SAN FRANCISCO -- Advanced Micro Devices said Monday that Chief Financial Officer Thomas Seifert was leaving the struggling Sunnyvale chipmaker to pursue other opportunities, sending its shares down almost 12 percent in after-hours trading.

After filling in as AMD's temporary CEO through August last year, Seifert now wants to find a permanent CEO position, a source close to the company said.

Seifert's departure is concerning because -- even though he had only been CFO for three years -- he had more experience at AMD than other top executives, Evercore Partners analyst Patrick Wang said.

"Thomas leaving is a big deal. He was pretty well-liked by investors. He was viewed as he rock throughout all the changes," Wang said.

Devinder Kumar, senior vice president and corporate controller, will serve as AMD's temporary CFO until a replacement is found.

"Seifert's departure is not based on any disagreement over the company's accounting principles or practices, or financial statement disclosures," AMD said in a statement.

PC-related companies have been hit by a slump in demand as smartphones and tablets grow in popularity, but AMD's woes are also seen as at least partly company-specific, with executives and analysts pointing to problems with execution.

In July, AMD slashed its outlook for second-quarter revenue after disappointing sales in China and Europe, becoming one of the biggest tech names to date to warn that a global economic slowdown is taking a harsher-than-expected toll.

AMD shares fell 11.72 percent in extended trading after closing up 2.82 percent at $4.01.

Before the announcement on Monday, Longbow Research analyst Joanne Feeney downgraded her ratings on AMD and larger rival Intel (INTC) Corp to "neutral" from "buy" due to concerns about slow PC sales.

"Thomas has been the one that many have trusted to keep operating expenses under control during a difficult time in the economy," she said later. "I suspect some of the concerns you're seeing in after-market trading reflect concerns about whether that kind of spending discipline will be maintained without his leadership."

After the board ousted previous CEO Dirk Meyer, over disappointment with AMD's limited success in developing products for the smartphone and mobile markets, Seifert stood in for several months until Read was hired.

Read's start at AMD was followed by the departure of senior executives, as well as some respected engineers.

To rebuild its talent pool, AMD hired former Apple (AAPL) executive Mark Papermaster last year to be Chief Technology Officer and Freescale Semiconductor executive Lisa Su as senior vice president and general manager, global business units.

In August, the company hired Jim Keller, formerly of Apple, as chief architect of its microprocessor cores.

Forty-eight year-old Seifert will stay on through Sept. 28, AMD said.