Sunday, November 16, 2008

Advanced Hybrid Die Attach Equipment

Hybrid die attach represents a convergence of advanced SMT and IC technologies. Equipment serving this market must be extremely flexible to bond many different components while applying two or more different epoxies, in a single pass. It must process the smallest to the largest, and also ultra-thin, die. Various single-pass component presentation methods are required and the equipment must work with many different substrate types. Key bond process parameters such as placement accuracy, bond-line thickness, die tilt, epoxy coverage and fillet, must be met.

A die-attach machine consists of the five major subsystems: pick-and-place, component presentation, substrate presentation, vision, and epoxy dispensing. All of them need to be evaluated under the specific terms of a given application.

Pick-and-place system
Compared to ball-screw or belt systems, linear-servo pick-and-place systems with positional feedback offer superior accuracy. To achieve this, encoder resolution must be about ten times the required die placement accuracy. If die placement accuracy is less than ±10μm@3 sigma, it is crucial how the system compensates for temperature change. At a minimum, a glass-board test is required to demonstrate bonding accuracy under controlled conditions.

Next consideration: mechanical stability at the point where the pick-up tool, requiring frequent changing during hybrid applications, and the bond-head engage by locking together. Additional evidence of mechanical stability can be found in the processes used to calibrate planarity and verify placement accuracy. Calibration should be required infrequently and guided by the operating software with simple step-bystep instructions. Accuracy should be verified by a glass board test that is well-documented with all calculations explained in detail.

The system must include bondforce calibration to maintain bond line thickness and prevent die damage due to excessive force. Automatic calibration via load cell provides a consistent bond process. A load cell is also used to automatically calibrate the needle for epoxy writing, which is critical for maintaining the required gap between the tip of the needle and the substrate. The operating range should cover 0 to 1000 grams of bond force. Lower force is important for bonding very thin die, or die with sensitive features such as air bridges and vias found on gallium arsenide (GaAs). High bond force (1000grams or more) is critical for very large die or applications requiring high viscosity epoxy.

Component presentation system
A die attach machine should support all standard formats of component presentation on one machine in a single pass, including all sizes of wafers, waffle packs, Gel-Paks and grip rings. Waffle vision (automatic component search), which provides searching for die in large waffle pack cavities, is also a critical feature. Depending on the application, total component capacity and/or total number of different components is a very important consideration.

Automatic wafer presentation systems should efficiently present large quantities and multiple types of components to the pick and place system, up to 200 of 50mm waffle packs or Gel-Paks, up to 25 different wafers or grip rings, or a combination of these, in a single pass. Easy loading and unloading is advantageous, as is detection of ink dots, missing corners and partial die rejects. Wafer mapping, which directs the pick and place system, will precisely locate good die on non-inked wafers. To maximize yield, and minimize cost, especially in higher-volume applications, the die should be picked directly from the wafer.

A hybrid die attach machine should accept tape and reel feeders for presenting SMD components. It should accommodate feeders for 8 to 44mm tape, 10 to 20 of those for 8mm. High-accuracy component centering is required, especially for 0201-component processing.

Most important here is flexibility. For hybrid applications, the handling of as many as possible, different components is the best solution. The die attach platform should be flexible enough to accept custom presentation systems and be ready for new requirements as markets change.

Substrate presentation system
There are several types of substrate presentation systems, including belt, gripper, walking beam and manual work holder. Belt-transport systems and manual work holders are most widely used for hybrid applications because of their overall flexibility.

Flexible tooling that is easy to use and can be changed over quickly is important, as is a touch probe that provides quantitative feedback so that adjustments can be made for planarity. Recommended is a touch probe with 1μm repeatability. Gripper and walking-beam indexers are less flexible, and are used for the more dedicated IC die attach.

In addition to flexible tooling, a substrate presentation system should accommodate a wide range of substrate sizes, from under 50mm to 200mm, on a standard edge belt, on Auer Boats or on custom carriers. Generally, presenting different sizes and types of substrates includes substrates under 50mm and flex over 50mm to 200mm as well as TO-Packages. Substrate presentation includes Auer Boats and custom carriers, standard edge belt and edge belt with custom carrier.

A substrate presentation system has to be SMEMA-compatible so that it can operate in-line with equipment including curing oven and conveyors. Options should include conveyor buffers for manual loading and unloading, single- or multiple-magazine input and/or output, in addition to leadframe/bare-board unloader.

Vision system
With vision systems, it is crucial to look 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: vision engine, wafer/component camera, substrate camera and upward-looking camera.

Minimal vision hardware should have an advanced 256-gray-level vision engine that utilizes a commercially available vision card. A component camera is required to provide inspection and align the die for ensuring proper orientation prior to picking. This camera is mounted either on the bond head or in a dedicated, fixed position. For maximum flexibility, some die attach machines provide both mounts.

