High Performance Printed Circuit Boards - Lecture #1
High Performance Printed Circuit Boards By Joseph Y. Lee Samsung Electro-Mechanics
Chapter - 1 High PerformancePrinted Circuit Boards - Introduction
Definitions Laminates – combination of a resin system and a reinforcement material. Glass transition temperature (Tg) – temperature such that resin behaving more like a gel or high viscosity liquid rather than a solid. Loss tangent or dissipation factors – power loss through the laminate material Epoxy - a thermosetting resin; used chiefly in strong adhesives and coatings and laminates
Cross section of FR-4 and CCL epoxy C u resinCC FR-4 glassL fibers void epoxy
Bismaleimide Tiazine (BT) Epoxy-Based Laminates Low cost Used in FR-4. Less water absorption compared to polyimide However, low Tg. Less tolerate of high temperatures. Tradeoffs- Difficulty in drilling causes microcracks Tradeoffs - brittle when lower % of resin
Table 1.3 Laminate Loss PropertiesLaminate Loss Tangent at 1MHzE-glass/epoxy 0.035E-glass/polyimide 0.035Aramid fiber (cross-plied)/cyanate ester 0.025E-glass/cyanate ester 0.015S-2 glass/cyanate ester 0.015Quartz/polymide 0.010E-glass/PTFE 0.001
PTFE (Teflon) Low dielectric constant and low loss tangent Nonpolar and hydrophobic E-glass widely use Costs cheaper than polyimide Tradeoffs – CTE problems caused plated-through holes failures z-axis expansion and contraction
Choice of Materials Considerations Cost Loss tangent must be low. CTE (Coefficient Temperature of Expansion) Tg (Glass Transition Temperature) Modulus of tensile strength Water absorption Dielectric constant – depends applications
Chapter – 2 Microvias, Built-UpMultilayers, and HighDensity Circuit Boards
Definitions Plated-through holes – connect together circuits on both sides of the board by means of a drill Multilayering – separate two-layer circuits could be laminated together and then connected to each other with plated-through holes Buried via – made by drilling and plating a two-sided board and then laminating into a multilayer Blind via – connects outer layer circuit to an interior layer without disturbing the layers below the interior layer Microvia – very small hole or via, generally by a non- mechanical means to connect two layers of circuitry.
Plated-through hole and buried vias Plated-through hole Buried via
Three Laser SystemsCO2 Fastest system Form vias in organic and glass reinforced dielectrics Cannot penetrate CuUV-YAG Slower Can penetrate CuExcimer Highest via resolution SlowestOverall – less stringent cleanliness requirements and no photomask is required. Cost is lower than photoprocess.
Sequential vs. Parallel Processes Sequential – circuit layers built up one at a time (laser process) Parallel – individual layers are separately made, then laminated all at once (B2IT process)Sequential yield = 0.95 * (0.92)4 = 68% yieldParallel yield = 0.95 * (0.92)2 = 80% yield
Microvia Much smaller Possible to pack much more functionality in smaller space More efficient circuit routing Less distortion to rise time Lower resistance by 17x Less signal delay at 100MHz and above
Three Processes for Microvia Via non-uniform,Both photovia and slow processlaservia have goodanistropic etching
Considerations Again, cost. Laser vs. Photo – depending on number of microvias Yield between photovia vs. B2IT Microvias is the way to go. Reduction of # of layers is required to reduce costs for microvias.
Chapter – 3Substrates for RF and Microwave Systems
What Are RF and Microwaves? They are analog signals or AC signals. Digital signals are 0’s and 1’s. 0 – 0 volt, 1 – 2 volts. Wavelengths between 0.1 to 30cm. Cell phones are hand held radios. They receive and transmit radio signals. Radio waves are electro-magnetic waves. An radio antennae is an inductor.
IC Chip – Parasitic Transmission Lines Passives on Printed Circuit Board used to correct reflectivity of parasitic transmission lines in IC chip. Why can we putEx. IC Chip with 4.5 km of metal lines in Pentium. the passives inMetal lines form parasitic transmission lines the IC Chip?
Via StructuresBlind pad Blind Buried Staggered Staircase Spiral Metal lines and vias embedded inside the PCB board do form parasitic transmission lines as well. Passives are used to correct this.
MicrostripAdvantage - Most widely used. Excellent integrationwith chip and lumped elements. Multilayers arepossible. Ground plane isolate the microstrip.Disadvantage - Line losses are somewhat higher withpoorer isolation between circuits. It is unshielded andsome radiation occurs for thicker substrates dependingon dielectric constant and frequency.
StriplineAdvantage - No fields extend outside ground plane. Workswell with multilayers and well-shielded.Disadvantage – It is susceptible to unwanted modes.Crosstalking may be a problem as well.
Coplanar WaveguideVery good integration with chip and lumped elements.Series and shunt connections are relatively easy.Disadvantage – thick substrates are required to keepstructure away from the chasis. Integration withmultilayers is poor.
Conductor-backed Coplanar WaveguideAdvantage – It offers less radiation loss than a puremicrostrip line. Connections can be made at the I/O’swith ground, signal, or ground probes.Disadvantages - It does require metal vias or plated-through holes to connect the coplanar grounds to thebackside ground. Structure is susceptible tounwanted parasitic modes.
Slotlines and Coplanar Strips Slotline Coplanar StripsEasy integration with chip and lump element.Combination with microstrip located on ground planeguarantee constant 180° phase shifts independent offrequency for certain circuits.However, they do not integrate well with multilayers.
Definitions – LTCC and HTCC LTCC (Low Temperature Co-fired Ceramic) – a multilayer ceramic technology, which processes the ability to embed the passive elements, such as resistors, capacitors and inductors into a ceramic interconnect package while the active elements are mounted on the top layer. HTCC (High Temperature Co-fired Ceramic) – differs from LTCC by high temperature of 1600°C while LTCC uses a temperature of 850 to 950°C.
LTCC vs. HTCC HTCC er=10 while LTCC er=4 to 8. tanδ HTCC < tanδ LTCC Resistivity of refractory metals like W and Mo for HTCC > resistivity of Ag metals for LTCC HTCC is made up of alumina particles while LTCC is made up of glass with some alumina. Firing oven heating, HTCC temperature > LTCC temperature Time to heat HTCC > Time to heat LTCC
General Design Recommendations Minimize # of conductor layers Use the widest possible lines and spaces. Avoid using tracks that are not 0 or 90° angles. Amount of metal should be evenly distributed.
Organic PWB’s FR-4 is the most easily manufactured laminate material. Thick metal ground plane provide high heat flow. Lower costs compared to ceramics. Dielectric constant and dissipation factors, same as ceramics if not better.