2.1 PCB Materials and Layers

The Printed Circuit Board (PCB) is not merely a passive holder for components; it is an active mechanical structure that expands, contracts, and absorbs heat. If the substrate material is mismatched to the thermal environment, the board will delaminate (tear itself apart) during the soldering process. Design the stackup not just for electrical connectivity, but for mechanical survival.

The Substrate: FR-4 and the "Tg" Factor

The vast majority of electronics are built on FR-4 (Flame Retardant 4), a composite of woven fiberglass cloth and epoxy resin. It acts as the non-conductive skeleton of the board. While "FR-4" is the generic category, the critical specification you must define is the Tg (Glass Transition Temperature).

Understanding Tg (The Softening Point)

Tg is not the melting point.4 It is the temperature at which the rigid epoxy resin becomes soft and rubbery. When the material crosses this threshold, it expands rapidly in the Z-axis (thickness).

• Engineering Reality: Lead-free soldering requires reflow temperatures up to 260˚C.
• Standard Tg (130˚C – 140˚C): Acceptable for simple consumer electronics with low layer counts.
• High Tg (170˚C+): Mandatory for industrial, automotive, or high-layer-count boards (8+ layers).
• If the resin expands too much (Low Tg) during reflow → Then the copper barrel inside the vias will rip apart (Via Cracking), causing open circuits.

The Stackup: Layers as a Sandwich

A PCB is built by laminating alternating layers of copper (conductive) and substrate (insulative). The "Stackup" is the specific arrangement of these layers.

Core vs. Prepreg

• Core: A rigid piece of FR-4 with copper already cured on both sides.
• Prepreg: Uncured "sticky" fiberglass sheets used as glue between cores. Heat and pressure melt the Prepreg to fuse the layers.

2-Layer Board

• Structure: Top Copper – Core – Bottom Copper.
• Use Case: Simple, low-speed circuits (Toys, breakout boards).
• Limitation: No shielding. Signals on the top interfere with signals on the bottom.

4-Layer Board (The Industry Standard)

• Structure: Top Signal – Ground Plane – Power Plane – Bottom Signal.
• Advantage: The internal solid copper planes act as electromagnetic shields. They provide stable voltage and reduce noise (EMI).
• If you have high-speed signals (USB, Wi-Fi) → Then a 4-layer board is the minimum requirement for signal integrity.

Multi-Layer (6, 8, 12+ Layers)

• Use Case: High density (Smartphones, Motherboards).
• Cost Driver: Each pair of layers adds a lamination cycle. Moving from 4 to 6 layers often increases bare board cost by 30–50%.

Copper Weight: Current vs. Definition

Copper thickness is measured in ounces per square foot (oz/ft²). This unit defines how much current the traces can carry.

• 1 oz (35 µm): The standard. Good balance between current capacity and etch precision.
• 2 oz (70 µm): High Power. Used for motor controllers or power supplies to handle high amps without overheating.
• 0.5 oz (18 µm): Fine Pitch. Used for dense digital boards where lines are very thin.

The Engineering Trade-Off

Thicker copper is harder to etch precisely.10

• If you specify 2 oz copper → Then you cannot have tiny 4 mil trace widths. The acid cannot etch straight down through thick copper without eating sideways ("undercut").
• Result: You must increase the spacing between traces, reducing the board density.

Final Checklist