What was the revenue for the fourth quarter of 2020 for TTM, excluding the Mobility business unit and the two E-MS plants that were closed 
revenue, due primarily to uncertainty around the timing and ramp of Phase 3 build for 5G in China. For the full year, networking communications declined 4%. We expect this market to grow, but be below the longer-term forecasts of 5% to 8% growth in 2021, due to the anticipated soft start in the early part of the year, followed by a ramp of 5G infrastructure needs in the back half of the year complemented by growth in networking.
Sales in the computing, storage, peripherals end-market represented 13% of total sales in the fourth quarter compared to 13% in Q4 of 2019 and in the third quarter of 2020. This end market was up 2% year-on-year as growth in our semiconductor customers' offset modest year-on-year declines from our data center customers. We expect revenues in this end market to represent approximately 14% of first quarter sales. For the full year, computing grew 9% as we saw growth across our data center and semiconductor customers. In 2021, we expect to be above the forecasted end market growth of 1% to 3%, driven primarily by data center growth.
Next, I'll cover some details from the fourth quarter. Note that all of the following operations metrics exclude the Mobility business unit and the two E-MS plants that we closed. This information is also available on page six of our earnings presentation. During the quarter, our advanced technology business, which includes HDI, rigid-flex, and RF subsystems and components accounted for approximately 31% of our revenue. This compares to approximately 27% in the year ago quarter and 29% in Q3. We are continuing to pursue new business opportunities and increase customer design engagement activities that will leverage our advanced technology capabilities in new markets.
Capacity utilization in Asia Pacific was 63% in Q4 compared to 61% in the year ago quarter and 63% in Q3. Our overall capacity utilization in North America was 58% in Q3 compared to 58% in the year ago quarter and 61% in Q3. Our top five customers contributed 34% of total sales in the fourth quarter of 2020 compared to 36% in the third quarter of 2020. Our largest customer accounted for 14% of sales in the fourth quarter. At the end of Q4, our 90-day backlog, which is subject to cancellations was $483.9 million compared to $402.8 million at the end of the fourth quarter of last year and $437.8 million at the end of Q3. Our PCB book-to-bill ratio was 1.19 for the three months ending December 28.
I'd like to conclude by again thanking our employees for continuing to contribute to TTM and our critical mission of inspiring innovation with our customers. Their efforts are particularly appreciated during these times by our customers in critical essential areas like defense and the medical industries. Despite the COVID-19 and currency related challenges we faced in 2020, our business performed better than we expected as a direct results of operational excellence, end market diversification and our employees concerted efforts to engage and support our customers. We've also taken positive strategic moves that will strengthen TTM for the long term.
Now, Todd will review our financial performance for the fourth quarter. Todd?
Todd B. Schull -- Executive Vice President and Chief Financial Officer
Thanks, Tom, and good afternoon, everyone. As Tom mentioned earlier, in 2020, TTM announced the closing of the sale of its Mobility business unit. As such, that the closure of TTM's GAAP results reflects the Mobility business unit as a discontinued operation. During this call, I will discuss non-GAAP financial information, which excludes the results of the Mobility business unit. The E-MS business unit is still included in both the GAAP and non-GAAP results we have reported. Please refer to the earnings schedule for additional details on exited businesses and continuing operations. Page seven of our earnings presentation and the appendix of our investor presentation also contain this information.
For the fourth quarter, net sales from continuing operations were $523.8 million, compared to $535.7 million in the fourth quarter of
revenue, due primarily to uncertainty around the timing and ramp of Phase 3 build for 5G in China. For the full year, networking communications declined 4%. We expect this market to grow, but be below the longer-term forecasts of 5% to 8% growth in 2021, due to the anticipated soft start in the early part of the year, followed by a ramp of 5G infrastructure needs in the back half of the year complemented by growth in networking.
Sales in the computing, storage, peripherals end-market represented 13% of total sales in the fourth quarter compared to 13% in Q4 of 2019 and in the third quarter of 2020. This end market was up 2% year-on-year as growth in our semiconductor customers' offset modest year-on-year declines from our data center customers. We expect revenues in this end market to represent approximately 14% of first quarter sales. For the full year, computing grew 9% as we saw growth across our data center and semiconductor customers. In 2021, we expect to be above the forecasted end market growth of 1% to 3%, driven primarily by data center growth.
Next, I'll cover some details from the fourth quarter. Note that all of the following operations metrics exclude the Mobility business unit and the two E-MS plants that we closed. This information is also available on page six of our earnings presentation. During the quarter, our advanced technology business, which includes HDI, rigid-flex, and RF subsystems and components accounted for approximately 31% of our revenue. This compares to approximately 27% in the year ago quarter and 29% in Q3. We are continuing to pursue new business opportunities and increase customer design engagement activities that will leverage our advanced technology capabilities in new markets.
 Capacity utilization in Asia Pacific was 63% in Q4 compared to 61% in the year ago quarter and 63% in Q3. Our overall capacity utilization in North America was 58% in Q3 compared to 58% in the year ago quarter and 61% in Q3. Our top five customers contributed 34% of total sales in the fourth quarter of 2020 compared to 36% in the third quarter of 2020. Our largest customer accounted for 14% of sales in the fourth quarter. At the end of Q4, our 90-day backlog, which is subject to cancellations was $483.9 million compared to $402.8 million at the end of the fourth quarter of last year and $437.8 million at the end of Q3. Our PCB book-to-bill ratio was 1.19 for the three months ending December 28.
I'd like to conclude by again thanking our employees for continuing to contribute to TTM and our critical mission of inspiring innovation with our customers. Their efforts are particularly appreciated during these times by our customers in critical essential areas like defense and the medical industries. Despite the COVID-19 and currency related challenges we faced in 2020, our business performed better than we expected as a direct results of operational excellence, end market diversification and our employees concerted efforts to engage and support our customers. We've also taken positive strategic moves that will strengthen TTM for the long term.
Now, Todd will review our financial performance for the fourth quarter. Todd?
Todd B. Schull -- Executive Vice President and Chief Financial Officer
Thanks, Tom, and good afternoon, everyone. As Tom mentioned earlier, in 2020, TTM announced the closing of the sale of its Mobility business unit. As such, that the closure of TTM's GAAP results reflects the Mobility business unit as a discontinued operation. During this call, I will discuss non-GAAP financial information, which excludes the results of the Mobility business unit. The E-MS business unit is still included in both the GAAP and non-GAAP results we have reported. Please refer to the earnings schedule for additional details on exited businesses and continuing operations. Page seven of our earnings presentation and the appendix of our investor presentation also contain this information.
For the fourth quarter, net sales from continuing operations were $523.8 million, compared to $535.7 million in the fourth quarter of