PCB layout in mobile phone design requires meticulous attention to minimize risks and ensure optimal performance. With multiple layers dedicated to various functions, specific guidelines help maintain signal integrity and reduce electromagnetic interference (EMI). Below are the detailed issues and wiring principles to consider in mobile phone PCB layout, especially for complex components like RF circuits and displays.

Layer Configuration

1. Layer 1 (Top Layer): Device placement.
2. Layer 2: Signal layer for most address and data signals, including some analog lines (layer 3 is the ground reference).
3. Layer 3: Ground (GND), with routing for keyboard signals and unplaceable layer 2 signals.
4. Layer 4: Critical routing layer for RF control, baseband analog, clock, and audio lines.
5. Layer 5: Ground (GND).
6. Layer 6: Power distribution (VBAT, VCORE, VSIM, etc.).
7. Layer 7: Signal layer for keyboard surface routing.
8. Layer 8 (Bottom Layer): Device placement.

Specific Wiring Guidelines

1. General Principles

• Routing Order: Prioritize RF stripline and control lines → Baseband RF analog interface → Audio and clock lines → Analog-digital baseband interface → Power lines → Digital lines.

2. RF Stripline and Control Line Routing

• Striplines (Layer 4):
  • Use a 3mil width.
  • Ensure surrounding ground on adjacent layers.
  • Avoid traces on other layers near vias.
• Top Layer RF Receiving Signals:
  • RX_GSM, RX_DCS: 8mil width.
  • Power amplifier outputs (e.g., GSM_OUT, DCS_OUT): 12mil width.

3. Analog Lines with RF Interface (Layer 4)

• Key Signals:
  • TXRAMP_RF, AFC_RF: At least 6mil width, shielded with ground.
• Differential Signals:
  • Equalize lengths and spacing (6mil width) for QN_RF/QP_RF and IN_RF/IP_RF.

4. Clock Lines (Layer 4)

• Minimize routing under noise-sensitive components (e.g., quartz crystals).
• Maintain short and grounded paths for SIN13M_RF, CLK13M_X signals (8mil width recommended).

5. Baseband Analog Lines (Layer 4)

• Differential signal pairs (e.g., RECEIVER_P/N, USB_DP/DN) should have equal length and spacing.
• Analog lines like BATID, TSCXP/M: Minimum 6mil width.

6. Grounding (AGND and GND)

• AGND Layout:
  • Near analog components, connect to GND with copper foil.
  • Ensure at least 50mil AGND width.
• Connections:
  • Use vias to connect ground planes across layers.

7. Digital Baseband to Peripheral Devices

• Reset and interrupt signals (e.g., LCD_RESET, SIM_RST): Minimum 6mil width.
• High-speed lines (e.g., VSDI, BSDO): Short, wide paths with surrounding copper.

8. Power Supply Routing

• High-Current Signals (Layer 6):
  • VBAT, CHARGE_IN: At least 40mil width.
  • LDO_2V8_RF, VMEM, and similar: Prioritize splitting across the power plane.
• Small-Current Signals: Use signal layers (e.g., VRTC, VMIC).
• Special Cases:
  • Charging and backlight circuits require wider lines (e.g., 16mil for high-current).

9. EMI Reduction

• Route sensitive signals (e.g., LPG networks) on inner layers with vias near output pins.
• Avoid parallel routing on adjacent layers.

10. Shielding

• Use a 0.7mm shielding strip with 0.3mm spacing between strips.
• Pads should maintain a 0.4mm clearance.

11. Additional Considerations

• Follow the 20H rule for power-ground plane indentation.
• Via sizes: Optimize based on layer connections (e.g., 0.3mm/0.1mm for 1-2, 7-8 layers).
• Edge Grounding: Maintain 1.5-2mm wide grounding strips with punched vias.

Optimized PCB Layout Practices

1. Minimize Crosstalk:
   • Avoid parallel routing on adjacent layers, especially layers 3 and 4.
2. EMI Shielding:
   • Ground large areas under components like SIM card connectors.
3. Copper Application:
   • Use vias to connect copper layers for thermal and EMI management.

Careful adherence to these guidelines ensures efficient and reliable mobile phone PCB designs. By prioritizing critical signal paths, ensuring proper grounding, and optimizing power distribution, potential risks in wiring are mitigated, resulting in better performance and reduced EMI issues.