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	(28,662)	0	(28,662)
Changes in cumulative translation adjustment	0	0	0	12,660	12,660
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	28	347	0	0	375
Shares issued for restricted stock units vested	614	(614)	0	0	0
Repurchase and retirement of stock	(189)	(2,076)	0	0	(2,265)
Share-based compensation expense	0	10,313	0	0	10,313
Shares issued under ESPP	114	1,488	0	0	1,602
Impact of sale of PCB Test business					0
Cash dividends	0	0	(2,486)	0	(2,486)
Balance at Sep. 26, 2020	41,962	442,648	11,369	(21,370)	474,609
Balance at Jun. 27, 2020	41,862	439,943	18,015	(33,062)	466,758
Net income (loss)	0	0	(6,646)	0	(6,646)
Changes in cumulative translation adjustment	0	0	0	11,692	11,692
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	6	80	0	0	86
Shares issued for restricted stock units vested	127	(127)	0	0	0
Repurchase and retirement of stock	(33)	(547)	0	0	(580)
Share-based compensation expense	0	3,299	0	0	3,299
Impact of sale of PCB Test business					0
Balance at Sep. 26, 2020	41,962	442,648	11,369	(21,370)	474,609
Balance at Dec. 26, 2020	42,190	448,194	26,230	(4,326)	512,288	[1]
Net income (loss)	0	0	146,436	0	146,436
Changes in cumulative translation adjustment	0	0	0	(12,562)	(12,562)
Adjustments related to postretirement benefits, net of tax	0	0	0	15	15
Changes in unrealized gains and losses on investments, net of tax	0	0	0	(25)	(25)
Exercise of stock options	250	2,260	0	0	2,510
Shares issued for restricted stock units vested	696	(696)	0	0	0
Repurchase and retirement of stock	(240)	(10,167)	0	0	(10,407)
Share-based compensation expense	0	11,371	0	0	11,371
Shares issued under ESPP	95	1,654	0	0	1,749
Impact of sale of PCB Test business	0	0	0	(2,515)	(2,515)
Sale of common stock, net of issuance costs	5,693	217,426	0	0	223,119
Balance at Sep. 25, 2021	48,684	670,042	172,666	(19,413)	871,979
Balance at Jun. 26, 2021	48,596	666,942	148,933	(14,240)	850,231
Net income (loss)	0	0	23,733	0	23,733
Changes in cumulative translation adjustment	0	0	0	(5,206)	(5,206)
Adjustments related to postretirement benefits, net of tax	0	0	0	44	44
Changes in unrealized gains and losses on investments, net of tax	0	0	0	(11)	(11)
Exercise of stock options	36	315	0	0	351
Shares issued for restricted stock units vested	76	(76)	0	0	0
Repurchase and retirement of stock	(24)	(853)	0	0	(877)
Share-based compensation expense	0	3,714	0	0	3,714
Impact of sale of PCB Test business					0
Balance at Sep. 25, 2021	$ 48,684	$ 670,042	$ 172,666	$ (19,413)	$ 871,979

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