A substrate camera is required for alignment to substrate fiducials and inspection of reject marks. For added functionality, the camera should align components prior to picking, perform pre- and postbond 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. Care must be taken as poor dicing can affect placement accuracy when aligning to the bottom edge of the die. As a remedy, there is an intermediate placement tool (IPT), which 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. Relying on the bottom edge of the die will often cause an alignment offset.

Camera systems should have motorized auto-focusing and support 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. Light levels for each program must be savable since they can be different for each algorithm used.

Vision system algorithms
A die attach machine should be supported by a comprehensive library of vision algorithms, and an engineering group willing to develop new algorithms for unique requirements, including pattern/template matching, circle matching, edge search, center-point search (blob analysis), symmetrical, and multi-search. Pattern or template matching enables the programming of unique features or dedicated fiducials. This does not require templates of any specific shape or construction to recognize features as small as 150μm, depending on camera magnification and the selected algorithm.

Circle matching is used to locate the center of a circular pattern, for example, the center of a TO-header. The edge-search algorithm allows the programming of component edges. This is useful when there is no unique pattern, or when component placement is edge-dependent. Centre-point search, or blob analysis, is a binary algorithm permitting a feature of a certain size be taught to the system as a ratio of dark and light pixels. The algorithm can be programmed to find the centre of a group of dark or light pixels. Software filters can be set to have the vision system reject features too large or too small. This is very useful in hybrid applications where ceramic thick-film substrates don’t always provide clear, consistent fiducials.

Multi-search capability allows linking multiple vision algorithms within the same field of vision (FOV). Thus, the first search could be a ‘coarse find’ of a unique feature. Subsequent searches would utilize pixel vectors from the first search to target specific features that may not be as unique. The multi-search function can use one vision algorithm for a group of searches, or be programmed to use various algorithms.

Additional vision features
Additional vision features for hybrid applications include threepoint substrate search, substrate check result measurement, relative placement, waffle vision, and preand post-bond inspection. Threepoint substrate search (best fit) corrects scaling for inconsistent materials such as ceramics subject to varying degrees of shrinkage after firing.

Substrate-check result measurement is used to set pass/fail criteria for inconsistent substrates. It measures the distance between two fiducials or search features, and compares the distance to the originally taught substrate. A failure threshold is settable down to 1μm. Substrates that do not meet this user-defined threshold are rejected.

Relative placement permits the programming of a local eye-point and of a defined position vector for placing a component in relation to a reference point. Relative placement is typically used with hybrid devices where a component must be placed in exact reference to a feature of another component already placed. Examples are die stacking and opto-applications, such as placement of emitter and receiver on the same substrate, or component arrays that must be precisely spaced.

Waffle vision, or component search, reliably locates components in waffle packs, where die size is significantly smaller than cavity size, regardless of die orientation. Waffle vision can make use a search pattern to find fiducials outside of the vision camera’s field of vision, in addition to finding patterns or templates of any orientation. This eliminates manual re-orienting of the die. Machine operation is more efficient if the die are oriented correctly in the optimal cavity size. Ideally, cavity size should be no more than 1.3 times die size.

Pre- and post-bond inspection is an important tool for observing component placement during production, at user-defined search intervals. This includes automatic on-the-fly placement correction, programmable warning and error limits, small-dot adhesive inspection and realtime statistical data such as mean placement, standard deviation and Cpk.

Dispensing systems
Ideally, a system would support many different dispensing techniques, including epoxy writing, stamping (dip and dab) and cross-needle dispensing, and it would dispense two or more different epoxies from independent dispensers in a single pass.

Preferred 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 and automatic programming should ensure a consistent epoxy pattern and volume, X-Y speed and auger speed. An epoxy low-level detector is also important for continuous operation.

High-performance volumetric screw-pump dispensers yield a highly repeatable epoxy dispensing from large patterns to ultrasmall dots, with the same needle. This eliminates multiple custom stamping tools, which can be costly and difficult to maintain. Additional features include solder-paste capability, easy removal and cleaning and automatic needle calibration.

Epoxy stamping is necessary because some epoxies cannot be dispensed. A rotary squeegee unit with a stamping tool can adjust the epoxy amount, doing fine adjustment via micrometer.

Adhesive stamping is an excellent option for printing small dots or patterns using standard stamping tools, custom designs or gang-stamping tools. The method utilizes a rotating stamping dish. Film thickness is controlled via micrometer-adjusted doctor blade. Adhesive stamping achieves repeatable results on warped substrates, and substrates with thickness variations. The dish and blade should be easily removable for cleaning.

