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From:
Yehuda Weisz - Netvision <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Wed, 31 Dec 2003 17:14:15 +0200
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Bruce,
I see that you have already received many good answers that touch various aspects of the subject and I'll try to add my two cents.
I assume that you can allow yourself to waiver the 2 mil requirement, since otherwise we wouldn't have this discusion.
The requirement of minimum annular-ring exists in the Mil-Spec and also in the IPC specs. that refer to the bare board (IPC-6012 et al.). I think that the main reason for this requirement to stay for so many years is because of its implications.
As Wolfgang has written, there is industry data that shows for most vias, that the removal of non-functional pads on innerlayers increase the reliability of the hole barrel and thus the reliability of the board. 
Your problem here is that the absence of sufficient annular ring will exist on nonfunctional as well as on functional pads and so - you will be facing (functional) interconnections that you cannot be sure of their reliability. If we'll imagine the innerlayer pad, we'll see that some of the plating is performed on the copper pad and some on the resin - at areas where the pad has faded away. It might be difficult to say how that would function under thermal cycling.
I agree with Edward that the coupons not always represent the combinations existing in your board. The problem is that many times - there exist worse cases inside the board than on the coupon (yes, it is so, unfortunately). If in their case it's accidentally the other way around (coupon is a non-reprentative worst case) - make them prove it to you beyond doubt.
And, yes, your manufaturer is right that the coupon is closer to the edge that the PCB. BUT, the coupon is also quite close to the PCB itself. 
So, if the coupon is non-representative of the center of the board, does it represent the edge of the board? Don't I want good registration there? Where would I draw my line?

And not only misregistration is the issue. I have seen cases where misregistration came hand in hand with excessive etching of innerlayers, thus reducing pad size. The combination of two killed the annular ring. If you have this - how do your lines look on the innerlayers? Would you allow THIS reduction if you knew about it?


To sum up - I also feel uneasy with their requirement. It feels that they had an error they want to cover up.

One last piece of information to confuse you - a few years ago IITRI (I think) has published a report in which they claimed to show that misregistration has low effect on board reliability (if any). I could not find the document number right now, but I can dig it up if you wish.
Nevertheless, I don't recall any big changes in the global views regarding the issue in the industry.

Have a Great New-Year !!!!!
Yehuda Weisz
Tel: (972)-3-6342045
Cel: (972)-53-556897

----- Original Message ----- 
From: "Bruce D Stilmack" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Tuesday, December 30, 2003 4:06 PM
Subject: [TN] Annular ring


> I have a PCB manufacturer requesting a waiver for the MIL SPEC requirement
> of a 2 mil annular ring.  Their position is that the coupons are from the
> edge of the laminate where the registration is worse.  The assumption is
> that the ring will be larger in the PCB itself.  I am leery to grant a
> waiver to allow this deviation, but would like to know how the community
> feels about this.
> 
> Bruce Stilmack
> GDLS-TO Manufacturing Engineer
> (850) 574-4773
> [log in to unmask]
> 
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