6. Intangible Assets

The gross carrying amount and accumulated amortization of the Company’s acquired identifiable intangible assets as of March 31, 2020 were as follows (in thousands):


	Weighted Average	Gross Carrying		Accumulated
	Amortization Period	Amount		Amortization		Total
Acquired technology	10 years	$	8,163	$	(2,971)	$	5,192
Customer relationships	10 years		260		(156)		104
Trademark	5 years		60		(60)		—
		$	8,483	$	(3,187)	$	5,296

The gross carrying amount and accumulated amortization of the Company’s acquired identifiable intangible assets as of December 31, 2019 were as follows (in thousands):


	Weighted Average	Gross Carrying		Accumulated
	Amortization Period	Amount		Amortization		Total
Acquired technology	10 years	$	8,244	$	(2,815)	$	5,429
Customer relationships	10 years		260		(150)		110
Trademark	5 years		60		(60)		0
		$	8,564	$	(3,025)	$	5,539

The change in the gross carrying amount of the acquired technology from December 31, 2019 to March 31, 2020 is due to changes in foreign currency translation.

In the second quarter of 2019, the Company acquired intellectual property from EnergyOr for $1.5 million. In addition, the Company agreed to pay the sellers a royalty based on future sales of relevant applications, not to exceed $3.0 million, by May 22, 2025. These royalties are added to the intangible asset balance, as incurred.

As part of the agreement to acquire the intellectual property from AFC, the Company shall pay AFC milestone payments not to exceed $2.9 million in total, if certain milestones associated with the production of components related to the acquired technology are met before April 2021. As of March 31, 2020, the Company paid $0.4 million and accrued $0.5 million in relation to the aforementioned milestones.

Amortization expense for acquired identifiable intangible assets was $0.2 million for the three months ended March 31, 2020 and 2019. Estimated amortization expense for subsequent years was as follows (in thousands):


Remainder of 2020	$	595
2021		793
2022		793
2023		793
2024 and thereafter		2,322
Total	$	5,296