High Performance Printed Circuit Boards - Lecture #2
High Performance Printed Circuit Boards By Joseph Y. Lee Samsung Electro-Mechanics
Chapter - 4Advanced Ceramic Substrates for Microelectronic Systems
Desired Substrate Properties High electrical resistivity High thermal conductivity Resistance to temperature Inert to chemical corrosion Cost
Selected CeramicsTable 4.1 Melting Point of Selected CeramicsMaterial Melting Point °CSiC 2700BN 2732AlN 2232BeO 2570Al3O2 2000
Substrate Fabrication Roll compaction; large parallel rollers to form sheet. Tape casting; moving belt that flows under a knife edge. Powder pressing; powder into hard die cavity with high pressure through sintering process. Isostatic powder pressing; flexible die surrounded with water or glycerin and pressed up to 10,000 lb/in2 Extrusion; forced through a die. Very economical and produces thinner part.
Definitions Surface roughness; measure of the surface microstructure Camber; measure of the deviation from flatness (red line). deviation
Substrates Rules: Thicker substrates – less camber Squares are better than rectangles Pressed methods are better.
Thermal Properties Thermal conductivity (W/m-°C): q = -k (dT/dx) Specific heat (W-s/g-°C): c = dQ/dT Temp. coeff. of expansion (ppm/°C): α= [L 2) – L 1)] /[L 1)(T –T)] (T (T (T 2 1
Mechanical Properties Modulus of elasticity: E = TCE * ∆T Modulus of rapture (MPa): σ = Mx/I This can be used to measure tensile strength of thinned Si wafers. Maximum stress at the tip of crack: SM = 2 So < (a / ρt)0.5 Ratio of max. stress to applied stress: Kt = SM/S0 = 2 (a / ρt)0.5 Plain strain fracture toughness: KIC = Z Sc (pi * a)0.5 Critical force to cause breakage: Sc = KIC / [Z (pi * a)0.5]
Metallization of Ceramic Substrates Thick film – additive process by which conductive, resistive, and dielectric patterns in the form of a viscous paste are screen printed, dried, and fired onto a ceramic substrate at an elevated temperature to promote adhesion of the film Thin film – subtractive process such that entire substrate is coated with several layers of metallization and unwanted material is etched away in a succession of selective photoetching processes. Copper 1) Direct bond copper 2) Plated copper 3) Active metal braze
Example of Thick Film Screen printing leaves metallic conductor, metal oxide resistor or dielectric insulator. Active element powders range from 1 to 10 µm in size. Adhesion element (glass and metal oxides) are used to bond active element on substrate. Organic binder is used to hold active and adhesion elements. Solvent is used to dilute thick organic binder.
Thin FilmsSeveral layers of metal are formed by vacuum deposition techniques –sputtering or evaporation, or by electro-plating. One example method isDC sputtering as shown in figure.
Copper Metallization Direct bond copper: bonded to alumina ceramic by placing Cu film heating to a temperature of 1065°C. Plated copper: Cu may be vacuum deposited by thin-film methods, screen printed by thick-film processes, or deposited by aid of a catalyst. Patterns formed by dry or wet photolithographic process. Active metal braze: one or more metals in the IV- B column are used. Braze formed of paste, powder, or a film. Ex. 70% Ti/15% Cu/15% Ni.
Substrate Materials Al2O3 – most common ceramic BeO – high thermal conductivity AlN – TCE matched with Si, high thermal conductivity Diamond – low specific heat BN – easily machinable, low TCE SiC – resistant to acids and bases, high thermal conductivity, low TCE
Composite Materials AlSiC – conductive with low TCE, softer than SiC, thermal conductivity 12 times greater than SiC Dymalloy – diamond with Cu20%/Ag80% alloy, melts at 800°C lower than CuComposite materials is combination of metal and ceramics.
