Larry, 

Since the fabrication shop must meet the Class 3 requirements (or scrap the batch) I would talk to them. 

We currently use Speedy Circuits and Murrieta Circuits in Orange County, CA for our Class 3 prototype and production boards and I don't design anything without first discussing it with them to see if they can build it and meet the Class 3 requirements.

Here are the fabrication notes we're using for ROHS Class 3 fabrication - 

GENERAL NOTES: UNLESS OTHERWISE SPECIFIED

1.	INSPECTION REQUIRED: EACH SHIPMENT OF PCBS SHALL CONTAIN A CERTIFICATE OF COMPLIANCE TO THIS SPECIFICATION.

2.	INSPECTION REQUIRED: THE PCB SHALL BE FABRICATED PER IPC-6012 (CURRENT REVISION) CLASS 3. 

3.	INSPECTION REQUIRED:  ROHS COMPLIANCE                                                                                                  
	THIS IS A LEAD-FREE PRODUCT.  ALL MATERIALS, FINISHES AND PROCESSES USED TO MANUFACTURE THIS PCB SHALL COMPLY WITH THE ROHS DIRECTIVE 	(CURRENT REVISION). 

4.	INSPECTION REQUIRED:  MARKING
	THE FABRICATOR MUST BE UL CERTIFIED AND THEIR TRADEMARK, "94V-0" AND "CTI 175" SHALL BE RENDERED IN ETCH ON THE BOTTOM SIDE OF THE BOARD 	APPROXIMATELY WHERE INDICATED.

5.	MATERIAL REQUIREMENTS:
	LEAD FREE APPLICATION WITH IPC-4101/126 OR /129.
	170 DEGREE C Tg MINIMUM, 340 DEGREE C DECOMPOSITION TEMPERATURE (Td) MINIMUM.

6.	FINISH:
	UNMASKED CIRCUITRY AND THRU HOLES SHALL BE PLATED WITH 2-10 MICROINCHES OF IMMERSION GOLD OVER 118-200 MICROINCHES OF ELECTROLESS NICKEL 	(REFER TO IPC-6012; TABLE 3-2).

7.	SOLDER MASK:
	APPLY LIQUID PHOTO IMAGEABLE (LPI) SOLDER MASK TO THE EXTERNAL LAYERS OF THE PCB OVER BARE COPPER IN ACCORDANCE WITH IPC-SM-840, CLASS H 	(REFER TO IPC-6012; SECTION 3.7). SOLDER MASK SHALL BE BLUE IN COLOR AND SHALL HAVE A THICKNESS OF 0.0005 - 0.002.
	SOLDER MASK SHALL WITHSTAND ALL STANDARD SMT ASSEMBLY PROCESSES.

8.	SILKSCREEN:
	USE WHITE EPOXY BASE INK WHICH SHALL WITHSTAND NORMAL PRODUCTION PROCESSES AS SILKSCREEN ON BOTH SIDES. 

9.	REGISTRATION:
	SOLDER MASK REGISTRATION TO BE WITHIN TRUE POSITION OF 0.004.
	SILKSCREEN LEGEND TO BE WITHIN TRUE POSITION OF 0.004.
	LAYER TO LAYER REGISTRATION TO BE WITHIN TRUE POSITION OF 0.003.

10.	TOLERANCES:
	BOW AND TWIST SHALL NOT EXCEED 1% OR 0.100 TOTAL, MEASURED IN ACCORDANCE WITH IPC-TM-650, METHOD 2.4.22 (REFER TO IPC-6012; SECTION 3.4.3).
	CONDUCTOR WIDTH OF ETCHED LINES AND SPACING BETWEEN LINES SHALL BE WITHIN 0.001 OF APPLIED MEDICAL SUPPLIED FILM OR GERBER FILES.

11.	HOLE REQUIREMENTS:
	MINIMUM ANNULAR RING FOR ALL LAYERS SHALL BE 0.002.
	HOLE LOCATION TO BE 0.003 OF TRUE POSITION.

12.	100% CONTINUITY TESTING:
	BARE BOARDS SHALL BE ELECTRICALLY TESTED USING PROVIDED IPC-D-356 NET LIST DATA FOLLOWING THE GUIDELINES ESTABLISHED BY IPC-ET-652.

13.	TEST COUPON:
	ONE METALOGRAPHIC SPECIMEN SHALL BE TESTED AND SUPPLIED WITH EACH SHIPMENT OF BOARDS. A REPORT OF MEASUREMENTS, SHOWING AVERAGE 	PLANE THICKNESS AND QUALITY SHALL BE SUPPLIED WITH EACH SPECIMEN. TESTING SHALL INCLUDE THERMAL STRESS AND MICROSECTIONING PER IPC-TM-650, 	METHODS 2.1.1 AND 2.6.8. EACH SPECIMEN SHALL BE IDENTIFIED AND TRACEABLE BACK TO THE ORIGINAL LOT.

14.	ALL DIMENSIONS INCLUDING HOLE DIAMETERS ARE AFTER ETCHING AND PLATING.


Tom


-----Original Message-----
From: DesignerCouncil [mailto:[log in to unmask]] On Behalf Of Larry Brophy
Sent: Monday, February 24, 2014 6:44 AM
To: [log in to unmask]
Subject: Re: [DC] Minimum Via size

Tom,

I am already using a 18mil pad with an 8mil hole on my current designs but we have a new 0.5mm pitch part that I need to use a laser drill of 4 mil and I just need to know if this is ok for Class 3?

Thanks,

Larry

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