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February 2011

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Subject:
From:
"Stadem, Richard D." <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Stadem, Richard D.
Date:
Thu, 17 Feb 2011 15:06:12 -0600
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Cedric,
I agree with Paul that the solder paste is the biggest contributor to voiding. So back to my original suggestion, can you pre-tin the bottom of the power transistor, and pre-tin the nickel-plated area on the aluminum, then reflow with a thin sheen of tacky flux such as Alpha WS319? Perhaps you do not even need to pre-tin the nickel-plated aluminum substrate, the solder on the bottom of the power transistor should be enough. This will greatly reduce the amount of voiding seen.

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Paul Edwards
Sent: Thursday, February 17, 2011 2:37 PM
To: [log in to unmask]
Subject: Re: [TN] Voids issue on Aluminium base plate

First question is how much voiding is acceptable and what is an acceptable distribution?

When bonding machined or cast Al substrates to flat surface with solder paste you will always get voids except when you do it in a vacuum furnace...

The big problem is solder paste is ~50% flux so unless you "pump" the flux out of the joint material you will have voids...

Where you have them and how big they are area function of size of the bonding area, surface finish, surface roughness, surface planarity, solder paste placement, solder paste type and reflow profiles...

You have to remember that in the best case, at reflow temperatures that reflowed paste is a high density fluid mixed with pockets of up to 30% the same volume of low density fluid trapped between 2 parallel plates.

SO the question is back to "where do voids cause the least harm?"...

If the top surface of the joint had a slight upward taper(or radius) with "radial grooves" such that the "groove" depthXwidth product increase toward the outside then the trapped fluids could flow to the outside of the joint..
Also if you coat the surface with a gradual increasing thickness of something like an OSP or a high melting point organic material with gas relief channels, the molten solder would wet to the center of the joint area and progress outward as the OSP/high melting point material releases exposing base metal solder to the solder melt. As long as the solder melt wets to the top surface of the joint starting from the inside and moving out, the wetting front should push the flux and gases out of the joint in front of the bulk solder melt.  

Paul


Paul Edwards
Process/Quality Engineering
Surface Art Engineering


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Cedric ORAIN
Sent: Thursday, February 17, 2011 11:53 AM
To: [log in to unmask]
Subject: Re: [TN] Voids issue on Aluminium base plate

George, 

It is in a recessed cavity.

Regards,

Cedric

-----Message d'origine-----
De : TechNet [mailto:[log in to unmask]] De la part de Wenger, George M.
Envoyé : jeudi 17 février 2011 17:48
À : [log in to unmask]
Objet : Re: [TN] Voids issue on Aluminium base plate

Cedric,

What is the size of the RF transistor base?  Is the transistor on a flat
aluminum surface or is it in a recessed cavity on an aluminum chassis?

Regards,
George
George M. Wenger
Senior Principal Reliability / FMA Engineer
Andrew Corporation - Wireless Network Solutions
40 Technology Drive, Warren, NJ 07059
(908) 546-4531 Office (732) 309-8964 Mobile
E-mail: [log in to unmask]

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Phil Bavaro
Sent: Thursday, February 17, 2011 11:27 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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