Wafer Backside Coating of Die Attach Adhesives
In wafer backside coating, the adhesive is supplied as a paste, which is applied to the back of a wafer and dried. Advantages include a 20-30% cost reduction compared with film, control of bond-line thickness and higher throughput.
Tony Winster and Craig Borkowski, Ablestik, Rancho Dominguez, Calif.; Alan Hobby, DEK Printing Machines, Weymouth, Dorset, UK -- Semiconductor International, 10/1/2006
For decades, the semiconductor industry has used adhesive pastes to attach die to leadframes and other substrates. Typically, adhesive has been dispensed through needles from syringes or, for smaller dots, applied by the pin-transfer method. Some manufacturers, notably in the automotive hybrid industry, have applied adhesive by screen printing.
What's the problem? Actually, there are several problems:
Speed — In the highly competitive semiconductor industry, time is money. In some cases, dispensing adhesives may be a limiting factor in the production rate. If adhesive can be applied faster, then production lines run faster and costs can be reduced. In principle, there are ways to speed up the dispense process, but none has made much impact thus far.
Size — With traditional paste die attach, the adhesive forms a fillet around the chip. This is a useful visual indicator to confirm that sufficient adhesive has been applied. It also increases the die shear strength by supporting the side of the die. However, the fillet effectively increases the footprint area of the chip, which means that any packaging must be slightly larger than the die itself. This extra area can be quite substantial after adding the tolerances that account for variations in fillet size caused by differences in paste volume, flow, and misplacement of paste and die (Fig. 1 ). Since all paste adhesives are prone to resin bleed, space must be left to ensure that surrounding bond pads are not affected by excessive bleed.
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| 1. Wafer backside coating (WBC) of adhesive over an entire wafer reduces irregular paste fillets, as shown in this SEM. |
What's changed?
As designers try to make packages smaller and thinner, and as manufacturing plants try to reduce costs by speeding up assembly, all the above issues become increasingly important. Solving one problem often only makes another one worse. For example, die-tilt variations may be solved with a higher-specification die bonder, but this may not be cheaper or run at higher speed than the existing bonder.
Another trend is to place more than one chip inside a package. These can sit side-by-side, in which case determination of the proper adhesive will permit optimal spacing of the die. Alternatively, chips can be stacked on top of each other. Here the die are usually very thin, and so tight control of fillet heights is essential to prevent adhesive from contaminating the die surface.
Clearly, an alternative to traditional paste adhesive application would be attractive. One solution already in place is to use adhesive in the form of a dry film. This is supplied at a controlled thickness. It can be either punched into preforms or applied to the back of a silicon wafer before dicing into individual die. When each die is picked, it carries its own adhesive with it to the leadframe or substrate. Some heat and pressure are needed to attach the adhesive to the leadframe, but the adhesive remains sufficiently hard to prevent the formation of large fillets and any significant die tilt.
This approach is widely used in die-stacking applications, where cost is not yet a major factor. Unfortunately, the cost of manufacturing and packaging a film is higher than a paste. Thus, this approach is difficult to implement in very high-volume, cost-sensitive applications.
What's the solution?
One practical solution is wafer backside coating (WBC). In WBC, the adhesive is supplied as a specifically designed paste, which is applied to the back of a wafer and dried (Fig. 2 ). This method leads to a number of practical advantages. These include:
- Paste is 20-30% cheaper than film.
- Bondline thickness can be controlled to customer specifications.
- Fillet control is very similar to that of a film product.
- Dispense operation is eliminated, which affords higher units per hour.
- Coated wafer can be stored until required.
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| 2. Screen or stencil printing can be used to coat the backside of semiconductor wafers in preparation for curing and subsequent die attach. |
The wafer can be attached to conventional dicing tape and diced with standard saws (Fig. 3 ). The only modification that may be required to a standard die bonder is to fit a heater under the stage to ensure that the adhesive softens sufficiently to wet the surface.
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| 3. This schematic provides an overview of the WBC process. |
The adhesive can be applied quickly using a screen-printing platform, such as those used in surface-mount production lines to deposit solder pastes (Fig. 4 ). The platforms are readily available at reasonable cost, and deliver sufficient accuracy and repeatability to enable complete coverage of the wafer with minimal waste.
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| 4. A dedicated screen or stencil printer can apply adhesive to the backside of an entire wafer in just a few seconds. |
Using such a printer, a concentric adhesive layer can be deposited on the wafer backside with little variation in thickness. The entire process typically takes 10-15 seconds to complete, using a suitable combination of stencil and squeegee. An emulsion screen may provide a suitable alternative to a metal stencil, depending on the adhesive characteristics.
