Jason Kilbride’s late winner sent Deeping Rangers into the Lincolnshire FA Senior Trophy final.
They will face Cleethorpes Town on Tuesday, April 11, at Lincoln City’s Sincil Bank.
Kilbride scored for the fourth successive game when he ended Boston Town’s bid for honours.
Deeping had to fight their way to a fifth consecutive win in the freezing fog against a well-drilled Poachers side on Tuesday night.
The Clarets took the lead through Scott Mooney on his return from a two-match ban.
But they were pegged back in first-half added-time to a well-placed shot from outside the box by Alex Revell.
Now we need to regroup after the cup semi-final and keep rolling.Michael Goode
In a frustrating second half, Kilbride struck with 10 minutes left.
Deeping are back in action on Saturday at Desborough Town who are just two places and six points behind them in the ChromaSport & Trophies United Counties League’s Premier Division.
Ar Tarn are unbeaten in 11 games since a 2-1 home loss to Yaxley at the end of November.
Rangers manager Michael Goode said: “Now we need to regroup after the cup semi-final and keep rolling.
“This is the start of a run of five games against tough opposition in the top seven over the next six weeks.
“It’s in our hands as we’ve done well since the defeat at Peterborough Sports.”