High-accuracy Mass Imaging for Semiconductor Die Attach
04/01/2006
More Speed and More Control
BY CLIVE ASHMORE, DEK
The most widely used die-attach technique is the application of wet, silver-loaded conductive epoxy to multiple substrates in strips (Figure 1), before placing the die using a pick-and-place type technology.
 Figure 1. Silver-loaded wet epoxy, deposited at high-speed using stencil printing technologies, will match leading-edge die placement throughput. |
Traditionally, adhesives were deposited using a dispense head integrated with the die-bonding machine. But placement capability has increased to beyond 40,000 units per hour (uph). Cascading several placement stations in series allows assemblers to drive uph rates even higher. Therefore, the dispense capability is becoming the bottleneck: it simply cannot keep up. Adding multiple dispensing stations will not deliver the throughput required, nor will it lower the cost of dispensing technology.
Package specialists want greater control over the characteristics of the adhesive deposit. Demands include closer tolerance on dimensions in X, Y, and Z axes; greater control over the deposit volume; and the ability to determine the optimal deposit shape to eliminate voiding. Tight volume control is essential to prevent excessive die-attach fillet, which may lead to die-surface contamination. On the other hand, depositing too little adhesive may cause die lifting or cracking, due to incomplete die-attach coverage or insufficient bond-line thickness. Excessive die-attach voids impair electrical and mechanical integrity, and can encourage die cracking during placement, or popcorning during subsequent component heating during reflow, for example. Voids can be introduced as the wet epoxy flows during die placement. Tests indicate that the shape of the deposited die-bond epoxy contributes to voiding, and a simple dot or ring is not the optimum shape. However, the multiple steps required to create a complex shape using a dispenser further slows the process.
Mass Imaging for Die Attach
Throughput has always been the greatest challenge to dispensing, which is being replaced by screen printing for deposition of materials such as solder paste. Thousands of solder paste deposits can be created simultaneously at high speed, which makes screen printing the only process that keeps pace with the latest pick-and-place technology, even when multiple placement machines are operated in series. If required, several thousand deposits can be made in a single excursion of the print head. Cycle time is independent from the number of deposits required (Figure 2), because the excursion time is constant for a given process. The optimal excursion speed depends on the material properties and stencil aperture characteristics, rather than the actual number of apertures. Cycle time is also independent of the deposit shapes of any deposits, which with die attach allows for complex shapes to be created at high speed, is optimized to minimize void formation during die placement, and maximizes electrical and mechanical integrity.
 Figure 2. Screen printing platforms are now capable of addressing wafer-level semiconductor applications at high levels of accuracy and repeatability. |
Next-generation die bonders are beginning to feature high-accuracy, mass-imaging capabilities tightly integrated with component placement, wafer handling, and die feeding solutions to deliver the step changes in throughput, accuracy, and repeatability that contract packaging specialists now require. These are well-suited to performing automated, in-line die attach at high speed using die-bond adhesives such as silver-loaded wet epoxy.
Adjusting the stencil or screen aperture dimensions and shape enables close control over deposit characteristics. The product of aperture area and stencil thickness defines the volume of adhesive deposited. This is repeatable from unit to unit, and wafer to wafer. The deposit has uniform thickness, which helps avoid die lifting, cracking, or contamination. Close cooperation with the materials manufacturers optimizes screen and stencil characteristics to ensure deposit thickness uniformity.
Screen Printing with Pre-mounted Passives
When no additional components are mounted prior to die placement, a metal screen printing mask may be used. These are typically laser-cut, stainless-steel stencils, but may also be electro-formed. However, small passives are often mounted on the substrate before the die is attached, adding topology to the die surface and preventing the use of a conventional metal stencil. An alternative stencil technology* delivers a solution to high throughput deposition for these substrates. Originally developed to deposit adhesives on the populated side of surface mount PCBs at high rates of throughput, this process uses acrylic stencils up to 8-mm thick that are rebated on the underside using a standard routing process to clear any small passives already mounted on the substrate. When used in conjunction with an enclosed print head, uniform aperture filling with materials displaying relatively high viscosity, such as a conventional die-attach epoxy, can occur.
B-stage Materials for Ultra-low-profile Devices
The success of extremely low-profile products such as ultra-thin phones has driven demand for thinner components, comprising thin substrate and silicon layers for a lower overall profile. Wafer thicknesses are reducing from 300 to 175 µm and thinner. Depositing a large volume of wet epoxy is not suitable in this context, so a new approach to die attach with a lower bond thickness is required.
 Figure 3. Fine mesh stencils deposit B-stage epoxy for die attach 50-µm thick, to within ±5 µm. |
By applying an epoxy directly to the wafer, either in the form of a film or direct deposit onto the wafer backside (Figure 3), a thin layer can be achieved over the entire surface. Because this layer is applied before dicing the wafer, a wet epoxy cannot be used. Instead, a B-stage epoxy - a material that part-cures upon deposition, and is fully cured by pressure and/or heat at a later stage - is used. By applying die-bond adhesive directly to the wafer, B-stage epoxies enable low component profiles, and deliver a solution for component types such as large memory devices or processors. The wafer-level process allows semiconductor manufacturers to move the die-bonding process back upstream to wafer houses, and eliminates one more complex process from the factory floor.
With the epoxy in its B-stage state, the wafer can be diced and prepared for presentation to the die-placement process. Placement of individual dice onto the desired substrate is accompanied by controlled placement pressure and - in some cases - a minor quantity of heat, fully curing the epoxy and creating a reliable electrical and mechanical bond between the two components.
Screen Printing B-stage Epoxies
To operate economically, wafer houses need a fast and cost-effective way to deposit the thin layers - usually 50 µm - of B-stage epoxy. Very tight tolerance, typically ±5 µm, is required to ensure coplanarity. With such a thin layer of an adhesive that is not designed to flow, any larger discrepancy between the highest and lowest point on the surface will likely cause the die to crack when placed onto the substrate.
High-accuracy screen printing techniques using fine mesh screens, which are optimal for use with semiconductor wafers, enable wafers up to 300 mm to be processed within a short cycle time. Tight volumetric control and repeatability are key attributes of high-accuracy industrial screen printing. Similar processes are used to deposit precision-resistive elements using conductive inks, and to produce other high-accuracy industrial products, such as shaft encoders for power steering controllers.
Conclusion
Die bonding using wet epoxy materials remains a mainstay of the semiconductor industry. However, available techniques to deposit these materials must be improved to maintain parity with the speed of the latest die-placement technology. Screen printing has demonstrated its ability to surpass throughput, repeatability, and flexibility of dispensing in almost every surface mount application and in component packaging processes.
At the cutting edge of package technology, the emergence of ultra-low-profile consumer and business electronic products emphasizes processes that enable thin components to further shrink the height of next-generation products. B-stage epoxies offer a solution, and can be handled by screen-printing technology already familiar to wafer processing houses.
Stencil or screen printing “high-accuracy mass-imaging” in wafer-level applications will be instrumental in delivering low-cost, high-yield, straightforward assembly technologies required for assembly using wet-type or B-stage epoxies.
CLIVE ASHMORE, global applied process engineer, may be contacted at DEK, 11 Albany Road, Granby Industrial Estate, Weymouth, Dorset DT4 9TH, England; 44/1305-760760; E-mail: [email protected].