Route planning for two-layer board in PCB Design - Full Explanation

Introduction

Designing a circuit board with only two layers can be tricky because you need to connect many parts without wires crossing. The challenge is to find paths for all connections while using just the top and bottom layers.

Explanation

Top and Bottom Layers

A two-layer board has a top layer and a bottom layer for routing electrical connections. The top layer is usually used for signal traces and components, while the bottom layer can be used for additional routing or ground planes. Designers switch between these layers to avoid crossing wires.

Using two layers allows more routing options than a single layer by switching connections between top and bottom.

Via Usage

Vias are small holes that connect traces from the top layer to the bottom layer. They let signals jump between layers to avoid crossing other traces. However, too many vias can increase cost and complexity, so they must be used carefully.

Vias enable layer switching but should be minimized for cost and reliability.

Routing Strategies

Common strategies include routing horizontal traces on one layer and vertical traces on the other. This reduces crossing and simplifies planning. Designers also prioritize critical signals and keep power and ground lines wide and short.

Organizing trace directions by layer helps prevent crossing and simplifies routing.

Design Rules and Constraints

Design rules set limits on trace width, spacing, and via size to ensure electrical performance and manufacturability. Constraints like component placement and board size also affect routing choices. Following these rules avoids shorts and signal problems.

Adhering to design rules ensures a functional and manufacturable board.

Real World Analogy

Imagine a two-story house where you need to run wires from switches to lights without crossing wires on the same floor. You can run wires on the first floor going left to right and on the second floor going front to back, using stairs (vias) to switch floors when needed.

Top and Bottom Layers → First and second floors of the house where wires run in different directions

Via Usage → Stairs that let wires move between floors to avoid crossing

Routing Strategies → Planning wire directions on each floor to keep them from crossing

Design Rules and Constraints → Building codes and space limits that guide how wires and stairs are placed

Diagram

Diagram

┌───────────────┐       ┌───────────────┐
│ Top Layer     │──────▶│ Horizontal    │
│ (Traces run  │       │ traces        │
│  left-right) │       └───────────────┘
│               │
│   ┌─────┐     │       ┌───────────────┐
│   │Via  │────▶│       │ Vertical      │
│   └─────┘     │       │ traces on     │
│ Bottom Layer  │──────▶│ bottom layer  │
│ (Traces run  │       └───────────────┘
│  up-down)    │
└───────────────┘

Diagram showing top layer with horizontal traces, bottom layer with vertical traces, and vias connecting layers.

Key Facts

Two-layer PCB → A printed circuit board with two conductive layers for routing signals.

Via → A plated hole that connects electrical traces between the top and bottom layers.

Trace → A conductive path on a PCB that connects components.

Routing Strategy → A planned approach to organize trace directions and layer usage to avoid crossing.

Design Rules → Constraints on trace width, spacing, and via size to ensure board functionality.

Common Confusions

Believing vias can be used without limit to solve routing problems.

Believing vias can be used without limit to solve routing problems. Vias add cost and can affect signal quality, so they should be used sparingly and strategically.

Thinking traces can cross on the same layer without issues.

Thinking traces can cross on the same layer without issues. Traces on the same layer cannot cross without causing shorts; layer switching or careful routing is needed.

Assuming design rules are optional guidelines.

Assuming design rules are optional guidelines. Design rules are mandatory to prevent electrical failures and manufacturing defects.

Summary

Two-layer boards use top and bottom layers to route connections by switching layers to avoid crossing wires.

Vias connect traces between layers but should be minimized to reduce cost and complexity.

Organizing trace directions and following design rules ensures a functional and manufacturable PCB.