Re: gEDA-user: Silver Epoxy: was Parts Manager Working Document

[Mark Stanley <mstanley@xxxxxxxxxxxxxxxx>]

On Monday 18 January 2010 19:30:55 Gabriel Paubert wrote:
> On Mon, Jan 18, 2010 at 06:26:41PM -0500, Mark Stanley wrote:
> > What kind of problems are you having?
>
> a) That I am a beginner with this technique.
>
> b) That I don't (yet) have the right tools to handle the dies, that
>    are small (square 15mil/side). They are ordered and should be
>    here early next month. I end up with either not enough epoxy
>    or too much with the die drowned in epoxy.

I use a ball pick for applying the epoxy.  The pick is normally used for 
cleaning the ball wedge on the bonding machine and appears to be almost 
molecular sharp.  I mount it in the handle of one of those small screw drivers 
used for repairing glasses.  I can apply a very precise amount of epoxy.  Oh 
yeah, and a microscope *really* helps when applying the epoxy. ;)

> c) That I still have to order the proper epoxy (the one I bought from
>    Farnell has a pot life of 10 minutes, which is annoyingly short).

The stuff I use can sit on my workbench at least half a day before it starts 
clumping.

> d) That the components are germanium based, therefore very sensitive to
>    temperature and I cannot use epoxies with cure temperatures above 125ÂC.

What I use can cure at temperatures as low as 50ÂC but will cure in 10 minutes 
at 100ÂC.

> e) That the distributors in Spain take outrageous margins ($200 becomes
>    â400), so I'm trying to find a cheap way to get the epoxy.

That's pretty rough.  Are you able to order from over seas companies?

> This may indicate that the gluing was not perfect (to put it mildly). The
> die is small so the force to remove it is rather small too.

I've had to use enough force during removal that I've dented the metal 
substrate.

> Besides that heating above Tg of the epoxy helps:

True.  Except that the brand I use releases at temperatures too high for the 
other dies on my modules so I generally remove the bad die by hand.

> What epoxy did you use (manufacturer and reference)?

I'll get you the information tomorrow when I get back to work.  For long term 
storage we keep it packed with dry ice (CO2) in the freezer and store the in-
service tube in the refrigerator.  We've had in-service tubes last up to six 
months.

> I've spent quite some time trying to find the best epoxy for my application
> over the last two weeks but am still undecided. Actually I was considering
> either Epotek H20E which seems to be an industry workhorse, or
> Alfaadhesives' E10-110 because of the convenience of the pouches (combined
> with high Tg and low cure temperature although the cure schedule is
> strange).

I think the stuff I use might be a bit expensive.  However, I only squeeze out a 
4mm drop and can keep it on my workbench for several hours while I am building 
modules.  With that one drop I can build 3 or 4 modules, each one having a VVA, 
2 amplifier dies, a diode, 2 hand wound inductors, and around 10 or 12 resistors 
and capacitors.  It cures at temperatures well below the limits of the dies and 
the only trade-off is that lower temperatures take a lot longer to cure.

Mark Stanley



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