3.2 Profiling Methods

Profiling is wherea distinct step from zone control. While Chapter 3.1 addressed the physics of heat distribution in the oven, this chapter addresses the methodology of measuring the thermal theoryexperience meets reality onof the productionPrinted floor.Circuit OvensBoard may(PCB) itself. Only a measured thermal profile reveals the true temperature of the solder joint, allowing the line to be settuned withfor preciseoptimal numbers, but only a well-placed thermocouple can reveal what the board itself actually experiences. The profile captures the ramp, soak, reflow,yield and cooling story in a single curve—exposing whether solder wets fully, plastics survive safely, and thermal imbalances are under control. Choosing between soak and ramp-to-peak strategies, and tuning time above liquidus with belt speed and zone adjustments, transforms reflow from a risky guess into a predictable process.

reliability.

3.2.1 WhatThe aPurpose “profile”of actuallythe isThermal Profile

A successful profile isplot justmust a temperature vs. time story told by a few well-placed thermocouples (TCs) as your board ridesconfirm the oven.assembly You’remeets provingthe fourrequirements thingsof atthe once:chosen solder paste and the most heat-sensitive component. Four critical process factors are proved simultaneously:

1. Ramp Rate: isn’tConfirms toothe aggressive,temperature increase is gradual enough to prevent thermal shock to ceramics or integrated circuit packages (typically 1–3˚C/second).
2. Soak/Dwell Time: Ensures temperature differences across the assembly equalize before reflow and provides sufficient time for flux activation and volatile outgassing.
3. SoakTime Above Liquidus (TAL): evensMeasures temperaturesthe andduration activatesthe flux,solder is molten. This is the wetting budget required to form a reliable intermetallic bond.
4. TALPeak Temperature and ∆T: Confirms the maximum temperature does not damage components while ensuring the Cross-Board Temperature Differential (∆T) is long enough to fully wet—but not so long you cook parts,
4. ΔTminimized (spread)typically at peak is tight across the board.≤10˚).

Keep that picture in your head; everything below is how to measure and shape it.

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3.2.2 ThermocouplesThermocouple thatPlacement telland Attachment Accuracy

The accuracy of the truthprofile (attachrelies likeentirely you mean it)

Pickon the TCquality spots first, thenof the adhesive.thermal Youmeasurement. wantThermocouples (TCs) must be attached directly to the copper/solder pad temperatures, not free-air.the solder mask or component body.

A) Critical TC Placement Strategy

WhereA minimum of six TCs should be used to placemap (6the TCsworst-case isthermal a sweet spot)performance:

• Cold spotSpot:: usually underTypically a densehigh-thermal-mass area, such as a ground plane, large BGA pad, or bigconnector groundpin plane.tied to heavy copper.
• Hot spotSpot:: A low-thermal-mass area, such as an isolated pad for a small chip jungle(0201) located near openthe copper.edge of the panel.
• Component Risk: Placed on the body of the most heat-sensitive component (e.g., plastic connector, electrolytic capacitor) to ensure its maximum temperature rating is not exceeded.
• QFN/LFPAKThermal Mass Check: Placed on a QFN thermal pad: voiding/tiltor liveslarge here.power MOSFET to monitor voiding risk and temperature rise.
• Edge vs. centerCenter:: TCs placed at the panel edge and panel center to see ΔT acrossmeasure the panel.oven's
• Plastic/connectorconvection risk: make sure you’re not exceeding its limit.
• Second-side riskuniformity (on∆T).

2-pass

B) builds)Attachment Methodology

The TC bead must have direct, reliable contact with the solderable copper pad. * Solder Dot (Best): underTin the pad, place the TC bead, and secure it with a talltiny partdot alreadyof soldered.high-temperature solder. This provides the most accurate reading of the pad metal temperature.

How to attach (ranked by accuracy)

• Micro-solderHigh-Temp dot to padEpoxy: (best): tin the pad, setPress the TC bead ininto a tiny solder dot. Measures pad metal directly. Keep thesmall dot tinyof so you don’t add mass.
• High-temp epoxy: TC bead pressed intohigh-temperature epoxy on the pad. VeryThis goodis ifthe solderbest isn’tmethod allowed;when curesoldering perdirectly datasheet.to the pad is prohibited.
• Kapton assistProhibition:: tape can holdTaping the wire, but don’t rely on tape alone—theTC bead will read air.
• Don’t: blob tape on mask, clip to component plastic, or let the bead float—those read low and late.

