Hot-air reflow profile for a 0.8 mm BGA
A BGA cannot be soldered with an iron. Every joint is under the package, so the entire process is thermal: the goal is to get all the balls above liquidus within a few seconds of each other and let the package settle under its own weight. A profile that is too fast leaves the outer perimeter reflowed and the centre still pasty.
Procedure
- 1
Tape a 0.1 mm thermocouple under the package edge and set the bottom pre-heater to 120 degrees C.
- 2
Run pre-heat at 1.5 to 2 degrees C per second to 150 degrees C, then soak for 60 to 90 seconds between 150 and 180.
- 3
Ramp at 1 to 2 degrees C per second to 235 to 245 degrees C and hold above 217 for 45 to 60 seconds.
- 4
Cap the peak at 245 degrees C for no more than 20 seconds, then cool at 2 to 3 degrees C per second to 150.
- 5
Record the curve, then repeat on the production board with the same settings and compare time above liquidus.
Workshop notes
Three phases and the numbers for each
Pre-heat from ambient to 150 degrees C at 1.5 to 2 degrees C per second, then soak at 150 to 180 degrees C for 60 to 90 seconds so the package and the board equalise. Ramp to 235 to 245 degrees C at 1 to 2 degrees C per second, hold above 217 degrees C for 45 to 60 seconds, and peak at 240 to 245 degrees C for no more than 20 seconds. Cool at 2 to 3 degrees C per second down to 150 - cooling faster than 4 degrees C per second warps thin packages and shows up as cracked balls at the perimeter.
Top and bottom heat must be balanced
A hot-air station with only top heat will reflow the package before the board, and the package will float on a liquid ball field. Set bottom pre-heat to 110 to 130 degrees C so the board is at temperature when the top nozzle opens up. The rule of thumb is that the board should reach 150 degrees C at the same time the package does; if the top thermocouple reads 200 while the bottom reads 90, the profile is going to fail at the perimeter regardless of what the top nozzle does.
Verifying with thermocouples, not with a stopwatch
Attach a 0.1 mm thermocouple under the package edge with high-temperature tape and run the profile once on a scrap board before touching the real one. Record the peak and the time above liquidus. Without this step you are guessing, and the failure mode of guessing is a joint that looks fine, passes continuity, and opens after 50 thermal cycles. Most BGA returns are profiled wrong, not placed wrong.
Practice drill
Run the same profile three times on scrap boards with the thermocouple at the package edge, at the centre, and on the opposite board face. Plot the three curves. If the spread between edge and centre exceeds 12 degrees C at peak, the soak is too short or the bottom heat is too low.
Failure modes
| Symptom | Cause | Treatment |
|---|---|---|
| Perimeter balls reflowed, centre balls still pasty | Ramp too fast with insufficient soak, or bottom heat too low so the board lags the package | Extend the 150 to 180 soak to 90 seconds and raise the bottom pre-heater to 120 degrees C, then re-run the profile with a thermocouple |
| Package warps and the corner balls crack | Cooling faster than 4 degrees C per second, usually from pulling the board straight off the pre-heater | Leave the board on the pre-heater through the cooling phase and let it fall to 150 degrees C naturally before removing it |
| Reflow looks perfect but the unit fails after burn-in | Profile never verified with a thermocouple, so time above liquidus was marginal | Verify every new board type with a thermocouple at the package edge and log the curve before running production units |
Supplies
- ROL1 gel flux for the pad field
- 0.1 mm thermocouple wire and high-temperature tape
- Isopropyl alcohol for post-reflow cleaning
- Scrap boards for profile verification
Tools
- Hot-air rework station with bottom pre-heater
- External thermocouple meter that can log curves
- 10x loupe or X-ray access for post-reflow inspection
- 3.2 mm chisel iron for pad dressing only
Safety
A 245 degrees C hot-air nozzle, a 120 degrees C bottom heater and lead-free alloy all in one process: the dominant injury is a hand burn rather than fume exposure. Use tweezers with long handles, and never bring fingers under the package to steady it.
Neighbouring cards
- Wicking and dressing a BGA pad field 360 °C / 1.5 s
- Corner-tack placement before BGA reflow 340 °C / 1.0 s
- Depositing paste on a BGA footprint with a mini stencil 300 °C / 1.0 s