Introduction
Wi-Fi modules or chips allow electronic devices to wirelessly connect to the Internet or other networked devices using the popular 802.11 Wi-Fi standards. They are widely used in products like smartphones, laptops, smart home appliances, industrial equipment and more.
The core component in a Wi-Fi enabled device is the Wi-Fi module which handles the wireless communication. This article will provide an overview of the key components within a Wi-Fi module and discuss the PCB design principles and layout techniques for implementing Wi-Fi modules to ensure proper functioning.
Wi-Fi Module Components
A Wi-Fi module contains the following key components and sub circuits:
Microcontroller (MCU)
A microcontroller such as ARM Cortex M3 or M4 runs the main firmware that controls the Wi-Fi module functionality. It interfaces with the host system over SPI, USB etc.
Wi-Fi Chipset
The RF chipset with baseband processor handles modulation, encoding, packetization and other signal processing related to the wireless transmission and reception.
Popular Wi-Fi chipsets are Cypress CYW4343W, Microchip ATWILC3000, Texas Instruments WL183x, Realtek RTL8723DS etc. High integration SoCs also combine the chipset with microcontroller.
Power Amplifier
The power amplifier boosts the output from the RF chipset to achieve the required wireless range. There are separate PAs for 2.4GHz and 5GHz bands.
Low Noise Amplifier
The LNA provides initial amplification of received signals with minimal noise. Improves the receiver sensitivity.
RF Matching Network
Carefully designed matching circuits using components like inductors and capacitors match the impedance between the RF ICs/PAs and the antennas. This maximizes signal power transfer.<img src=”https://drive.google.com/uc?export=view&id=1Am9E_9lTg1o7OdnbUGFZbnrSAeqxTNW3″ alt=”WiFi module block diagram” style=”width:500px;height:300px;”>
Oscillators
Precision oscillators provide the required clock signals to the microcontroller and RF chipset. TCXOs (Temperature Compensated Crystal Oscillator) offer stable frequency over temperature variations.
Filters
EMI filters, harmonic filters reduce noise emission and improve signal quality.
Power Management
Voltage regulators and DC-DC converters generate clean power rails from the input supply.
Flash Memory
Stores the module firmware. External serial flash may be used for higher capacity.
PCB Antenna
Printed antennas etched on the PCB itself are commonly used. Or provisions for external antenna connectors.
Indicators
LEDs to provide visual indication of Wi-Fi connectivity status.
Debugging Support
Test points, JTAG/SWD interfaces help debug and program the module.
Host Interface
USB, SD card, SPI etc. interface to communicate with the main host system.
Additional Components
Such as baluns, resistors, capacitors, crystals, buttons, fuses complete the circuitry.
Wi-Fi Module PCB Design Guidelines
The PCB design and layout for a Wi-Fi module is crucial for its functioning and requires careful implementation. The key guidelines are:
Placement
Optimal component placement minimizes track lengths and noise coupling. Group related sub-circuits together with adequate spacing. Keep RF traces short and direct.
Stackup Selection
A 4-layer or 6-layer PCB stackup with adequate copper thickness enables effective shielding, transmission lines and heat dissipation.
Impedance Control
Controlled impedance routing (50Ω or 100Ω) required for RF traces like chipset to connector, chipset to antenna.
Bypassing and Decoupling
Liberal use of bypass/decoupling capacitors next to each IC suppresses noise – typical values are 0.1μF, 1μF, 10μF etc.
Transmission Lines
Controlled impedance lines required from RF ICs to antennas with proper width/space based on stackup. Avoid 90° bends.
Grounding
A continuous ground plane provides low impedance reference. Use several vias to connect ground pads. Have separate analog and digital grounds which join at a single point.
Supply Filtering
Additional ferrite beads, capacitors and pi-filters on supply lines prevent noise coupling.
Shielding
Use coplanar waveguides with ground planes or full copper fills to isolate RF and noise sensitive parts.
Antenna Design
On-board printed antennas like Inverted F, meander line perform adequately for many devices. Provide 50Ω tracks for external antenna connectors.
Thermal Relief
RF ICs dissipate significant heat. Thermal reliefs under pads, copper fills and vias help transfer heat to bottom layer.
Wi-Fi Module Layout Considerations
Here are some key layout techniques that must be followed for a Wi-Fi PCB:
Component Placement
Place components with care for short traces. Keep RF chipset near antenna. Group decoupling caps. Place heat sources on edge near connectors. Have a symmetrical layout.
Routing
Use 45° traces for RF lines. Avoid 90° turns. Do not run RF trace along board edge. Keep RF and digital routes separated. Avoid noise coupling.
