Add custom button to navigation controller without border

For Swift 4

let backButton = UIButton(frame: CGRect(x: 0, y: 0, width: 25, height: 25))
backButton.setImage(UIImage(named: "back.png"), for: .normal)
backButton.addTarget(self, action: #selector(backAction), for: .touchUpInside)
self.navigationItem.rightBarButtonItem = UIBarButtonItem(customView: backButton)

And action (selector) should be like the following :

@objc func backAction () {
        // do the magic 
}

Try this.

UIButton *backButton = [[UIButton alloc] initWithFrame:CGRectMake(0, 0, button_width, button_height)];
[backButton setImage:[UIImage imageNamed:@"back.png"] forState:UIControlStateNormal];
[backButton addTarget:self action:@selector(backAction) forControlEvents:UIControlEventTouchUpInside];
self.navigationItem.leftBarButtonItem = [[UIBarButtonItem alloc] initWithCustomView:backButton];