Die bonding for high-power devices
05/01/2002
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Thermal management for broadband component assembly
BY EDWARD J. WILLS
Die bonding with eutectic solder processing is becoming more important as thermal challenges increase with the proliferation of laser diode assemblies. A solder joint functions as both the electrical contact and the primary heat transfer interface between the die and its substrate. Present-day pump laser diodes (Figure 1) produce power at levels as high as 200 W/cm2. Lasers also require precise temperature control during operation, and the required drive current rises rapidly with increased operating temperatures. Without a robust thermal interface, increased device temperatures severely degrade light output performance and device longevity.
Soldering Background
In a eutectic solder bond, solder reacts with a small amount of the base metal to form an intermetallic, covalent bond. The intermetallic compound is typically hard and brittle, and it is the source of strength of the solder joint.
Eutectic Temperature: The term "eutectic" is from the Greek word eutektos, meaning "easily melted." It is defined as the temperature and percent mixture (point "E" on Figure 2) where it will go directly from pure solid to pure liquid, just like a pure metal would. Any point on the phase diagram above the line "AEB" is above the eutectic temperature and is referred to as the "liquidus" state, i.e., where the mixture is 100 percent liquid.1
 Figure 1. Typical semiconductor laser diode assembly. |
Heat: The thermal conductivity of the substrate must be such that there is some means of heating the joint area where the solder is placed. Without applied heat, nothing (barring some esoteric solder methods) will melt the solder. Conversely, the heat applied must not be so excessive that it damages surrounding components.
Wetting: The joining surfaces must permit molten solder to wet and spread during the available time window without significant de-wetting (Figure 3). For best wetting, the contact angle q must be minimized.1 Oxides and contaminants reduce the contact angle. The dividing line between "good" and "bad" wetting is usually considered to be a contact angle of 90°. Passivation layers affect wetting by forming a barrier between the solder and base metal, and these layers are typically oxide films that may be only a few angstroms thick. Other elements, such as nickel, can form passivation films that require fluxes (solvents) or plasma cleaning to remove. Oils, silicones and organics are frequent contaminants that can also form barriers to wetting and act as a passivation layer.2
 Figure 2. Lead-tin phase diagram. |
Many factors contribute to the correct metallurgical, thermal and wetting conditions for a robust solder joint, including die metallization, temperature and the substrate material itself.
Die Metallization
Gold metallization patterns are applied during the last steps of the semiconductor fabrication process. It is at this point that the bond pads for wirebonding, the fiducial etch markings, and the bottom-layer metallization are created. The metallization thickness, composition, contaminants and coverage applied during this process are all key factors to solder the die.
 Figure 3. Wetting vs. non-wetting conditions. |
Usually the equipment used to solder the die must adapt all process variables to accommodate the existing die fabrication and metallization properties.
Temperature
Temperature control is important to solder joint formation for several reasons. An excessive temperature can blister the thin optical anti-reflective coating on the emitting edge of the laser, rendering the module useless.3 Solder reaction rates increase exponentially with temperature, and higher temperatures promote the formation of oxides that are barriers to wetting. Another problem is bond pad "leaching," in which the composition of thin film deposits are corrupted because of diffusion and other transport phenomena that occur at elevated temperatures. The two primary methods of applying heat to a sub-assembly are pulse heat profiling and steady-state heating with a scrubbing action.
Pulse Heat Profiling
Pulse heat profiling is a new method of applying a controlled temperature with a time-varying profile to a substrate. The information (bonding profile) is typically entered through a graphical user interface, and the temperature vs. time can be displayed and tracked using a closed-loop feedback system. Time is an important element in the profile because there must be sufficient time for the solder to wet and spread around the joint through capillary action referred to as "wicking."
 Figure 4. Eutectic die collet with two-sided scrub tool. |
Pulse heat profiling is a good method for heating substrates with low thermal mass (typically ceramic or alumina substrates less than 1 cm2). Using parts with low thermal mass permits rapid temperature ramp times (faster than 50°C per second), which minimizes wait time between eutectic bonds and maximizes throughput.
Pulse heating larger substrates (such as the size of butterfly packages) is an option, but heating times are usually longer. This is because of two main factors. First, the butterfly package is, by design, an excellent heat sink and is difficult to heat from the bottom. The heating system must overcome the heat sink effect through both the package and thermo-electric cooler on its way to heating the bonding area. Second, the package is larger, and the power required to heat a package increases in direct proportion to the area required, not the linear dimensions.4
Steady-state Heat and Scrubbing
Scrubbing is a process used at a constant temperature to aid in solder wetting and drive out voids (Figure 4). Force is applied over time, with either a linear or an orbital motion in the X, Y and sometimes Z directions. Surface variations in the substrate generate friction during the scrub, and this combination of force, friction, and high temperature force the solder material into the base material and forms the intermetallic bond. Scrubbing may be used in combination with any heating method, including stand-alone steady-state hotplates, pulse heat profiling and heated rail conveyor systems. There are situations where scrubbing may not be a good option, specifically:
- Some semiconductor materials (InP and GaAs) are brittle.
- Edge collets are required to "grip" the die on its sides to facilitate the scrub motion.
