Patent Description:
Sintered silver die attach films combine unique physical properties of nanosilver powder and innovative chemical formulations into innovative products that allow joining various electronic devices to produce extremely reliable high thermal and electrical conductivity interfaces. Reference can be made to <CIT> for processes in which sintered silver die attach films are uniquely positioned to fit into existing manufacturing equipment and processes to enable high volume manufacturing. This technology covers wide variety of devices and applications from large area thyristors and power modules for electrical and automotive equipment to miniature discrete components for mobile technology and LED lighting. The technology improves performance of existing devices by either increasing power or light output or reliability as compared to traditional joining techniques. The sintering films enable use of the new high temperature SiC and GaN semiconductors and new device designs that produce electrical efficiency unattainable with the existing technologies.

Silver die attach pastes and films are the joining materials that attach an electronic device to a passive substrate or to another device. A silver film is unique because it can be applied or laminated to an individual die, to the back side of the die or to the wafer. In one embodiment, the die is placed on the preheated substrate with a force sufficient to densify the film and establish intimate connection between the material and the connected parts. Under the applied heat and pressure, the film sinters and connects the die to the substrate. The resulting joint between the die and the substrate is metallic silver with the structure and properties shown with reference to <FIG>.

In many semiconductor packages, an electrical connection of the device is provided through clip bonding to the top of the die. This arrangement ensures low electrical resistance of the package and improved performance compared to wire bonding. Clip bonding is done by soldering a copper clip to one or both bond pads and their corresponding leads. The process typically uses high lead solder material and requires a good control of the soldering process, especially the positioning of the die and clips during the molten phase of the solder reflow process.

<NPL> discloses a pressure sintering process, in which a substrate, die and clip are sintered simultaneously using sintering films, with the help of a sintering press.

<CIT> discloses noble metal bonding means in the form of foil which comprises a solid fraction and liquid fraction with corresponding mixing ratio in vol. % of <NUM>:<NUM>-<NUM>:<NUM>, wherein the solid fraction contains at least <NUM> vol. % of silver, as first component, and the liquid fraction comprises a solvent component and at least an organic binder dissolved in the solvent, and the first component of the solid fraction exists as particles with a length dimension of <NUM> to <NUM>. The solvent is used to wet the surface of the bonding partner to be bonded with the silver foil.

<CIT> discloses that a sinter paste comprising sintered metal particles and solvent is applied on a contact area of a substrate. A sintering layer is formed by heating the sinter paste and removing the solvent. A liquid, e.g., a short-chain polyvalent alcohol such as glycerol is applied on the sintering layer. A power semiconductor component is placed on the sintering layer, and is fixed with the substrate through the liquid for subsequent handling. The sintering layer is converted into a sintered metal layer by application of temperature and pressure.

The present invention provides a method of die and clip attachment according to claim <NUM>.

The dependent claims define embodiments of the present invention.

Accordingly, the present invention can provide a process for die and clip attachment using sinterable silver films which is well suited for high volume manufacturing of discrete packages. The process utilizes the technology of a dispensable tack agent to position the dies and clips on the lead frame before sintering. The clips can also be used in a matrix connected to a lead frame. The following sintering step can be done in any commercially available sinter press.

A clip can enable a uniform transfer of pressure to a die and a foot pad of a substrate.

Where technical features in the figures, detailed description or any claim are followed by references signs, the reference signs have been included for the sole purpose of increasing the intelligibility of the figures, detailed description, and claims. Accordingly, neither the reference signs nor their absence are intended to have any limiting effect on the scope of any claim elements. The figures are provided for the purposes of illustration and explanation and are not intended as a definition of the limits of the invention. In the figures:.

Sinter technology is an excellent alternative to the clip soldering since sintering provides void-free, high electrical and thermal conductivity bond and eliminates use of toxic lead containing solders. This disclosure describes a process uniquely suited for high volume manufacturing of clip attached power packages through the sintering process. The process of embodiments of the present invention utilizes capabilities of sinterable silver films and special tack agent to ensure accurate positioning and reliable connection of the components. A process of the die and clip attachment in a single step using pre-applied sinterable silver films is disclosed herein. The composition and use of the silver films are described in <CIT>. The process is described schematically with reference to <FIG>, which shows a die <NUM> and a clip <NUM>.

