Learning objectives
- Walk through the four phases of a reflow profile and what each phase does
- Explain why profile values come from the paste datasheet and component limits
- Match profile faults to the defects they create, such as tombstoning and cold joints
- Describe how a profile is proven using thermocouples on a real board
Lesson
Reflow is where printed paste and placed parts become solder joints. The board rides a conveyor through a convection oven of commonly 8 to 12 zones. Some ovens run a nitrogen atmosphere to protect wetting, and a vacuum assisted variant exists to pull voids out of large joints. The board does not simply get hot. It follows a profile, a planned journey of temperature over time with four phases, and the operator’s job is to understand what each phase is for.
The ramp and preheat phase raises the board at a controlled rate. Heating too fast can shock components and can turn trapped moisture inside moisture sensitive parts into steam, cracking packages in the very failure Stage 1 works to prevent. The soak phase holds the board at a steady warmth so the flux activates and cleans the surfaces, and so large and small components reach a similar temperature, closing the temperature difference across the board. The reflow phase takes the solder above its liquidus point, where it melts, flows and wets the pads and terminations. Two values rule this phase: the peak temperature and the time above liquidus, long enough for every joint to form, short enough to avoid damage. The cooling phase then brings the board down at a controlled rate so joints solidify with a sound structure instead of a grainy, weakened one.
Lead free SAC solders peak at temperatures materially higher than the older tin lead solders, which is why nobody on a competent line works from one remembered number. Peak, soak and time above liquidus are taken from the paste datasheet and checked against component limits, then proven by running a profiler, a recorder carrying thermocouples fixed to a real board, through the oven. James Onsomu teaches in his PCB quality book that every soldering method has quality parameters that must be monitored closely, including temperature profiles, to ensure reliable connections. Defect patterns read like a profile diary. Tombstones stand up when one end of a small part heats and wets before the other. Head in pillow on a BGA means the ball and the paste never melted together. Graping, solder balling, cold grainy joints, dewetting and board discolouration each point back to a phase that ran too hot, too cold, too fast or too long.
Key numbers
- Oven size: commonly 8 to 12 convection zones, with nitrogen and vacuum assisted variants in use
- Peak temperature: set from the paste datasheet, with lead free SAC peaks materially higher than tin lead peaks
- Time above liquidus: set from the paste datasheet, long enough for every joint to form fully
- Profile proof: measured with thermocouples on a real board, never assumed from zone settings alone
Defects to spot
- Tombstoning: one end of a small part heats and wets before the other, from uneven heating or uneven paste
- Head in pillow: the BGA ball and the paste deposit never melt together, often from warpage or a weak profile
- Voiding: gas trapped inside the joint as the solder solidifies
- Graping: paste only partly coalesces, from an exhausted flux or a poor soak
- Solder balling and spatter: moisture or too fast a ramp throwing solder off the pads
- Cold, grainy joints: peak temperature or time above liquidus too low
- Component cracking and popcorning: moisture loaded parts heated too fast
- Board discolouration and warpage: profile too hot or too long
Stage check, self test
Answer each question, then open the answer to check yourself. Passing standard: 80 percent, at least 4 of 5 correct.
1. What happens during the soak phase of a reflow profile?
A. The solder melts and flows
B. The flux activates and large and small parts even out in temperature
C. The board is cooled to room temperature
D. The components are placed
Show answer
Answer: B
2. Where should peak temperature and time above liquidus values come from?
A. A number remembered from a previous job
B. The oldest operator on the shift
C. The paste datasheet, checked against component limits
D. The packing specification
Show answer
Answer: C
3. A row of small resistors is standing upright on one end like tombstones. What is the most likely family of causes?
A. Uneven heating or uneven paste between the two ends of the part
B. Too much nitrogen in the building
C. Excessive cooling at the packing bench
D. A wrong label printer setting
Show answer
Answer: A
4. Joints come out cold and grainy. What does the profile most likely need?
A. A lower peak temperature
B. A shorter time above liquidus
C. More time in storage
D. A higher peak or longer time above liquidus, within datasheet limits
Show answer
Answer: D
5. How is a reflow profile properly proven?
A. By reading the numbers set on the zone controllers
B. By running a profiler with thermocouples attached to a real board through the oven
C. By touching the board as it exits
D. By asking the placement operator
Show answer
Answer: B
Previous: Stage 5: Component placement