Eutectic die bonding attaches a die to its substrate with a metallurgical joint — no polymer, no filler — formed by melting a eutectic alloy at a single, sharply defined temperature. A eutectic alloy is the one composition of two metals that melts at one temperature instead of over a range, and that sharpness is what makes the process controllable and the joint dense. It is the attach you reach for when the part runs hot, has to stay dry, or has to survive later assembly steps.
Why the alloy is chosen first
A eutectic attach is defined by its alloy, because the alloy sets the process temperature and the temperature has to fit everything that happens after. The named systems we run cover most work:
- AuSn (80Au/20Sn) — the workhorse. Its eutectic point sits near 280 °C, and that high remelt is the whole point: the joint survives a later lid seal and board-level reflow without the die floating. Run fluxless, it leaves no residue in the cavity.
- AuSi — a higher-temperature gold-silicon eutectic, near 363 °C, that bonds a bare-silicon die backside directly with no separate preform metal.
- AuGe and AuIn — gold-germanium and gold-indium systems for other backside metallurgies and lower-temperature steps in a stepped assembly.
A stepped build sequences these from highest melt to lowest, so each attach survives the one after it. Choosing the alloy is really choosing where the part sits in that thermal ladder.
What the process actually requires
A known die backside
Eutectic solderability depends on the metal on the back of the die. AuSi wants bare silicon; the solder eutectics want a solderable backside stack such as Ti/Ni/Au. The single most useful thing an RFQ can include is the die backside metallization, and it is the thing most often left out. Get it wrong and the joint will not wet — the process window looks fine until the shear test.
Clean surfaces and a controlled atmosphere
Unlike a fluxed solder process that can chemically clean as it reflows, a fluxless eutectic attach has nothing to clean the interfaces for it. It wants clean, properly metallised surfaces on both die and substrate, and it is run under an inert or reducing atmosphere with a controlled thermal profile, often with mechanical scrubbing to break up surface oxide and let the metals wet.
Void control you can see
A eutectic joint's job is usually to carry heat, and a void is a hole in the thermal path. We verify die-attach void content by X-ray when void measurement is required by the part or the specification, alongside automated optical inspection and metrology, with cross-section available where a programme needs destructive confirmation. The useful question about any attach is what void content was measured against what limit — not whether it is void-free by assertion.
When eutectic is the right call
Eutectic earns its extra process cost when the part runs hot, has to be hermetic with nothing organic in the cavity, or has to survive downstream reflow. When none of those apply, an epoxy die attach is lower-temperature, more forgiving and cheaper — the trade is laid out in eutectic versus epoxy die attach.
We run AuSn, AuSi, AuGe and AuIn eutectic die attach — plus tin-silver-copper, indium-based, high-lead and silver-sinter systems, subject to the die backside metallization — in Halethorpe, Maryland, on US soil. More on our die bonding services page.
Answered.
What is eutectic die bonding?
It is die attach using a eutectic alloy — the one composition of two metals that melts at a single fixed temperature rather than over a range. The alloy melts, wets the die backside and the substrate, and solidifies into a dense metallurgical joint with no polymer or filler. It is used where the attach has to carry heat, stay hermetic, or survive later assembly steps.
What is the difference between AuSn and AuSi die attach?
AuSn (80Au/20Sn) is a gold-tin eutectic with a remelt near 280 °C, run fluxless and used where a joint must survive a later lid seal or board reflow. AuSi is a higher-temperature gold-silicon eutectic near 363 °C that bonds a bare-silicon die backside directly. The alloy is chosen to fit the part's place in the assembly's thermal sequence.
Why does the die backside metallization matter for eutectic attach?
Because eutectic solderability depends on it. AuSi wants bare silicon; the solder eutectics want a solderable backside such as Ti/Ni/Au. If the backside metal is wrong for the alloy the joint will not wet, and the problem often does not show up until the shear test. It is the first detail to confirm before quoting a eutectic attach.
Does eutectic die bonding need flux?
It is typically run fluxless, under an inert or reducing atmosphere with a controlled thermal profile and often mechanical scrubbing to break up surface oxide. Because there is no flux to clean the interfaces, clean and properly metallised surfaces on both die and substrate are required.
Which eutectic die-attach alloys do you run, and where?
AuSn, AuSi, AuGe and AuIn, plus tin-silver-copper, indium-based, high-lead and silver-sinter systems, subject to the die backside metallization — in Halethorpe, Maryland, on US soil, at prototype and low volume.