Distance for Connectors
Distance/Air Gap, the path of least resistance
travels along the path of least resistance. Determining
the best path for electricity involves a means of measuring
the distance from one conductor to another conductor. The
shortest distance is through air (Air Gap) or along the
surface of the insulation (Creepage Distance).
in a cable assembly are where this "path of least resistance"
exists. This least resistance path is above, not through
the insulation. It is measured in either "air gap"
or "creepage distance. To measure and determine the
creepage distance you want to answer the question of "How
far does electricity need to travel" between two pins
in air, or along the insulating surface between two conductors,
to arc over?
the Pins on the Connector
an example, a 15 pin high-density d-Sub miniature connector
has contacts on .090" spacing. The subsequent rows
are spaced .078" away and the pins of the three rows
are offset from each other by .045". This provides
.090" spacing between all diagonal contacts. (If they
were not equal, we would need to use the smaller dimension.)
So, what is the creepage distance of this connector?
you guessed .090", you forgot to subtract the diameter
of the pin at the surface of the connector. The size 22
pin has a .03" diameter so half a pin diameter is subtracted
from the distance from center to center of two adjacent
pins for .090" -.030"/2 -.030"/2 = .060".)
Lesson #1: Subtract the pin diameter.
The Backside of the Connector:
real creepage distance may still be less than measured on
the connector face. It is likely that the termination end
is larger in diameter than the pin at the face. A #22 contact
at the termination side can be .044" in diameter. Now
our creepage distance is .090" - .044" = .046"
(Many connectors such as those with crimp and poke contacts
have insulation from the connector housing on the insertion
side that completely covers the backside of the contact,
so in those connectors the creepage path will likely be
longer than that on the face of the connector and won't
be the area of shortest creepage).
#2: Check the creepage on the backside of the connector
Check Female Contacts as Well:
about the socket? The end that accepts the pin can be .062
in diameter. This would leave .090"-.062 = .038"
of gap. Usually this is not where the path is shortest.
Why? The socket is slightly recessed in the connector housing
providing additional distance for this creepage path.
Lesson #3: Check creepage for the female contact too.
Check For Hidden Creepage Paths Inside
the creepage distance is hidden. D-Sub miniatures are typically
manufactured with two plastic insulators, one in front and
one in back, that capture the contacts in-between. Lets
see how big the contact shoulder is that holds the contact
in place by separating the 2 halves. The shoulder measures
.073" so the internal creepage distance is .090"-.073"
= .017" According to our arc gap calculator, this dimension
could fail at around 1425 VDC.
Lesson #4: Look for hidden creepage paths inside
Check for Hidden Creepage Paths Outside of the Connector
of the other connector creepage distances that can be hidden:
If the connector solders to a PCB, inspect adjacent PCB
pads for the connector contacts both on the top and bottom
of the PCB. Conformal coatings can provide good protection
and eliminate the path for breakdowns. Solder mask does
not provide an effective barrier to creepage.(As you can
see in the photo below the mask has worn away in spots.)
Lesson #5: Look for hidden creepage paths outside
As the brief analysis above illustrates, checking all creepage
variables can be time consuming! However, as you diligently
explore each of your test failures, you will come to better
understand the test challenges that different style connectors
present. The design insights gleaned from this practice
will enable you to justify lowering test voltage specifications
to meet your "real world" test needs. After all,
you cannot test cables more stringently than the physics
of the cable will allow.
Subtract the pin/socket diameters
Check the creepage on the backside of the connector
Check creepage for the female contact too.
Look for hidden creepage paths INside the connector.
Look for hidden creepage paths OUTside the connector.