The embodiments of the present disclosure relate to semiconductor device packaging, in particular the reducing of stress of bond connections in the packaging.
The electronics industry continues to rely upon advances in semiconductor technology to realize higher-function devices in more compact areas. These higher-function devices have to survive the stresses incurred during manufacturing and those stresses imposed by their ultimate application by the end user.
The present disclosure has been found useful in the packaging of semiconductor devices having increased strength against the breaking of wire bonds owing to lead bouncing and the subsequent lift-off of the ball bond on the device die bond pad.
There are ways of reducing bouncing of a lead, for example by using clamping techniques and lead frame structure design. However, these techniques may add to the cost of the lead frame materials and subsequently increase device cost. Thus, there is a need to overcome and reduce lead bouncing.
The bouncing of lead frame material can be decreased by sandwiching a nonconductive material sheet between the underside surface of the device die and the topside surface of lead frame structure. A resilient material sheet is put onto lead frame structure by appropriate techniques. In various embodiments, a pattern shape and size of the material sheet may be designed to address to different levels of bouncing lead as well.
In an example embodiment, a semiconductor device comprises a lead frame assembly having a non-conductive material (NCM) sheet placed on a location of the lead frame assembly, a device die having a length, width, and thickness is attached to the NCM sheet, the device die being attached to the NCM with an adhesive. The NCM sheet has a length and width greater than the length and width of the device die and the NCM sheet has a thickness less than the thickness of the device die. A feature of this embodiment is the NCM sheet mitigates wire bond lifting at device die bond pads by reducing bouncing of the wire bond leads owing to stress and movement of the lead frame assembly underneath the device die.
In an example embodiment, a semiconductor device is assembled in a leadless package. The leadless package comprises a device die having a length, width, and thickness, the device die having a top-side surface with active circuit elements and an underside surface. A non-conductive material (NCM) sheet has a length and width greater than the length and width of the device die, the NCM is attached to the underside surface of the device die. There is a lead frame assembly, the lead frame assembly having been characterized such that the semiconductor device die attached to the NCM is placed in a cover area and where bonding areas and not-allow bonding areas are defined. The NCM sheet mitigates wire bond lifting at device die bond pads by reducing bouncing of the wire bond leads owing to stress and movement of the lead frame assembly underneath the device die.
In an example embodiment, there is a method for assembling a semiconductor device. The method comprises selecting a lead frame assembly, attaching a NCM sheet in a device die position on the lead frame assembly, attaching a device die onto the NCM sheet, wire bonding the device die to the lead frame assembly, and encapsulating the device die and lead frame assembly. A feature of this embodiment is wherein the pre-determined device die position on the lead frame assembly has been found by a characterization of the lead frame, whereby the pre-determined device die position is a location having substantially reduced stress and displacement with respect to other positions on the lead frame assembly.
The above summaries are not intended to represent each disclosed embodiment, or every aspect, of the present disclosure. Other aspects and example embodiments are provided in the figures and the detailed description that follow.
The invention may be more completely understood in consideration of the following detailed description of various embodiments of the invention in connection with the accompanying drawings, in which:
While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.
The present disclosure has been found useful in reducing the stress placed upon wire bonds of a device die encapsulated into a lead less package. The stress causes the lead frame to bend thereby causing lead bouncing. The lead bouncing results in bump ball lift off at the wire bonds. To reduce the likelihood of bump ball lift-off, a device die is placed onto a lead frame assembly onto which a resilient non-conductive material sheet is placed. Bump ball lift-off is the result of a weak inter-metallic bond between the device die bond pad and the bump ball.
The device die may be glued onto this resilient sheet or a die-attach film (DAF) may be used. In a current wire bonding process, placement of device die in a suitable position may reduce lead bouncing, but may be problematic for the variety of devices which may be assembled in a given package type. The present disclosure obviates the need to find a particular optimal position, but enables the user to place the die in more positions on the lead frame structure and yet, have minimal lead bouncing. Further, with some characterization of the lead frame structure (in finding the optimal die placement area) a combination of a preferred placement in combination with the use of the resilient non-conductive material underneath the device die, will substantially enhance the optimization and thereby reduce the incidence of bump ball lift-off. In an example embodiment, a series of design rules may be developed based on empirical data obtained in studies of lead frames, NCM configurations, device die characteristics, etc.
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For characterizing a lead frame, XRF (energy dispersive X-ray fluorescence) analysis would be suitable for determining the elements and thickness of the plated layers, or the lead frame manufacturing process (e.g., etching or stamping).
