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Chapter 18 Fundamentals of Packaging Materials and Processes

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Chapter 18 Fundamentals of Packaging Materials and Processes Jason Mucilli Vincent Wu Thick Film Screen Printing A widely used thick-film process for applying films ... – PowerPoint PPT presentation

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Title: Chapter 18 Fundamentals of Packaging Materials and Processes


1
Chapter 18 Fundamentals of Packaging Materials
and Processes
  • Jason Mucilli
  • Vincent Wu

2
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.

3
18.1 Role of Materials in Microsystems Packaging
cont.
4
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 ultra-low loss is
    required.

5
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6
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.

7
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).

8
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.

9
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 Youngs
    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.

10
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.

11
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.

12
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.

13
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).

14
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 materials physical properties such as
    volume and modulus.
  • Glass transition temp. is characterized from
    thermochemical analysis (TMA) and dynamic
    mechanical analysis (DMA).

15
18.2 Packaging materials and properties cont.
Glass transition temp. phenomena in polymers.
16
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.

17
18.2 Packaging materials and properties cont.
  • Youngs 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
18.2 Packaging materials and properties cont.
19
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.

20
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.

21
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.

22
18.3 Materials Processing cont.
23
18.3 Materials Processing cont.
  • Ceramic
  • Ceramic are generally regarded as
    high-performance 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.

24
18.3 Materials Processing cont.
25
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 polymer-based
  • 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 arent stable at higher temperatures

26
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

27
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

28
Organic Thick Film Cont.
  • Common organic materials

29
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

30
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
31
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.

32
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

33
Thin-Film Processes
  • Increased integration demands more layers on
    thick-film 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

34
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

35
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

36
Thin-Film Processes Cont.
  • Chemical Vapor Deposition (CVD)
  • Process in which chemicals in vapor phase react
    to form a solid film on a surface

37
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.

38
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

39
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

40
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

41
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

42
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

43
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)

44
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

45
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

46
Summary and Future Trends
  • Interconnections
  • What other metals?
  • Have to consider many aspects
  • Melting temperature
  • Health risks
  • Wettability
  • Mechanical strength

47
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48
Summary and Future Trends
  • Interconnections Cont.
  • For low cost electronic assembly, research has
    narrowed down to few binary eutectic alloys

49
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

50
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

51
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

52
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

53
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

54
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