Patent Publication Number: US-11658098-B2

Title: Leadframe package with side solder ball contact and method of manufacturing

Description:
BACKGROUND 
     Technical Field 
     The present disclosure is directed to a leadframe package having a side solder ball contact in order to improve solder wettability of the lead sidewall without additional post manufacturing plating. 
     Description of the Related Art 
     A typical leadframe package includes a die having its backside coupled to a leadframe and its active side coupled to various electrical contacts. An encapsulant is then used to cover the combined die and leadframe to create the leadframe package. The resulting combination can be connected to a circuit board, such as a printed circuit board (PCB), with solder using surface mount technology (SMT). 
     Although SMT allows for smaller packages, it also creates some disadvantages. In particular, the solder joints between the package and the PCB can be weakened due to the PCB and the package having different coefficients of thermal expansions (CTE). Thus, the reliability of the package may, in some cases, depend on the integrity of the solder joints. But, most surface mount leadframe packages only have solder on the bottom of the package and do not have solder wettable material to form connections between the package and the circuit board. In such cases, the solder joints are weakened or cannot be formed because there is no adhesion between the solder and the sidewall of the package. This results in less contact area, a weaker bond and increased resistance with the net outcome being a less reliable package. As packages reduce in size, the available space for solder joints is further limited. Thus, strong solder bonds between the package and the PCB are desired. 
     Past responses to this issue have been to add a plating layer on the side of the leadframe package after manufacturing to provide for sidewall solder contact. However, plating after manufacturing requires expensive equipment and results in a less efficient manufacturing process. Further, these post plating techniques do not guarantee adequate coverage of the lead sidewall and also do not allow the resulting package-substrate combination to be inspected by automated solder inspection. Without the assistance of automated solder inspection, the solder joints cannot be properly inspected, which increases the likelihood that products will leave the manufacturing facility with problems that will manifest in a lower cycle life for the product. 
     BRIEF SUMMARY 
     Embodiments of the present disclosure are directed to leadframe packages with a side solder ball contact and methods of manufacturing the same. In one embodiment, the package has a solder ball exposed on a sidewall of the package that extends from the leadframe and into the encapsulant. When the package is attached to a substrate, solder flows between the leadframe and the substrate and forms a connection with the solder ball to create an integral solder joint that covers a sidewall of each lead. This enables strong solder joints between the leads of the package and the substrate. This also increases the solder contact area between the package and the substrate, which results in less resistance and a lower thermal load per solder pin. This reduced thermal load allows for a reduction in the number of pins required to handle the power supply load. If fewer pins are required to carry the positive and negative power supplies, then the chip can be made with fewer pins, saving money. It also allows for an increase in the number of signal and data pins in the resulting device, if needed. 
     Other embodiments are directed to methods of manufacturing a leadframe package with these characteristics. In an embodiment, a copper leadframe is plated on both sides before portions of the plate are removed and a plurality of recesses are formed on one side of the copper leadframe. Then, solder balls are attached to selected ones of these recesses, the die is coupled to the leadframe and wires are coupled between the die and the leadframe. An encapsulant is placed to cover the die, the wires, and the leadframe. After placing the encapsulant, the package is separated by cutting through the solder balls to produce a package with a portion of a solder ball exposed on the sidewall of the package. In other alternative embodiments of the process, the cutting process may cause a portion of the solder ball to spread onto a sidewall of the leadframe. Yet another embodiment of the process involves flowing the solder ball during the original placement in order to form a bond between the metal plating layer on the leadframe and the solder ball before continuing manufacturing. An additional alternative embodiment includes plating the leadframe after forming the plurality of recesses so that the solder balls can form a stronger connection with the metal layer. A final alternative embodiment describes plating a sidewall of the leadframe after singulation. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       In the drawings, identical reference numbers identify similar elements or acts unless the context indicates otherwise. The sizes and relative positions of elements in the drawings are not necessarily drawn to scale. 
         FIG.  1    is a cross-sectional view of an exemplary embodiment of a leadframe package having a side solder ball contact; 
         FIGS.  2 A- 2 I  are cross-sectional views of various stages of an assembly process of leadframe packages, such as the package of  FIG.  1   , in accordance with an embodiment of the present disclosure; 
         FIG.  3    is an alternative embodiment of a leadframe package with a portion of the side solder ball contact smeared onto a sidewall of the leadframe; 
         FIGS.  4 A and  4 B  are cross-sectional views of an alternative embodiment of various stages of a leadframe package assembly process where the solder ball is flowed over a portion of a metal plating layer on the leadframe during assembly; 
         FIGS.  5 A- 5 D  are cross-sectional views of an alternative embodiment of various stages of a leadframe package assembly process where a plurality of recesses are plated with a metal plating layer before a solder ball is attached to select recesses; and 
         FIG.  6    is an alternative exemplary embodiment of a leadframe package having a metal plating layer on a sidewall of each of a plurality of leads in the package. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, certain specific details are set forth in order to provide a thorough understanding of various embodiments of the disclosure. However, one skilled in the art will understand that the disclosure may be practiced without these specific details. In other instances, well-known structures associated with electronic components and fabrication techniques have not been described in detail to avoid unnecessarily obscuring the descriptions of the embodiments of the present disclosure. 
