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April 1998

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Subject:
From:
Jana Carraway <[log in to unmask]>
Reply To:
Date:
Fri, 3 Apr 1998 09:48:18 PST
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Steve,  this is a really good question.  I know consistency of line width is
important with RF, I do not know if you are using fine lines or the required
copper thicknesses.  In PCB manufacturing, the thinner the base copper, the
tighter the tolerance capable on the line width.
If you refer to design rules from different manufacturers, they may not state
the tolerance, but they will state line width capability with respect to
copper thickness, for example:
1/8 oz copper   0.002"          (1/8 oz copper is ~.17 mils thick
or ~ 4 micron cu, if I didn't slip a dec pt)
1/2 oz copper   0.003"          (1/2 oz copper is ~ .7 mils thick)
1 oz copper     0.004"          (1 oz copper is ~1.4 mils thick)
(internal etched features)

However, there is a significant difference between a 0.003" line in 1/2 oz cu
and a 0.005" line in 1/2 oz cu.  For example:
0.005" line @ +/-20% =        .004 - .006"
0.003" line @ +/-20% =        .0024 - .0036".

20% of .005" is a 2 mil range whereas 20% of .003" is a 1.2 mil range.  Most
manufacturers can easily hold the .005" line +/- .0005".  The latter case
depends on their fine line capability.

Another question to ask, and one that is bothering me alot, is: using the
tools in most PCB manufacturing facilities and the magnifications given in IPC
specifications, can the shop even measure the variation in line width
accurately?  At 30X or 60X, they can measure at best in 1 mil increments.
They cannot accurately measure .0024 to .0036" without going to 100X or
greater.  Most shops do not have such measuring equipment available for
routine measurements, if at all.

Steve, I have probably not answered your question, but you did give me a
format to air my concern.  I hope you get a few better answers!
Best Regards,
Jana Carraway


"Steve Collins" <[log in to unmask]> Wrote:
|
| This message is pointed to those of you involved in RF
| circuitry I think.
|
| The IPC-6012 specification for trace with tolerance of ±
| 20% is coming under
| fire here. Typically what can most PCB fabricators hold
| for trace width
| tolerance regardless of what IPC says. How do other RF
| designers specify
| trace width tolerances in critical circuits.
|
| Steve Collins
| PCB Design Supervisor
|
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Jana Carraway
Maxtek -- the Maxim/Tektronix Multichip Module facility
tel 503.627.2063, fax 503.627.4651
email [log in to unmask],  www.maxtek.com

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