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Fluxless Die Attach by Activated Forming Gas



Fluxless Die Attach by Activated Forming Gas
As demonstrated in the study, the fluxless and oxide-free technology using EA has made it possible to achieve a high quality die attach with zero or near-zero voids.
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Authored By:


C. Christine Dong and Russell A. Siminski
Air Products and Chemicals
Allentown, PA USA

Summary


Eutectic god-tin (Au80Sn20) is widely used as the die-attach material for making radio-frequency (RF) and microwave devices. The metallic bonding is typically achieved by soldering using a gold-tin preform in a forming gas containing hydrogen (H2) and nitrogen (N) to a peak temperature at 300°C. The performance and reliability of the devices are strongly dependent on the quality of the die-attach layers.

It is always expected to make the die-attach layers as free of voids as possible since the voids are poor thermal and electrical conductors and also are stress concentration centers. There are three major causes for void formation, which include poor solder wetting due to surface oxides, vapor out-gassing by flux decomposition, and gas entrapment from preform melting. It is known that the gas entrapment is more significant for larger dies and can be managed by gas evacuation before the melting of the solder. However, a good fluxless and oxide-free technology for metal die attach is still lacking.

The organic fluxes used in conventional soldering not only induce void formation but also leave residues, which contaminate the dies and are corrosive. In addition to being costly and inconvenient to clean, the residues at the bonding interfaces of the die-attach layer are trapped, thus degrading interfacial bonding over time. The current study introduces a novel technology of using electron attachment (EA) to activate H2 for fluxless die attach. EA is a new concept for ambient-pressure gas activation, which brings several advantages compared with plasma-based gas activation. Results obtained in this study demonstrate that solder oxides and organic contaminations on the surfaces to be bonded can be effectively removed by EA-activated forming gas, which leads to better solder wetting compared to flux-based process.

The EA technology also shows a feasibility of reducing peak temperature of die attach from 300°C to 290°C, thus minimizing high-temperature induced damages. These advantages are believed to be attributed to the higher surface tension of the oxide-free molten solder compared with that of an organic flux, thus leading to a larger wetting force. As demonstrated in the study, the fluxless and oxide-free technology using EA has made it possible to achieve high quality die attach with zero or near-zero (less than 5% voids.

Conclusions


A novel technology of using EA to activate a forming gas containing H2 and N for fluxless die attach with eutectic gold-tin preform has been developed. Our study introduces applying the EA-activated forming gas to clean surface oxides and organic contaminations at a temperature below the solder's melting point. This is then followed by an inert soldering either under ambient pressure or a pressure variation involving vacuum. The results obtained demonstrate that oxides on the preform surface and organic contaminations on the gold termination can be successfully removed by the EA-assisted precleaning.

If an inert soldering under ambient pressure is applied after the EA-assisted pre-cleaning, a zero or near-zero (less than 5%) voids can be achieved for the size of the dies around 3 mm X 4 mm or below. For relatively large dies, the EA-assisted precleaning can be combined with the vacuum contained pressure variation. A result in this study shows that by using the recommended approach, a die-attach layer with a near-zero voids can still be achieved even for the die as big as 10 mm X 10 mm.

Comparing with the conventional flux-based die attach, the fluxless and oxide-free technology using EA has the following major advantages: 1) it eliminates the flux-vapor induced voids, 2) it removes the flux-residues related problems, 3) it provides a reduced peak temperature for die attach (e.g. 290°C versus 300°C), and 4) it leads to an even better solder wetting, thus decreasing the overall voiding tendency. Therefore, the developed technology is believed to be a promising solution for achieving high-quality die attach, thus satisfying the needs for applications requiring high reliability.

Initially Published in the IPC Proceedings

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