Basic Concepts

Simply put, the main difference between positive and negative processes lies in PCB plating and image transfer methods. Once image transfer (creating the circuit pattern) is completed, the subsequent processes (such as solder mask, legend printing, etc.) are largely similar.

The fundamental difference between positive and negative processes lies in whether it's "what you see is what you get" or "what you see is the opposite (reverse)", and whether it uses "subtractive" or "additive" methods.

Below, I'll use a typical 4-layer board as an example to provide a detailed analysis covering definitions, process flows, advantages/disadvantages, and application scenarios.

1. Negative Process Detailed Explanation (Tenting Process)

Negative Film

After applying photosensitive dry film, exposure, and developing, the copper surfaces revealed after developing are the ones we don't need. These exposed copper areas are subsequently etched away by acidic etching solution. The circuit patterns we need are protected by the dry film. After etching is completed and the protective dry film is stripped away, the circuit is formed.

Industry Terminology:

• This typically corresponds to Panel Plating or Tenting processes.
• Characteristic: Negative process usually involves plating to thicken the copper layer across the entire board before starting circuit image transfer.

Negative Process Basic Flow (DES Process)

Front-end Design → Panel Cutting (Material Preparation) → Inner Layer Circuit Fabrication (Negative) → Lamination → Drilling → Electroless Copper (PTH) → Panel Plating (Primary Copper, plated to full thickness) → Pre-treatment → Lamination → Exposure → Developing → Acidic Etching → Stripping → Back-end Processes (AOI Optical Inspection)

Memory Point:

• Negative process follows the DES process
• Transparent areas on the film are circuits (dry film hardened and preserved)
• Black light-blocking areas are regions to be etched away

2. Positive Process Detailed Explanation (Pattern Plating)

Positive Film

After lamination, exposure, and developing, the copper surfaces revealed after developing are exactly the circuit areas we need. The areas covered by dry film are the unwanted copper.

The exposed copper after developing is typically thin, so we perform Pattern Plating (secondary copper) to thicken the circuits, followed by plating a layer of tin or tin-lead as an anti-etching protective layer.

After plating copper and tin, first strip the dry film to reveal the unwanted copper surfaces, then use alkaline etching solution to etch away these unprotected copper areas. Finally, strip the tin layer protecting the circuits to complete circuit fabrication.

Industry Terminology:

• This corresponds to Pattern Plating process.
• Characteristic: Positive process involves applying a thin copper layer (primary copper) first as a base, creating the pattern, then performing thickening plating specifically on circuit areas.

Positive Process Basic Flow (SES Process)

Front-end Design → Panel Cutting → Inner Layer Fabrication → Lamination → Drilling → Electroless Copper (PTH) → Panel Plating (Primary Copper, thin copper base) → Pre-treatment → Lamination → Exposure → Developing → Pattern Plating (Secondary Copper Thickening) → Tin Plating (Protective Layer) → Stripping → Alkaline Etching → Tin Stripping → Back-end Processes (AOI)

Memory Point:

• Positive process follows the SES process
• Black areas on the film are circuits (light-blocking, dry film not hardened and washed away during developing, exposing copper surface for plating)
• Transparent areas are unwanted regions

3. Image Transfer Principle: Like Developing Photos

You can think of PCB manufacturing process as similar to traditional camera "photo developing":

1. Positive: "What You See Is What You Get"

• Like regular photographs
• Black areas on the film (light-blocking), dry film doesn't harden, copper surface exposed after developing, these copper surfaces eventually become PCB circuits (preserved through plating thickening)
• Black patterns on film = circuits to be preserved

2. Negative: "Reverse Operation"

• Like photographic negatives (film)
• Transparent areas on the film (light-transmitting), dry film photosensitizes and hardens, protecting underlying copper
• Black areas on the film, dry film washed away during developing, underlying copper subsequently etched away
• Transparent patterns on film = circuits to be preserved

4. Process Comparison and Application Scenarios

This difference mainly occurs during the "image transfer" and "etching" stages.

Positive Process (Pattern Plating)

• Core Concept: Circuit tin plating protection, alkaline etching
• Process Complexity: High (additional secondary copper, tin plating, tin stripping steps)
• Cost: Higher (more process steps, more consumables)
• Circuit Precision: High. Because it's formed by plating, and tin plating layer provides good anti-etching protection, less side etching, better line width control
• Application Scenarios: Outer layer circuits. Particularly suitable for boards requiring component holes (PTH holes), fine lines, or high-density interconnect (HDI)

Negative Process (Panel Plating / Tenting)

• Core Concept: Direct circuit protection by dry film, acidic etching
• Process Complexity: Simple (direct etching after developing)
• Cost: Lower (high efficiency)
• Circuit Precision: Relatively lower. Acidic etching prone to side etching, line widths may become thinner
• Application Scenarios: Inner layer circuits. Mostly used for power planes, ground planes, or layers with large copper areas

5. Summary and Memory Tips

• Positive (for outer layers): Like "addition". First determine positions, then "put armor on circuits" (tin plating), then eliminate enemies without armor (excess copper) with alkaline etching. High precision, suitable for outer layer routing and through-holes.

• Negative (for inner layers): Like "subtraction". First cover areas to be preserved with dry film, then eliminate all unwanted copper at once with acidic etching. Low cost, fast speed, suitable for inner layer fabrication.

In modern PCB manufacturing (especially multi-layer boards), mixed usage is common:

• Inner Layers: Use negative process (simple, inexpensive)
• Outer Layers: Use positive process (for PTH holes and ensuring circuit precision)

Hope this detailed explanation helps you thoroughly understand the difference between positive and negative processes!

Special Note

The above discussion is for reference only. Each PCB manufacturer has different chemical characteristics, process capabilities, equipment parameters, and product structures, so don't apply these concepts rigidly. All effective process improvements must be based on data and verified through DOE experiments - this is the professional approach for engineers.