I've been in the microwave mystery and magic game for many years. You may
get into a. interface impedance problem  b. production consistency issues.
From 30 to 60 GHz we prefer ribbon bonding. I have tested the method and
believe I have some stuff. Start with this:
http://www.microwavejournal.com/articles/print/9650-a-comparison-study-of-high-frequency-characteristics-for-ball-and-ribbon-bonding

Some of my friends belong to the best on what you work with. If you get
problems, I can contact them.

Inge

* *

On 23 October 2013 19:46, Guy Ramsey <[log in to unmask]> wrote:

> Excellent input Mike. Targeted all my concerns.  But, I did mislead.  The
> foil is copper, nickel, gold. The gold is an electroplated flash. We are
> measuring between .25um and .35um  of gold over the nickel. So, even with
> the thin flash, the thin solder makes the gold a concern, our goal is 15um
> to 20um of bulk solder for the interface.
>
> We are even having issues with Type 4 in a 50 um stencil. We scoop some of
> it out, on some pads.  But, maybe a 75 um stencil . . . But, then I do, as
> you mentioned face variation from wetting and spreading (now boys careful
> with the comments).
>
> Fortunately, we are printing and reflowing first on an OSP pad,
>
> I think I'll try a thicker stencil with a smaller aperture. If it helps,
> I'll try to remember to post back to this thread.
>
> Guy
>
>
> -----Original Message-----
> From: Mike Fenner [mailto:[log in to unmask]]
> Sent: Wednesday, October 23, 2013 2:03 PM
> To: 'TechNet E-Mail Forum'; 'Guy Ramsey'
>  Subject: RE: [TN] requirement for thin solder joint.
>
> Guy
> Type 3 powder size won't hack it. In practice Type 4 particle size is not
> that much different from type 3 from some suppliers, but in theory you
> should be able to print it.
> T5 would enable you to fiddle around with stencil thickness and reductions
> but will be relatively expensive, could limit your effective supplier base
> and alloy choice and likely will put out lead times.
> Assuming you get an even print you will end up with the right volume of
> solder for a 25um reflowed deposit but you might have some problems with
> actual reflowed height according to how the surfaces wet and the spread you
> get. I imagine you are doing this lead free so this will tend to wet in
> place on the non gold surface but will spread on the gold side. Depending
> on
> the thickness of the foil. You haven't dimensioned that so you are anywhere
> from getting a solder joint, dissolving Au completely to having high
> intermetallics and so on depending on everything including time over
> liquidus (as short as possible) and cool down rate( as quick as possible).
> A
> laser zap as mentioned seems attractive. You might need to go to a tin free
> alloy. No tin and no lead alloys do exist, but perhaps not in T5 powder.
> I imagine your assembly does not consist of just the one solder joint so
> what are you doing with all the others, are they regular alloy joints?
> There are some alloy and process combinations that would likely work
> without
> the above issues but would need to have a better understanding of your
> application than you might be prepared to share on an open forum.
>
> Regards
>
> Mike Fenner
> BS&P
> M: +44 [0] 7810 526 317
> T: +44 [0] 1865 522 663
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Guy Ramsey
> Sent: Wednesday, October 23, 2013 3:04 PM
> To: [log in to unmask]
> Subject: [TN] requirement for thin solder joint.
>
> Hi all,
>
> We would like to solder a thin gold foil, with some magic on top, to a PWB.
>
>
> The foil and pad are round, about 560 um in diameter.
>
> We want the solder connection to end up less than 25 um thick.  So, we are
> trying to print with a 50um thick stencil, and 20% reduction, the stencil
> aperture is 460 um diameter.  So, area ratio, or aspect ratio look great,
> printing shouldn't be a problem.
>
>
>
> Print results were inconsistent.
>
>
>
> This morning I realize that many of solder paste spheres are over half the
> thickness of the stencil.  And the rule of thumb that five spheres should
> fit across the aperture for good paste release.   Well, what about the
> thickness of the stencil versus the diameter of the spheres?
>
>
>
> Has anyone out there wrestled with this problem?  Maybe I should be using a
> thicker stencil, and push the aspect ratio a bit more.
>
>
>
> Thoughts?
>
>
>
>
>
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