We are often asked what the ‘preferred’ method is for attaching MMIC die to the substrate is. We use, and prefer, silver epoxy die attachment, because epoxy die attach
- is a low temperature process (epoxy can dry at room temperature, vs. very high temperatures for solder or eutectic)
- is easy to do, with a low learning curve and minimal waste of parts
- does not require a solderable die surface (not all chip based MMICs have a nickel solder barrier layer)
- has a low probability for voids underneath the chip
- is a reversible process, where the part can be removed without damage if performed carefully.
For these reasons, we use silver epoxy (specifically Epotek H20E), place a minimal amount beneath the die, and place the chip into position so that a thin epoxy fillet is created around the chip. The part is then cured at temperature according to the epoxy instructions.
However, while this is the method that we prefer, other methods are not precluded. Generally a solder or eutectic die attach is used because of material cost, thermal conductivity, and because it is suitable for hierarchical assembly. Generally the cost of the attachment material (solder or epoxy) is a small percentage of the cost of our parts, so for our customers the risk of losing parts and the time to develop the process are more important than the cost of silver epoxy. Further, MMIC mixers have very low power dissipation (typically just the LO power, which is below 100 mW), and so they do not have stringent thermal conductivity requirements.
So that means the only reasons to go with solder instead of epoxy is for hierarchical assembly. This means that another high temperature process must be performed after the die attach. If this cannot be avoided, then we expect that there is no reason that certain MMIC chips (with solder barrier) would not qualify, but we recommend contacting the factory for further guidance for this procedure.
