07/01/2010, Irvine, California

 

New Wafer Backside Coating Innovation Stacks Up Against Film

Consumers continue to drive demand for smaller, thinner and more capable electronic devices which, then, require integrated packages that can cope with today’s new product footprints. For packaging specialists, that means the processing of thinner wafers and stacking of much thinner die. The catch? All of this has to be achieved at an end unit cost that is palatable and marketable to tech‐savvy consumers.

Without question, advances in die attach material technology have been central to the proliferation of highly miniaturized devices. And, while traditional die attach pastes and newer die attach films address many of these challenges for certain applications, the drawbacks to these options have encouraged an innovative response in the form of Wafer Backside Coatings (WBC) for stacked packages.

Widely regarded for its throughput advantages over traditional die attach pastes and lower cost as compared to film, WBC materials have been used successfully on leadframe package applications for the past several years. Conventional paste materials rely on dispensing for deposition, which is a serial process and, therefore, limits throughput. Additionally, the bondline thickness control is highly dependent on die placement force which can result in either insufficient coverage (too little force), bleed out (too much force) or die tilt (non‐uniformity). For today’s thinner wafers, much of the market has turned to die attach films for their handling capability, thin coverage and coating uniformity. But, like pastes, films also have some shortcomings, not the least of which is their higher cost as compared to paste mediums. The WBC materials developed for leadframe packages can be applied via screen printing, stencil printing or spin coating, which dramatically improves throughput over dispensing processes and provides a lower‐cost solution to film.

Even though these materials have been successful with leadframe applications, WBCs haven’t historically been perceived as a viable option for stacking of modern, thinned wafers for several reasons: High throughput deposition methods such as screen‐ and stencil‐printing that are extremely viable for wafers thicker than 75 microns are challenged on thinner wafer dimensions. In addition, the mesh marks left from screens and the “scooping” effect often induced with squeegee traverse may lead to non‐ uniformity of the much thinner coatings. The other alternative, spin coating, commonly resulted in material waste of greater than 70%, which countered the material cost savings as compared to films.

Henkel’s perseverance in combination with new developments in spray coating technologies, however, have led to a new WBC material, Ablestik WBC‐8901UV, that overcomes many of the issues associated with films and traditional pastes for stacked die applications. The new material has been designed to address the demanding requirements of multiple die stack applications for the memory market segment, including packages such as TSOPs, MCPs and FMCs (Flash Memory Cards).

Uniquely formulated as a solution for in‐line processing, Ablestik WBC‐ 8901UV offers a robust and cost‐ effective alternative to current film‐ based solutions for die stacking techniques, reducing the total cost of ownership as compared to film by as much as 30% to 50%. Process flexibility is also enhanced with Ablestik WBC‐8901UV, as packaging specialists can now adjust die attach thickness based on specific manufacturing requirements and can also select their dicing tape of choice. Film die attach materials are generally supplied in pre‐determined thicknesses as a bundled product which incorporates the dicing tape.

Applied via a spray coating method following the wafer thinning process, Ablestik WBC‐ 8901UV is precisely deposited across the back of the silicon wafer following which the material is B‐staged using a UV irradiation process. After this step, dicing tape is laminated to the wafer, backgrinding tape is removed and the wafer is diced in preparation for die pick‐up and placement. Henkel is currently partnering with spray technology and backgrinding equipment manufacturers to deliver an integrated, in‐line process solution for this unique WBC advance.

Ablestik WBC‐8901UV has successfully proven to deliver a precise wafer coating as thin as 10 microns with a total thickness variation across the wafer of +/‐10% and remarkably low material waste of less than 20%. Wafers as thin as 50 microns have been processed using this method.

In keeping with Henkel’s forward‐looking approach, further developments to Ablestik WBC‐8901UV are already underway and the company expects to achieve uniform, 5 micron coating thicknesses or better within the next year.

For more information on Ablestik WBC‐8901UV or any of Henkel’s WBC portfolio materials, log onto www.henkel.com/electronics or call 949.789‐2500.