Flexible Hybrid Die Attach
Key Requirements to Consider When Selecting a System
BY DAVID R. HALK
Flexibility is key when it comes to die attach capabilities critical for hybrid microelectronics, especially in North America's automotive, medical and military markets. An educated hybrid equipment buyer makes the best decisions, based on knowledge and insight into the key technical features and capabilities required of flexible die attach equipment. This article provides details about the key requirements that you should look for in each of five major die attach equipment subsystems, including full MCM capabilities, tool changing, wafer and waffle pack presentation systems, and multiple adhesive deposition solutions (including stamping and adhesive dispensing).
Hybrid Market Overview
The hybrid die attach market represents a convergence of advanced SMT and advanced IC die attach technologies (Table 1). Equipment serving this market must be extremely flexible, combining the best technical features and capabilities of each. A flexible die attach machine should have the capability to:
- Bond many different components in a single pass;
- Apply two or more different epoxies in a single pass;
- Process the smallest to the largest die, in addition to ultra-thin die;
- Support different component presentation methods in a single pass;
- Process many different types of substrates;
- Meet all key bond process parameters: placement accuracy, bond-line thickness, die tilt, epoxy coverage, and fillet, etc.
All die attach machines consist of five major subsystems. These include: pick-and-place, component presentation, substrate handling, vision, and epoxy dispensing. As an equipment buyer, what critical features and technical specifications are important for each of these systems?
Table 1. Pick-and-place market.
In general, when evaluating die attach machines, ask questions such as "What features do I require?," "What issues do I have with my existing equipment?," "Are there applications I cannot handle?" "What features would make my life easier?" Focus on the fundamental building blocks, the subsystems within the machines you are evaluating, and their technical features, specifications, and mechanical stability.
Linear servo pick-and-place systems with positional feedback offer greater accuracy than ball screw or belt systems. A rule of thumb is to look for encoder resolution that is about 10× the required die placement accuracy. In addition, if you require die placement accuracy less than ±10 µm at 3 sigma, you should investigate how the system compensates for temperature changes. At a minimum, request a glass board test to demonstrate bonding accuracy under controlled conditions.
Look for designed-in mechanical stability, where the pick-up tool, requiring frequent changes during hybrid applications, engages the bond-head by locking together like a v-block. Additional evidence of mechanical stability can be found in the processes used to calibrate planarity and verify placement accuracy. Calibration should be infrequently required and guided by the operating software with simple step-by-step instructions. Accuracy should be verified by a glass board test that is well documented with all calculations explained in detail.
The pick-and-place system must also include a method to calibrate bond force to maintain bondline thickness and prevent die damage due to excessive force. Automatic bond force calibration via load cell provides a consistent bond process. A load cell also is used to automatically calibrate the needle for epoxy writing, critical for maintaining the required gap between the tip of the needle and the substrate. Look for an operating range from 0 to 1,000 g of bond force. Lower bond force is important when bonding very thin die or die with sensitive features such as air bridges and vias found on gallium arsenide (GaAs) die. High bond force in the range of 1,000 g or greater is critical for large die or applications requiring high-viscosity epoxy.
Component Presentation Systems
Look for a die attach machine that supports all standard forms of component presentation on one machine in a single pass. Waffle vision (automatic component search), which provides the capability to search for die in large waffle pack cavities, also is a critical feature. Depending on the application, the total component capacity and/or the total number of different components the system can handle may be important.
An automatic wafer handling system, such as the one shown in Figure 1, can efficiently present large quantities and multiple types of components to the pick-and-place system. Look for a system that can present up to 200 2-in. waffle packs, or as many as 25 different wafers or grip rings, or a combination of these, in a single pass. Always look for a system that is designed for easy loading and unloading.
Figure 1. Automatic wafer changing system.
The wafer presentation system should have the capability to detect ink dot, missing corner and partial-die rejects to ensure the highest possible yield. Also look for wafer mapping, which directs the pick-and-place system, so that it can precisely locate good die on non-inked wafers. In general, to maximize yield and minimize cost, especially for higher-volume applications, always pick die directly from the wafer.
