Jim,

The subject of conductor width measurement has been the subject of much 
controversy over the years, due to the inherent geometry of conductors 
resulting from common printed board manufacturing processes.  The 
definition that you quoted from the IPC Terms and Definitions (IPC-T-50) 
publication is the result of an industry aggreement after much discussion at
IPC meetings. I am not sure how you are accomplishing your measurement, 
but IPC-TM-650 Test Methods Manual, method 2.2.2 A describes the method 
for measuring conductor width. This method states measurement using 
"Any illuminated eyepiece or microscope having a reticle and a 
magnification capability of at least 5X magnification", "recordrd to the 
nearest 0.0001 in." In addition, IPC-RB-276: Qualification and 
Performance Specification for Rigid Printed Boards, describes "Conductor 
Definition" in PP 3.8, which allows for certain "allowable width 
reductions" (Too lengthly to repeat here). I can FAX the allowable 
reduction info. to you if you need it; E-mail or call the IPC office.
All of the above mentioned publications are available from IPC; Contact 
Customer Service for ordering information.

Be aware that although the inherent geometry of conductors through 
processing has been tolerable for many years, many of the newer designs 
requiring controlled impedance may have much tighter requirements on 
width, thickness, and shape in order to achieve the desired impedance.

Hope this helps

Vivian Vosburg, E-mail: [log in to unmask]
IPC, 2215 Sanders RD., Northbrook, IL. 60062-6135
708-509-9700,  FAX 708-509-9700 
 


On Wed, 4 Oct 1995, Jim McNeal wrote:

> As a manufacturer of bare printed circuit boards, it is common
> for us to see minimum line widths called out on fabrication 
> drawings.  A question has been raised as to the proper method 
> for measurement of finsished line width.
> 
> When the circuit is etched, the copper is typically thinner at 
> the top than at the surface of the laminate. (Bare copper inner
> layer)
> 
> (Crude cross-section graphic to follow)
> 
> 
> 
> 
> 
>             |______6.6 mils______|   
>             |                    |
>             | |___ 5.5 mils____| |
>             | |________________| |
>             | /                \ |
>             |/                  \|
>  -----------'--------------------'----------------
> 
> 
> 
> In the above graphic, the line width at the base of the circuit is
> 6.5 mils.  At the top of the circuit, it measures 5.5 mils.  If the
> drawing requirement is 6.0 mil minimum, would this meet the drawing 
> requirments?
> 
> In the past, I have always considered this to be a 6.5 mil circuit.
> Is this correct thinking?
> 
> A review of IPC-T-50 terms and definitions says that the conductor 
> width is "The observable width of a conductor at any point chosen at
> random on the printed wiring board normally viewed from directly 
> above unless otherwise specified."  
> 
> If this circuit is viewed from above with backlighting, the 
> observable width is 6.5 mils.  When viewed from above with top
> lighting, the appearance is 5.5 mils.  Is this issued clarified
> in any of the IPC specifications?  Is there a consensus as to 
> the meaning of "observable" as it applies to conductor width 
> measurements?
> 
> Jim McNeal
> 
> =====================================================
> 
> Jim McNeal  Q.A.and Eng. Mgr.
> Electro Plate Circuitry, Inc.
> Carrollton, Tx
> Ph. 214-466-0818
> EMAIL  [log in to unmask]
> 
>