Hi Steve! The typical method used to avoid dezincification is to use a barrier
metal such as copper or nickel to prevent the zinc from diffusing into the
solder so yes - your 0.0002 - 0.0004 copper plate will prevent any
dezincification issues. And I'll venture a guess that your "highly variable"
solder wetting is possibly due to the formation of tin oxides that are impacting
soldering. A SERA or wetting test can be useful to see if an intermetallic
problem exists - or process a cross section to see if you really have the
100-300 microinches of tin that would prevent the copper/tin intermetallic phase
from reaching the surface and becoming unsolderable. I would investigate the
oxidation issues before chasing any other possible cracking causes - you have to
make a good solder joint or all other efforts are useless. Good Luck.

Dave Hillman
Rockwell Collins
[log in to unmask]





"Meschter, Stephan J" <[log in to unmask]> on 02/26/2000 11:02:45 AM

To:   "'TechNet E-Mail Forum.'" <[log in to unmask]>, David D
      Hillman/CedarRapids/Collins/Rockwell@Rockwell
cc:

Subject:  RE: [TN] brass pins



Dave H./Group
What is the latent failure mode due to de-zincification of brass?
I have a "hermetic" filter that has a brass case with a 0.0002 to 0.0004
inch Cu plate and a 0.0001-0.0003 inch tin plate. The filter is 3/8 inch in
diameter 3/4 inch long with a 1/4-28 threaded header at one end. We are
seeing the case to header joint crack in some instances during assembly.
Examination of the joint shows that the solder wetting is highly variable.
The supplier is concerned about solder balls and is sparingly applying
solder (just enough to go through leak test).

The a capacitor is soldered to the center of the case is soldered to a
threaded header with Sn62Ag2Pb36 solder. The header is steel with a nickel
plate, Cu Plate and Sn plate. The assembly sees an initial thermal exposure
when the center capacitor is soldered initially (125 +/-10 degC preheat for
20 +/-5 min. oven soldered at 200 to 210degC for 11 to 13 minutes) and then
the header and end cap are soldered to the case with  solder using an iron.
The assembly then sees a 24-48 hour drying cycle at 125C. The part is
supposed to be a full military rated temperature part.  The intended service
qual temp is 71C if we can manage to put it together.

I know that I have CTE differences between the header and the case. I am
also concerned about the amount of side load seen on the filter when it is
potted.

Is the Cu underplate thickness adequate for this application? What other
long term concerns do I have with the brass?

Thanks in advance for your help.
Steph

Stephan Meschter               [log in to unmask]
Lockheed Martin Control Systems  Phone  :(607)770-2332
600 Main Street, MD R52F         FAX    :(607)770-2056
Johnson City, NY 13790-1888      MARCALL: 8 * 255-2332


> ----------
> From:   David Hillman[SMTP:[log in to unmask]]
> Reply To:    TechNet E-Mail Forum.;[log in to unmask]
> Sent:   Thursday, February 24, 2000 7:09 PM
> To:     [log in to unmask]
> Subject:     Re: [TN] brass pins
>
> <<File: att1.htm.html>>
>
>
> Hi Brent! You should be able to solder to brass - check with you flux
> supplier
> to get their recommendations. The flux you are using may not have enough
> activation to breakdown the oxides on the brass surface. On word of
> caution -
> brass (copper and zinc alloy) can undergo 'dezincification' if there is no
> barrier plating between the brass and your solder joint. In some use
> environments this becomes a latent defect. Good Luck.
>
> Dave Hillman
> Rockwell Collins
> [log in to unmask]
>
>
>
>
> gelon <[log in to unmask]> on 02/24/2000 04:48:15 PM
>
> Please respond to "TechNet E-Mail Forum." <[log in to unmask]>; Please
> respond to
>       gelon <[log in to unmask]>
>
> To:   [log in to unmask]
> cc:
>
> Subject:  [TN] brass pins
>
>
>
> Has anyone ever tried to flow solder brass pins before? We are attempting
> to do
> this and not having much luck. If anyone has done this before and has some
> suggestions that might help I would appreciate it
>
> thanks
> Brent Green
>
>

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