In addition to multiple die presentation systems, a hybrid die attach machine must accept tape-and-reel feeders to present SMD components. Look for a system that accommodates 8- to 44-mm tape feeders with a capacity for 10 to 20 8-mm feeders, and offers high-accuracy component centering, especially if 0201 capability is required.
Again, flexibility is the key. The system that handles the most components and component types is always the best solution for hybrid applications. In addition to the die and SMD component presentation systems mentioned here, the die attach platform should be flexible enough to accept custom presentation systems. This provides a built-in safety net, making it possible to meet new requirements and market changes.
Substrate Handling Systems
There are several basic types of substrate handling systems, including belt, gripper, walking beam, and manual workholder. Belt transport systems and manual workholders are widely used for hybrid applications because of their overall flexibility (Figure 2). Tooling design is important. Look for flexible tooling that is easy to use and can be changed over quickly. Also look for a tool, such as a touch probe, that measures and provides quantitative feedback so that adjustments can be made for planarity. A touch probe with 1-µm repeatability is recommended. Gripper and walking beam indexers are more specialized and less flexible, and are used extensively in the more dedicated IC die attach market.
Figure 2. Automatic belt substrate transport system.
In addition to flexible tooling, look for a substrate handling system that accommodates a wide range of substrate sizes from under 2- to 8-in. on a standard edge belt, Auer boats, or custom carriers.
The substrate handling system must be SMEMA compatible so that it can be put in-line with other equipment, including a curing oven and conveyors. Available options should include conveyor buffers for manual loading and unloading, single or multiple magazine I/O, in addition to a leadframe/bare board unloader.
Look beyond the equipment itself for a dedicated, experienced vision engineering group with a commitment to continuous improvement of vision hardware, software, illumination, and optics. A vision system is composed of four major elements: the vision engine, the wafer/component camera, the substrate camera, and the upward-looking camera.
An example of a vision system with minimum hardware is an advanced 256 gray-level vision engine that uses a commercially available vision card. A component camera is required to provide inspection capability and align the die to ensure proper orientation prior to picking. This camera is mounted on the bond-head, or in a fixed position. Some die attach machines provide both for maximum flexibility.
A substrate camera is required to align to substrate fiducials and inspect for reject marks. For added functionality, substrate cameras can align components prior to picking, perform pre- and post-bond inspection with automatic offset adjust, and read 2-D code on the substrate.
An upward-looking camera, mounted in a fixed position inside the machine, is required to align the die after pick-up. It is important to note that poor dicing can affect placement accuracy when aligning to the bottom edge of the die with the upward camera. A feature that overcomes this problem is an intermediate placement tool (IPT). An IPT uses the substrate camera and a temporary placement station to accurately align and place components relative to the patterned portion on top of the die, instead of relying on the bottom edge of the die where irregular shapes often cause alignment offset (Figure 3).
Figure 3. Poorly diced 10-mil die with irregular edges.
Camera systems should provide motorized focus with auto-focus capability, support for all available algorithms and, most importantly, programmable vertical and oblique light level control. The three cameras should work independently to enable individual optical and lighting settings that result in more efficient machine operation.
Vision System Algorithms
Die attach machines that include a comprehensive library of vision algorithms, and a vision engineering group willing to develop new algorithms to solve any new and unique requirements, are critical. The vision system should support all available algorithms, including pattern/template matching, circle matching, edge search, center point search (blob analysis), symmetrical, and multi-search.
Pattern or template matching enables programming of unique features or dedicated fiducials, and does not require templates of any specific shape or construction. This method recognizes features as small as 150 µm (6 mils), depending on camera magnification and the selected algorithm. Circle matching is used to find the center of a circular pattern, for example, to find the center of a TO header. The edge search algorithm allows programming of component edges and is useful when there is no unique pattern, or when component placement is edge dependent (Figure 4).
