Printed Circuit Board

Only available on StudyMode
  • Topic: Printed circuit board, Copper, Solder
  • Pages : 16 (2376 words )
  • Download(s) : 37
  • Published : August 23, 2011
Open Document
Text Preview
PCBs/Overview
– Printed Circuit Boards (PCB)
• Introduction • Conductors. Supply Planes. Dielectric. • Vias • PCB Manufacturing Process • Electronic Assembly Manufacturing Process

29/09/2005

EE6471 (KR)

263

PCBs/Overview
– For electronic assemblies PCBs are substrates providing mechanical support as well as electrical interconnect – PCB: Rigid or flexible substrate with single or multiple layers of conductors separated by insulating layers Note: PCBs are sometimes also referred to as PWBs (Printed Wiring Boards) 29/09/2005 EE6471 (KR) 264

PCBs/General
– PCB Origin: United States – Therefore: Non-SI units (oz, mil, in) have been universally adopted for specifying PCBs: • Board dimensions in inches
(1in = 25.4mm)

• Dielectric thicknesses and conductor widths/spacing in mil (1mil = 0.001in = 25.4µm)

• Conductor (commonly copper) thicknesses in ounces (oz)
The weight of conductor metal in a square foot of material. Typical copper thicknesses are: 0.5oz (17.5µm), 1oz (35µm), 2oz (70µm), 3oz (105µm) µ µ µ µ

29/09/2005

EE6471 (KR)

265

PCBs/General
– PCB General Dimensional Specifications:
• Finished thicknesses
– Standard: 31mil, 39mil and 62mil (0.8mm, 1.0mm and 1.6mm) – Non-standard: » Readily available for high-volume orders » Board thicknesses: 10mil-125mil (many PCB manufacturers stop at 20mil - depending on plating finish)

• Maximum dimensions typically 16in x 20in • Irregular shapes/slots etc readily available (routing)

29/09/2005

EE6471 (KR)

266

Prepreg Core

PCBs/Stackup
Shown: Cross-section of a typical 8layer PCB Stackup

Prepreg Core Prepreg Core

– Typical PCB Stackup:

Prepreg

• Alternating layers of core and prepreg • Core: Thin piece of dielectric with copper foil bonded to both sides. Core dielectric is cured fiberglass-epoxy resin • Prepreg: Uncured fiberglass-epoxy resin. Prepreg will cure (i.e. harden) when heated and pressed • Outermost layers are prepreg with copper foil bonded to the outside (surface foils) • To avoid crosstalk: Wires on adjacent signal layers are routed mostly orthogonally • Stackup is symmetric about the center of the board in the vertical axis to avoid mechanical stress in the board under thermal cycling 29/09/2005 EE6471 (KR) 267

PCBs/Conductors
– Conductor:
• Material: Typically Cu • Number of layers:
– Single or multilayer (up to 20 layers, and more) – Dedicated supply layers (also called “ground layers”, “ground planes”) – Most popular: 4-8 signal layers plus 4-8 ground layers

• Material dimensions:
– Thicknesses: 0.5oz-3oz typically. 0.5oz/1oz standard for inner layers. – Trend: towards 0.25oz (particularly for laminated IC packages) – Width and spacing: ≥ 5mil

29/09/2005

EE6471 (KR)

268

Prepreg Core

PCBs/Stackup
Shown: Cross-section of a typical 8layer PCB Stackup

Prepreg Core Prepreg Core Prepreg

– Power Planes:
• Power planes are typically built on thinnest core available from a fabrication vendor to maximize the capacitance between the planes • Power planes often use thicker copper layers than signal layers to reduce resistance

– Why power planes?
• Provide stable reference voltages for signals • Distribute power to all devices • Control cross-talk between signals 29/09/2005 EE6471 (KR) 269

PCBs/Conductors
– PCB sheet resistances
• Cu resistivity ρ=1.7*10-8Ωm • Remember: Sheet resistance… PCB Sheet Resistances at T=300K
(TC Copper: +3930ppm) Conductor PCB Copper Track (0.5oz Cu) PCB Copper Track (1oz Cu) PCB Copper Track (2oz Cu) PCB Copper Track (3oz Cu) Rs in µΩ 971 486

h W

L

L = R= = Rs h ⋅W W A
Compare to Semiconductor Rs figures:
Sheet Resistances
Material Metal (Aluminium) (top layer) Metal (Aluminium) (lower layers) Polysilicon (silicided) Diffusion (n+, p+, silicided) Polysilicon (doped) Diffusion (n+, p+) Rs Ω 0.05 0.1 6 10 30 100 5k several k several Meg in

ρ ⋅L

ρ⋅L

243 162

n-well Nichrome Mixed signal IC resistor material....
tracking img