In certain embodiments, the materials used in the process (<FIG>) include a die, a clip and a substrate, e.g., a lead frame. In a first step (<FIG>), two components of the package, namely a die <NUM> and a clip <NUM>, are laminated with the sinterable silver film <NUM> according to the process described in <CIT>. In certain embodiments, the laminating pressure can vary from <NUM>-<NUM> MPa at a temperature between <NUM>-<NUM> for less than <NUM> second to <NUM> seconds. In a particular embodiment, the lamination parameters may include a pressure of <NUM>-<NUM> MPa or higher, and a temperature of <NUM> applied for <NUM> seconds. The dies can be laminated individually or as a whole wafer followed by dicing. Similarly, the clips can be laminated individually or in the form of a copper plate followed by dicing or stamping. The lamination step can be done in a lamination press capable of providing specified pressure and temperature or in the die bonding equipment, such as a Datacon™ <NUM> evo multi-chip die bonder offered by BE Semiconductor Industries N. , or similar machine. The laminated dies are collected and stored in a waffle pack or on a dicing tape. The laminated clip are collected and stored on the tape, reel or waffle pack.

In a second step (<FIG>), a tack agent <NUM> is dispensed on a substrate (or lead frame) <NUM> to place and hold in position the components of the package before the parts have been moved to the sintering press. The role of the tack agent <NUM> is to provide temporary attachment of the die <NUM> and the clip <NUM> to the lead frame <NUM>. During sintering process tack agent evaporates without interference with the sintering process.

In the next step (<FIG>), the die <NUM> is placed on the lead frame <NUM> and secured in the position due to the tack agent <NUM>. The die placement can be done in an ESEC™ <NUM> die bonder offered by BE Semiconductor Industries N. , or similar equipment.

In the fourth step (<FIG>), the clip <NUM> is placed on the lead frame <NUM> using standard pick and place equipment or equivalent epoxy die bonder, such as an ESEC™ <NUM>. The clip <NUM> is secured in the position due to the tack agent. The clip placement can be done in a pick and place machine, such as a Fuji® pick and place machine, or similar equipment.

In the final step (<FIG>), the lead frame <NUM> populated with all the dies and clips is moved to the sintering press (heated platen <NUM>). In certain embodiments, the sintering pressure can vary from <NUM>-<NUM> MPa at a temperature between <NUM>-<NUM> for <NUM>-<NUM> seconds. In one embodiment, the parts are sintered at a pressure of <NUM> MPa, and at temperature of <NUM> for <NUM> seconds. The sintering step can be done in Sinterstar™ Innovate-F-XL press machine from Boschman Technologies, or similar equipment.

The shape of the clip can vary and depends on the die and lead frame design to accommodate electrical connections. The clip design can also account for thermal stress relieve due to the sintering process. Examples of clip designs are in <FIG> illustrates clip <NUM>. <FIG> illustrates clip <NUM>. <FIG> illustrates clip <NUM>. As shown, clips <NUM> and <NUM> are each configured with one leg to engage the lead frame. Clip <NUM> is configured with one leg to engage the lead frame. In a particular embodiment, the clip design may embody the design shown in <FIG>. As shown, clip <NUM> is designed to secure die <NUM> and lead frame <NUM>. It should be understood that any number of clip designs may be employed depending on the size and shape of the die and/or the lead frame.

<FIG> illustrates a package having a die and clip attachment manufactured by an embodiment of the process of the present invention.

As shown, the package includes clip <NUM> and die <NUM>, which are configured to be secured to lead frame <NUM>.

<FIG> illustrates an exemplary lead frame, <FIG> illustrates exemplary clip designs, and <FIG> illustrates exemplary dies for use in embodiments of the present invention.

Silver films and tacking agents are also provided.

The dies and the clips are laminated with sinterable silver film Argomax® <NUM> from Alpha Metals, Inc. The tack agent was dispensed on the lead frame at the positions shown below. In one embodiment, the tack agent is sold under DATA <NUM> and is a commercial product of Alpha Metals, Inc.

<FIG> show a tack agent dispensed on a lead frame to hold a die and clip in place.

The dies and the clips are placed at eight positions on the lead frame and sintered in the sinter press Sinterstar™ from Boschman Technologies. The optical view from the top and the side of the packages indicated good sintered connection between the die and the substrate and the clip and the die.

The clip design is unique and allows for uniform pressure to transmit from a top of the clip to a die area and to a foot pad of the clip. Conversely, the die tool is unique and designed to apply uniform pressure across a top area of the clip. In this way the clip, die and substrate or lead frame can be sintered in one step.

In addition, the die tools, termed dynamic inserts, can be clustered in an array to apply uniform pressure across each individual die, a group of dies or all dies at once. Packages that are flat and three-dimensional packages can be processed effectively with the dynamic insert tool configuration. An array of lead frames or packages can be sintered in parallel with the only limitation being the press area.

