Alan,

Two immediate thoughts on your problem. First, have you cross sectioned
any solder joints prior to the cycling to make sure they were properly
formed and Second, brass usually has a very thin (flash) layer of copper
over it BEFORE plating the Ni and Au. If the Cu flash is skipped, you
can delamination between the brass and the Ni/Au platings. This could
propagate into the solder easily during cycling.

Regards,

Paul Terranova

Here's my MS Outlook98 virtual business card:

Analytical and Environmental Test Services Lab
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                -----Original Message-----
                From:   Alan Kreplick
[mailto:[log in to unmask]]
                Sent:   Tuesday, October 13, 1998 8:04 AM
                To:     [log in to unmask]
                Subject:        [TN] Soldering Brass to Copper

                2nd Technet (I don't think the 1st email went through)

                Hello Technetters:

                I have a small surface-mount connection with the
following criteria:
                   The lead is 1/2 hard brass with .00005/.00015" gold
over .000005/.00015" of
                   nickel plating.  It is L-shaped with the bottom of
the L making the contact
                   with the substrate.  Also, the bottom surface has
been cut such that the
                   brass is exposed.  The bottom surface dimension is
                    .012" x .112".
                   The substrate is copper with OSP and is .025" wide.
The copper is the middle
                   layer of a beryllium/Kapton/copper/Kapton/beryllium
flex circuit.
                   The L-shaped lead is forming a lap connection onto
the substrate, and is held
                   in place via tooling.  The tooling currently holds
the lead directly onto the
                   substrate (no gap) which squeezes the solder paste to
the sides.
                   The solder paste is 2% (62/38) no-clean.  It is
deposited with a Camalot (3
                   dots/lead) and reflowed with a hot-air gun.
                   The top of the L-shaped lead acts as a pogo in its
application and needs to
                   (would like to) withstand 500,000 cycles
(depressions) at 85C (and possibly
                   150C next year).

                The problem is that the solder joint has exhibited
cracking (at the toe) after
                only 5000 cycles.

                Two questions:

                What solder paste (alloy and flux) is recommended for
soldering brass to copper
                (Note the mechanical cycling and the cycling
temperature)?

                What should be the gap between the bottom of the
L-shaped lead and the
                substrate?


                Thanks in advance for any help,

                Al Kreplick
                Teradyne, Inc.
                617-422-3726


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