2.13 Profiling Methods

Profiling is a distinct step from zone control. While Chapterprevious 3.1chapter addressed the physics of heat distribution in the oven, this chapter addresses the methodology of measuring the thermal experience of the Printed Circuit Board (PCB) itself. Only a measured thermal profile reveals the true temperature of the solder joint, allowing the line to be tuned for optimal yield and reliability.

3.2.13.1 The Purpose of the Thermal Profile

A successful profile plot must confirm the assembly meets the requirements of the chosen solder paste and the most heat-sensitive component. Four critical process factors are proved simultaneously:

Ramp Rate: Confirms the temperature increase is gradual enough to prevent thermal shock to ceramics or integrated circuit packages (typically 1–3˚C/second).
Soak/Dwell Time: Ensures temperature differences across the assembly equalize before reflow and provides sufficient time for flux activation and volatile outgassing.
Time Above Liquidus (TAL): Measures the duration the solder is molten. This is the wetting budget required to form a reliable intermetallic bond.
Peak Temperature and ∆T: Confirms the maximum temperature does not damage components while ensuring the Cross-Board Temperature Differential (∆T) is minimized (typically ≤10˚).

3.2.13.2 Thermocouple Placement and Attachment Accuracy

The accuracy of the profile relies entirely on the quality of the thermal measurement. Thermocouples (TCs) must be attached directly to the copper/solder pad, not the solder mask or component body.

A) Critical TC Placement Strategy

A minimum of six TCs should be used to map the worst-case thermal performance:

Cold Spot: Typically a high-thermal-mass area, such as a ground plane, large BGA pad, or connector pin tied to heavy copper.
Hot Spot: A low-thermal-mass area, such as an isolated pad for a small chip (0201) located near the 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.
Thermal Mass Check: Placed on a QFN thermal pad or large power MOSFET to monitor voiding risk and temperature rise.
Edge vs. Center: TCs placed at the panel edge and panel center to measure the oven's convection uniformity (∆T).

B) Attachment Methodology

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

High-Temp Epoxy: Press the TC bead into a small dot of high-temperature epoxy on the pad. This is the best method when soldering directly to the pad is prohibited.
Prohibition: Taping the TC bead over the solder mask or attaching it to component plastic is prohibited, as this measures air temperature, introducing significant error.

3.2.13.3 Profile Styles: Soak vs. Ramp-to-Peak

The choice of profile shape is determined by the complexity of the board and the requirements of the solder paste alloy.

Profile Style	Primary Feature	Application and Benefit	Risk and Trade-Off
Soak	Holds temperature constant (e.g., 150–180˚C) for 60–120 seconds before the final ramp to peak.	Equalization. Essential for boards with high-variance thermal mass (large BGAs and small chips). Minimizes ∆T before reflow.	Flux Exhaustion. Too long a soak can prematurely deplete flux activators, leading to poor wetting and increased risk of solder balls.
Ramp-to-Peak	Near-linear temperature climb directly to peak, bypassing the soak phase.	Speed and Cleanliness. Ideal for boards with uniform thermal mass, low-temperature alloys, and fine-pitch components where minimizing time is critical.	High ∆T. If thermal mass variance is high, cold corners may not reflow adequately before the hot spots exceed limits.

Strategy: The Ramp-to-Peak profile is generally preferred for throughput if the cross-board ∆T can be maintained ≤12˚C.

3.2.13.4 Tuning Time Above Liquidus (TAL)

TAL is the most critical factor for joint reliability. It must be long enough to achieve full intermetallic formation but short enough to prevent excessive component cooking and intermetallic thickness (which causes brittleness).

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: Defects like Head-in-Pillow (HIP) on BGAs or poor wetting on massive copper planes are observed. HIP often indicates insufficient time at high temperature to collapse the joint fully.
Shorten TAL When: Degradation of plastics, excessive joint discoloration, or high-risk second-side reflow (where component damage is a major risk) is a concern.

3.2.13.5 The Repeatable Profiling Routine

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

Seed Recipe: Start with a template from the paste vendor or the recipe used for the most similar board mass/alloy.
TC Wiring: Wire 4-6 TCs to the determined hot/cold/risk spots on a scrap ~~board (Section 3.2.2).~~board.
First Run: Run the board and log the raw data.
TAL Adjustment: Adjust the belt speed to bring the TAL into the required range (40–80 seconds for SAC alloys is typical).
Peak Adjustment: Adjust the final 1–2 reflow zone setpoints to bring the peak temperature within component limits.
∆T Tightening: Adjust the soak zone temperature and blower fan speeds to pull the cold and hot spots closer together, ensuring ∆T is minimized.
Lock and Document: Once all metrics are within tolerance, lock the zone settings and save the Golden Plot.

Final Checklist: Profile Acceptance

Metric	Requirement	Tuning Action
TAL	Must be within paste vendor limits (e.g., 40–80s for SAC).	Belt speed is the primary lever.
Peak Temp	Must be ≤ component maximum ratings.	Final zone setpoints are the primary lever.
∆T	Cross-board temperature difference ≤ 10˚C (at peak or soak).	Soak zone settings and blower speeds.
Ramp Rate	≤ 3˚C/second to prevent thermal shock.	Preheat zone settings and conveyor speed.
Documentation	Golden Plot and TC map saved and linked to the Golden Recipe.	Recipe control system (MES).