This article is based on an original publication by Gyan Dutt, Ravi Bhatkal and Alpha, an Alent plc company
Several considerations should be given to the selection of die attach material in LED applications. These include packaging process, performance, reliability and cost. While eutectic gold-tin, silver-filled epoxies, solder, silicones and sintered materials are all used for LED die attach, they result in varying trade-offs.
This white paper discusses those trade-offs as well as the fit between the die attach materials, different chip structures and their operating power levels. It demonstrates all material platforms have a place in LED die attach, and a diverse portfolio of materials provides options for LED device manufacturers.
A trend in mid-to-high power LEDs operating at increasingly higher current and power levels brings the need for robust thermal dissipation to the forefront. When heat dissipation is not managed properly, LEDs can experience loss of radiant flux, change in forward voltage, wavelength shift and reduced lifetime.
Thermal performance and stability of the die attach layer directly impact LED light output, light extraction and lumen maintenance over time. The material and the process also have a significant impact on the cost of ownership of the light engine.
There are three main LED chip structures: lateral, vertical and flip-chip. The lateral structure, consisting of laterally spaced electrodes, is used in low power applications. For most high- and super-high power applications, the vertical structure is used. It consists of a conductive substrate on the bottom, and the current flows vertically. The flip-chip has both electrodes on one side. It is put face down on the substrate and provides the highest lumen density at cost lower than Vertical structure. When the three structures are mounted next to each other on a board they form modules called chip-on-board.
Eutectic gold-tin (80/20 Au/Sn by weight) has a high cost of ownership because of its proven thermal performance (57W/mk) and reliability for high power applications. For LED die attach, it is used as a precoated layer on LED backside, preform or as a solder paste, all of which involve different processes and performance.
Conductive adhesives, mostly silver filled epoxies, are the most common of thermal die attach materials and are used for LEDs as well as all semiconductor packages. Compatible with existing back-end packaging equipment, they are popular for providing a balance of cost and performance (50W/mK thermal with second reflow compatibility). Conductive adhesives are preferred for dies without backside metallization because they don’t stick to bare silicon.
Sintered silver forms nano-porous, mostly metallic silver joints. It consists of micro/nano scale silver particles that fuse together at 180-300°C. Sintered silver can be applied as either a paste or film. Sintering happens in a press, which requires new equipment, or a regular oven. These materials are moderately priced and provide superior mechanical reliability and higher thermal performance than AuSn. Sintered materials also improve the lumens output by more than 30 percent for red and green LEDs and UV LEDs.
Solder, mostly SAC based, is known for its low cost, fast assembly process and reasonable thermal performance (50-60W/mK). SAC solder use is limited to applications in which either high temperature stability is not required in operating conditions or during additional processing.
Three considerations for selecting LED die attach materials include thermal resistance, second reflow compatibility and cost of ownership.
Sintered silver has the highest bulk thermal conductivity among the materials previously mentioned (>100W/mK) and has a lower thermal resistance than AuSn and silver epoxies.
Second reflow compatibility is the next key consideration. LED packages assembled on submount go through an additional reflow step. Whereas AuSn, conductive adhesives and sintered silver withstand secondary reflow, only low temperature solders such as Sn-Bi based solders are used for second reflow.
Thirdly, cost of ownership should be considered when selecting material for die attach in an LED application. The die attach step is typically the most capital intensive step in LED packaging, making it imperative that die attach material and process is compatible with the existing high throughput dispense/pin transfer bonders.
Die attach selection usually begins during the end application and design. End application determines the operating environment. Design determines the number and power levels of the dies. The design and end-use decisions should be made prior to selection based on thermal resistance, second reflow compatibility and cost of ownership. The power level of the die determines the heat dissipation requirements. The higher power ones require high thermal conductivity die attach to keep the thermal resistance low. Low thermal die attach is sufficient for low power dies. High powered and super-high powered vertical LEDs are suited for pressure-assisted and pressure-less sintered materials and eutectic gold-tin. This is because they lower the thermal resistance and keep the junction temperature manageable for optimal performance of the LED. Mid power dies can use solder and high thermal epoxy, and low power lateral LEDs can use lower end epoxies or silicone.
Solder is the preferred die attach material for assemblies that do not go through second reflow, and AuSn or sintered materials are generally selected for in-package attach silver epoxy. Process equipment and throughput or cost of ownership is the final consideration. Eutectic AuSn and pressure-assisted sintered materials are not compatible with traditional die bonding equipment. Conductive epoxies and pressure-less, sintered silver materials are easily adopted on existing lines. Solder, however, is compatible with semiconductor packaging and SMT lines.
This paper discusses various materials in the Alpha LED die attach portfolio and their performance during different applications on different substrates. Argomax, for example, has the highest thermal and reliability performance for ultra-high power applications like projection and entertainment lighting. For most high power applications, like vertical UV, flip-chip on ceramic and lead frames, and laser diodes, pressure-less sintering Fortibond™ provides higher thermal and reliability performance than silver epoxy. Conductive adhesive Atrox™ meets the requirements for most mid power vertical dies in general lighting applications. Lastly, solder is preferred for low-mid power applications requiring die attach directly on the board.
A broad portfolio provides different price and performance options for die attach solutions given the diversity of LED structures, power levels, applications and process equipment considerations.