- There is the possibility that completed die bonds may shift or "swim" during the wait time between placement and before the entire substrate has cooled down. "Swim" can be minimized using solders with a "steep" alpha-phase curve like gold-tin (Figure 5). During heating, gold from the substrate or pad is rapidly absorbed into the solder during scrub, which makes the solder mixture gold-rich. This means that the gold part of the solder mixture is solid, while the tin part of the mixture may still be liquid.1 This ensures that the solder now has a new higher liquidous temperature and that swim will not result in a post-placement shift of more than 5 - 10 µm.
Substrate and Pad Material
Ceramic carriers are popular as substrates because of their availability, cost and heat transfer characteristics. They are mostly alumina-based compounds. For thin-film conductors created by evaporation or sputtering, 99 percent Al2O3 is frequently used, while 96 percent Al2O3 is typically used for thick-film materials.
Most bond pads on these ceramic substrates are gold. Because of the gold layers can be quite thin, sometimes material from both the substrate and the bond pad can migrate upward into the solder material, which is referred to as "leaching." Leaching can be minimized by using a gold-platinum alloy or by depositing a thicker bond pad layer.
 Figure 5. Gold-tin phase diagram. |
In attempts to drive broadband packaging costs lower, some customers are migrating to copper-plated substrates. Unfortunately, copper materials at high temperatures rapidly form a thick copper oxide layer if exposed to the atmosphere. To solder to a copper base, aggressive cleaning processes before bonding and an inert forming gas environment during bonding are required.
Solder Material
The selection of solder material is usually driven by the thermal limitations of the package or substrate. If this is not a factor, then cost and environmental factors come into play. Most European and U.S. companies are trying to do away with "soft solder" lead in all forms and are migrating to indium-based solders as a replacement for these applications. Indium also has its drawbacks, because it oxidizes rapidly (less than one hour at room temperature).
For these reasons, the majority of eutectic laser diode attachments are completed using "hard" gold-tin solders (80/20) because of good electrical conduction, good thermal conduction and low oxidization rates of reaction. Another reason why gold-based solders are popular is their resistance to oxidation. Gold-based solders can usually be bonded with good results in an inert (N2) environment. Lead solders often require use of a forming gas (nitrogen or argon with 8 to 12 percent hydrogen) to purge all oxygen and avoid formation of an oxide layer.
Solder Size and Presentation
To achieve proper wetting, optimal solder thickness and footprint must be chosen. The following are good rules of thumb for small die being joined with both soft solder (lead- or indium-based) and hard (gold-based) solder:
- Thickness = 25 - 40 µm
- Length = 90 percent of die length
- Width = 90 percent of die width.
There are many methods to present the solder for bonding.
Preforms: Preforms are pre-cut shapes of solders placed at the joint area before heating. This is the most common method because of its quick set-up time. Singulated preforms can be expensive to procure, but they enable flexible production because substrate designs and feed mechanisms do not need to be changed. They can be of all shapes and sizes, but usually they are rectangular, resembling the aspect ratio of the die that will be soldered. Preforms are very difficult to handle at very small sizes.
Backside Die Metallization: A metallization layer is applied to the back of the wafer as a part of the wafer fabrication process. This can save money both in material and time. The downside to die metallization is that it is usually not an option on InP material, but can usually be done on Si-based die.
Pre-Deposition of Solder on the Substrate: The solder is pre-deposited or sputtered onto the bond pad when the substrate is produced. This is a cost-effective method, but it is possible to pre-deposit on only one bond area per substrate. Depositing solder on additional bond pads is not feasible because the entire substrate is usually heated during eutectic bonding, and thus all pads will melt at the same time.
Solder Ribbon or Automatic Preform Feeder: This is a high-volume method of producing custom preforms using a ribbon spool feeder and a punch mechanism. This method brings the cost of preforms down if they are produced in a low-mix/high-volume environment and if the scrap solder ribbon is recycled. It is important to remember that most preforms are only about 40 µm thick, and that ribbon less than 125 µm thick is difficult to feed because the material frequently folds over itself and jams the feeder (commonly referred to as "shingling").
Conclusion
Process and design engineers looking to lower broadband packaging costs must carefully consider the ramifications of design decisions on eutectic assembly processes. Scrub attachment is a very quick, straightforward, and reliable process, but among its drawbacks are mechanical agitation of the die. Pulse heat profiling is mechanically less stressful to III-V components but may involve some additional time and meticulous process planning if it is to be employed successfully. Understanding which bonding process matches a particular package design leads to insightful thermal management decisions for maximum yielded throughput and profit. AP
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References
- ASM Handbook, Vol 6: Welding, Brazing, and Soldering, ASM International, 1997.
- F.G. Yost, F.M. Hosking, D.R. Frear, The Mechanics of Solder Alloy Wetting and Spreading, KAP, 1993.
- O. Ueda, Reliability and Degradation of III-V Optical Devices, Artech House, 1996.
- Y. A. Cengel, Thermodynamics: An Engineering Approach. McGraw-Hill, 1989.
Edward J. Wills, product marketing manager of component assembly systems, can be contacted at Palomar Technologies Inc., 2230 Oak Ridge Way, Vista, CA 92083; 760-931-3600; Fax: 760-931-5191; E-mail: [email protected].