Introduction to Set Top Boxes
A set top box (STB) is an electronic device that converts digital signals into audio-video content that can be displayed on a ordinary analog television set. Set top boxes are used to receive and decode broadcast signals from sources like cable TV, satellite TV, terrestrial TV, IPTV etc. They essentially act as a mediator between the broadcast provider and the user’s TV.
The major components of a set top box include:
- Printed Circuit Board (PCB)
- TV tuner
- AV outputs to connect to TV
- Storage such as RAM, ROM
- multimedia processors
- Operating system such as Linux
- Power supply
- Casing/housing
The PCB is the heart of an STB which holds all the major electronics. STB PCB design, manufacturing process and cost play a crucial role in the overall set top box development.
Design Considerations for STB PCB
Designing a PCB for set top boxes requires attention to many factors:
Functional Requirements
The STB PCB must support required functions like:
- Tuning digital broadcast signals
- Demodulating and decoding video and audio
- Graphics processing and overlay
- AV output generation
- Memory interfacing
- Connectivity for smart features
Supported Features
Advanced features like digital video recording, multimedia playback, web browsing etc. demand additional circuitry.
Performance Specifications
Specifications like frequency range, video resolution, processing power, network bandwidth affect component selection.
Size Constraints
STB PCB sizes are typically 6 to 8 inches but can be larger. Compact multilayer boards allow fitting more functions.
Cost Targets
Rigorous cost control is needed as STBs are mass produced consumer devices.
Reliability Requirements
STB PCBs must be reliable enough for permanent installation and use.
Regulatory Standards
STB design must incorporate safety, EMI and compliance considerations.
Manufacturability
Designing for ease of manufacturing and assembly optimizes cost and yield.
Block Diagram
A typical block diagram for a set top box PCB is shown below:
It consists of the following key blocks:
- Tuner & Demodulator: Receives broadcast signal and extracts video/audio channels
- MPEG Decoder: Decodes compressed digital multimedia data
- AV Output: Generates analog video & audio outputs for TV
- External Interfaces: Ethernet, USB, HDMI for connectivity
- Video Processor: Graphics engine for overlays and UI
- Storage: Flash, RAM for buffering data
- Microcontroller: Controls overall STB operation
- Power Supply: Generates required DC voltages
Key Components
The major electronic components in a typical STB design include:
Tuner
Tuners for modern STBs support both analog (terrestrial) and digital (DVB-T/T2, DVB-C, DVB-S/S2) television standards. Popular tuner chips are Rafael Micro R820T/R828D, Fitipower FC2580, MxL661.
Demodulator
Demodulators extract the modulation signal to recover the original video and audio data. ICs like Rafael Micro R850, Sony CXD2861ER handle demodulation.
MPEG Decoder
Decoder chips decompress the MPEG-2/MPEG-4 streams from the demodulator. Examples are Realtek RTD1295, Sigma Designs SMP8656.
AV Encoder
Audio/video encoders like Analog Devices ADV7183 encode the signals into analog CVBS, YPbPr, Audio L/R etc. for connecting to the TV.
External Interfaces
USB, Ethernet, HDMI etc. provide future expandability. Chips like Realtek RTL8211F (Ethernet) and Genesys Logic GL850G (USB 2.0) implement these.
Graphics Processor
Graphics processing units like Sigma Designs SMP8654 plus DDR3 memory drive the on-screen menus and overlays.
Microcontroller
A SoC like Broadcom BCM7356 runs the software stack and controls the STB. External Flash and SDRAM provide program and data storage.
Power Supply
AC input is converted to DC voltages like 12V, 5V, 3.3V using power supply ICs such asPulse Electronics PE-A109N.
PCB Design & Layout
The printed circuit board design for the STB involves:
Schematic Capture
The schematic shows the connectivity between all components. CAD software like Altium, Eagle, or OrCAD is used.
PCB Layout
The physical layout of traces, pads, vias, shapes on board layers is done. Constraints like high speed routing, controlled impedance, EMI reduction are considered.
Stackup Selection
Layer count (4 to 8), dielectric materials, copper thickness, and stackup sequence are optimized for cost and performance.
Component Placement
ICs, connectors, and other parts are intelligently placed to minimize track lengths, noise and crosstalk. Heat dissipation areas are allocated.
Routing
Connections between pins are routed on the appropriate layers. Area fill shapes provide ground/power planes.
Design Rule Checks
The physical and electrical constraints are verified through design rule checking before sign off.
PCB Technology Choices
Key PCB technology aspects for a set top box design are:
Layer Count
4 to 6 layer boards are commonly used. High end STBs may use 8 layers for more routing channels.
Board Thickness
1.6mm is typical. 2.4mm can provide better stiffness but increases cost.
Copper Thickness
1 oz copper foil allows cost effective board fabrication while providing adequate current capacity.
Minimum Trace/Space
6/6 mil lines and spaces yield optimal cost and reliability balance.
