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

Chapter 18Fundamentals of Packaging

Materials and ProcessesJason 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.


• 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.


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.


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).


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.


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.


Coefficient of Thermal Expansion• Dimensional change that occurs during

heating or cooling of a material is characterized by its coefficient of thermal expansion (CTE).


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).


Glass transition temp. phenomena in polymers.


Mechanical PropertiesMaterials in electronic system packages

are always subjected to large forcesForces 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.


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.


Surface Tension and WettingAll 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.


AdhesionAdhesion 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 interfaceIncreased 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.


CeramicCeramic 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.

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 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 insulatorsWidespread 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 sidesSurface 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 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

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


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


Chemical Vapor Deposition (CVD)Process in which chemicals in vapor phase

react to form a solid film on a surface


Solution Based: PhysicalSpin coating: Thin-film is obtained by

rotating the substrate at a high speed. Yields thicknesses from 2-20 microns.


Solution Based: PhysicalMeniscus 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 tubeDip Coating-involves the vertical motion of

the substrate after being dipped in a reservoir


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


Solution Based: ChemicalElectroless 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


Solution Based ChemicalElectroplating-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

InterconnectionsLead is highly toxicStrong 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


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


InterconnectionsWhat other metals?

Have to consider many aspects: Melting temperature Health risks Wettability Mechanical strength


Interconnections Cont.For low cost electronic assembly, research

has narrowed down to few binary eutectic alloys


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


Non-conductive adhesive:Concept is relatively newAdhesive does not by itself contribute to the

electrical conduction. The contact area has a metallic surface which,

permits conduction by electron-tunneling


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


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


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??

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

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