3.1 Heat Transfer & Zone Control
Conduction, convection, radiation—the heat trio—and how to tune heaters, blowers, and belt speed so reflow is controlled, not mysterious.
Reflow profiling controls how heat moves into a PCB so solder melts evenly without damaging parts. Heat comes from conduction (through contact points), convection (hot air or nitrogen flow), and radiation (infrared from heaters). The process follows four stages—preheat, soak, reflow/TAL, and cool — tuned by adjusting zone temperatures, blower speeds, and belt speed. Zone temps shape the profile and set peak, blowers balance heat across the board, and belt speed controls how long the board spends in each stage. Thermocouples placed at representative hot and cold spots guide adjustments, and nitrogen can improve wetting, cosmetics, and voiding performance when needed. Once tuned, save the recipe with clear naming, comments, and a golden profile plot so the oven repeats the result consistently.
3.1.1 The physics in one page (no stress)
- Conduction: heat flowing through contact—board on conveyor fingers/rails, parts into pads. Good conduction evens out temperatures but can also steal heat from the center of a thin panel.
- Convection: hot air/N₂ blown across the board. This is the main act in modern ovens; blower speed sets how hard the air scrubs heat into every corner.
- Radiation: IR from heaters. It’s always there, but on convection ovens it’s the supporting cast, smoothing things out.
Think of your oven like weather: setpoints are the climate (how hot the zones are), blowers are the wind (how fast heat moves), and belt speed is time (how long the board stays in each climate).
3.1.2 The four reflow segments (and the knobs that matter)
Typical lead-free SAC guidance—always check your paste datasheet.
Two guardrails:
- Keep ΔT across the board tight (aim <10–12 °C at peak for most builds).
- Don’t break datasheet ramp/TAL limits (especially for large BGAs and plastic connectors).
3.1.3 Zone control—what each dial is really for
- Zone setpoint (temperature): raises/lowers the ceiling in that segment. Use it to hit peak and shape soak.
- Blower/fan speed: boosts convection (heat transfer coefficient). Use it to reduce ΔT across big/heavy boards or dense areas. Too high can kick parts or dry flux early—nudge, don’t slam.
- Top vs bottom balance: top a little hotter for heavy top-side copper or tall parts; bottom up a touch when large ground planes live underneath.
- Belt speed: your time knob. Faster belt → shorter soak/TAL; slower belt → more time everywhere.
Rule of thumb:
- Missed peak / short TAL? Slow the belt a bit or lift late-zone temps.
- Overheating / long TAL? Speed the belt or drop late-zone temps.
- Hot edges, cold center? Increase blower and give a calmer, longer soak.
3.1.4 A simple tuning playbook (start here)
- Load the vendor’s starter recipe for your alloy.
- Run a profile board with 4–6 thermocouples (see 9.1.5).
- Use belt speed to get TAL in range.
- Use late-zone temps to nail peak.
- Use blower speed and mid-zone temps to tighten ΔT (edges vs center, light vs heavy components).
- Lock it, label it, and rerun once the oven is heat-soaked (after a few boards).
3.1.5 Thermocouples that tell the truth
Where you stick TCs matters more than how many you use.
- Attach well: high-temp epoxy or small solder dots; Kapton tape only as a helper.
- Pick smart spots:
- “Cold spot” (usually a shadowed BGA center or large copper area)
- “Hot spot” (small passive cluster near open copper)
- Heavy thermal pad (QFN/LFPAK slug)
- Edge rail vs board center (to see ΔT)
- Connector / plastic risk (watch peak)
- Do both sides on double-sided builds—second pass often needs a softer recipe to protect the first side’s joints and plastics.
3.1.6 Air vs nitrogen (what changes)
- Nitrogen (low O₂) improves wetting/cosmetics, often trims voiding on big thermal pads, and can help marginal pastes at lower peaks. Heat transfer is similar enough that your zone temps won’t jump wildly—but you may be able to lower peak a few degrees or shorten TAL while keeping quality.
- Air is cheaper and fine for most assemblies if printing and profiles are solid. Save N₂ for dense BGAs/QFNs, tight cosmetics, or finicky flux.
3.1.7 Common symptoms → fastest fixes
3.1.8 Don’t fight warp with heat
Warp/bow makes TCs lie and parts skate. Fix mechanically first: edge rails, center fingers, and board supports in printer and PnP. Then profile. Using heat to “bend a board flat” usually backfires.
3.1.9 Keep the oven happy (little things that pay)
- Warm-up & soak: give the oven a few boards to reach steady state before final profiling.
- Exhaust & filters: clogged filters change convection—clean them on schedule.
- Recipe names: encode alloy, side, panel mass, N₂/air in the name (e.g., “SAC_Top_4upHeavy_Air_v3”).
- One change at a time when tuning; note what you touched and why in the recipe comments.
3.1.10 Release checklist (tape this to the oven PC)
- Profile board run with 4–6 TCs (hot/cold spots covered)
- Ramp, soak, TAL, peak within paste datasheet limits
- ΔT at peak within target (≤10–12 °C typical)
- Top/bottom balance set; blower tuned for evenness
- Recipe saved (clear name + comments); Golden plot attached
- Air/N₂ choice documented; second-side recipe (if needed) created
Bottom line: use belt speed for time, zone temps for shape/peak, and blowers for evenness. Place thermocouples where the board struggles, not where it’s easy to tape. Do that—and profile to the paste’s limits—and your oven stops being mysterious. It just does the same good thing, every pass.