Finishes
Immersion silver provides excellent solderability and shelf life. ENIG is used for connecting to gold fingers.
Vias
Blind/buried vias minimize layers used while reducing cost over thru-hole vias.
Solder Mask
High quality solder mask with 150 micron thickness ensures protection against environmental exposure.
Silkscreen
Permanent white silkscreen allows clear identification and markings on both sides.
Lead-free
Lead-free assembly and RoHS compliance are mandated due to environmental regulations.
Impedance Control
For high speed HDMI, Ethernet, SATA traces, controlled 100Ω differential impedance is necessary.
HDI Capability
For denser designs, microvias and thinner dielectrics facilitate HDI PCB fabrication.
PCB Assembly
Set top box PCB assembly involves:
Solder Paste Printing
Solder paste is applied on pads using stencils. Solder paste inspection ensures print quality.
Component Placement
Parts are accurately placed using automated pick and place machines.
Reflow Soldering
A convection reflow oven heats the PCB to solder all joints simultaneously.
Wash & Dry
Any flux residue is washed off with deionized water. PCBs are then dried.
Conformal Coating
A protective acrylic coating may be applied for environmental protection.
Press Fit Connectors
For tuner/demodulator modules, press fit connectors provide reliable mechanical and electrical connectivity.
Testing & Inspection
Testing validates the board functionality. Automated optical inspection and x-ray inspection check for defects. Failed boards can be reworked before final configuration.
PCB Cost Structure
The manufacturing cost of set top box PCBs depends on:
Board Complexity
Layer count, component density, fine features and tight tolerances increase fabrication costs.
Board Size
Larger PCB sizes reduce cost per area but increase raw material utilization.
Quantity
High volume orders bring down cost through better utilization of fixed costs.
Testing
Rigorous testing and inspection adds cost but ensures quality.
Lead Time
Standard lead times around 2-3 weeks are more economical than rush orders.
Location
Manufacturing region impacts labor, logistics and regulatory costs.
Cost Comparison
A sample cost estimate for a typical 6 layer STB PCB with 300mm x 200mm size in quantities around 5000 pieces per month:
| Region | PCB Cost per board |
|---|---|
| USA | $28 – $38 |
| Europe | €22 – €32 |
| China | $13 – $19 |
| Taiwan | $16 – $23 |
| Southeast Asia | $15 – $21 |
The China and Southeast Asia options provide the best value while also offering advanced technology and reliable quality.
Key PCB Manufacturers
Some leading suppliers with expertise in STB PCB manufacturing include:
- Rayming Technology – China
- Shengyi Technology – China
- Advanced Circuits – US
- Eurocircuits – Europe
- NCAB Group – Sweden
- Unimicron – Taiwan
- AT&S – Austria
Case Study Examples
Here are two examples highlighting real-world STB PCB implementations:
Liberty Global Virgin Box
- 6 layer board measuring 210mm x 143mm
- Utilizes Rafael Micro R828D tuner
- Includes MPEG-4 HD decoder
- Provides both IP and RF connectivity
Dish Network Wally STB
- 8 layer PCB with 270mm x 180mm dimensions
- Supports UHD 4K along with HD and SD
- Incorporates both satellite and over-the-top receivers
- Includes wireless connectivity for multi-room streaming
- Powered by Broadcom BCM7252S processor
Both these examples use high performance multilayer PCBs to pack multiple functions into the STB.
Conclusion
Designing and manufacturing the PCB is a key part of developing set top boxes. With careful consideration of requirements and selection of optimal PCB technologies, STB providers can create boards that offer advanced functionality, robust performance and long term reliability at a reasonable cost point. Global PCB manufacturers that understand STB electronics can deliver quality boards tailored to the exact needs. As STBs continue adding more features and connectivity, the PCB design and fabrication processes will evolve in tandem to enable next generation devices.
FAQs
What are some key factors in choosing PCB material for STBs?
FR-4 glass epoxy is universally used. Key considerations are Tg, thermal conductivity, Dk/Df values, moisture absorption and CTE. High Tg (>130°C) materials ensure reliability.
How is shielding achieved in STB designs?
Sensitive analog and digital circuits are separated. Individual metal shield cans, guard traces, ground planes and stitching vias provide shielding against EMI/RFI interference.
What special capabilities are required for manufacturing STB PCBs?
Tight impedance control, excellent RF signal integrity and minimal crosstalk noise are essential. This demands advanced fabrication processes.
What are some alternatives to using a PCB in an STB?
Some options are – wirewrap boards using point-to-point wiring, discrete construction, MCM substrates combining ICs, and FPGA/CPLD solutions combining multiple functions.
How can costs be reduced in STB PCB manufacturing?
Standard FR-4, moderate layer counts, optimal board sizes, panel utilization, scaled test/inspection and Engineering involvement help achieve cost reductions.
