Abstract:
A packaged electronic device has first and second lead frame leads and a passive electronic component mounted, across a gap between the leads, on the top sides of the leads, using an adhesive. Facing lateral sides of the leads each include a recess that receives the adhesive. The recess promotes adhesion between the electronic component and the corresponding lead while limiting spread of the adhesive on the bottom side of the electronic component. The adhesive in the recesses promotes adhesion of the component to the leads by inhibiting cracking, and enhances inspection capability at the device backside.

Description:
BACKGROUND 
     The invention relates to packaged electronic devices and, more particularly, to a lead frame and a method of mounting passive electronic components on the lead frame. 
     In the field of packaged electronic devices, it is known to mount an electronic component, such as a capacitor, onto two adjacent leads of a lead frame such that the capacitor spans the gap between the leads, where one terminal of the capacitor is electrically connected to one lead and the other capacitor terminal is electrically connected to the other lead. When the capacitor is mounted using an adhesive, such as an electrically conductive adhesive, cracking can occur within the adhesive interface between the component and each lead that can jeopardize the electrical connections between the component and the leads as well as the mechanical bonding of the component to the leads. 
     It would be advantageous to have a method of mounting passive devices to leads of a lead frame that provides for a secure mechanical and electrical connection. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention will become more fully apparent from the following detailed description, the appended claims, and the accompanying drawings in which like reference numerals identify similar or identical elements. 
         FIG. 1A  is a top plan view of a portion of a lead frame used to assemble a packaged electronic device (not shown), according to one embodiment of the invention; 
         FIG. 1B  is a bottom plan view of the portion of the lead frame of  FIG. 1A ; 
         FIG. 2A  is a top plan view of a portion of a sub-assembly formed by mounting an electronic component onto the top sides of the two adjacent leads of the lead frame of  FIGS. 1A and 1B  using a suitable adhesive; 
         FIG. 2B  is a bottom plan view of the portion of the sub-assembly of  FIG. 2A ; 
         FIG. 2C  is a cross-sectional side view of a portion of the sub-assembly of  FIGS. 2A and 2B ; 
         FIG. 3  is a top plan view of the portion of the lead frame of  FIGS. 1A and 1B  after the adhesive has been applied to the top sides of the first and second leads; 
         FIGS. 4-10  are top plan views of lead frames according to alternative embodiments of the invention; and 
         FIG. 11  is an isometric view of an example packaged electronic device according to one embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Detailed illustrative embodiments of the invention are disclosed herein. However, specific structural and functional details disclosed herein are merely representative for purposes of describing example embodiments of the invention. The invention may be embodied in many alternate forms and should not be construed as limited to only the embodiments set forth herein. Further, the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments of the invention. 
     As used herein, the singular forms “a,” “an,” and “the,” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It further will be understood that the terms “comprises,” “comprising,” “includes,” and/or “including,” specify the presence of stated features, steps, or components, but do not preclude the presence or addition of one or more other features, steps, or components. It also should be noted that in some alternative implementations, the functions/acts noted may occur out of the order noted in the figures. For example, two figures shown in succession may in fact be executed substantially concurrently or may sometimes be executed in the reverse order, depending upon the functionality/acts involved. 
     In one embodiment, the invention is a sub-assembly for a packaged electronic device. The sub-assembly comprises first and second leads of a lead frame and an electronic component. The first lead comprises a top side, a bottom side, and a first lateral side. The second lead is separated from the first lead by a gap and comprises a top side, a bottom side, and a first lateral side. The electronic component comprises a top side, a bottom side, and first and second lateral sides. The electronic component is mounted, across the gap, onto the top sides of the first and second leads using an adhesive. 
     A first portion of the adhesive is located between the bottom side of the electronic component and the top side of the first lead, a second portion of the adhesive is located between the bottom side of the electronic component and the top side of the second lead, a third portion of the adhesive spans between the bottom side of the electronic component and the first lateral side of the first lead, and a fourth portion of the adhesive spans between the bottom side of the electronic component and the first lateral side of the second lead. 
     In another embodiment, the invention is a method for assembling a sub-assembly for a packaged electronic device. The method comprises providing a lead frame comprising (i) a first lead comprising a top side, a bottom side, and a first lateral side and (ii) a second lead separated from the first lead by a gap, the second lead comprising a top side, a bottom side, and a first lateral side. 
     Adhesive is applied to the top sides of the first and second leads, and an electronic component comprising a top side, a bottom side, and first and second lateral sides is applied onto the adhesive. The electronic component is mounted, across the gap, onto the top sides of the first and second leads. 
