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February 2011

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Subject:
From:
Tan Geok Ang <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Tan Geok Ang <[log in to unmask]>
Date:
Tue, 22 Feb 2011 08:45:06 +0800
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Try pre-form solder instead of solder paste. Of course, process controls
of flux still a key parameter. Vacuum oven will further reduce the
issue.

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Phil Bavaro
Sent: Friday, 18 February 2011 12:27 AM
To: [log in to unmask]
Subject: Re: [TN] Voids issue on Aluminium base plate

Cedric,

Lots of things come to mind....plating thicknesses, paste patterns,
wetting, profiles.

Is there any pattern to the voiding? Does it resemble the printed paste
shape?   

I have soldered lots of RF transistor bases down, but I haven't ever
soldered them to NiAg plated aluminum (always to Ni/Au plated copper
slugs).

You don't mention the paste patterns used but that will contribute to
large voiding if you don't give the gases a path to get out.  I would
reduce the solder paste coverage to 45%, use a checkerboard pattern of
dots, and then observe the voiding pattern.  

This assumes that the voiding is not relative to wetting as that will
change everything.  To do this, check the solderability of both surfaces
being soldered to independently.


Phil
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Cedric ORAIN
Sent: Thursday, February 17, 2011 6:26 AM
To: [log in to unmask]
Subject: [TN] Voids issue on Aluminium base plate

Dear Technetters,

We have to solder a RF power transistor onto an aluminium base plate and
we
are facing with a very challenging issue.

We encountered a high voids rate (>30% or 40%) underneath the component
body
(which is about 35x10mm). This is not acceptable for this application.

The component is Nickel/gold plated whereas the aluminium baseplate is
plated with Nickel and Silver and then cleaned with solvent.

We tried to change/adapt:
- soak time of the profile, 
- 2 difference solder pastes (SAC305), 
- change the screen design and thickness

Finally none of these parameters looks to have significant effect on
voids
rate. We are now wondering if the solderability of the Ni/Ag plating is
good
enough and even if it is convenient for that purpose.

Is somebody could give us advises? How to enhanced NiAg plating
solderability (acid ?!, ...)? Which flux could help to compensate the
poor
wettability? Which kind of plating would be preferable? ...

Thanks and regards,

Cedric



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