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3.1 Heat Transfer & Zone Control

Heat in aThe reflow oven is boththe final, irreversible step in Surface Mount Technology (SMT), where the sculptormechanical andplacement theis saboteur of solder joints. Controlled correctly, it melts alloys evenly, activates fluxes, and locks components in place with clean, reliable joints. Left unchecked, the same forces warp boards, split BGAs, and leave weak, inconsistent connections. By breaking down conduction, convection, and radiation into manageable levers—zone temperature, blower speed, and belt timing—the process shifts from a black boxconverted into a repeatablereliable recipe.electrical Whatconnection. emergesControl over the oven is notparamount, justas soldering,the butentire thermalprocess choreographywindow thatis makesdictated high-yieldby manufacturingthe possible.

transfer

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 stealof heat from the centeroven's ofzones ato thinthe panel.
  • Convection:PCB hotassembly. Stable, repeatable 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 speedtransfer is timeessential (howto longprevent soldering defects, minimize component stress, and ensure the boardmetallurgical staysintegrity of the solder joint. The design and discipline of zone control directly impact energy efficiency (OpEx) and thermal profile consistency.

3.1.1 Heat Transfer Methods in each climate).

Reflow


Thermal
energy


is

3.1.2transferred The four reflow segments (andto the knobsPrinted thatCircuit matter)

Board

TypicalAssembly lead-free(PCBA) SACprimarily guidance—alwaysthrough checkthree yourmechanisms. pasteControl datasheet.and uniformity are dominated by the forced convection system.

SegmentMethod

What you wantMechanism

“Typical”Role targetsin SMT Reflow

WhichControl knob moves it mostComplexity

PreheatForced (ramp)Convection

GentlyHot warmgas everything(air or nitrogen) circulated by fans across heating elements and onto the PCBA surface.

Primary mechanism. Provides the most uniform and repeatable heating, minimizing temperature differences across the panel.

~0.7–2.0High °C/s— torequires ~150–170precise °Cfan speed, damper control, and PID loop tuning.

Infrared (IR) Radiation

Heat emitted from heating elements (radiant heat).

Belt speedSecondary/Supplemental. first,Used thenprimarily earlyin older ovens or when high thermal mass components require rapid heating.

Low — less uniform. Can cause shadowing, non-uniform heating, and component damage (e.g., melting plastic connectors).

Conduction

Heat transfer via physical contact (e.g., conveyor rails, support pins).

Minor/Negligible. Mainly contributes to heat loss or localized rail heating effects.

Minimal.

Process Control Note: Modern SMT demands pure forced convection to achieve the tight temperature tolerances necessary for complex boards with varying thermal mass (BGAs, large heatsinks, small 0201 chips).

3.1.2 The Profile Zones and Their Purpose

Reflow is divided into four functional zones, each achieving a specific metallurgical and chemical goal.

Zone

Goal

Control Metric

Risk of Failure

1. Preheat/Ramp

Raise the board temperature gradually.

Ramp Rate (e.g., 1–3˚C/second).

Thermal shock, component cracking, and paste slump.

2. Soak/Dwell

Equalize the temperature difference across all components.

Time in Soak (e.g., 150-180˚C).

Failure to activate flux or excessive temperature differential before reflow.

3. Reflow/Peak

Achieve the molten state (liquidus) and form the joint.

Time Above Liquidus (TAL) and Peak Temperature.

Bridging, Tombstoning, Intermetallic Growth (reliability risk).

4. Cooling

Solidify the solder joint quickly and uniformly.

Cooling Rate (e.g., -2 to -6˚C/second).

Weak grain structure, cracking, and excessive intermetallic thickness.

3.1.3 Zone Control and Energy Management (OpEx)

Effective zone tempscontrol relies on the precision of the temperature regulation system and smart energy management.

  • PID Control: Each heating zone operates under a PID (Proportional-Integral-Derivative) control loop to maintain the setpoint temperature despite varying thermal load (different board masses). Regular calibration of thermocouples and zone sensors is mandatory.
  • Zone Isolation: Zones must be physically and thermally isolated to prevent temperature bleed-over, ensuring that a setting change in one zone does not significantly affect the adjacent zone.
  • Energy Efficiency: Oven design (e.g., insulation, heating element efficiency) dictates OpEx. Regular fan and filter maintenance is critical: dirty fans reduce airflow, forcing the heating elements to work harder, increasing energy consumption and thermal instability.

3.1.4 Metrics for Profile Stability

Profile stability is measured by two primary factors that reflect the oven's capability:

  1. Cross-Board Delta (∆T): The maximum temperature difference between the hottest and coldest points on the board at any given time (especially during soak). A tight process requires ∆T ≤ 10 ˚C. This is the primary measure of the oven's convection uniformity.
  2. Lane-to-Lane Delta: In dual-lane ovens, the temperature difference between the same zone in the left and right lanes. This must be tightly controlled to ensure parallel production yields consistent quality.

Final Checklist: Reflow Zone Setup

Parameter

Mandate

Control Point

Heat Transfer

Forced Convection must be the primary method for uniformity.

Regular fan speed and damper calibration.

Preheat

Ramp Rate controlled to prevent thermal shock (typical 1–3˚C/sec).

Profile software lock.

Soak

Cross-Board ∆T ≤ 10 ˚CEven outat temps,soak activate fluxtemperature.

~60–120Board ssupport, aroundzone 150–180settings, °C

Zonefan temps in middle zones, blower balancespeeds.

Reflow/TALCooling

FullyCooling melt, wet, collapse

Time Above LiquidusRate (217must °Cbe forfast SAC):enough ~40–80to s;ensure Peakstrong ~235–250joint °C

Peakgrain zone temps and belt speed

Cool

Solidify without stressstructure.

~2–4Integrated °C/scooling down through 180→100 °C

Cooler/blower settings, exit fans

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)

  1. Load the vendor’s starter recipe for your alloy.
  2. Run a profile board with 4–6 thermocouples (see 9.1.5).
  3. Use belt speed to get TAL in range.
  4. Use late-zone temps to nail peak.
  5. Use blower speed and mid-zone temps to tighten ΔT (edges vs center, light vs heavy components).
  6. 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

Symptom

First move

If still there…

Hot edges, cool center

Raise blower, lengthen soak

Add center supports in oven; lower early-zone setpoints to reduce edge preheatfans/chillers.

BGA HIP (head-in-pillow)Maintenance

SlightlyFilters longerand TALfan motors cleaned/inspected weekly to protect OpEx and steadierthermal soakstability.

ConsiderPM N₂;schedule verify paste and VIPPO fill; check warpage support

QFN voiding

Longer, gentler soak; try small peak reduction

Switch paste or add N₂; revisit stencil windowing (7.4)

Skew/tombstones

Smooth ramp (≤2 °C/s), tighten ΔT

Revisit aperture balance (7.4), paste health (7.2)

Solder balls/splatter

Shorten soak, reduce peak a touch

Paste age/handling; understencil cleaning cadence (7.5)

Dull joints/cosmetics

Raise peak a little or add N₂

Check flux family (7.1), finish (2.4)enforcement.



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



A disciplined approach to profiling—balancing thermal physics with paste limits, smart thermocouple placement, and clear recipe management—turns reflow ovens into consistent production tools. Done well, it prevents defects, preserves components, and ensures that every board leaving the line meets both electrical and cosmetic expectations.


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