Multilayer Substrates Structures Thick films – limited to 3 layers Thin films – expensive Cu – single layersTypes of Multilayer substrates- HTCC- LTCC- AlN
Chapter – 5 Flexible Printed CircuitsThey are printed circuits that are fabricated on a thin flexible-based material and are usually constructed with a non-reinforced polymeric material.
Traditional flexible circuits Traditional flexible circuits #Single side #Double side #Multi-layer rigid/flex with MIL #Multi-layer rigid/flex for consumer (#Multi-layer flex) #Flex with stiffeners
Fluorocarbons Unmatched chemical inertness High thermal resistance Outstanding dielectric properties Tough mechanical properties Plated through-hole process difficult Electroless Cu do not adhere well Dimensional stability not as good as polyimide film
PET (Polyethylene terephthalate) Low cost compared to polyimide films Higher insulation resistance Greater tear strength Lower dielectric constant Lower moisture absorption Better dimensional stability Lower lamination temperature Melts below soldering temperature Difficult to bond to other laminates
PEN (Polyethylene Naphthalate) Tg = 120°C. Can withstand soldering temperature of 260°C for exposure of 5 to 10 seconds. Reaction product of 2,6 naphthalate dicarboxylate with ethylene glycol
Polyimide Ability to withstand heat of manual and automatic soldering Excellent thermal resistance Good insulators and high voltage barriers Absorb moisture Weak link in the polyimide laminate
Aramids Low initiating and propagation tear strength Dielectric constant of 1.6 to 2 Dissipation factor of 0.0015 Good dimensional stability Absorbs water or else it blisters seriously
Copper foils Electrodeposited copper foils Starts with Cu solution being plated at very high deposition rates. Rolled anneal copper Starts with Cu ingot and that are hot rolled to an intermediate gauge Good flexural endurance Resistance to fracturing in dynamic applications Low temperature anneal foils Better flexural properties compared to RAC. Better handling higher yield strength, resist denting and crazing Resist foil damage
5 Distinct Layers Base dielectric film Conductor layer Cover-coat film Two layers of adhesives Accounts of 50% of total flexural thickness
Adhesives- Mixtures of solvents, polymers, and curing agents Must be tack-free Must be thin – 0.001 to 0.002 inch Must have low residual volatile content Must be resistant to attack by chemicals Thermosetting to some degree Cured in reasonable time Limited shelf-life must be monitored
Types of Adhesives Acrylic adhesives High heat resistance and good electrical properties Dimensional stability and small hole drilling decrease yield. Polyimide and epoxy adhesives Better heat resistance and electrical properties as good as acrylics Better dimensional stability, better process, lower overall thickness Increased laminate stiffness Polyester and phenolics Cost effective Good electrical properties, flexibility, fair heat resistance Butryal phenolics Better heat resistance than polyester Electrical properties poorer though, not as flexible
Problems with Adhesives Adhesives become insulator rather than insulator film Lacking in resistance delamination Poor moisture absorption Poor resistance to heat aging, discoloration, embrittlement, outgasing Poor insulation at elevated temperature
Adhesiveless Films Polyimide resin and Cu metal devoid of any nonpolyimide adhesive Thinner – 4 mils of savings Better thermal conductivity Greater flexibility and thinner circuits Thermal stress resistance of higher layer count rigid flexes is better as well.
Classification of Adhesiveless Cast to foil Involves casting a liquid solution of polyamic acid onto surface of metal foil Vapor deposition on film Cu is vaporized in a vacuum chamber and the metal vapors are deposited on polyimide film Limited to a Cu thickness of 2µm. Sputter to film (vacuum chamber with Cu cathode) Base metal undercoating of Cr, Ni, oxide Slower process Plated on film (electroless metal chemistries) Do not induce any thermal stresses Much easier to handle
Cover Coat Protects circuit against corrosion and contamination Affords protection against mechanical damage Puts Cu on a single-sided flex on the neutral bending axis Helps anchor the terminal pads to the base dielectric