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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