Subject:                              Time:  1:48 PM
  OFFICE MEMO         RE>Exposed Copper on Assemblies       Date:  2/23/96

Denis ([log in to unmask])

You asked "On some assemblies, we use grounded contact pads to ground     
shields/bulkheads, etc. The contact between the shield and the board is     
simply a metal tab on the shield that presses against a contact pad on     
the board. Is such a connection reliable if the pad is exposed copper (or
copper with residual OCC after a couple of reflow cycles and an aqueous wash)
as opposed to HASL? Pasting those pads is not possible in all applications due
to the size, number of pads, etc."

The use of exposed "bare" copper for mating surfaces for electrical or
grounding contacts is a poor design practice and should not be used, unless
there is sufficient clamping pressure to exclude air (oxygen and other
corrosion causing materials such as sulfides and nitrides).

In the electrical world, bare copper mated surfaces can be used provided the
contact pressure is great enough and the current density exceeds some number
(from memory about 300 A/sq.in.).  Otherwise, there must be some form of
corrosion/oxidation protection.

In the case of EMC (electromagnetic compatability) for good shielding and
bonding, the mating surfaces shall be suitably corrosion resistant and the
joint shall be coated if exposed to high himidity/condensurate.

In our world of printed board of electrical interconnections, we generally use
"gold" plating for at least the mating surfaces of electrical contacts for all
interconnections having electrical signal requirements of about 100 millivolts
or a few milliamperes or less.  We as an industry have and are using other
metal coatings (elemental, alloys & compositions), but their application and
use environments are controlled and limited.

Two concerns - one major and two functional:

A major concern (ELECTRICAL SAFETY RELATED) is that because of the possiblity
of poor electrical contact, if the shield is conductive and could become
electrified (in the parlance of the USA's National Electrical Code) then
you've got a serious electrical safety problem if a person becomes electrified
between the shield and a "good" ground.

If the shield is used for EMC, then the shield could be ineffective and create
more (intermittent) EMC problems than it cures.

If the shield is used for cosmetic or physical barrier purposes -- then who
cares.

Denis, if you need some more info. or references feel free contact me.

Ralph Hersey
Lawrence Livermore National Laboratory
Phn.  510.422.7430
FAX.  510.424.6886
e-mail:  [log in to unmask]