Outer Layer Circuit Process: A Complete Pattern Plating Walkthrough

Outer Layer Circuit Process: A Complete Pattern Plating Walkthrough

Overview

After drilling and hole metallization, all layers are electrically connected. What remains is to form the final traces on the outer copper. The outer layer process has two parts: the D stage, outer layer dry film (providing the pattern), and the E stage, outer layer circuit (pattern plating + etching).

Unlike the inner layer’s “negative” logic, the outer layer uses pattern plating (the positive process): first plate thick copper and a protective tin layer on the traces, then etch away the unwanted copper. This article walks the whole line.

Process Chain Overview

Outer layer process chain (D stage + E stage)
D stage (pattern prep):  Pretreat → Lamination → Exposure → Developing
E stage (trace forming): Pattern Plating → Tin Plating → Strip → Etching → Tin Stripping
                           │                 │            │        │            └─ HNO₃ removes tin, revealing traces
                           │                 │            │        └─ etch away copper lacking tin protection
                           │                 │            └─ NaOH removes dry film, exposing thin copper
                           │                 └─ tin layer acts as etch resist
                           └─ build copper up to the customer-specified thickness

Part 1: Outer Layer Dry Film (D Stage)

The logic is identical to the inner layer (pretreat → lamination → exposure → developing); the purpose here is to provide the pattern for the outer layer circuit.

• Pretreat: remove contaminants from the copper surface and increase roughness for lamination; main material: abrasive brushes
• Lamination: hot-press dry film tightly onto the copper surface; main material: dry film (photo resist)
• Exposure: transfer the required trace pattern onto the dry film by photoimage transfer; main material: film / photomask
• Developing: wash away the un-polymerized regions with developer; the exposed (polymerized) portions remain on the copper as the resist film for etching or plating; main material: weak alkali K₂CO₃

Part 2: Outer Layer Circuit (E Stage)

Pattern Plating (Second Copper)

• Purpose: thicken the copper on the exposed surfaces after developing (the future trace areas) to reach the customer-specified copper thickness
• Main material: copper balls

Note: the exposed areas after developing are the trace areas — this echoes the positive logic where “what the dry film covers is the non-trace region” (see the comparison below).

Tin Plating

• Purpose: plate a tin layer over the pattern-plated copper as the protective agent during etching
• Main material: tin balls

Strip (Film Removal)

• Purpose: chemically strip the dry film that served as the anti-plating resist
• Main material: stripping solution (NaOH)

After stripping, the regions previously covered by dry film (the thin base copper of the non-trace areas) are exposed, while the trace areas are protected by tin.

Circuit Etching

• Purpose: etch away the copper in the non-conductive regions
• Main material: etching solution, ammonia

Alkaline etching solution “etches copper but not tin”: the exposed thin copper is etched away while the tin-protected trace copper remains.

Tin Stripping

• Purpose: strip the protective tin from the conductive regions
• Main material: HNO₃ tin-stripping solution

After tin stripping, the finished outer layer traces are complete.

Positive vs Negative: The Full Picture

	Inner Layer (Negative Logic)	Outer Layer (Positive / Pattern Plating)
What the dry film pattern represents	Traces to keep	Regions to etch away
Core action sequence	Laminate → expose/develop → etch → strip	Laminate → expose/develop → plate copper + tin → strip → etch → strip tin
How traces are protected	The dry film itself is the etch resist	The plated tin layer is the etch resist
Why	Inner-layer base copper is the final copper; etching alone forms the pattern	Outer-layer final copper = base copper + pattern-plated copper; must plate first, etch later

One-line mnemonic: inner layer “where film stays, traces stay”; outer layer “where copper is bare, traces are there”.

Summary

• The outer layer process = dry film (D stage) + pattern plating (E stage), a “plate first, etch later” positive route
• Five forming steps: pattern plating (thicken) → tin plating (protect) → strip (expose non-trace areas) → etch (remove thin copper) → tin strip (reveal finished traces)
• Each chemical has its job: K₂CO₃ develops, NaOH strips film, ammonia etches copper, HNO₃ strips tin
• With all traces formed, it is time to “dress the board in green”: solder mask and legend

Next up: silkscreen — solder mask, legend printing, and curing.

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