     A first portion of the adhesive becomes located between the bottom side of the electronic component and the top side of the first lead, a second portion of the adhesive becomes located between the bottom side of the electronic component and the top side of the second lead, a third portion of the adhesive spans between the bottom side of the electronic component and the first lateral side of the first lead, and a fourth portion of the adhesive spans between the bottom side of the electronic component and the first lateral side of the second lead. 
     Referring not now to  FIG. 1A , a top plan view of a portion of a lead frame  100  used to assemble a packaged electronic device (not shown), according to one embodiment of the invention, is shown.  FIG. 1B  is a bottom plan view of the portion of the lead frame  100  of  FIG. 1A . Note that, as used in this specification, the terms “top,” “bottom,” and “lateral” are relative terms based on a typical orientation of the described structures and do not refer to inherent characteristics of those described structures. 
     The lead frame  100  has (at least) a first lead  110  and a second lead  120  separated from the first lead  110  by a gap  130 . On one lateral side, the shape of the first lead  110  defines a curved recess  112 , while, on one lateral side, the shape of the second lead  120  defines another curved recess  122 . 
       FIG. 2A  is a top plan view of a portion of a sub-assembly  200  formed by mounting an electronic component  210  across the gap  130  onto the top sides of the two adjacent leads  110  and  120  of the lead frame  100  of  FIGS. 1A and 1B  using a suitable adhesive  220 .  FIG. 2B  is a bottom plan view of the portion of the sub-assembly  200  of  FIG. 2A . 
     The electronic component  210  can be any suitable passive or active electronic component, and, depending on the particular type of component, the adhesive  220  can be either a suitable electrically conductive adhesive or a suitable electrically non-conductive adhesive. In one implementation, the electronic component  210  is a capacitor, and the adhesive  220  is an electrically conductive adhesive that mechanically bonds the capacitor to the top sides of the leads  110  and  120  and electrically connects one terminal of the capacitor to the first lead  110  and the other capacitor terminal to the second lead  120 . 
       FIG. 2C  is a cross-section side view of a portion of the sub-assembly  200  of  FIGS. 2A and 2B .  FIG. 2C  shows the electronic component  210  mounted onto the top sides of the first and second leads  110  and  120  using the adhesive  220 .  FIG. 2C  also shows the  FIG. 2C  also shows the following six different portions  220 ( a )- 220 ( f ) of the adhesive  220 . A first portion  220 ( a ) located between the bottom side of the electronic component  210  and the top side of the first lead  110 . A second portion  220 ( b ) located between the bottom side of the electronic component  210  and the top side of the second lead  120 . A third portion  220 ( c ) that spans between the bottom side of the electronic component  210  and a first lateral side  114  of the first lead  110  that is substantially orthogonal to the bottom side of the electronic component  210 . A fourth portion  220 ( d ) that spans between the bottom side of the electronic component  210  and a first lateral side  124  of the second lead  120  that is substantially orthogonal to the bottom side of the electronic component  210 . A fifth portion  220 ( e ) that spans between a first lateral side  212  of the electronic component  210  and the top side of the first lead  110 , which is substantially orthogonal to the first lateral side  212 ; and a sixth portion  220 ( f ) that spans between a second lateral side  214  of the electronic component  210  and the top side of the second lead  120 , which is substantially orthogonal to the second lateral side  214 . The fifth and sixth adhesive portions  220 ( e ) and  220 ( f ) are optional, and therefore one or both might not exist in some embodiments of the invention. 
     The third and fourth adhesive portions  220 ( c ) and  220 ( d ) strengthen the mechanical bonding between the electronic component  210  and the leads  110  and  120  beyond the mechanical bonding that would be provided by only the first and second adhesive portions  220 ( a ) and  220 ( b ). The third and fourth adhesive portions  220 ( c ) and  220 ( d ) are visible in the bottom plan view of  FIG. 2B , that is, visible from below the sub-assembly  200 . This visibility enables inspection of the sub-assembly  200  to verify that the third and fourth adhesive portions  220 ( c ) and  220 ( d ) are properly configured. 
     The fifth and sixth adhesive portions  220 ( e ) and  220 ( f ) further strengthen the mechanical bonding between the electronic component  210  and the leads  110  and  120 . Note that the fifth and sixth adhesive portions  220 ( e ) and  220 ( f ) are visible from above the sub-assembly  200 . This visibility enables inspection of the sub-assembly  200  to verify that the fifth and sixth adhesive portions  220 ( e ) and  220 ( f ) are properly configured. 
