Our company is a manufacturer of industrial turbine and reciprocating
engine control systems, used a variety of environments.   We are in the
process of converting from  home grown (Mil driven) printed wiring board
design and inspection standards to IPC standards.  In particular, we are
utilizing IPC-D-275, IPC D-279, IPC-A-600 and IPC-A- 610.

We in engineering are told by purchasing there is a premium of around
15% to be paid for non standard specifications, which we are told, means
deviation from IPC Class 2.  We have noted several exceptions where we
feel Class 3 or better is necessary, primarily in the specifications
affecting circuit spacings.   Specifications affecting  inner layer and
outer layer spacings at Class 2 will require substantially  increasing
spacings by design to allow for spacings reductions in manufacturing
allowed by Class 2 .  For example IPC -A-600 2.1.10.2 allows a 20%
reduction in spacings in isolated areas.  We design to minimum spacings
required by UL and IEC standards reflecting dielectric strength,
impulse, and EMC surge test requirements.  Spacings violated by spikes,
rough edges, conductive islands and inclusions will  result in  a
failure to meet these requirements.  Therefore,  to allow even a Class 3
reduction in spacings requires compensation of the designed spacings in
the event some of the allowed violations do occur.   We can't afford the
real estate, especially for higher voltages where spacings are required
to be over 3mm.

If you have  industrial market experience as a board supplier or user,
please comment.
Do you use IPC standards as controlling documents ?
Do you find there is a substantial cost adder for exceeding Class 2
requirements on some specifications?
Do you accept the added cost ?
Do you inspect for spacings violations on purchased Class2 bare boards?


Thank you very much.

Randy Johnson
Reliability & Safety Compliance Engineer
Woodward Governor Company
Loveland Colorado
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