The die attach problem in laser packaging
Eutectic solder die attach is the dominant method for bonding laser diode chips to ceramic submounts. The joint must be thin, void-free, and mechanically stable over temperature cycling. For a single-mode edge-emitting DFB laser at 1310 nm or 1550 nm, even a 5% void in the solder layer can raise junction temperature by several degrees Celsius — enough to shift lasing wavelength, reduce output power, or accelerate degradation.
The most common solder used is Au/Sn 80/20 (80% gold, 20% tin by weight), a eutectic alloy with a melting point of 280°C. It offers high strength, good thermal conductivity (~57 W/m·K), excellent resistance to creep at elevated temperatures, and compatibility with gold-metallized surfaces. The challenge is getting it onto the submount in the right quantity, at the right location, without contamination or oxidation.
Traditionally, this is done with a separately handled solder preform — a thin stamped or rolled sheet of Au/Sn that is picked and placed onto the submount immediately before die attach. This process has several failure modes that pre-deposited submounts eliminate entirely.
What is a pre-deposited AuSn submount?
A pre-deposited AuSn submount has the Au/Sn 80/20 solder layer applied directly to the bonding pad during the metallization process at the submount manufacturer — before the part ships to you. The solder is deposited by electroplating or PVD sputtering in controlled thicknesses (typically 3–5 µm) with tight composition control (±1% by weight).
The base metallization stack is typically Ti/Pt/Au or Ti/Ni/Au, with the Au/Sn selectively deposited only on the laser die bonding pad — not on wire bond pads, not on the sidewalls, and not on areas adjacent to optical alignment features. This selectivity is what makes pre-deposited parts precision components rather than simply coated substrates.
FerraLink supplies ALN submounts with selectively pre-deposited Au/Sn 80/20 (3–5 µm) on the die bonding pad, with bare Ti/Pt/Au on wire bond pads and edge faces. Custom pad geometries are available to match your die layout.
5 ways pre-deposited AuSn improves assembly yield
Eliminates preform pick-and-place errors
Solder preforms are thin (20–50 µm), fragile, and prone to misalignment during pick-and-place. A misaligned preform causes asymmetric solder coverage, producing a tilted die that shifts the output beam. Pre-deposited solder is already in the correct position and geometry — there is nothing to pick or place.
Reduces oxidation and contamination risk
Au/Sn preforms exposed to air oxidize gradually, raising liquidus temperature and reducing wetting. Preforms stored incorrectly or reused from opened reels are common sources of assembly variation. Pre-deposited AuSn on a sealed submount maintains a pristine solder surface until the point of reflow.
Enables tighter solder volume control
The exact quantity of solder needed for a specific die area can be calculated from die size and target bond line thickness. Electroplated or sputtered pre-deposited layers achieve ±0.5 µm thickness control across the pad area — far tighter than stamped preform thickness tolerances of ±5–10 µm. This translates directly to more consistent bond line thickness and void fraction.
Improves void-free reflow at smaller pad areas
As laser die attach pads shrink below 500 µm × 500 µm, placing a discrete preform without overlap or tearing becomes increasingly difficult. Pre-deposited solder wets uniformly from the pad surface up, with no trapped air pockets from preform edges. Typical void fraction with pre-deposited AuSn and optimized reflow profiles is below 2% by area, compared to 5–15% with manually placed preforms on the same geometry.
Reduces process steps and cycle time
Pre-deposited submounts remove a dedicated preform placement station from the assembly line. For high-volume production of telecom or datacom modules, this reduces die attach cycle time by 15–30% and eliminates a source of variability that required monitoring, inspection, and occasional scrapping.
Reflow profile for pre-deposited AuSn submounts
Au/Sn 80/20 melts at 280°C. A typical reflow profile for laser die attach to a pre-deposited ALN submount uses:
| Stage | Temperature range | Ramp rate | Dwell time |
|---|---|---|---|
| Preheat | 25°C → 150°C | 5–10°C/s | 30–60 s |
| Soak | 150°C → 260°C | 2–5°C/s | 20–30 s |
| Reflow peak | 280°C → 305°C | 3°C/s | 5–10 s |
| Cool down | 305°C → 150°C | −5°C/s max | controlled |
Reflow should be performed in a forming gas (N₂/H₂ mix, typically 95/5) or high-purity N₂ atmosphere to prevent oxidation of the gold-tin surface. Force bonding with 5–50 g applied during peak temperature further improves void fraction and bond line uniformity.
When to choose pre-deposited vs. separate preform
| Factor | Pre-deposited AuSn | Separate preform |
|---|---|---|
| Assembly volume | Low to high — always beneficial | High volume only (justifies tooling) |
| Die pad size | Best for <1 mm × 1 mm | Practical above 1.5 mm × 1.5 mm |
| Process control | Higher — no pick-and-place step | Lower — depends on handler precision |
| Void fraction | Typically <2% | Typically 5–15% |
| Cost per part | Higher unit cost (built in) | Lower unit cost but added process step |
| Storage | Shelf life: 12+ months sealed | Preform oxidation risk after opening |
For prototype and R&D stages, pre-deposited submounts are almost always the right choice: they remove one process variable, deliver consistent results with minimal process development, and require no special tooling. For high-volume production above 50,000 units per year, both approaches are valid — the decision shifts to cost and process capability analysis.
FerraLink ALN submounts with pre-deposited AuSn
FerraLink supplies aluminum nitride submounts with selectively pre-deposited Au/Sn 80/20 at 3–5 µm thickness on the die bonding pad. The base metallization is Ti/Pt/Au (standard) or Ti/Ni/Au (on request). Wire bond pads are left with bare gold. Edge gap for edge-emitting laser assembly is available down to 5 µm.
Parts ship with material certification, lot traceability, and XRF thickness measurement data per lot. Custom pad geometry, size, and thickness available on request. Standard sample part number: FL-ALN-035455-001 — 3.5 × 4.55 mm, 0.30 mm thick, Ti/Pt/Au + Au/Sn.