Cardiff Blues will play Gloucester in the final of the EDF Energy Cup after winning a cracking second semi-final.
Despite dominating the first half the Blues could only manage a Ben Blair penalty and Jason Spice try to take an 8-0 lead into half-time.
Northampton hit back through Joe Ansbro's well-taken but unconverted try in the corner straight after the break.
Stephen Myler missed the chance to kick the Saints into the lead and a late Blair penalty sealed a Blues victory.
Their win ensures the final on 18 April will be an Anglo-Welsh affair for the fourth year in a row, but had Myler not missed two penalties for Northampton it could easily have been a different result at the Ricoh Arena.
Young hails Blues' display
Former All Black Blair kicked the Blues into an early lead with a penalty and Saints spurned the chance to level when Myler missed a very kickable effort after 10 minutes.
Blair then failed with a long-range effort as both sides, despite trying to play open rugby, struggled to create try-scoring chances.
The Blues were enjoying much the better of territory and possession and, with England hooker Dylan Hartley conceding a couple of penalties at the breakdown, they should have been able to establish themselves in the Saints half.
However, Blues number 10 Nicky Robinson twice missed touch and Northampton were able to relieve the pressure.
The game was being played at a ferocious pace and just after the half-hour mark the Blues went ahead as scrum-half Spice sniped over for an unconverted try after a surging attack by the Welsh side.
Mallinder proud of Northampton effort
It gave them an 8-0 lead at the break but it was a poor reward for their control, and within two minutes of the restart their lead was down to just three points.
Myler made a fine break and when he fed Ansbro the centre did superbly to ride the final tackle and just stay in touch before diving over in the corner.
Myler failed to nail the conversion from the touchline but the Saints were in business and they took control of the game for a spell.
Winger Paul Diggin was called back for a forward pass when clear and Myler knocked on to halt another searching attack.
The Blues were under huge pressure but they lifted the siege with a searing run from Leigh Halfpenny which took him three-quarters of the length of the field.
A minute later, fellow Blues winger Tom James cut the Saints defence to shreds but the English side weathered the storm and should have hit back after they destroyed the Blues scrum to win a penalty, but Myler missed his second penalty of the game.
It took the wind out of the Northampton sails and when Blair slotted his second penalty it gave the Blues a six-point lead with just a few minutes to play.
Northampton refused to give up and hammered away but the Welsh songs rolling round the stadium told their own story as the Blues booked a final trip to Twickenham.
Cardiff Blues: Blair; Halfpenny, Shanklin, J Roberts, James; N Robinson, Spice, Jenkins, G Williams, Filise, D Jones, Tito, Molitika, Sowden-Taylor, Rush.
Replacements: T Thomas for G Williams (63), Yapp for Filise (67), Davies for D Jones (70), Morgan for Rush (77).
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