Patent Publication Number: US-9892936-B2

Title: Packaged semiconductor device having leadframe features preventing delamination

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This divisional application claims priority to and benefit of U.S. patent application Ser. No. 14/592,011, flied on Jan. 8, 2015, the entirety of which is incorporated herein by reference. 
    
    
     FIELD 
     Embodiments of the invention are related in general to the field of semiconductor devices and processes, and more specifically to the structure and fabrication method of leadframes with mechanical and metallurgical features for preventing delamination of packaged semiconductor devices. 
     DESCRIPTION OF RELATED ART 
     Moisture-induced failures of plastic packaged semiconductor devices have been observed and investigated for many years. It is well known that plastic packages made, for instance, by epoxy-based molding compounds can be penetrated by discrete water molecules within a time period of about one day. However, this penetration does not lead to a problematic situation as long as there is good adhesion inside the package between the plastic compound and the device components (semiconductor chip, metallic leadframe, etc.), and the penetrated water molecules cannot accumulate to form films of water on free surfaces. 
     In contrast, when some interfacial delamination has happened and water films have been able to form, quick rises of temperature may vaporize the water and initiate expansive internal pressures between the components and the package material. The expansive pressure may be high enough to bulge the plastic material at thin spots and eventually cause a crack of the package. As an example, the temperature may rise quickly beyond the water boiling point when the packaged device is heated in order to reflow the device solder balls for attaching the device to a board. In the literature, the phenomenon of local package cracking by steam pressure has been dubbed popcorn effect. 
     A variety of methods have been tried to enhance adhesion between device components and prevent delamination and cracking. Among the efforts have been chemically purifying the molding compounds; activating metal surfaces, for instance by plasma, just prior to the molding process; enhancing the affinity of leadframe metals to polymeric compounds by oxidizing the base metal or by depositing special metal layers (such as rough tin); coining the leadframes for creating dimples and other three-dimensional surface features and roughness for interlocking the plastic with the metal surfaces. However, the success of all these efforts has only been partial and limited. 
     SUMMARY 
     The polymerization process, or molecular cross linking, of thermoset compounds causes a volumetric shrinking of the polymeric material. As a consequence, the polymerized material exerts an inward-directed pressure on bodies enclosed by the thermoset compounds. In plastic encapsulated semiconductor products, this effect causes an inward directed force on the enclosed pad with the assembled semiconductor chip. 
     Applicants discovered that the volumetric shrinkage of packaging compound and the correlated inward-directed pressure can be exploited to clamp leadframe parts, such as the assembly pad, between portions of the package positioned on opposite sides of the pad. This clamping pressure can thus be utilized to counteract and compensate the expanding pressure of vaporized water. 
     For plastic encapsulated semiconductor devices with metallic leadframes, applicants solved the problem of package delamination and cracking when they applied the concept of clamping pressure by volumetrically shrinking compounds to the large assembly pad of the package. By forming the pad so that a plurality of opening through-holes and through-windows can be distributed along the perimeter of the pad, the molding compound filling these holes and windows can interconnect package portions on the top and the bottom sides of the pad and thus clamp the portions strongly against the pad. Experience has shown that this clamp force is stronger than the force of expanding water vapor. 
     It is advantageous to supplement the concept of through-holes and through-windows with additional leadframe features such as ditches, grooves, and indents, which provide locations for anchoring packaging compound the contours of leadframes. In addition, as much of the leadframe base metal surface as possible should be made available for adhesion to packaging compound. It is, therefore, advantageous to restrict any areas of additional metal layers to local spots needed for attaching the wire stitch bonds. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a perspective view of a leadframe strip according to the invention, the strip having a plurality of device sites. 
         FIG. 2  depicts a perspective top view of a portion of a discrete device site of the leadframe strip in  FIG. 1 , detailing embodiments of the invention. 
         FIG. 3  illustrates a perspective bottom view of the discrete device site of  FIG. 2 . 
         FIG. 4  shows a perspective top view of a discrete device site of the leadframe strip of  FIG. 1 , with a semiconductor chip assembled on the pad and wire bonded. The outline of the package is depicted in dashed lines. 
         FIG. 5  illustrates a perspective view of the leadframe strip of  FIG. 1  after the process of encapsulating the assembled devices in packages and plating the un-encapsulated strip portions with a metal layer. 
         FIG. 6  depicts a perspective view of a packaged and trimmed device with formed leads. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An exemplary embodiment of the invention is a leadframe as illustrated in  FIG. 1 . The leadframe is based on a strip  100  stamped or etched from a flat sheet of metal selected from a group including copper, copper alloys, aluminum, iron-nickel alloys, and Kovar™. When the metal sheet is made of copper, the preferred thickness of the sheet is between 100 and 300 μm. For some applications, the sheet may be thicker or thinner. As  FIG. 1  shows, leadframe strip  100  includes a plurality of device sites  101 . Each site includes the components proper for assembling and supporting a semiconductor chip, such as assembly pad  110  and leads  120 , as well as rails and connection strips needed for integrating the units into a strip suitable for batch processing. 