For example, a lead frame having a base metal of copper (Cu) may be pre-plated with NiPd, NiPdAu, etc. The pre-plating has a nickel (Ni) thickness of about 1.5 um, the palladium (Pd) thickness of about 0.152 um, and a thin layer of gold (Au), gold/silver (Au/Ag) or palladium/silver (Pd/Ag).
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Determining the placement of the NCM sheet on the lead frame is a result of characterizing the lead frame by placing a device die at a predetermining location and measuring the forces applied to the lead frame with a device having a NCM sheet attached and with a device not having one. Refer to
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In an example embodiment, a particular lead frame may be characterized so as to locate areas having reduced stress and displacement. Within that area, the NCM sheet may be included in the device die attach process to minimize stress even further.
In configuring a lead frame assembly according to the present disclosure, as discussed in reference to
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To even further reduce the stress/displacement, in step 920, a NCM sheet material is applied to the selected area (i.e., the area of minimal stress/displacement) of the lead frame assembly. With a standard force applied, the resulting stress/displacement are measured. In a lead frame assembly whose stress/displacement has been characterized and an optimum area is location has been found and is useable, the NCM sheet material will further enhance the stability of the die attach and subsequent wire bonds.
In some circumstances, however, the design of the lead frame may not exhibit sufficient resistance to stress/displacement no matter which location is chosen. Due to system requirements, a particular lead frame configuration must be employed. These requirements may include lead frame area and vertical thickness, for example. Thus, a location with the least stress/displacement may not be available.
The use of a NCM sheet material, according to the present disclosure, may be sufficient to reduce the stress/displacement in these situations. In characterizing the NCM sheet, a particular shape may reduce stress/displacement more than other shapes. For a given device die size, rectangular or square shapes may be useful; for others, hexagonal, octagonal, or more sides, etc. may be appropriate. In other cases, the NCM sheet material may be circular and circumscribe the device die. The dimensions of these shapes will exceed the boundary edges of the device die by a prescribed amount; the prescribed amount being determined by characterizing the process.
In step 930, having characterized the lead frame assembly and NCM sheet material, the results are incorporated into the device assembly process.
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The embodiments of the present disclosure may be applied to any micro-electronic packages that have the device die undergo wire bonding or ribbon bonding. The reducing of displacement and stress is applicable to any size bond wired, any loop shape of wire bonding, forward and reverse bonding technique with and without bump balls. Metals such as gold (Au), silver (Ag), copper (Cu), aluminum (Al) and their alloys may be used, but are not limited to theses.
In combination with locating the area on the lead frame exhibiting a minimal amount of stress and displacement, the present disclosure serves to reduce the stress and displacement even further.
It will be readily understood that the components of the embodiments as generally described herein and illustrated in the appended figures could be arranged and designed in a wide variety of different configurations. Thus, the following more detailed description of various embodiments, as represented in the figures, is not intended to limit the scope of the present disclosure, but is merely representative of various embodiments. While the various aspects of the embodiments are presented in drawings, the drawings are not necessarily drawn to scale unless specifically indicated.
The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by this detailed description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.
Reference throughout this specification to features, advantages, or similar language does not imply that all of the features and advantages that may be realized with the present invention should be or are in any single embodiment of the invention. Rather, language referring to the features and advantages is understood to mean that a specific feature, advantage, or characteristic described in connection with an embodiment is included in at least one embodiment of the present invention. Thus, discussions of the features and advantages, and similar language, throughout this specification may, but do not necessarily, refer to the same embodiment.
Furthermore, the described features, advantages, and characteristics of the invention may be combined in any suitable manner in one or more embodiments. One skilled in the relevant art will recognize, in light of the description herein, that the invention can be practiced without one or more of the specific features or advantages of a particular embodiment. In other instances, additional features and advantages may be recognized in certain embodiments that may not be present in all embodiments of the invention.
Reference throughout this specification to “one embodiment,” “an embodiment,” or similar language means that a particular feature, structure, or characteristic described in connection with the indicated embodiment is included in at least one embodiment of the present invention. Thus, the phrases “in one embodiment,” “in an embodiment,” and similar language throughout this specification may, but do not necessarily, all refer to the same embodiment.
Numerous other embodiments of the invention will be apparent to persons skilled in the art without departing from the spirit and scope of the invention as defined in the appended claims.