In short: A reflow profile is a controlled temperature-versus-time recipe in four stages (preheat, soak, reflow, cooling) that must melt lead-free solder at about 217 to 221 °C without exceeding component limits, and every defect from tombstoning to voiding traces back to a mistake in one of those stages.
Why the profile matters more than the oven
Surface-mount assembly prints solder paste on pads, places components, and sends the board through a multi-zone oven. The paste is a suspension of tiny solder spheres in flux. Whether the result is a shiny, wetted fillet or a grainy, voided joint depends on the temperature the joint itself experiences over time, not on the oven's set points. That curve is the reflow profile, and it is measured with thermocouples attached to a real board that runs through the oven.
Think of it as a thermal budget. The paste needs enough heat to activate flux and melt; the laminate, the ceramic capacitors and the silicon packages need to be spared too much. The profile is the negotiation between those two limits. If you are new to board construction, start with what a PCB is, because the board's copper and layer count directly decide how it heats.
Stage 1: Preheat
The board climbs from room temperature to roughly 150 °C at no more than about 3 °C per second. Too fast and two things go wrong. Solvents in the paste boil violently and spit solder out from under the stencil aperture, leaving solder balls beside the pad. And different materials expand at different rates, so ceramic chip capacitors can crack from thermal shock.
Moisture is the other hidden risk. Plastic packages that absorbed humidity can pop their moulding compound when the vapour expands, the classic "popcorning" failure. This is why parts carry a moisture sensitivity level (MSL) under IPC/JEDEC J-STD-020 and why opened reels are baked or kept in dry cabinets.
Stage 2: Soak (thermal equilibrium)
The soak holds the assembly between roughly 150 and 200 °C for 60 to 120 seconds. Its job is to let a heavy BGA and a tiny 0402 resistor reach nearly the same temperature, and to let the flux do its chemistry: strip oxide from pad and ball surfaces so molten solder can wet them.
A soak that is too short leaves large parts behind. They then reach liquidus late, so small neighbours melt first and pull the part sideways. That is the root of tombstoning: one end of a chip melts, surface tension lifts the other end off the pad. A soak that is too long or too hot burns off flux activity before melting, which causes head-in-pillow defects on BGAs, where ball and paste touch but never coalesce.
Key fact: The goal of the soak is a small temperature difference across the board at the moment it enters reflow, typically under 10 to 15 °C between the coolest and hottest joint.
Stage 3: Reflow (the only stage that makes a joint)
Temperature now rises through the liquidus. Common SAC305 (Sn96.5/Ag3.0/Cu0.5) melts at about 217 to 221 °C, while eutectic tin-lead melts at 183 °C. The typical lead-free window is a peak of 240 to 250 °C with a time above liquidus (TAL) of 45 to 90 seconds. Tin-lead needs only a 210 to 220 °C peak.
The peak has a hard ceiling: each component datasheet cites its maximum body temperature, classified by package thickness and volume in J-STD-020. Small thin parts may tolerate 260 °C, while large thick BGAs and electrolytic capacitors are often limited to 245 °C or less. The coldest joint must still clear the liquidus by around 10 to 15 °C, and the hottest component must stay under its limit, so the usable window on a complex board can be only 15 to 20 °C wide.
Too little time above liquidus gives cold, dull, poorly wetted joints. Too much grows a thick brittle intermetallic layer (Cu6Sn5 and Cu3Sn) at the pad interface, which fractures under vibration or drop. Slow, long reflow also lets flux gases escape too late and increases voiding.
| Parameter | SnPb (63/37) | SAC305 |
|---|---|---|
| Liquidus | 183 °C | 217–221 °C |
| Typical peak | 210–220 °C | 240–250 °C |
| Time above liquidus | 45–75 s | 45–90 s |
| Soak | 120–170 °C | 150–200 °C |
| Max cooling rate | ≈ 4 °C/s | ≈ 6 °C/s |
Stage 4: Cooling
Once the solder drops below liquidus it solidifies and the microstructure is locked in. A moderate cooling rate (around 2 to 6 °C/s) produces fine grains and a strong joint. Cooling too slowly coarsens the grains and thickens intermetallics; cooling too fast induces thermal stress, warps boards and can crack joints or ceramic parts.
Key fact: Most field failures that appear "random" after months of thermal cycling were set during reflow: coarse grain structure, thick intermetallics or hidden voids created in the first few minutes of the product's life.
Design choices that change the profile
The profile is not just a manufacturing setting; your layout feeds it. A large copper pour or ground plane sinks heat, so a pad tied to it lags behind an isolated pad, which is why thermal reliefs exist: they slow heat leaking out of the pad so it melts in step with its neighbours. Board stack-up matters too; a heavy plane-rich stack takes much longer to equalise than a light one, as you can see when comparing single and multilayer PCBs. Asymmetric pad sizes or traces on a two-pad chip are a direct recipe for tombstoning.
Layout tools help catch this earlier. Cadence Allegro and Siemens Xpedition with Valor-style DFM checks can flag unbalanced pads and copper imbalance before fab, and Synopsys-style thermal analysis can estimate hot spots when a package dissipates power. For hands-on design-service support, the engineers at Avecas review layouts for manufacturability.
Animation: one joint travelling through the oven
How engineers tune a profile
- Attach thermocouples to the coldest spot (a BGA centre ball, a heavy connector) and the hottest (a small passive near the board edge).
- Start from the paste maker's recommended window, then adjust belt speed and zone set points until all probes sit inside the limits.
- Check the process window index, then inspect with X-ray for voids (many specifications target under 25 % per BGA ball area) and cross-sections for intermetallic thickness.
- Use nitrogen only if oxidation is the proven cause; it improves wetting but adds cost.
These skills sit alongside layout knowledge from topics such as via aspect ratio and plating, since plated barrels also react to reflow heat. To go deeper, browse our CourseTron courses, and see hands-on training from ChipXpert.
Ready to design boards that survive the oven? Explore the PCB design track in our CourseTron courses and learn layout, stack-up and DFM step by step.
Frequently asked questions
What is the typical peak temperature for lead-free reflow?
Most SAC305 assemblies peak between 240 and 250 °C, which is 20 to 30 °C above the liquidus, while respecting each component's J-STD-020 limit.
Why do chip resistors stand up on one end?
That is tombstoning. One pad melts first, and surface tension pulls the part upright. Balanced pads and copper, and a proper soak, fix it.
What does time above liquidus mean?
It is the number of seconds the joint spends hotter than the solder's melting point, typically 45 to 90 seconds for lead-free alloys.
Can I use one profile for every board?
No. Thermal mass, copper area, component mix and paste all change the curve, so each product needs its own measured profile.