What Is Reflow Soldering?
Also known as: reflow oven, reflow profile, SMT reflow
Definition
Reflow soldering is the process that melts pre-applied solder paste in a controlled-temperature oven so surface-mount components bond to the board pads. The time and temperature curve is called the reflow profile.
Reflow Soldering Explained
After pick-and-place, boards travel on a conveyor through an oven with several heating zones. A typical profile has four stages: preheat to ramp temperature gently, soak to activate flux and equalise temperature across the board, reflow where the solder alloy melts above its liquidus, and cooling to solidify the joint. Lead-free SAC alloys reflow at higher peak temperatures than tin-lead, commonly in the 235 to 250 C range at the joint.
Profiles are set by measuring actual board temperature with thermocouples and a profiler, not just oven setpoints. A profile must keep every component below its maximum body temperature while still fully melting the solder on large thermal-mass parts. Poor profiles cause tombstoning, voiding, cold joints, warpage or component damage.
Moisture sensitive components can crack or delaminate in reflow if they were exposed to humidity beyond their allowed floor life, which links reflow quality back to storage and handling controls.
In ERP and MES terms, the reflow profile is a controlled routing parameter. A work order operation references the approved profile ID, and the MES records oven recipe, line and time for each lot. This lets quality teams link a defect cluster to a specific oven run in the lot record.
Why It Matters
- Reflow is where most solder defects are created, so profile control directly drives first pass yield.
- Profile and oven recipe data forms part of the process record auditors ask for in aerospace and medical builds.
- Mixed lead-free and tin-lead builds in the same factory need strict segregation to avoid contamination.
- Component moisture exposure before reflow is a planning and storage issue the ERP inventory process can help enforce.
In Practice
A new board with a large ground-plane connector shows voiding in X-ray. The process engineer re-profiles with thermocouples on the connector and extends the soak by 20 seconds. The new profile is saved as revision B and attached to the routing, and the ERP work order operation is updated so only the approved recipe is used on subsequent lots.
Frequently Asked Questions
What are the stages of a reflow profile?
Preheat, soak, reflow and cooling. Each stage has controlled ramp rates and times to protect components and form reliable joints.
What defects come from a bad reflow profile?
Tombstoning, solder balling, voids, cold joints, warpage and thermal damage to components.
How is a reflow profile controlled in a factory system?
It is stored as an approved recipe linked to the routing operation, and the MES records which recipe ran for each lot.
Related Terms
Surface Mount Technology (SMT)
Surface Mount Technology (SMT) is the method of placing and soldering electronic components directly onto the surface of a printed circuit board, instead of inserting leads through holes. It is the dominant way modern electronics are assembled.
Moisture Sensitivity Level (MSL)
Moisture Sensitivity Level (MSL) is a rating, defined in the J-STD-020 and J-STD-033 standards, that states how long a surface-mount component can be exposed to factory air before it must be baked, to avoid cracking during reflow.
First Pass Yield
First pass yield is the percentage of units that complete a process correctly the first time, without rework, repair, or retest. Units that eventually pass after rework are excluded, so the metric captures hidden factory loss.
Rework
Rework is action taken on nonconforming product to bring it into full conformance with the original specification. It differs from repair, which makes product acceptable for use without meeting the original requirement.
SPC (Statistical Process Control)
SPC (Statistical Process Control) is the use of control charts and statistical rules to monitor a process in real time, distinguishing normal common-cause variation from special-cause variation that signals a genuine change requiring action.
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