Two pro tips

• Strain-relief the wire so it doesn’t peel mid-run.
• Trimover the solder mask windowor slightlyattaching ifit youto must;component betterplastic contactis beatsprohibited, prettyas tape.this measures air temperature, introducing significant error.
  

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3.2.3 TwoProfile profile styles:Styles: Soak vsvs. Ramp-to-Peak

BothThe canchoice beof “right.”profile Chooseshape is determined by the onecomplexity thatof fitsthe your board, paste,board and defectthe risks.requirements of the solder paste alloy.

Profile Style	WhatPrimary it doesFeature	WhenApplication itand shinesBenefit	Watch-outsRisk and Trade-Off
Soak	Holds intemperature a mid-rangeconstant (≈e.g., 150–180 °180˚C) tofor equalize60–120 tempsseconds andbefore outgasthe volatiles,final then goesramp to peak.	Equalization.Mixed- Essential for boards with high-variance thermal mass boards,(large bigBGAs copperand imbalance,small QFNchips). voidingMinimizes risk.∆T before reflow.	Flux Exhaustion. Too long →a soak can prematurely deplete flux exhaustion,activators, leading to poor wetting and increased risk of solder balls; too hot → early oxidation.balls.
Ramp-to-Peak	Smooth,Near-linear near-lineartemperature climb directly to peak;peak, shortbypassing dwellthe abovesoak liquidus.phase.	Speed and Cleanliness.Even-mass boards,Ideal for boards with uniform thermal mass, low-temperature alloys, and fine-pitch components where tombstoning/bridgesminimizing hatetime longis soaks; low-temp alloys.critical.	High ∆T. If ΔTthermal mass variance is large,high, weakcold corners may under-reflow.not reflow adequately before the hot spots exceed limits.

HowStrategy: toThe pickRamp-to-Peak fast

profile

• Ifis generally preferred for ΔT at peakthroughput isif alreadythe <10–12cross-board °C, a ∆ramp-to-peakT iscan usuallybe cleanermaintained and faster.≤
• If 12˚Cvoiding or cold corners haunt you, try a gentle soak to equalize and vent before peak.
  

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.

3.2.4 Tuning Time Above Liquidus (TAL):

when

TAL is the most critical factor for joint reliability. It must be long enough to lengthen,achieve whenfull intermetallic formation but short enough to shortenprevent

TALexcessive =component time above the alloy’s melt pointcooking and intermetallic thickness (≈217which °Ccauses for SAC, ≈183 °C for SnPb, lower for Bi-based)brittleness). It’s your wetting “budget.”

• Tuning TAL: Belt speed is the primary tool for adjusting TAL. Slowing the belt lengthens TAL; speeding it up shortens it. Zone temperature settings primarily control the peak temperature.
• Lengthen TAL When: whenDefects you seelike head-Head-in-pillowPillow (HIP) on BGAs or unevenpoor wetting on massive copper.copper Keepplanes peakare modest;observed. extendHIP time.often indicates insufficient time at high temperature to collapse the joint fully.
• Shorten TAL When: whenDegradation cosmeticsof degrade,plastics, componentsexcessive arejoint heat-sensitive,discoloration, or fluxhigh-risk is aggressive (water-soluble): hit peak cleanly, exit sooner.
• Second-second-side reflow: keep(where component damage is a major risk) is a concern.

3.2.5 The Repeatable Profiling Routine

A standardized, iterative approach is required to establish a stable profile.

1. Seed Recipe: Start with a template from the paste vendor or the recipe used for the most similar board mass/alloy.
2. TC Wiring: Wire 4-6 TCs to the determined hot/cold/risk spots on a scrap board (Section 3.2.2).
3. First Run: Run the board and log the raw data.
4. TAL Adjustment: Adjust the TALbelt just enoughspeed to reflowbring jointsthe withoutTAL over-cookinginto first-sidethe plastics—oftenrequired arange slightly(40–80 seconds for SAC alloys is typical).
5. Peak Adjustment: Adjust the final 1–2 lowerreflow zone setpoints to bring the peak temperature within component limits.
6. ∆T Tightening: Adjust the soak zone temperature and shorterblower TALfan works if printing is solid.

