Accounts receivable include amounts that are unbilled at the end of the period that are expected to be billed and collected within a 12-month period. Unbilled accounts receivable, included in accounts receivable, totaled $5.7 million and $7.2 million as of June 30, 2021, and December 31, 2020, respectively. Unbilled accounts receivable that are not expected to be billed and collected during the succeeding 12-month period is recorded in other non-current assets and totaled $1.6 million and $2.0 million as of June 30, 2021, and December 31, 2020, respectively.

Test equipment includes systems assets at customer sites that are contributing to DFI™ systems revenues. The construction-in-progress balance related to construction of DFI™ systems assets totaled $20.9 million and $18.9 million as of June 30, 2021, and December 31, 2020, respectively. Depreciation and amortization expense was $1.6 million and $1.7 million during the three months ended June 30, 2021 and 2020, respectively, and $3.3 million and $3.4 million during the six months ended June 30, 2021 and 2020, respectively.

The change in the carrying amount of goodwill during the six months ended June 30, 2021, was as follows (in thousands):

(1)

Goodwill adjustment was recorded within the measurement period with a corresponding reduction in the Holdback Amount. See Note 4, Business Combination.

There were no impairments to goodwill during the six months ended June 30, 2021.

Intangible assets, net, consisted of the following (in thousands):


		June 30, 2021						December 31, 2020
	Amortization	Gross				Net		Gross				Net
	Period	Carrying		Accumulated		Carrying		Carrying		Accumulated		Carrying
	(Years)	Amount		Amortization		Amount		Amount		Amortization		Amount
Acquired identifiable intangibles:
Customer relationships	1-10	$	9,407	$	(5,719)	$	3,688	$	9,407	$	(5,398)	$	4,009
Developed technology	4-9		30,000		(16,150)		13,850		30,000		(14,987)		15,013
Tradename and trademarks	2-10		1,598		(759)		839		1,598		(706)		892
Patent	7-10		1,800		(1,620)		180		1,800		(1,600)		200
Noncompetition agreements	3		848		(165)		683		848		(24)		824
In-process R&D	*		3,635		—		3,635		3,635		—		3,635
Total		$	47,288	$	(24,413)	$	22,875	$	47,288	$	(22,715)	$	24,573

* Non-amortizing intangible asset

The Company expects annual amortization of acquired identifiable intangible assets to be as follows (in thousands):


Year Ending December 31,	Amount
2021 (remaining six months)	$	1,523
2022		3,013
2023		2,990
2024		2,592
2025		2,427
2026 and thereafter		6,695
Total future amortization expense	$	19,240

Intangible assets are amortized over their useful lives unless these lives are determined to be indefinite. The weighted average amortization period for acquired identifiable intangible assets was 7.0 years as of June 30, 2021. Intangible asset amortization expense included under "Cost of revenues" and "Amortization of other intangible assets" account in the Condensed Consolidated Statements of Comprehensive Loss was $0.9 million and $0.3 million during the three months ended June 30, 2021 and 2020, respectively, and $1.7 million and $0.6 million during the six months ended June 30, 2021 and 2020, respectively. 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 six months ended June 30, 2021, there were no indicators of impairment related to the Company’s intangible assets.