase the mean free path of the evaporate
particles
 Reduce the vapor pressure
 Remove atmosphere and other contaminants
Thin-Film Processes Cont.
Physical Vapor Depositon (PVD)
 Sputtering-low pressure process where a
target is bombarded with energetic positive
ions. When the ions hit, particles are
ejected from the target and hit the substrate
that is to be covered.
 The target material is torn off by the energy
released and it deposits on the substrate
 Typical deposition rate is 100-1000 angstroms/min
Thin-Film Processes Cont.
Chemical Vapor Deposition (CVD)
 Process in which chemicals in vapor phase
react to form a solid film on a surface
Thin-Film Processes Cont.
Solution Based: Physical
 Spin coating: Thin-film is obtained by
rotating the substrate at a high speed.
Yields thicknesses from 2-20 microns.
Thin-Film Processes Cont.
Solution Based: Physical
 Meniscus Coating-a liquid polymer solution
is pumped out of a narrow slit on the top of
a tube over which the substrate slides.
 Material may be collected under the tube and
re-circulated into the center of the tube
 Dip Coating-involves the vertical motion of
the substrate after being dipped in a
reservoir
Thin-Film Processes Cont.
 Solution Based: Chemical
 Sol-Gel Deposition-allows for the deposition of
films with a high degree of chemical homogeneity
at relatively low temperatures
 Hydrothermal Deposition-involves the dissolution
of reactants and precipitation of products in hot,
pressurized water.
 A Standard technique used to form fine powders with
superior physical and chemical properties
Thin-Film Processes Cont.
Solution Based: Chemical
 Electroless plating-is a metal deposition
process, usually in an aqueous solution
medium, which proceeds by a chemical
exchange reaction between the metal
complexes in the solution and the particular
metal to be coated
 DOES NOT require external current
Thin-Film Processes Cont.
Solution Based Chemical
 Electroplating-process of depositing an
adherent metallic coating onto a conductive
object immersed in an electrolytic bath
composed of a solution of the salt of the
metal to be plated
 Depositon occurs by passing DC current
through the electrolyte
 Cheap and low temperature process
Photolithography
 SINGLE MOST IMPORTANT process
enabling the semiconductor and electronic
industry
 Used for transfer and definition of fine patterns that
are not amenable by screen printing
 Process is generated on CAD and is then
transferred onto photographic film (photomask)
 Photoresist- thin photosensitive material-used for
transferring the pattern
 The mask is then aligned with respect to the prior
patterning on the substrate
Photolithography Cont.
 Classified as negative or positive depending
on whether light initiates cross-linking in the
polymer making the illuminated portion difficult
to dissolve in the developer (negative resist)
or light breaks the molecules, making the
illuminated portion easier to dissolve in the
developer (positive resist)
Summary and Future Trends
Interconnections
 Lead is highly toxic
 Strong drive to replace lead in solders with
other elements and yet retain its
advantages
 2 approaches to lead free solders:
 Lead free metallic solders
 Conductive polymers
Summary and Future Trends
 Interconnections Cont.
 Rely on tin as base metal
 Tin
 considered one of least toxic metals, relatively
inexpensive, sufficiently available and has desirable
physical properties
 Interacts very strongly with a wide range of metals, forming
strong bonds.
 Tin by itself is unacceptable because it whiskers,
migrates under e-fields, has a high melting
temperature and forms brittle grain structure at
cold temperatures
Summary and Future Trends
Interconnections
 What other metals?
 Have to consider many aspects:




Melting temperature
Health risks
Wettability
Mechanical strength
Summary and Future Trends
Interconnections Cont.
 For low cost electronic assembly, research
has narrowed down to few binary eutectic
alloys
Summary and Future Trends
Organic based electrical interconnections:
 Polymers:
 Generally non-conductive
 Low die stress because of low modulus of the
adhesives compared to solders and low
processing temperature
Summary and Future Trends
Non-conductive adhesive:
 Concept is relatively new
 Adhesive does not by itself contribute to the
electrical conduction.
 The contact area has a metallic surface which,
permits conduction by electron-tunneling
Summary and Future Trends
 Anisotropic Conductive Adhesive (ACA) Adhesive consisting of conductive particles
dispersed in an adhesive matrix.
 Low processing temperature: Mostly used to
attach LCD display drivers since solder reflow
temperatures would destroy the LCD
 Isotropic Conductive Adhesives (ICA) ICA is an epoxy filled with silver particles
 The adhesive is conductive in all directions, and
much care must be taken to avoid short-circuiting
between neighboring pads.
 Limitations
 High initial contact resistance, unstable contact
resistance and inferior impact strength
Summary and Future Trends
 Low Dielectric Constant Dielectrics
 Fluorinated polyimides- possess good planarizing
capabilities but have several disadvantages:
 moisture absorption, low break down potential, increased
leakage currents, poor adhesion and corrosion of metal
components.
 MSK and carbon-doped silicon dioxide-provide the
thermal stability and strength of inorganic materials
Summary and Future Trends
 Underfill Materials
 Successful no-flow underfill material should meet
the following requirements:
 Minimal curing reaction at temperatures below the solder
reflow temperature
 Rapid curing reaction after maximum solder bump reflow
temperature
 Good adhesion of underfill to chip
 Lower shrinkage of the material during curing, lower CTE
and reasonable modulus to minimize the thermal stress
from the curing process
 Self-fluxing capability, passivating the substrate
conductor oxides prior to the solder reflow
Questions??