Consolidated Statements of Stockholders' Equity (Unaudited) - USD ($) $ in Thousands	Common Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	AOCI Attributable to Parent [Member]	Total
Balance at Dec. 28, 2019	$ 41,395	$ 433,190	$ 42,517	$ (34,030)	$ 483,072
Net income (loss)	0	0	(22,016)	0	(22,016)
Changes in cumulative translation adjustment	0	0	0	968	968
Adjustments related to postretirement benefits, net of tax					0
Changes in unrealized gains and losses on investments, net of tax					0
Exercise of stock options	22	267	0	0	289
Shares issued under ESPP	114	1,488	0	0	1,602
Shares issued for restricted stock units vested	487	(487)	0	0	0
Repurchase and retirement of stock	(156)	(1,529)	0	0	(1,685)
Impact of sale of PCB Test business					0
Share-based compensation expense	0	7,014	0	0	7,014
Cash dividends	0	0	(2,486)	0	(2,486)
Balance at Jun. 27, 2020	41,862	439,943	18,015	(33,062)	466,758
Balance at Mar. 28, 2020	41,686	435,402	22,755	(34,959)	464,884
Net income (loss)	0	0	(4,740)	0	(4,740)
Changes in cumulative translation adjustment	0	0	0	1,897	1,897
Adjustments related to postretirement benefits, net of tax					0
Changes in unrealized gains and losses on investments, net of tax					0
Shares issued under ESPP	114	1,488	0	0	1,602
Shares issued for restricted stock units vested	84	(84)	0	0	0
Repurchase and retirement of stock	(22)	(266)	0	0	(288)
Impact of sale of PCB Test business					0
Share-based compensation expense	0	3,403	0	0	3,403
Balance at Jun. 27, 2020	41,862	439,943	18,015	(33,062)	466,758
Balance at Dec. 26, 2020	42,190	448,194	26,230	(4,326)	512,288	[1]
Net income (loss)	0	0	122,703	0	122,703
Changes in cumulative translation adjustment	0	0	0	(7,356)	(7,356)
Adjustments related to postretirement benefits, net of tax	0	0	0	(29)	(29)
Changes in unrealized gains and losses on investments, net of tax	0	0	0	(14)	(14)
Exercise of stock options	214	1,945	0	0	2,159
Shares issued under ESPP	95	1,654	0	0	1,749
Shares issued for restricted stock units vested	620	(620)	0	0	0
Repurchase and retirement of stock	(216)	(9,314)	0	0	(9,530)
Impact of sale of PCB Test business	0	0	0	(2,515)	(2,515)
Share-based compensation expense	0	7,657	0	0	7,657
Sale of common stock, net of issuance costs	5,693	217,426	0	0	223,119
Balance at Jun. 26, 2021	48,596	666,942	148,933	(14,240)	850,231
Balance at Mar. 27, 2021	48,411	661,984	53,837	(14,758)	749,474
Net income (loss)	0	0	95,096	0	95,096
Changes in cumulative translation adjustment	0	0	0	2,890	2,890
Adjustments related to postretirement benefits, net of tax	0	0	0	151	151
Changes in unrealized gains and losses on investments, net of tax	0	0	0	(8)	(8)
Exercise of stock options	22	215	0	0	237
Shares issued under ESPP	95	1,654	0	0	1,749
Shares issued for restricted stock units vested	94	(94)	0	0	0
Repurchase and retirement of stock	(26)	(951)	0	0	(977)
Impact of sale of PCB Test business	0	0	0	(2,515)	(2,515)
Share-based compensation expense	0	4,134	0	0	4,134
Balance at Jun. 26, 2021	$ 48,596	$ 666,942	$ 148,933	$ (14,240)	$ 850,231

[1] Derived from December 26, 2020 audited financial statements