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Summary of significant accounting policies (Tables) | 12 Months Ended
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Dec. 31, 2020
Text block [abstract] |
Summary of Ownership Interests in Subsidiaries |
|  |  |  |  |  |  |  | Percentage of Ownership (%) |  |  |  |
|  |  |  |  |  |  |  | December 31, |  |  |  |
Name of investor |  | Name of investee |  | Main business |  | Location |  | 2019 |  |  | 2020 |  |  | Note |
The Company |  | ChipMOS U.S.A., Inc. (“ChipMOS USA”) |  | Marketing of semiconductors and electronic related products |  | San Jose, USA |  |  | 100 |  |  |  | 100 |  |  |  |  |
The Company |  | ChipMOS TECHNOLOGIES (BVI) LTD. (“ChipMOS BVI”) |  | Holding company |  | British Virgin Islands |  |  | 100 |  |  |  | 100 |  |  |  |  |
ChipMOS BVI |  | ChipMOS SEMICONDUCTORS (Shanghai) LTD. (“ChipMOS Shanghai”) |  | Marketing of semiconductors and electronic related products |  | Shanghai, People’s Republic of China (“PRC”) |  |  | — |  |  |  | 100 |  |  |  | Note |
Estimated Useful Life for Depreciation of Individual Assets |
Buildings |  | 5 to 51 years
Machinery and equipment |  | 2 to 8 years
Tools |  | 2 to 4 years
Others |  | 2 to 6 years

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