The bottom line: flexibility
In the fast-paced hybrid market, customized jobs are the order of the day. An equipment vendor should be in a position to accommodate these custom projects. For low volume/high mix production, a basic manual-load die attach system will be sufficient. For highvolume, or high-mix, production the die attach system must be operable in-line with ovens, wire bonders, screen printers, laser markers, plasma cleaners, etc.

If the procurement budget permits just basic die attach, it is good policy to include all the technical features for easy retrofitting later when requirements change and business grows. To accommodate ultra-thin die down to 50μm, features such as needle-less ejection, synchronous ejection and an ultra-light tool are needed. To accommodate new materials, there may be a need for heated pick-up tools and substrates, heated press, tool changers, carriers, component presentation systems, illumination, dispensers, etc. Important are examples, references and an organization chart that illustrates how a custom project will be handled.

Any vendor specializing in custom projects should provide a flexible, modular die bonder that accommodates flip-chip die sizes up to 50mm, includes a well-designed cavity fluxing system, and offers 10μm placement accuracy. Flexibility is a must for all five of the die bonder subsystems: pick and place, component presentation, substrate presentation, vision and dispensing. Last but not least: the installed base of some of the equipment selected should include major IDMs and subcontractors. In conclusion, only the most flexible hybrid assembly solutions will safeguard hybrid production well into the future.

by Manfred Glantschnig,
Datacon Technology

Sunday, November 02, 2008

The back-end process: Step 4 - Die attach Today's challenges


In back-end semiconductor manufacturing, the die attach process is a critical step. In simple terms, die attach is picking a chip from a wafer and placing it onto a substrate or metal lead frame. The way the chip is bonded defines the die attach process - epoxy, soft solder, eutectic and flip chip are the most widely used techniques. Die attach seems to be a simple process step in the semiconductor manufacturing chain. However, the continuously escalating requirements of today's applications create difficult challenges in die bonding.

Typical Die Attach Process Flow

In high-volume production, die attach is performed on fully automated assembly equipment. The basic die attach steps, some of which are performed simultaneously, are:

* A robotic loader picks up a lead frame from a stack and places it on the input area of the workholder.
* The lead frame is moved from the input position to the dispense position. Depending on the required placement accuracy, mechanical or optical alignment points are used to define the dispense position. Epoxy is dispensed in a pattern and volume appropriate for the chip size.
* A sophisticated vision system inspects the lead frame, dispensing pattern and bond pads before the substrate is transported to the bonding position.
* In the meantime, a pattern recognition system locates a good die on the sawn wafer.
* A vacuum pick-up tool mounted on a bond head grabs the aligned die from the wafer and places it on the programmed and pre-dispensed bond position on the substrate.
* Appropriate bonding time and bonding force result in a strong bond, according to the specified process requirements. An additional optical inspection is performed to ensure that placement position and epoxy bleed-out requirements are met.
* Each bond pad on the lead frame or substrate goes through this process before it is unloaded into an output magazine.

The Most Common Processes

There are four major die attach processes used in semiconductor packaging.

Eutectic Die Attach: The eutectic die attach process is a well-established bonding technique, having been used in the early days of semiconductors for the first transistors and integrated cicuits (IC). It is still widely used for small signal products - so called "jelly beans" - manufactured by the millions every day. Currently, eutectic die attach has regained importance in the field of packaging optoelectronic components and high-power communication devices. Under high temperature, the silicon-gold combination forms the eutectic bond. A scrub motion during the bond process increases the strength and quality of the intermetallic connection between the chip and substrate. For large chips, additional gold in ribbon form is used. Because of the high temperature of the process (up to 450°C), a protective forming gas atmosphere (a nitrogen/ hydrogen combination) is needed to prevent oxidation of the lead frame.

Soft Solder Die Attach: This process uses a solder material to bond the die to the lead frame. The solder is introduced as a wire preform and melted onto the hot lead frame surface as a liquid solder dot. A chip is placed on the hot solder and as soon as the solder cools down, a solid connection is established. These solders are typically lead- and tin-based alloys. A controlled temperature profile is required to define the liquidus/ solidus transition. Again, a protective forming gas atmosphere is required to prevent oxidation of the lead frame. Soft solder applications are typically used in automotive and high-power devices.

Flip Chip Bonding: A process similar to soft solder die attach is used in flip chip bonding. Here, the chip is flipped before being attached, and solder bumps between the chip and substrate serve as both an electrical and mechanical interconnection. Drivers for this process are high I/O counts and increased electrical performance requirements for high-speed, high-frequency applications.

Tape and Other Die Attach Processes: For some special applications, such as large memory devices, different tapes are used as the adhesive (epoxy, polyimides and thermoset/thermoplastic materials). The latest trend is to apply the adhesive directly to the backside of a wafer instead of using tape as an adhesive carrier. Wafer backside coating provides clear advantages for thin die and stacked die applications, because the adhesive is spread evenly on the bottom of the chip and it is always in the right position.

