What is the effect of die attach epoxy on the RF performance of a millimeter wave MMIC?
Die Attach Material at mmWave
Die attach material selection is one of those "small details" that can make or break a mmWave module. The material must satisfy RF, thermal, mechanical, and process requirements simultaneously.
Frequently Asked Questions
Does die attach material matter for a receive MMIC (LNA)?
Less than for a PA but still important: (1) Thermal: the LNA dissipates much less power (50-200 mW vs 1-10 W for a PA). The thermal resistance of the die attach is less critical. (2) RF performance: if the LNA uses microstrip (BackVia technology): the die attach conductivity affects the ground return quality, which affects the NF and gain. A poor ground connection adds loss in the input network, directly increasing the NF. Even 0.1 dB of additional input loss increases the NF by 0.1 dB (significant for a state-of-the-art mmWave LNA targeting < 2 dB NF). (3) For GCPW LNAs: the die attach has minimal effect on RF performance (the signal does not flow through the die attach). Standard conductive epoxy is adequate.
How do I minimize voiding in the die attach?
Voids are air bubbles trapped in the die attach layer. Voids are problematic because: they create local thermal hot spots (no heat conduction through air), they weaken the mechanical bond (stress concentrators), and they can shift position during thermal cycling (moving voids can crack the die or the bond). Minimization techniques: (1) Vacuum reflow: perform the die attach in a vacuum chamber. The reduced pressure draws out trapped air. Void content: < 5% with vacuum (vs 15-30% without). (2) Scrub motion: after placing the die, scrub it back and forth (0.1-0.5 mm displacement, 2-5 cycles). This breaks up air pockets and improves wetting. (3) Epoxy rheology: use epoxy with low thixotropy (flows easily to fill gaps). The epoxy viscosity at the dispense temperature should be optimized for the die size and gap. (4) Preform rather than paste: for solder die attach, use a preform (thin sheet) rather than solder paste. The preform melts uniformly and produces fewer voids than paste (which contains flux that outgasses during reflow). (5) X-ray inspection: after die attach, use X-ray transmission imaging to inspect for voids. Reject units with > 10% void area or any single void > 25% of the die width.
Can I use non-conductive die attach?
Yes, for some applications: if the MMIC uses GCPW (all signal and ground connections on the top side): the die attach is only mechanical and thermal. A non-conductive, high-thermal-conductivity die attach (e.g., silver-filled non-conductive epoxy: k = 10-20 W/m·K, not electrically conductive) can be used. Advantage: no galvanic corrosion between the die and carrier (because there is no electrical connection through the die attach). Used in: some optical and MEMS packaging where electrical isolation between the die and carrier is desired. For microstrip MMICs: non-conductive die attach is NOT acceptable (the backside metallization must be grounded through the die attach to the carrier).