Hi Alan! One issue you may want to investigate is the possibility that the
solder joint is being contaminated by zinc (if I understand your situation
you are soldering directly to brass for a portion of the solder joint). If
one solders to brass that does not have a barrier layer (e.g. nickel,
copper) then the zinc contained in the brass will diffuse overtime into the
solder joint resulting in degradation. The higher the temperature, the
faster the zinc contamination can progress. A metallographic cross section
analysis would provide the information you need to check for the
contamination. Good Luck.

Dave Hillman
Rockwell Collins
[log in to unmask]





Alan Kreplick <[log in to unmask]> on 10/13/98 07:03:56 AM

Please respond to "TechNet E-Mail Forum." <[log in to unmask]>; Please respond
      to Alan Kreplick <[log in to unmask]>

To:   [log in to unmask]
cc:
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

################################################################
TechNet E-Mail Forum provided as a free service by IPC using LISTSERV 1.8c
################################################################
To subscribe/unsubscribe, send a message to [log in to unmask] with following
text in the body:
To subscribe:   SUBSCRIBE TechNet <your full name>
To unsubscribe:   SIGNOFF TechNet
################################################################
Please visit IPC's web site (http://www.ipc.org) "On-Line Services" section
for additional information.
For technical support contact Hugo Scaramuzza at [log in to unmask] or
847-509-9700 ext.312
################################################################

################################################################
TechNet E-Mail Forum provided as a free service by IPC using LISTSERV 1.8c
################################################################
To subscribe/unsubscribe, send a message to [log in to unmask] with following text in the body:
To subscribe:   SUBSCRIBE TechNet <your full name>
To unsubscribe:   SIGNOFF TechNet 
################################################################
Please visit IPC's web site (http://www.ipc.org) "On-Line Services" section for additional information.
For technical support contact Hugo Scaramuzza at [log in to unmask] or 847-509-9700 ext.312
################################################################