WILSONVILLE, OR -- Carl Icahn was at it again yesterday, spelling out in even greater detail his case for shaking up the leadership of Mentor Graphics.

In a letter to fellow shareholders, the notorious activist said the EDA company was stifled by aging, ineffective executives who have failed to move the stock over the past 17 years. He added that the EDA company's annualized revenue growth over that time has been 5.3%, lower than its annualized SG&A growth of 5.7%.

The expenses are the albatross, said Icahn, who accused Mentor of lagging competitors like Cadence and Magma Design Automation in that area. "SG&A as a percentage of total revenues [has increased] to 46%. With 5.3% annualized revenue growth it seems absurd to us that Mentor could not better leverage its SG&A expenses," Icahn said.

In his letter, Icahn laid out a defense of his nominees for director, saying they would form a committee to assess and improve the company's SG&A expenses.

He also argued that, relative to its peers Cadence and Synopsys, Mentor has been diluting its stock during CEO Wally Rhines' 17-year tenure.

Contrary to Mentor's position that Icahn would break up or sell the company, Icahn, who is the printed circuit board and semiconductor design software development firm's largest shareholder, said a sale is "not our only plan." He said he a second plan on the table is to "hold management accountable" to lower the SG&A ratio and to use its cash flow to buy back its stock. Doing so would enhance the company's earnings per share, he said.

He saved his harshest criticism for Rhines, who he said has received $65 million in compensation despite no movement in Mentor's stock. "It is time for a change on the board of directors," Icahn wrote. "Eight years have passed without a single director joining or leaving. All three of the directors we seek to replace have been on the board since 1994, just after Walden Rhines became CEO. One of the directors we seek to replace is actually the lead independent director, despite spending over 17 years on the board with the CEO and the president. ... This board has failed to hold management accountable to shareholders in our opinion."

Mentor's Revenues and SG&A, 1994 and 2011

Source: Carl Icahn, Mentor filings