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

[Gabriel Paubert <paubert@xxxxxxx>]

On Mon, Jan 18, 2010 at 06:26:41PM -0500, Mark Stanley wrote:
> On Monday 18 January 2010 16:46:59 Gabriel Paubert wrote:
> > And yes, I do use tunnel diodes, in bare dies actually. I bonded a couple
> > of them last Friday, but am still fighting problems with the attachment of
> > the backside, which is the cathode for that particular model, on the
> > substrate with conductive (silver) epoxy.
> 
> 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.

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).

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

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.


> I've used silver epoxy to build hundreds 
> of modules with bare dies and the only problem I have is replacing the dies.

There is only one bare die in the circuit, and I've not had any problem removing
one previously glued (enough room around the die, we're speaking of a single
50 ohms coplanar line reaching the diode, backside to ground plane). 

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. Besides that heating
above Tg of the epoxy helps:

http://epotek.com/SSCDocs/techtips/Tech%20Tip%208%20-%20Reworking%20Cured%20Epoxy.pdf

It's point 4 in the above document, the other solutions are not selective
enough and may cause collateral damage. With only 1 die I wouldn't care, but 
I don't like exotic chemicals or sulfuric acid on my PCBs.

What epoxy did you use (manufacturer and reference)?

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).

> It's a real pain to get off the substrate without damaging other parts.

	Gabriel


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