The clip and tool design allows for high production throughput and yield. Therefore, cost targets can be achieved, with the benefit of high reliability and a lead-free system.

<FIG> are photographic representations of completed die and clip attachment assemblies manufactured by embodiments of the process of the present invention.

Scanning acoustic microscopy revealed uniform and full connection at both interfaces, namely the clip and the die and the die and the substrate.

<FIG> are photographic representations of joint analysis by CSAM.

The attached components are cross sectioned and the connection examined using a scanning electron microscope. Both die and the clip are connected. Uniform fully sintered silver bond of about <NUM> is formed between the connected parts.

<FIG> are photographic representations of joint analysis by SEM. <FIG> are photographic representations of joint analysis of completed die and clip attachment assemblies.

Clips can be laminated with a sinterable silver layer individually as was described above or in a form of a matrix connected to the lead frame. In this case, the spacing between the clips is matched to the dies positioned on the substrate or lead frame. <FIG> show an example of such a design before and after lamination. To attach the clips to the top side of the dies, the clip will be positioned and placed on the top of the substrate lead frame with the previously attached dies. Following sintering step is conducted similarly to what is described above.

<FIG> is a view of a lead frame with clips prior to lamination. <FIG> is a view of clips prior to lamination. <FIG> is a view of a lead frame with clips after lamination. <FIG> is a view of clips after lamination.

Alternatively, flat copper foil, with or without a silver plating layer, can be laminated with sinterable silver film. In a following step, copper foil can be trimmed and stamped to produce a lead frame with the clips shown in <FIG>.

It is to be appreciated that embodiments of the methods and apparatuses discussed herein are not limited in application to the details of construction and the arrangement of components set forth in the following description or illustrated in the accompanying drawings. The methods and apparatuses are capable of implementation in other embodiments and of being practiced or of being carried out in various ways. Examples of specific implementations are provided herein for illustrative purposes only and are not intended to be limiting. In particular, acts, elements and features discussed in connection with any one or more embodiments are not intended to be excluded from a similar role in any other embodiments.

Any references to embodiments or elements or acts of the systems and methods herein referred to in the singular may also embrace embodiments including a plurality of these elements, and any references in plural to any embodiment or element or act herein may also embrace embodiments including only a single element, insofar compatible with the remaining disclosure.

References in the singular or plural form are not intended to limit the presently disclosed systems or methods, their components, acts, or elements. The use herein of "including," "comprising," "having," "containing," "involving," and variations thereof is meant to encompass the items listed thereafter as well as additional items. References to "or" may be construed as inclusive so that any terms described using "or" may indicate any of a single, more than one, and all of the described terms. Any references to front and back, left and right, top and bottom, upper and lower, and vertical and horizontal are intended for convenience of description, not to limit the present systems and methods or their components to any one positional or spatial orientation.

Claim 1:
A method of die and clip attachment comprising:
providing a clip (<NUM>), a die (<NUM>) and a substrate (<NUM>);
laminating a sinterable silver film (<NUM>) on the clip (<NUM>) and the die (<NUM>) in a first step;
depositing a tack agent (<NUM>) on a side of the substrate (<NUM>) in a second step:
placing the die (<NUM>) on the tack agent (<NUM>) on the substrate (<NUM>) in a third step, with the sinterable silver film (<NUM>) on a side of the die (<NUM>) facing the substrate (<NUM>) and placed on the tack agent (<NUM>) on the substrate (<NUM>);
placing the clip (<NUM>) on the die (<NUM>) and on the tack agent (<NUM>) on the substrate (<NUM>) to create a substrate, die and clip package in a fourth step, with the sinterable silver film (<NUM>) on the clip (<NUM>) facing the die (<NUM>) and the substrate (<NUM>) and placed on the tack agent (<NUM>) on the substrate (<NUM>) and on a side of the die (<NUM>) opposite to the side of the die (<NUM>) facing the substrate (<NUM>);
moving the substrate, die and clip package to a sintering press; and
sintering the substrate, die and clip package in the sintering press in a final step;
wherein the tack agent (<NUM>) is dispensed on the substrate (<NUM>) to place and hold in position the die (<NUM>) and the clip (<NUM>) on the substrate (<NUM>) before the substrate, die and clip package has been moved to the sintering press,
wherein the tack agent (<NUM>) provides temporary attachment of the die (<NUM>) and the clip (<NUM>) to the substrate (<NUM>), wherein the tack agent (<NUM>) evaporates during the sintering of the substrate, die and clip package without interference with the sintering process.