On Mon, Jan 18, 2010 at 09:00:35PM -0500, Mark Stanley wrote: > 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. ;) I don't have that tool, since my bonder is a wedge bonder, which I got second hand virtually for free but had a few problems for restarting after staying unused for over 10 years. For the diode, I need very small wedges (1.5 mil length and width) and 0.7mil gold wire. The microscope is no problem, otherwise I would not even seen how to orient the die (square, with 4 bonding pads marked by roman numeral, that's paleoelectronics), fully symmetric. On each waffle pack, the manufacturer specifies which of the pads has to be used for that particular set of diodes. Ok, I have attached a photo of the diode; the quality is not great, it's a cheap Dino-lite microscope, but consider that the sides of the square are 15mil (381Âm). The bonding pads are the crosses. > > > 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. That's much better. Ten minutes is really too short, especially when learning. > > 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. 100ÂC is fine. > > > 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? Yes, at least the ones that don't have a distributor in Spain (this happens but they sometimes redirect to France or Germany). > > 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. Not my case by far. > > > 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. > That might be the problem if I use Alfadhesive's stuff. But my epoxy was not very hard. > > 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. That's a single component, right? They typically need -20 or -40ÂC for conservation from what I've seen. I have a very small fridge to keep chemicals, but it does not have a freezer compartment. On the other hand, we use liquid nitrogen by the tens of liters, but that's too cold. > > > 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. I'll certainly reach the expiration date before the product is exhausted, therefore it becomes a fixed cost every year for the lab. > 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. I only have to fix the diode, and it is the last step, the rest is already soldered by standard reflow procedures (tin/lead, non RoHS) for SMD and hand soldering for through hole components (there's a strange mixture of SMD on both sides, through hole components and a bare die on the same board). Gabriel
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