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Subject:	Re: Soldering Paladium-Silver-Platinum terminations
From:	[log in to unmask]
Reply To:	TechNet E-Mail Forum.
Date:	Wed, 31 Mar 1999 17:44:31 EST
Content-Type:	text/plain
Parts/Attachments:	text/plain (49 lines)

In a message dated 31/03/99  05:17:24PM, you write:

> I don't think my first message went through.   Anyone out there that can
tell
> me
>  how to best achieve less grainy solder joints when working with
>  Paladium-Silver-Platinum terminations when used Kester R596 solder paste.
>
>  Thanks,
>
>  Shaun Dalton

It's not clear - to me at any rate - if  the Pd/Ag/Pt terminations are
thickfilm metalisation or plated metal, or  what solder alloy is in the Kester
paste. Also I presume other visual criteria such as low contact angle/joint
profile and wetting are OK as you did not mention them.

I guess you are seeing growth of intermetallics in the solder joint.

Thick film and Sn62 together can easily give a grainy or frosted appearance.
Ordinary Sn63 on high silver content surfaces can do the same.
In either case it's counter intuitive (silver is a bright shiny metal,
therefore things with silver in them will be bright and  shiny also) .
Long times over alloy melt point and/or long cool down times allow
intermetalics to form and grow in multiple sites in the solder joint, given
time they can achieve a size where they disrupt the smooth surface of the
joint giving a frosty or grainy appearance. The extra 1.5 - 2.0 percent Ag in
the Sn62 will contribute handily to this effect.

If this is only happening on certain other wise apparently identical
terminations and not on others, it could be that you have a profile problem or
maybe the components / board layout is giving a heat reservoir effect to just
the problem joints.


Mike Fenner

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