       FIGS. 2 and 3  display a portion of a discrete unit  200  of exemplary leadframe  100  (portion as marked in  FIG. 1 );  FIG. 2  illustrates the top surface (assembly surface) of the unit, and  FIG. 3  depicts the opposite bottom surface of the unit it should be pointed out that herein the top surface is referred to as first surface  201   a , and the bottom surface as second surface  201   b . As  FIG. 2  shows, unit  200  includes a pad  210  for assembling a semiconductor chip. Pad  210  has in first order a rectangular outline with four sides. In other devices, the assembly pad may have the shape of a square or a circle, or any other suitable geometrical configuration. Pad  210  exhibits the base metal of the leadframe (for example, copper) in naturally oxidized mode, free of plated metal layers. For copper leadframes, oxidized copper is characterized by its affinity to adhesion to polymeric encapsulation compounds such as epoxy-based molding compounds. 
     As  FIG. 2  indicates that two opposing sides of pad  210  include a plurality of through-holes or via-holes  220  from the first pad surface  201   a  to the second pad surface  201   b . Through-holes  210  may have square cross section, as in  FIG. 2 , or may have round or any other cross section. There may be two holes on each side, as shown in the example of  FIG. 2 , or more holes. During the encapsulation process, through-holes  210  will be filed with packaging compound, such a molding compound; after hardening by polymerization, the filled vies enable strong mechanical locking of the compound portions on the top leadframe side with the compound portions on the bottom leadframe side. Due to the volumetric shrinkage of polymeric compounds, the leadframe is clamped between the compound portions on top and on the bottom of leadframes. Strong clamps inhibit delamination of compound and leadframe. 
     The exemplary device of  FIG. 2  illustrates another pad side, which has an elongated window or hole  221  between the first and the second pad surfaces. There may be one or more elongated windows, and they may have a smooth or a wavy configuration. The elongated windows  221  serve the same purpose as the through-holes  220 . After filling the openings with encapsulation compound and volumetric shrinking and hardening the compound during the polymerization process, the leadframe is clamped between the compound portions and thus inhibited from delaminating from the leadframe. In other devices, windows and through-holes may interchange positions, shape, and numbers. 
     Other contributions to anchoring polymeric packaging compounds into the metallic leadframe are achieved by a plurality of grooves and ditches into the surfaces of the pad, and by indents into the sidewalls of leads. Grooves and ditches can be produced by coining the leadframe metal or by partial etching into the leadframe thickness, so-called half-etching.  FIG. 2  shows an example of ditches  222  cut into the top surface  201   a  of pad  210 . In this example, the ditches are parallel to the pad sides; in other devices, the ditches may have other orientations. Viewing the bottom surface  201   b  of the leadframe in  FIG. 3 , pad  210  has a plurality of grooves  223  cut into the bottom surface. They are sized and distributed to offer superior mechanical anchoring for the packaging compound into the leadframe pad. In addition, pad  230  exposes the (slightly oxidized) base metal surface free of plated layers of additional metals. 
       FIGS. 2 and 3  illustrate a plurality of leads  230  of leadframe  230 , which have opposite elongated sides. As the figures indicate, these elongated and opposite sides are castellated by indents  231  into the base metal of the leadframe. These indents, therefore, allow the molding compound to grip the leads for mechanical support and thus counteract pressures of delamination and separation. 
     In  FIG. 2 , areas  240  of leadframe first surface  201   a  indicate spots, which have received additional plating by a metal facilitating wire stitch bonding. The preferred choice for the additional metal is silver. However, as  FIG. 2  shows, these areas are minimized in order to retain as much of the oxidized leadframe base metal as possible for adhesion to the encapsulation compound. Consequently, these areas surrounding spots intended for stitch bonding are selective and locally restricted. 
     Another embodiment of the invention is a packaged semiconductor device, which uses a metallic leadframe with features described above. An example of a device with low lead count is illustrated in  FIG. 4 . The leadframe has a first and a second surface, and an assembly pad  410  bordered by two opposing sides; the first surface is the surface for chip assembly, the opposite surface is the second surface. Each pad side includes a plurality of through-holes  420  from the first to the second pad surface. Pad  410  is further bordered by another side, which includes one or more elongated windows  421  between the pad surfaces. In addition, the first surface of pad  410  has a plurality of ditches  422 , which extend certain lengths across the first surface. The opposite second pad surface includes a plurality of grooves. The leadframe further has a plurality of leads  430  with opposite elongated sides; the sides are castellated by indents  431 . The leads have layers  440  of bondable metals deposited on the leadframe base metal, but layers  440  are restricted to localized areas surrounding bond spots. 
     In  FIG. 4 , a semiconductor chip  450  is attached to pad  410 . The terminals of chip  450  are connected by bonding wires  460  to the bondable localized spots  440 . Chip  450 , wires  460 , pad  410 , and portions of the leads  430  are encapsulated by a packaging material  470 , such as a polymerizable epoxy-based molding compound. During the process of applying the packaging compound—for instance by a transfer molding technique—, through-holes  420 , windows  421 , indents  431 , ditches  422 , and the grooves on the second surface of pad  410  are filled with packaging material. After curing and polymerizing the material, the leadframe is securely anchored in the packaging material. 
