You indicated that you are experiencing micro-balls on a printed circuit board that has NO solder paste on it. My concern is that the circuit board may have some residual solder alloy on it that was left behind during the HASL process.

With nitrogen, these residues of alloy may coalesce and form micro balls. I would suggest that you look at the board under magnification to observe this residue. Or have your board manufacturer try to clean the boards prior to shipping to you.

I hope this will assist you in processes issues.

It sounds like your reflow process is too aggressive. Try reducing the ramp rate so the fluxes don’t burn off as fast.

Consider repeating the test following a baking cycle in a batch oven at 125C for 8 hours to determine whether moisture is a factor.

Micro-balling can be attributed to the print process, placement process, reflow process, the environment or the solder paste itself.

Verify solder paste is properly stored, and allowed to sufficiently warm to ambient temperature prior to being used. Verify proper stencil design guidelines have been followed, especially stencil thickness and aperture size, and verify deposition registration is accurate and repeatable. Verify that the reflow process parameters; (especially the first two minutes) are correct for the solder paste being used.

Too low of a rate-of-rise will contribute to micro-balling by allowing the solder paste to hot-slump. High-humidity, and high-temperature environments can contribute to the cold slump characteristics of solder paste. If boards are being printed and allowed to sit for an extended period of time prior to placement and reflow, cold-slump could be a contributing factor.

Consider evaluating a solder paste with a slightly higher viscosity and/or metal load. This will help combat the affects of both cold and hot slump. Check placement pressures to verify components are not being driven too far into the paste deposit.