Burnback is when the arc climbs the wire and fuses it solid into the contact tip — it ruins the tip, stops you cold, and usually traces to one of five things: the wire feeds slower than the arc melts it, the stickout is too short, the feed stutters, the tip sits wrong in the nozzle for the current, or the ground is flaky. It is the same family of problem as a wire that won't feed (an erratic feed causes burnback), so if yours burns back constantly, fix the feed first. Here are the five causes and the fix for each.
Burnback happens worst right at arc start, and the arc sounds uneven. The arc is melting wire faster than the rolls deliver it, so it melts back to the tip.
Raise the wire-feed speed (or drop the voltage) until the arc note evens out to a steady sizzle. A feed speed a touch slow for the heat is the single most common burnback cause.
You're welding with the nozzle almost on the work, very little wire showing past the tip.
Hold roughly 3/8–1/2 in of stickout (wire past the tip). Crowding the work puts the contact tip right in the arc heat, which melts the wire back into it. Back off to a normal stickout.
The bore of the tip is grooved or oval, drive-roll tension is off, or the liner drags — any of which makes the wire feed in stutters, and each momentary starve burns back.
Fix the feed path first: replace a grooved/oversized tip with the correct ID for your wire, set drive-roll tension by the half-turn-past-slip method, and clear or replace a fouled liner. See the full wire-won't-feed guide for the step-by-step.
Burnback on higher-amp work with the tip sitting flush with — or proud of — the nozzle.
Recess the tip deeper into the nozzle as the current rises, to keep it out of the arc's radiant heat; set it back near flush for low-amp short-arc work. Matching tip recess to amperage is a common fix overlooked on spray and high-current settings.
The work clamp is on a painted or dirty bench, or arc is intermittent while the wire keeps feeding steadily.
Clamp the work lead directly to clean base metal, not the welding table. A flaky ground drops arc voltage while the wire keeps coming, and the wire fuses to the tip. Clean the clamp contact point to bright metal.