What: Shares of semiconductor packaging and test services provider Amkor Technology (NASDAQ:AMKR) tumbled on Friday following the company's fourth-quarter earnings report. Amkor missed analyst estimates across the board, and weak guidance for the first quarter added fuel to the fire. At 11:50 a.m. ET, the stock was down about 19%.

So what: Amkor reported quarterly revenue of $671 million, down 21.3% year over year and about $17 million below the average analyst estimate. Revenue from advanced products fell 28% year over year, despite a slight rise in the number of advanced product packaged units. This discrepancy may be explained by a shift from mobile devices to automotive and industrial products during the fourth quarter. Fifty-four percent of Amkor's business came from communications devices during the quarter, including smart phones and tablets, down from 60% during the prior-year period.

Non-GAAP EPS came in at $0.02, down from $0.38 during the prior-year period and a penny short of analyst expectations. Amkor posted a $0.04-per-share GAAP loss, down from a $0.06-per-share gain during the fourth quarter of 2014.

Now what: Amkor pointed to sluggish market conditions in the first quarter, particularly related to the high-end smartphone market. This weakness led it to guide for revenue between $785 million to $835 million, up sequentially and year over year but shy of the average analyst estimate of $864 million. Amkor also expects to post a non-GAAP loss between $0.03 and $0.15 per share, compared to analyst expectations of a $0.01 gain.

Slowing sales of high-end smartphones, including the iPhone, are weighing on Amkor's results and guidance. A mix shift toward other categories appears to be hurting the company's profitability, despite the revenue growth expected during the first quarter. With Amkor missing estimates and providing guidance well below expectations, it's no surprise that the stock sold off on Friday.