David,
From my experience and the examination of quite a few hundreds of microsections - YES, you should evaluate the smallest holes too.
The tolerance of many of the manufacturing processes shrinks as you proceed towards smaller hole diameters. Beginning with the drilling operation - parameters for small drill bits, drilling machine accuracy, design of the drill bits etc. . Through the desmear process and the entire wet processes that are based on a good fluid flow through the holes (among others - PTH and electrolytic plating). And, of course, there are all the registration problems - because smaller holes come usually with smaller lands. This list could go on and on but I think I made my point.

I agree with you that there are small holes that cannot be accurately microsectioned but I think these are much smaller than 0.030 inches (below 0.010 inches - and I mean it for the DRILLED hole, without plating). For these we should find another evaluation method. But remember that you can always expect for trouble from the small holes so it would not be wise to ignore them.

If I remember well, the PC Met Jr. has diamond stops to set the correct grinding depth and it is quite accurate. Is there a chance that the reference plate that you are using to set the diamond stops is not set-up properly? This could cause you great trouble in getting to the hole center. Check if you are getting above or below the center. Getting above the hole center can usually be corrected by secondary polishing, whereas if you missed the center - the specimen is dead.

Well, these are my two pennies about "PCB reliability and small hole inspection". Expecting for other comments.

Yehuda

    ************************************* 
    *          Yehuda E. Weisz              
    *  e-mail: [log in to unmask]
    *  Tel: 972-3-5240362                    
    ************************************




-----Original Message-----
From:   David Tyler [SMTP:[log in to unmask]]
Sent:   Wed, August 06, 1997 7:51 PM
To:     [log in to unmask]
Subject:        [TECHNET] (Fab) IPC "B" Coupon

Hello Technet
I'm having discussions with some of our PCB suppliers concerning the IPC
"B" coupon spec. which requires the holes to be drilled with the smallest
hole size used on the associated PCB.  The problem is that hole sizes have
been decreasing and we no longer feel that the smallest hole is
representative of the board.  Not only that but we (the end user) are
finding it increasingly difficult to microsection these coupons for quality
assurance purposes.  It is very difficult to get to the center of holes
below .030 inches.  We are using a PC Met Jr. to fixture the coupons for
grinding and an Ecomet to do the grinding.  So the question is, should we
spec. the most numerous component holes to be drilled instead, is the IPC
considering a different spec. or should we consider other equipment?  If
there is anyone who has encountered (and overcome) this problem I would
appreciate the feedback.

Thanks
David Tyler
Inspection Services
Teradyne Inc.
179 Lincoln St.
Boston, MA 02111