     In an implementation in which the electronic component  210  is a capacitor having opposing terminals respectively mounted onto the leads  110  and  120  using an electrically conductive adhesive  220 , it is important that the third and fourth adhesive portions  220 ( c ) and  220 ( d ) are mechanically and electrically isolated from one another to prevent electrical shorting between the capacitor terminals. The curved shapes of the recesses  112  and  122  on the lateral sides of the leads  110  and  120  shown, for example, in  FIGS. 1A and 1B , help to prevent such electrical shorting by providing additional volume for receiving the adhesive  220  that limits the spread of the adhesive  220  on the bottom side of the capacitor  210  to prevent direct contact between the third and fourth adhesive portions  220 ( c ) and  220 ( d ), while still promoting the additional mechanical bonding to the leads  110  and  120  provided by the third and fourth adhesive portions  220 ( c ) and  220 ( d ). 
     In one embodiment, the process of assembling the sub-assembly  200  of  FIGS. 2A-2C  begins by applying a desired amount of the uncured adhesive  220  in a viscous liquid or gel form onto the top side of each of the first and second leads  110  and  120  of the lead frame  100  of  FIGS. 1A and 1B .  FIG. 3  is a top plan view of the portion of the lead frame  100  after the adhesive  220  has been applied to the top sides of the leads  110  and  120 . 
     The next step is to place the electronic device  210  onto the adhesive-covered portions of the first and second leads  110  and  120  and apply a sufficient amount of pressure, during which the uncured adhesive  220  flows to form all six portions  220 ( a )- 220 ( f ) of the adhesive  220  shown in  FIG. 2C . Note that each of the portions  220 ( c )- 220 ( f ) corresponds to adhesive meniscus. Depending on the implementation, the adhesive  220  is then cured with the application of heat, radiation, and/or time, to form the sub-assembly  200  of  FIGS. 2A-2C  with the electronic device  210  securely mounted onto and, if appropriate, electrically connected to the first and second leads  110  and  120 . 
       FIGS. 4-10  are top plan views of lead frames  400 - 1000  having leads  410 - 1010  and  420 - 1020 , respectively, according to alternative embodiments of the invention. The leads  410 - 1010  and  420 - 1020  define differently shaped recesses  412 - 1012  and  422 - 1022  that can be employed to achieve the similar results of good mechanical bonding and short-circuit prevention achieved by the recesses  112  and  122  of the leads  110  and  120  of  FIGS. 1A-2C . In general, leads of the invention may have recesses with any suitable shapes. 
       FIG. 11  is a perspective view of an example packaged electronic device  1100 , according to one embodiment of the invention. Those skilled in the art will understand that the invention can be implemented in a wide variety of different types of packaged electronic devices. 
     Those skilled in the art will understand that the lead frames of the invention may be made of any suitable materials, including, but not limited to, copper. 
     As used in this specification, the term “adhesive” covers epoxies, glues, and other types of adhesives that are applied in a viscous liquid or gel form and then cured. In addition, the term “adhesive” also covers solders and other connecting materials that are re-flowed by the application of heat to form the different adhesive portions  220 ( a )- 220 ( f ) of  FIG. 2C . 
     Unless explicitly stated otherwise, each numerical value and range should be interpreted as being approximate as if the word “about” or “approximately” preceded the value or range. 
     It will be further understood that various changes in the details, materials, and arrangements of the parts which have been described and illustrated in order to explain embodiments of this invention may be made by those skilled in the art without departing from embodiments of the invention encompassed by the following claims. 
     A lead frame is a collection of metal leads and possibly other elements (e.g., power bars, die paddles also known as die pads and die flags) that is used in electronic packaging for assembling one or more integrated circuit (IC) dies into a single packaged electronic device. Prior to assembly into a packaged device, a lead frame may have support structures (e.g., a rectangular metal frame and tie bars) that keep those elements in place. During the assembly process, the support structures may be removed. As used herein, the term “lead frame” may be used to refer to the collection of elements before assembly or after assembly, regardless of the presence or absence of those support structures. 
     In this specification including any claims, the term “each” may be used to refer to one or more specified characteristics of a plurality of previously recited elements or steps. When used with the open-ended term “comprising,” the recitation of the term “each” does not exclude additional, unrecited elements or steps. Thus, it will be understood that an apparatus may have additional, unrecited elements and a method may have additional, unrecited steps, where the additional, unrecited elements or steps do not have the one or more specified characteristics. 
     It should be understood that the steps of the exemplary methods set forth herein are not necessarily required to be performed in the order described, and the order of the steps of such methods should be understood to be merely exemplary. Likewise, additional steps may be included in such methods, and certain steps may be omitted or combined, in methods consistent with various embodiments of the invention. 
     Although the elements in the following method claims, if any, are recited in a particular sequence with corresponding labeling, unless the claim recitations otherwise imply a particular sequence for implementing some or all of those elements, those elements are not necessarily intended to be limited to being implemented in that particular sequence. 
     Reference herein to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment can be included in at least one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments necessarily mutually exclusive of other embodiments. The same applies to the term “implementation.”