Figure 4. Edge search algorithm.
Center point search, or "blob analysis," is a binary algorithm that allows a feature of a certain size to be "taught" as a ratio of dark and light pixels. The algorithm can be programmed to find the center of a dark or light group of pixels. Software filters can be set to cause the vision system to reject features that are either too large or small. This is useful in hybrid applications where thick-film, ceramic substrates do not always provide clear, consistent fiducials.
A multi-search capability allows multiple vision algorithms to be linked together within the same FOV. In one example, the first search could be a 'coarse find' used to find a specific unique feature. Subsequent searches could use pixel vectors from the first search to target specific features that may not be as unique. The multi-search function can use the same vision algorithm for a group of searches, or it can be programmed to use various algorithms.
An ideal dispensing system supports many different dispensing techniques, including epoxy writing, stamping (dip and dab), and cross-needle dispensing, with the ability to dispense two or more different epoxies from independent dispensers in a single pass.
Desirable dispensing techniques include volumetric (auger screw) dispensing, time/pressure dispensing with multi-needle shower head, stamping, gang stamping, and epoxy writing with programmable dispensing patterns. Process control features and automatic programming also are desirable to ensure consistent epoxy pattern and volume, X-Y speed and auger speed. An epoxy low-level detector is important for continuous operation.
Look for a high-performance volumetric screw pump dispenser that enables highly repeatable epoxy dispensing from large patterns to ultra-small dots for 8-mil die with the same needle, as shown in Figure 5. This eliminates the requirement for multiple custom stamping tools, which can be costly and difficult to maintain. Additional features to look for include: solder paste capability, easy removal and cleaning, and a method for automatic needle calibration.
Figure 5. Epoxy dispensing large patterns to ultra-small dots with a single needle.
Epoxy stamping is necessary because some epoxies cannot be dispensed. Rotary squeegee units with a stamping tool enable adjustable epoxy amount with fine adjustment via micrometers.
Adhesive stamping is a suitable option for printing small dots or patterns using standard stamping tools, custom designs, or gang stamping tools. This method uses a rotating stamping dish, where film thickness is controlled via micrometer-adjusted doctor blade. Adhesive stamping enables repeatable results on warped substrates and substrates with thickness variations. The dish and blade should be easy to remove for cleaning.
In the fast-paced hybrid market, you may be required to customize your next job. To be prepared, look for an equipment vendor that is organized and positioned specifically to accommodate these types of projects. Today, you may require low-volume, high-mix production, and a basic manual load die attach system will be sufficient. Down the road, you may need a high-volume or high-mix production, so be sure your die attach system can be integrated into a line complete with ovens, wire bonders, screen printers, laser markers, plasma cleaners, etc.
Your budget now may only allow for a basic die attach machine — just make sure that all the technical features that may be needed in the future can be retrofitted as your requirements change. For example, ultra-thin die down to 50 micrometers requires features like needle-less and synchronous ejection, and an extremely low pickup and bond force tool (0 to 6 g). Also, as new materials are developed, look for customized features such heated pick-up tools and substrates, heated press, tool changers, carriers, component presentation systems, illumination, dispensers, etc. Finally, always ask for examples, references, and an organization chart that shows how your project will be handled.
From a vendor that specializes in custom projects, look for a flexible, modular die bonder that also accommodates flip chip die sizes up to 50 mm, includes a well designed cavity fluxing system, and offers 10-µm placement accuracy. Look for flexibility in all five systems within the die bonder: pick and place, component presentation, substrate handling, vision, and dispensing. Last, but not least, make sure that the installed base for the equipment you select includes all major IDMs and subcontractors.The bottom line is flexibility — a flexible supplier and their flexible hybrid assembly solutions will enable you to meet your hybrid production goals now and into the future.
DAVID R. HALK, general manager, may be contacted at Datacon North America Inc., Seven Neshaminy Interplex, Suite 116, Trevose, PA 19053; (215) 245-3052.