     Another embodiment of the invention is a method for fabricating semiconductor devices with leadframes having safeguards against delamination of leadframe and encapsulation material, and against package cracking. Certain steps of the method are depicted in  FIGS. 1 through 6 . The method starts by providing a leadframe strip as depicted in  FIG. 1  by exemplary strip  100 , which has a first and a second surface and a plurality of device sites  101 . Each site includes an assembly pad  110  bordered by two opposing sides; each side has a plurality of through-holes  220  from the first to the second pad surface. Pad  110  is further bordered by another side, which includes one or more elongated windows  221  between the pad surfaces. In addition, the first surface of pad  210  has a plurality of ditches  222 , which extend certain lengths across the first surface. The opposite second pad surface includes a plurality of grooves. The leadframe further has a plurality of leads with opposite elongated sides; the sides are castellated by indents  231 . The leads, and in some devices portions of the pad, have layers  240  of bondable metals deposited on the leadframe base metal, but layers  240  are restricted to localized areas surrounding bond spots in order to retain as much surface of oxidized base metal as possible for adhesion toe packaging compound. 
     In the next process, indicated in  FIG. 4 , a semiconductor chip  450  is attached to the pad  410  of each leadframe site. Thereafter, the terminals of each chip are connected by bonding wires to the bondable spots of respective leads. In the next process, indicated in  FIGS. 4 and 5 , a plurality of packages is formed by encapsulating the chip, wires, pad, and certain lead portions of each site in a packaging material. The preferred process is a transfer molding technique, which uses an epoxy-based polymeric compound filed with inorganic filler particles. During the transfer molding process, the packaging compound fills through-holes  420 , windows  421 , ditches  422 , and grooves on the second pad surface, and interlocks with indents  431 . After polymerizing and hardening the compound, the leadframe of each site is securely anchored into the package. Since the polymerization process involves a volumetric shrinkage of the packaging compound, the leadframe is actually clamped by the polymerized packaging compound, wherein the clamping force counteracts the expansion force of steaming water and thus prevents delamination of packaging compound and leadframe. 
       FIG. 5  shows a leadframe strip  500  after encapsulating the plurality of assembled chips of device sites  501 . The packaged devices of the strip are subjected to a heat treatment at about 175° C. for several hours in order to polymerize the plastic compound by cross linking the polymeric molecules. The polymerization process of thermoset compounds causes a volumetric shrinking of the polymeric material. As a consequence, the polymerized material exerts an inward-directed pressure on leadframe portions and assembled chips enclosed by the thermoset compounds, effectively camping the polymerized compound against these parts. The clamping pressure can counteracts and at least partially compensates the expanding pressure of a vaporized water film formed during prolonged storage of the packaged device. 
       FIG. 5  further indicates that it is economical for certain devices to plate the un-encapsulated portions of metal strip  500  with layers of solderable metal such as tin or a tin alloy. 
     Due to the interlocking of leadframe portions with indents and recesses with the packaging compound, cracking of the hardened (polymerized) compound during the mechanically stressful trim-and-form processes can be avoided.  FIG. 6  shows a singulated discrete unit  600  of the plurality of devices, resulting from trimming the leadframe strip of  FIG. 5 . Finally, the un-encapsulated lead portions  680  are formed. With the plating of solderable metal at the process mentioned in  FIG. 5 , the formed leads are ready for attachment to boards. 
     The concept of exploiting the clamping pressure of volumetrically shrinking compounds of a plastic-packaged device to a large pad with its assembled chip has been applied to the construction of the pad so that a plurality of opening through-holes and through-windows are distributed along the perimeter of the pad; the molding compound filing these holes and windows interconnect package portions on the top and the bottom sides of the pad and thus clamp the package portions strongly against the pad. Experience has shown that this clamp force can be stronger than the force of expanding water vapor. Any still missing force difference to prevent delamination and cracking can be provided a combination of features for anchoring and tightly locking the packaging compound with leadframe parts. Examples are indents on opposite sides of leads; ditches in the pad close to the assembled chip; and grooves in the pad surface opposite the assembled chip. In addition, as much of the leadframe base metal surface as possible can be made available for adhesion to the packaging compound. This can be achieved by restricting any areas of additional metal layers to local spots needed for attaching the wire stitch bonds. 
     While this invention has been described in reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. As an example, the invention applies not only to active semiconductor devices with low and high pin counts, such as transistors and integrated circuits, but also to combinations of active and passive components on a leadframe pad. 
     As another example, the invention applies not only to silicon-based semiconductor devices, but also to devices using gallium arsenide, gallium nitride, silicon germanium, and any other semiconductor material employed in industry. 
     As another example, the invention applies to leadframe pads, where the pad extends on one side into more than one lead. As yet another example, the invention apples to pads which are offset from the plane of the leadframe. 
     It is therefore intended that the appended claims encompass any such modifications or embodiments.