Automated wafer loaders/unloaders have also been developed to support wafer-level, screen/stencil printing processes. Available units are compatible with wafers in all standard sizes up to 300 mm and larger. These integrate directly with standard wafer-storage formats such as JEDEC cassettes or FOUPs, which are widely used throughout the semiconductor processing industry. After unloading the coated wafer, it can be returned to the cassette/pod or transported directly into the curing oven. A typical loader/unloader uses industry-standard SMEMA protocols to communicate with related equipment.
The design and construction of suitable stencils have been adapted from technology developed in the surface-mount industry. Highly satisfactory results have been achieved using a stainless steel stencil with a single, large aperture. The effectiveness of stencils as thin as 37 µm has been achieved. A purpose-designed squeegee, which is very rigid to avoid scooping adhesive out of the aperture as it passes, produces a thin and uniform coating.
Modifications to the tooling and handling mechanisms of a standard printer enable WBC. These include tooling revisions to support extremely thin wafers. The wafer must remain very flat throughout the process in order to prevent excessive variations in the thickness of the adhesive layer. A pallet, having comparable flatness to a surface plate for metrology applications, is used to hold the wafer. A vacuum is applied to the wafer through porous ceramic inserts in the pallet to hold the wafer without incurring damage (Fig. 5 ).
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| 5. This wafer pallet features porous vacuum inserts to hold the wafer without incurring damage. |
Non-contact measuring equipment can accurately calculate the thickness of the adhesive layer, but it is not suitable for rapid inspection of each wafer in a volume production environment. Thus, after calibrating the process using a profilometer, it is most practical to test samples at regular intervals during production.
Once correctly set up, there is no reason for the process to vary widely, since screen- or stencil-printing equipment and processes are very robust. A complete turnkey system, including printer, wafer handler and inline oven, costs ~$320,000 and provides a throughput of 60-85 wph, based on the use of a 200 mm wafer.
In addition to solutions to the basic problems outlined, WBC also affords additional benefits. These include:
- Reduction of paste adhesive volume by a factor of 10 on die <0.5 × 0.5 mm by eliminating the fillet.
- Improved inventory control by concentrating the application of wet adhesive in a dedicated print area within the user's facility. This also reduces the need for expensive engineering and safety controls where chemicals are being handled. In fact, it may be possible to print the adhesive at the wafer fab facility, thus eliminating the need to handle wet paste adhesives in the packaging or “back-end” facility.
- Additional inventory control advantages because only one paste formulation is required rather than the multiple thicknesses and widths currently necessary with films.
What is the status of WBC?
First-generation WBC products are currently available in both electrically conductive and electrically insulating versions. Second-generation materials with improved printability and thermal properties will be available this year. In parallel with materials development, improvements are being made in wafer handling to allow faster throughputs and enable processing of thinner wafers. In fact, 150 mm wafers that are just 30 µm thick have been successfully coated. Thus, current indications are that, as the process is improved even further, WBC will take its place in the pantheon of standard semiconductor processing.
| Author Information |
| Tony Winster is responsible for providing technical support to Ablestik customers. In the past year, he has specialized in wafer coating process support; he has been with Ablestik for 17 years. Prior to his technical advisor position, he was responsible for design and development of hybrid microcircuits. Winster studied metallurgy and materials science, and is a member of the International Microelectronics and Packaging Society (IMAPS). Phone: +44 1223 893771 E-mail: tony.winster@ablestik.com |
| Craig Borkowski is the product manager and leadframe business team leader for the leadframe adhesives business unit at Ablestik; he joined the company in 1999. Prior to his team leader position, he held positions in manufacturing, engineering and technical service. Borkowski has an MBA from Pepperdine University and a B.S. in chemical engineering from Purdue University. Phone: 1-310-764-4600 E-mail: craig.borkowski@ablestik.com |
| Alan Hobby is in applied applications engineering, process support at DEK Printing Machines. He has been with DEK for 22 years, and has worked in a variety of printing applications including semiconductor, SMT, displays and thick-film hybrids, and multichip modules. Prior to this position, he was in thick-film hybrid manufacture, CRT manufacture, and development of targets and detectors in nuclear physics research. Hobby holds a Higher National Certificate (HNC) in applied physics, and is a member of the SMART Group technical committee. Phone: +44 1305 760760 E-mail: ahobby@dek.com |
