Reflow Soldering Profiles: SAC305, Lead-Free & Leaded

Reflow soldering melts solder paste to form permanent joints between surface-mount components and PCB pads. The thermal profile — the time-temperature curve the board follows through the reflow oven — is the single most important process variable. An incorrect profile causes cold joints, tombstoning, component cracking, or electrolyte boiling; all of which pass visual inspection and fail in the field. Profile verification is a key part of quality inspection for any PCB assembly project.

Overview

A conveyor reflow oven has 6–14 temperature zones. The board travels through each zone at a controlled speed, experiencing a rising and falling temperature profile. The profile must heat the board enough to fully melt and wet the solder, while staying below the damage limits of every component on the board. For mixed assemblies (through-hole pins wave-soldered into a board with existing reflow SMD components), the reflow profile must also not re-melt joints from the previous process.

Two dominant solder systems in China production:

• SAC305 (Sn96.5/Ag3.0/Cu0.5): lead-free, RoHS compliant, liquidus 217°C
• Sn63Pb37: leaded, liquidus 183°C — still used for industrial, military, and some high-reliability applications exempt from RoHS

Key Parameters

Zone-by-Zone Profile Breakdown

Zone 1–3: Preheat (25°C → 150°C) Goal: raise board temperature gradually to avoid thermal shock, which cracks multilayer ceramic capacitors (MLCCs). Ramp rate must not exceed 3°C/sec (SAC305) or 2°C/sec (leaded). At this stage, residual moisture on the board or in component packages is driven off. If moisture is not driven off before the soak zone, it vaporizes in the solder paste, creating voids.

Zone 3–5: Soak/Activation (150–200°C for SAC305) The solder paste flux activates and cleans the pad and component surfaces (removing oxides). The paste must stay in this zone long enough for full activation — 60–120 seconds. Too short: poor wetting on oxidized pads. Too long: flux burns off before reflow, also causing poor wetting. During soak, the board temperature equalizes across the panel — critical for boards with large thermal mass differences between zones.

Zone 5–7: Reflow (above liquidus, 217–250°C for SAC305) Solder melts, wets pad and component termination, and forms the joint. Time above liquidus (TAL) of 30–90 seconds allows complete wetting without overheating components. Peak temperature is measured at the coldest point on the board (under the heaviest thermal mass, such as a power connector or large heatsink pad). Many factories measure peak at the easiest thermocouple location — require that the thermocouple is placed under the most thermally challenging component.

Cooling zone: Peak → 100°C Rapid but controlled cooling (<4°C/sec). Too slow: intermetallic layer grows thicker, reducing joint strength over time. Too fast: thermal shock, or solder joint structure has insufficient time to organize — can produce grainy, brittle joints. Nitrogen atmosphere (if used) prevents re-oxidation during cooling.

Low-Temperature Alternatives

SnBiAg (e.g., Sn42/Bi57/Ag1): Liquidus ~138°C, peak profile 150–165°C. Used for heat-sensitive components that cannot withstand 250°C SAC305 peaks — some thin-die QFNs, certain display modules, components with plastic connectors rated to 180°C. Lower peak temperatures reduce PCB warpage in large-format boards. Trade-off: lower tensile strength and thermal fatigue resistance than SAC305. Not recommended for high-vibration or thermal-cycling applications.

J-STD-020 Moisture Sensitivity Levels (MSL)

Components absorb moisture during storage. If moisture-saturated components enter the reflow oven, water vaporizes and expands inside the package faster than it can escape, causing internal delamination (“popcorning”) — often invisible externally but devastating to reliability. J-STD-020 classifies components by how quickly they absorb moisture to unsafe levels:

BGAs and QFNs are frequently MSL 3 or higher. If they have been in open storage for more than 168 hours (or the factory can’t confirm storage conditions), they must be baked before reflow: 125°C for 8–24 hours depending on package thickness and MSL level, per J-STD-033.

Ask the factory: “How do you track MSL for components? Do you log when reels are opened?” If they don’t have a procedure, you are at risk.

Nitrogen Atmosphere Reflow

Replacing air with nitrogen (O₂ <100 ppm) inside the oven prevents solder and pad oxidation during reflow. Benefits: better wetting, lower defect rates on fine-pitch components, ability to use no-clean flux with minimal residue. Cost: adds $0.10–0.30/board for nitrogen consumption. Required for: some lead-free no-clean processes on 0.3 mm pitch BGAs; recommended for high-reliability Class 3 assemblies.

What to Specify When Ordering from China

• Solder alloy: SAC305, Sn63Pb37, or specific low-temp alloy — state which and reference J-STD-006 alloy designation
• MSL handling procedure: require factories to document component MSL levels and bake history; make this a factory audit checklist item
• Profile characterization on your board: require thermocouple log from the first production run placed at the worst-case thermal location, not a generic saved profile
• Nitrogen atmosphere: specify “yes” or “no” — if not specified, most factories default to air
• First article inspection (FAI): require one board from the first panel to be X-rayed and cross-sectioned to verify joint quality before approving mass production

Quality Checks

The definitive check: thermocouple-logged profile for your specific board, with the thermocouple taped to the underside of the most thermally massive component. Compare measured peak temperature, TAL, and ramp rates against the specification table above. Request this as part of first article inspection documentation.

Cross-section microscopy on BGA joints reveals: intermetallic layer thickness (1–3 µm ideal), void percentage, and joint geometry. For production, X-ray AXI confirms joint formation on every board.

Common Issues

Cold solder joints on SAC305: Peak temperature insufficient or TAL too short. SAC305 requires higher peak than leaded — factories running SAC305 paste through a profile tuned for leaded solder will produce cold joints that may test OK electrically (contact resistance) but fail thermomechanically under vibration or thermal cycling.

Cracked MLCCs: Ramp rate exceeded 3°C/sec in preheat, or board dropped into a cold fixture immediately after reflow. MLCCs (especially 0402 and 0201 X7R/X5R) are brittle; thermal gradient across their body causes mechanical stress fractures. Microcracks often pass electrical test at room temperature but fail at elevated temperature or after mechanical stress.

Electrolytic capacitor failure: Peak temperature exceeded component rating (typically 260°C for 10 seconds per J-STD-020). Electrolyte boils, venting the capacitor. More common on high-density boards where airflow in the oven is uneven and hot spots form. Verify component temperature ratings against worst-case oven profile.

Related Resources

• SMT Assembly Process — complete SMT line flow
• IPC-A-610 Acceptance Criteria — joint quality classification
• DFM Guidelines — design rules that affect thermal profile behavior
• Factory Audit Checklist
• Quality Inspection Services
• PCB Manufacturing & SMT Sourcing
• PCB Assembly China Buyer’s Guide