Colleagues,

Please take a moment to review the following request by industry peers for the determination of minimum conductor width allowances per IPC-6012A, section 3.5.1.  Personally I think they are missing the point in that we're not attempting to define how much a conductor's width can be reduced through etching processes but rather how much reduction is allowed by the spec beyond the width values supplied in the procurement documentation.  That's the mistake I think they're making in their last paragraph, but maybe I'm misinterpreting them.  

What do you think of the following:

Having a technical discussion with colleagues, regarding the final width of narrow conductors 0.127 mm (0.005") and 0.10 mm (0.004"), I was surprised to find that there is no special attribute to this type of cond. in Para. # 3.5.1 of IPC-6012.
The general requirement is that the final width of a cond. shall not be reduce by more than 20% (with some isolated more relieves).

I think it is time to update this item to be more rational and suitable to the present design parameters.

Actually the capability of the etching process, especially in external layers, (where the ammonical base material is in common use), is depended on the base copper thickness and not the conductor width :
A conductor of 0.50 mm (0.020") width and a conductor of 0.025 mm (0.010")  and even a conductor of 0.1 mm (0.004" ) supposed to be reduced by the same absolute figure which is depended on the basic copper thickness.
On 0.5 Oz. (0.0175 mm) basic copper thickness their width can be  reduced by 0.035 mm. And on  1.0 Oz. (0.035 mm) basic copper thickness their width can be reduced by 0.070 mm, with no connection to their nominal initial width.
I use the term "can be reduced" instead of "will be reduced", because most of the manufacturers used to enlarge all the conductors in the customer designed files by some factor (Each vendor does it in accordance with his own experience), to compensate part of the etching process width reduction. 

The basic rule should allow always a conductor width reduction rate which is twice the copper base thickness.
This value is coming out of the chemistry character of the etching process, that while it dissolve the  copper in the "Z" axis (which is the wanted direction), we get on the same time copper dissolving result in the "X-Y" axis. And in the same time that the material is dissolving a certain thickness in the "Z" direction, the same length is dissolved in each side of the conductor (At the upper part of the copper base material. And it becomes less and less as we are coming towards the base material, getting the trapezoid shape)

I am sure I haven't innovate nothing up to now, all these facts are well known in the industry of PCBs manufacturing, but yet it surprise me that we are still using the traditional request for min. cond. width which is completely irrelevant for 0.127 mm and 0.1 mm cond. width.
Lets take the case of 0.1 mm conductors width built on 0.0175 mm base copper on external lys. They can be normally reduced by 0.035 mm (35%!) without any fault in the manufacturing process. So how can we limited it to 20% only?















John Perry
Technical Project Manager
IPC
2215 Sanders Road
Northbrook, Il 60062
1-847-790-5318 (P)
1-847-509-9798 (F)
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