Rule-of-thumb bands (validate for your paste):

• SAC: TAL ~40–80 s, peak 235–250 °C.
• SnPb: TAL ~30–60 s, peak 205–220 °C.
• Low-temp Bi: TAL tighter, ~30–60 s, peak per datasheet (often 165–185 °C).
  




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3.2.5 A quick, repeatable profiling routine

1. Seed recipe: start from your paste vendor’s template or your last similar board.
2. Wire 4–6 TCsspeeds to realpull padsthe (8.2.2),cold labeland them.hot spots closer together, ensuring ∆T is minimized.
7. FirstLock passand Document:: runOnce all metrics are within tolerance, lock the board,zone settings and save the rawGolden plot.
4. Set TAL with belt speedPlot: too short → slow belt; too long → speed up.
5. Set peak with late-zone temps: adjust the final 1–2 reflow zones.
6. Tighten ΔT with soak/blowers: nudge mid-zones and blower speeds to pull corners/center together.
7. Second-side (if needed): clone recipe, drop peak a bit, re-prove TAL..

Final Checklist: Profile Acceptance

Metric	Requirement	Tuning Action
TAL	OnlyMust changebe onewithin knobpaste atvendor alimits time(e.g., re-run,40–80s andfor annotate the recipe (“+10 mm/min belt; TAL 48→62 s”)SAC).	3.2.6 Fast DOE when you’re chasing defects If you’re stuck between “almost good” profiles, run a pocket DOE over a handful of boards: Factor A: Belt speed (−10%is /the nominalprimary / +10%)lever. Factor B:
Peak Temp	Must be ≤ component maximum ratings.	Final zone setpointsetpoints (−5are °the primary lever.
∆T	Cross-board temperature difference ≤ 10˚C /(at nominal / +5 °C) Optional C: Blower (−1 / 0 / +1 step)peak or soak).	AtmosphereSoak zone settings (air vs N₂)  Measure BGA HIP, QFN voiding, tombstones, and AOI/AOI-related calls. Keep the combo that fixes the Pareto without cooking everything else. 3.2.7 Special cases (where profiles get touchy) Massive thermal pads (QFN/LFPAK): prefer soak + modest peak; windowed apertures help more than heat (see 7.4). Fine-pitch BGA/CSP: HIP hates short, spiky TAL—smooth ramp and steadyblower speeds.
Ramp Rate	≤ 3˚C/second to prevent thermal shock.	Preheat zone settings TAL. N₂ can buy you margin. Tall connectors/plastics: guard maximum part temperature; consider heat shields or a cooler second-side profile. Low-temp alloys: narrower window—avoid long soaks; gentle ramp-to-peak is your friend. Warp-prone panels: fix mechanics (supports/rails) before you chase with heat (9.1.8). 3.2.8 What “good” looks like on the plot Ramp ≤ ~2 °C/s (no saw-teeth). Soak (if used) flat and calmconveyor speed.
Documentation,	Golden 60–120 s in range; ΔT shrinking before reflow. TAL inside target, smooth through liquidus (no dip). Peak inside paste + part limits, ΔT at peak ≤10–12 °C typical. Second-side: same shape, slightly gentler. 3.2.9 Lock it and make it findable Save as: Alloy_Side_PanelMass_Air/N2_vX (e.g., SAC_Top_4upHeavy_Air_v3). Attach the golden plotPlot and TC map to the recipe. Note belt speed, zone temps, blower %,saved and atmosphere. Storelinked withto the Golden Recipe.	Recipe control system (8.5) so night shift loads the same truth. 3.2.10 Release checklist (tape this to the oven PC) TCs soldered/epoxied to pads; spots labeled (cold/hot/QFN/edge/connector/2nd-side)MES). Ramp, soak (if used), TAL, peak inside paste + part limits. ΔT at peak within target (≤10–12 °C typical). Second-side recipe proven (if applicable). Golden plot + recipe saved with clear name/comments.

A disciplined profiling routine—grounded in solid thermocouple attachment, clear recipe documentation, and careful tuning of TAL—ensures stable reflow performance across boards and shifts. Done well, it prevents hidden defects, protects sensitive parts, and creates a reliable thermal recipe that production can trust run after run.