3. BALANCE SHEET COMPONENTS

Accounts receivable include amounts that are unbilled at the end of the period that are expected to be billed and collected within 12-month period. Unbilled accounts receivable are determined on an individual contract basis. Unbilled accounts receivable, included in accounts receivable, totaled $18.5 million and $11.5 million as of September 30, 2016 and December 31, 2015, respectively. Unbilled accounts receivable that are not expected to be billed and collected during the succeeding 12-month period are recorded in other non-current assets and totaled $7.7 million and $0.1 million as of September 30, 2016 and December 31, 2015, respectively. The increase in other non-current asset is due to the increase in the long-term unbilled portion of two of the Company’s Design-to-silicon-yield solutions contracts. Deferred costs balance was $0.8 million and zero as of September 30, 2016 and December 31, 2015, respectively. The balance was included in prepaid expense and other current assets and other non-current assets in the accompanying balance sheets.

Property and equipment, net consists of (in thousands):

	September 30 , 2016			December 31, 2015
Property and equipment, net:
Computer equipment	$	10,828		$	9,188
Software		1,835			1,713
Furniture, fixtures and equipment		1,140			907
Leasehold improvements		1,157			1,126
Test equipment		10,799			7,214
Construction-in-progress		8,588			4,777
		34,347			24,925
Less: accumulated depreciation		(16,155	)		(13,600	)
Total	$	18,192		$	11,325

Depreciation and amortization expense was $1.0 million and $0.7 million for the three months ended September 30, 2016 and 2015, respectively. Depreciation and amortization expense was $2.6 million and $1.9 million for the nine months ended September 30, 2016 and 2015, respectively.

Property and equipment related to our DFI solution was $9.6 million and $2.7 million as of September 30, 2016 and December 31, 2015, respectively.

As of both September 30, 2016 and December 31, 2015, the carrying amount of goodwill was $0.2 million. The following is a rollforward of the Company's goodwill balance (in thousands):

	September 30 , 2016
Balance as of December 31, 2015	$	215
Adjustment		—
Balance as of September 30, 2016	$	215

Intangible assets balance was $4.4 million and $5.0 million as of September 30, 2016 and December 31, 2015, respectively. Intangible assets as of September 30, 2016 and December 31, 2015 consist of the following (in thousands):

				September 30 , 2016							December 31, 2015
	Amortization Period (Years)			Gross Carrying Amount		Accumulated Amortization			Net Carrying Amount		Gross Carrying Amount		Accumulated Amortization			Net Carrying Amount
Acquired identifiable intangibles:
Customer relationships	1	-	9	$	5,920	$	(3,756	)	$	2,164	$	5,920	$	(3,547	)	$	2,373
Developed technology	4	-	6		14,100		(12,263	)		1,837		14,100		(11,976	)		2,124
Tradename	2	-	4		610		(570	)		40		610		(533	)		77
Backlog		1			100		(100	)		-		100		(46	)		54
Patent	7	-	10		1,800		(1,430	)		370		1,800		(1,400	)		400
Other acquired intangibles		4			255		(255	)		-		255		(255	)		-
Total				$	22,785	$	(18,374	)	$	4,411	$	22,785	$	(17,757	)	$	5,028

The weighted average amortization period for acquired identifiable intangible assets was 6.61 years as of September 30, 2016. For both the three months ended September 30, 2016 and 2015, intangible asset amortization expense was $0.2 million. For the nine months ended September 30, 2016 and 2015, intangible asset amortization expense was $0.6 million and $0.2 million, respectively. The Company expects annual amortization of acquired identifiable intangible assets to be as follows (in thousands):

Period Ending September 30 ,
2016 (remaining 3 months)	$	188
2017		728
2018		701
2019		701
2020		701
2021 and thereafter		1,392
Total future amortization expense	$	4,411

Intangible assets are amortized over their useful lives unless these lives are determined to be indefinite. Intangible assets are reviewed for impairment whenever events or changes in circumstances indicate that the carrying amount of an asset or asset group may not be recoverable. During the three and nine months ended September 30, 2016, there were no indicators of impairment related to the Company’s intangible assets.