Detailed Steps for PCB Baking in SMT Processing

1. Pre-Baking Preparation

1.1 PCB Condition Check

• Identify PCB material: FR‑4, aluminum substrate, flexible PCB (FPC), high‑frequency board, etc.
• Check packaging:
  • Vacuum‑sealed with humidity indicator card (HIC) ≤30% and storage time <3 months: no baking required.
  • Broken vacuum package, HIC >30%, exposed to air, or stored >3 months: mandatory baking.
• Inspect for surface contamination, oil, oxidation, or physical damage. Clean contaminated boards before baking.

1.2 Oven Preparation

• Use a forced hot-air circulating oven for uniform heating.
• Preheat and calibrate the oven; ensure temperature uniformity ≤±5 °C across the chamber.
• Verify ventilation is unobstructed to allow moisture vapor to escape.

1.3 PCB Placement

• Place PCBs horizontally in a single layer on ESD-safe racks or stainless steel trays.
• Do not stack boards; leave 5–10 mm gaps between boards for air circulation.
• Thin PCBs (≤0.6 mm) and large boards must be fixed with jigs to prevent warping.

2. Formal Baking Process

2.1 Loading & Gradient Heating

• Load trays into the oven and close the door.
• Use gradient heating to avoid thermal shock and warping:
  1. Rise from room temperature to 80 °C and hold for 30 minutes.
  2. Continue heating to the target baking temperature.

2.2 Constant-Temperature Baking

• Normal moisture exposure (3–6 months): 125 ±5 °C for 2–4 hours
• Severe moisture exposure (>6 months, exposed): 125 ±5 °C for 4–8 hours
• Thick or high-layer-count boards: add 1 hour per additional 0.5 mm thickness

• Do not open the oven door to avoid temperature drop and re‑absorption of moisture.
• Monitor for blistering, delamination, or discoloration.

2.3 Controlled Cooling

• After baking completes, turn off heating but keep the door closed.
• Allow PCBs to naturally cool to room temperature (25 ±5 °C) inside the oven, typically 1–2 hours.
• Important: Do NOT remove hot PCBs directly into open air — this causes rapid re‑absorption of moisture.

2.4 Unloading & Visual Inspection

• Once cooled, remove PCBs and inspect:
  • No warping, blistering, or delamination
  • No yellowing or charring from over‑baking
  • No solderability degradation

3. Post-Baking Handling

• Baked PCBs must be used for SMT printing within 4 hours.
• If not used immediately, seal in moisture‑barrier bags with fresh desiccant and HIC.
• Transport using sealed ESD containers to avoid re‑absorption in high-humidity environments.

4. Adjustments for Special Board Types

Aluminum substrates

• Temperature: 100 ±5 °C
• Time: 2–4 hours
• Use fixtures to prevent separation of metal and dielectric layers

Flexible PCBs (FPC)

• Temperature: 80–90 °C
• Time: 1–2 hours
• Lay flat on silicone trays; do not fold or overlap

High-frequency boards (Rogers, PTFE)

• Temperature: 125 ±5 °C
• Time: 4–6 hours (deeper drying required)

PCBs with pre-mounted components

• Reduce temperature to 80 °C for 2–3 hours.
• For MSL 3+ moisture-sensitive devices, remove components before baking.

5. Key Rules & Avoidance of Common Defects

• No over-temperature or over-time baking — causes brittleness, delamination, and blistering during reflow.
• No stacking — leads to uneven drying and popcorning during soldering.
• No hot unloading — the most common cause of re‑absorption.
• No mixed baking of different materials.

• Warpage: caused by fast heating, uneven temperature, or unsecured thin boards.
• Blistering / popcorning: insufficient baking, re‑absorption, or poor ventilation.
• Yellowing: over‑temperature or over‑time baking.

6. Environmental Requirements

• SMT shop: temperature 25 ±3 °C, humidity 30–60%.
• Baking area should be dry, away from water sources and air vents.
• Minimize distance between baking area and SMT print line to reduce exposure time.