Copper Fills
Use copper pours for shielding and heat dissipation. Maintain clearance from traces and pads. Add stitching vias for ground continuity.
Vias
Minimize via stubs. Any open vias should be back drilled. Use caps/epoxy to seal unused vias. Place vias around perimeter of grounds for stability.
Board Shape and Size
Avoid long and thin PCB shapes. Allow margin from edge for manufacturing tolerance and case mounting. Standard rectangular sizes help reduce cost.
Antenna Clearance
Keep clearance below antenna area on bottom layer for optimal radiation. Do not route noisy traces below antenna.
Text Markings
Use smaller text size. Avoid text under components. Place reference designators and markings intelligently to assist assembly and testing.
Wi-Fi Module PCB Fabrication
Wi-Fi PCBs can be fabricated using industry standard process:
- Lamination – Metal and dielectric layers are stacked up under high pressure and temperature. 2.4GHz modules typically use FR408 material while 5GHz modules require higher performance RF laminates like Rogers 4003C.
- Drilling – Holes are precisely drilled for vias and component mounting. Some RF modules require laser drilling for very small microvias.
- Metallization – Electroless copper and direct plate copper create the conductive traces on layers along with plating the drilled holes for interconnection between layers.
- Photolithography – Desired track patterns are printed using lithographic techniques. Several alignment steps add more trace layers.
- Etching – Exposed unwanted copper is etched away leaving only the protected copper to form the designed circuitry pattern.
- Solder mask – The solder mask layer is applied for electrical insulation and mechanical protection with openings left only at points requiring soldering.
- Silkscreen – Identifying textual and graphical markings are printed using the silkscreen legend layer.
- Routing – Individual PCBs are cut apart from panels using routers or v-score.
- Testing – 100% electrical testing and automated optical inspection ensure quality.
Wi-Fi Module Assembly
Assembly of components on the fabricated PCB comprises:
- Stencil Printing – A metal stencil is used to apply the solder paste pattern on pads prior to component placement.
- Pick and Place – Surface mount components are accurately placed on pad locations using automated assembly machines.
- Reflow Soldering – The PCB passes through the reflow oven thermal profile to form reliable joints by melting the solder paste.
- Cleaning – Any post-soldering flux residues are cleaned off to prevent contamination.
- Conformal Coating – A protective plastic coating may be applied for environmental protection and preventing short circuits.
- Curing – For epoxy-based coatings, a heat cure cycle ensures complete polymerization.
- Testing – Comprehensive testing validates RF performance along with electrical functionality and program operation. Failed boards can be reworked.
- Certification – Final regulatory certification for emissions, safety compliance etc. needs to be completed.
The assembled Wi-Fi module can then be integrated into the final product enclosure using mounting points, connectors, cables etc.
Conclusion
Wi-Fi connectivity has become an essential feature in many electronic systems today. The Wi-Fi module is the core enabler, housing the RF, baseband and microcontroller components required for wireless communication along with supporting circuitry within a small PCB. Careful schematic design and PCB layout applying the specific guidelines outlined here are crucial to develop a reliable, high performance Wi-Fi board. With the availability of proven RF chipsets and contract electronics manufacturing services, companies can now readily build Wi-Fi capability into their products with speed and cost efficiency. As Wi-Fi standards and chips continue to evolve with higher speeds, greater range and advanced capabilities, efficient implementation using proven PCB design principles will remain key to creating the compact, robust Wi-Fi modules powering tomorrow’s connected world.
FAQs
What are some key factors in choosing a Wi-Fi module?
Key parameters are protocol support, operating frequency, data rates, tx power, sensitivity, interfaces, certifications, power consumption, operating temperature, and packages.
What is the difference between a Wi-Fi chipset and a Wi-Fi SoC?
A chipset consists of separate ICs – one for RF/baseband processing and one for the microcontroller. An SoC integrates both functions into a single chip.
How much does a Wi-Fi module typically cost?
Simple Wi-Fi modules with PCB antenna can cost under $5. High performance multiprotocol combo modules with precertification can cost $15-$20 or more.
What kind of testing is required for Wi-Fi modules?
Important tests are frequency/channel accuracy, modulation quality, tx power, rx sensitivity, bandwidth, error vector magnitude, interference handling, security, and regulatory compliance.
What are some key PCB design tools used for Wi-Fi layouts?
Allegro PCB Editor, Mentor Xpedition, Cadence Allegro/OrCAD, Altium, and Zuken CR-8000 are some leading PCB design platforms.