Epoxy Die Attach: Epoxy die attach is the most commonly used process, and the term generally encompasses the use of other adhesives, such as polyimide- or silicone-based materials. The adhesive is dispensed in paste form on the bond pad of the lead frame or substrate before die placement. Typical applications using epoxy cover a wide range of devices, from simple transistors to high power processors, memory and ASICs.

Today's packaging technologies create challenges for epoxy die attach processes. The key criteria for success (such as productivity and material cost) are not as crucial in advanced packaging. The challenges in this segment lie within the process and its execution.

Stacked Die

The major market drivers for stacked die applications are reduced space, weight savings and enhanced electrical performance of the devices (which are mainly used in portable consumer products). Stacking of chips, in which two or more ICs of different types are placed at the same coordinates in the x-y plane, is an alternative to silicon integration. The memory industry has discovered this opportunity to minimize package size and cost by stacking one die on top of the other in a single package. Such a system usually consumes less power and features higher speed than separate components. Stacked die applications provide flexibility in combining different devices without touching the design level of the silicon. Time to market can be drastically reduced. Additionally, the functionality of the device can be doubled or tripled in the same package size.

The vertically integrated system in a package has a much higher package integration ratio compared to the single die solution. In addition, the electrical performance and reliability of stacked die is improved because only one package has to be tested, and established IC assembly methods can be used.

Challenges of Stacked Die

To stay within standard package heights, the stacked chips need to be thinned. The backgrinding process is used to reduce the die thickness to the range of 50 - 125 µm. As a result, wafer handling needs special attention. Thus, a gentle and controlled die pick-up procedure is needed. Thin large die have a tendency to warp, which causes problems during the die bond process. Special tooling and techniques are required to keep the die flat on the substrate.

For the stacked die process, high die placement accuracy is essential to mount the upper chip accurately onto the lower one. Inaccurate die placement can lead to electrical failure (wire shorts) and impacts the epoxy bleed out. Since another cause of epoxy bleed-out is poor dispense quality, the position and volume of the dispensed epoxy needs to be consistent. Excessive epoxy may cover the wire bonding pads, preventing a proper interconnection. Using backside-coated wafers eliminates this problem since the adhesive is evenly spread on the backside of the top die and no epoxy/die offset can occur. This benefit makes the backside wafer process very attractive for stacked die. It is important to consider, however, that this process needs immediate curing, which may reduce productivity. Depending on the process applied (paste form adhesive), a curing step is required before the second die can be attached. However, curing often warps the substrate. Special downhold methods are required to keep the substrate in place for an accurate second die placement. For wire bonding applications, the substrate design rules and the loop height have to be carefully monitored to prevent a wire from shorting between the two die.

Small Thin Die on High BLT

The main drivers for small die applications on a high bondline thickness (BLT) are high-power RF amplifiers in small packages for wireless applications. BLT is the thickness of the resulting adhesive between substrate and chip after die placement. For high-power applications, a relatively high and constant BLT is essential for long and reliable performance of the device. Precise dispensing (amount and location of the epoxy) and a repeatable small bond force of 5 to 10 grams guarantee a high and constant BLT. The die attach equipment must be capable of compensating for the substrate thickness variations, which is very common, to achieve a consistent BLT. Today, with current high-end die bonding equipment, a standard deviation of 1 µm for a target BLT of 18 µm using a 1 mm square die is achievable.

Technology Trends

The technology trends in the back-end industry can be summarized as "more functionality in a smaller space." Future devices will contain - in the same package - chips bonded with traditional techniques in combination with the flip chip process. Passive components may be added in the same package to increase functionality. Vertical or horizontal package integration seems to be the logical evolution. Complicated packages with seven-stacked die beside a flip chip device and a few passive components packaged in the same small unit could soon be real. Space, flexibility and cost considerations will push the limits of package design and die attach technology.

In the future, high-volume processing of very thin die (50 - 100 µm) will challenge the chip assembly industry. Packaging equipment needs to be optimized to meet the new requirements. Die pick-up, bonding process and epoxy dispensing technologies need to be fine-tuned.

In contrast to small die, there is also a trend in the industry toward very large die used in microprocessors and digital signal processors. Attaching large die requires different methods. Void-free epoxy dispensing for a large area is crucial. An increased and controlled bond force is a prerequisite for consistent chip planarity and BLT.

All of these new technology trends will only take off and survive in high-volume production if economic aspects are considered. In the area of die attach, increasing productivity without compromising placement accuracy will be key to meet adequate cost of ownership. It is essential that equipment manufacturers, material suppliers and packaging designers collaborate closely to meet future packaging requirements.