TL;DR
Solder balls are usually caused by a ramp rate too high, moisture in the PCB, or solder paste slump from printing to reflow delay. Check the preheat profile and board storage first.
What you might see
- solder balls visible around pads on AOI
- solder splatter on PCB surface
- defect rate correlated with high board moisture
- solder ball increase after long staging time
Likely causes
Preheat ramp rate too fast, causing solder paste flux to outgas rapidly and spatter
Moisture absorbed in the PCB laminate, boiling during reflow
Solder paste printed too long before reflow allowing paste slump
Contaminated or degraded solder paste with exceeded shelf or open-time
Required tools
- Thermocouple profiler board
- AOI system for inspection
- Humidity indicator cards for PCB storage
- Solder paste expiry documentation
Safety first
- Solder flux fumes contain rosin and organic acids. Ensure adequate fume extraction at the oven exit.
- Do not handle boards exiting the oven reflow zone with bare hands. Allow boards to cool on a heat-safe rack.
Procedure
- 1
Pull an AOI inspection sample and document the solder ball location pattern on the board.
- 2
Run a profiler board and verify the preheat ramp rate. IPC standards recommend a ramp rate of 1 to 3 degrees C per second for most SAC alloy pastes.[1]
- 3
If the ramp rate exceeds 3 degrees C per second, reduce the conveyor speed or lower the preheat zone set-point incrementally and retest.
- 4
Check the PCB storage log. Boards stored outside of controlled humidity for more than 24 hours should be baked at 120 degrees C for 1 to 4 hours before reflow.
- 5
Check the time between solder paste print and oven entry. Keep the interval below the paste manufacturer's specified open time.
- 6
Verify the solder paste lot expiry date and storage temperature. Discard any paste past its expiry or that was stored outside the temperature range.
- 7
Run a test board after any profile or process change and inspect on AOI. Accept only when solder ball count is below the production defect limit.
Sources
Heller Industries Heller 1900 / 1936 MK7 Reflow / Wave Solder Oven general technical documentation, Heller Industries
Heller Industries reflow oven and IPC-7530 reflow profile general guidelines (general)
More guides for Heller Industries Heller 1900 / 1936 MK7
How to clear a conveyor jam on a Heller 1900 / 1936 MK7 reflow oven
Conveyor jams are usually caused by a warped PCB, a worn chain link, or a drive sprocket particle buildup. Stop the machine, cool the zone, and remove the board before inspection.
How to fix reflow profile deviation on a Heller 1900 / 1936 MK7 reflow oven
Profile deviation is usually a failed heating element, a blower motor fault, or a thermocouple out of calibration. Run a profiler board and compare to the baseline before changing set-points.
Stop fixing the same fault twice.
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