     Unless the context requires otherwise, throughout the specification and claims that follow, the word “comprise” and variations thereof, such as “comprises” and “comprising,” are to be construed in an open, inclusive sense, that is, as “including, but not limited to.” 
     The use of ordinals such as first, second and third does not necessarily imply a ranked sense of order, but rather may only distinguish between multiple instances of an act or structure. 
     Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. 
     As used in this specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the content clearly dictates otherwise. It should also be noted that the term “or” is generally employed in its sense including “and/or” unless the content clearly dictates otherwise. 
     The present disclosure is generally directed to providing a package with a side solder ball contact. An exemplary embodiment of a package  20  with a side solder ball contact is shown in  FIG.  1   . In this embodiment, a die  22  is coupled to a metal plating layer  23  positioned on a first surface of a die pad  24 . The die pad has a first surface  64  and a second surface  66 , which are covered with the metal plating layer  23 . A plurality of leads  26  are spaced apart from the die pad  24 . Each of the plurality of leads  26  has a first side  28 , a second side  30  and a sidewall  62 . Each sidewall  62  has a concave region  32  on an outermost surface of each of the plurality of leads  26 . The first side  28  and the second side  30  are covered by the metal plating layer  23 . In other embodiments, as will be described below, the metal plating layer  23  is also positioned on the concave region  32  and on the sidewall  62  of each of the plurality of leads  26 . A plurality of wires  34  coupled between the die  22  and the leads  26  provide an electrical connection between the die  22  and the leads  26 . The concave region  32  on each of the leads  26  has a size and a shape for receiving one of the plurality of solder balls  36 . The die  22 , the wires  34  and the solder balls  36  are covered by an encapsulant  38  to form a leadframe package  20 . In this embodiment, the leadframe package  20  is coupled to a substrate  21  with solder  25 , although the package  20  can be manufactured and sold separately. 
     In the leadframe package  20 , the solder balls  36  extend outward from each of the plurality of leads and into the encapsulant  38 . In addition, the package  20  has a sidewall  27  with a first portion of the solder balls  36  covered by the encapsulant  38  and a second portion of the solder balls  36  exposed to an exterior environment on a first side and abutting both the encapsulant  38  and the concave region  32  of each of the plurality of leads  26  on a second side. This allows the plurality of solder balls  36  to act as a preliminary solder material to ensure maximum solder coverage of the sidewall  62  of each of the plurality of leads  26  when the package  20  is mounted to the substrate  21 . Upon combining the package  20  with the substrate  21 , the solder used for the coupling flows and combines with the plurality of solder balls  36  to form a plurality of integral solder portions  54  that cover the entire sidewall  62  of each of the plurality of leads  26 . This coverage ensures a stronger solder joint, which helps resists the effects of differing CTE between elements in the system, thus increasing the expected cycle life of the finished package  20  over known packages. In addition, the presence of solder on the exterior of the package  20  allows the package  20  to be inspected with automated solder inspection, which enables the manufacturer to detect defects in the soldering process that could ultimately lead to lower cycle life. 
     An exemplary embodiment of a method of manufacturing the package  20  is shown in  FIG.  2 A . In this embodiment, a leadframe  29  is plated on a first side  31  and a second side  33  with the metal plating layer  23 . The leadframe  29  is typically comprised of copper or a copper alloy due its conductivity and cost advantages, although other metals may be used. The metal plating layer  23  typically consists of at least one of nickel, palladium or gold, although each of these alternatives may be used exclusive of the others in addition to other alternatives. The main reason for plating the leadframe  24  is that most solders typically form a weak mechanical bond with the copper of the leadframe. In other words, most solders used for packages form a weak mechanical bond with copper. However, nickel, palladium and gold have strong mechanical compatibility with both the solder and the copper. Therefore, the plating layer  23  acts as an intermediary between the copper and the solder in order to increase adhesion and provide for a strong bond between the solder, the plating layer  23  and the copper while maintaining desirable electrical properties. However, using different solder or leadframe materials may reduce or eliminate the need for plating. 
     Once the plating is complete, portions  44  of the plating layer  23  are removed on both surfaces of the leadframe  29  as shown in  FIG.  2 B . These portions  44  can be removed with either a masking process followed by wet or dry etch, a laser, or a mechanical blade. There are many known ways to pattern a plating layer on a leadframe. In the spaces left by removing the portions  44 , a first plurality of recesses  46  and a second plurality of recesses  48  are formed in a body of the leadframe  29  on the first surface  31 , as in  FIG.  2 C . In this embodiment, forming the first plurality of recesses  46  and the second plurality of recesses  48  is done by wet etching the leadframe  29  with an etch that is selective to remove copper and not the plating layer, although other cutting, etching or forming techniques may be used in other embodiments. The first plurality of recesses  46  have a size and a shape configured to receive a solder ball. 
     Once the recesses are formed, the plurality of solder balls  36  are coupled to each of the first plurality of recesses  44  as in  FIG.  2 D . In this embodiment, the solder balls  36  form an intermetallic connection with a peripheral edge  35  of each of the first plurality of recesses  44  and are placed before encapsulation and singulation. Once the solder balls  36  are in place, a die  22  is coupled to the metal layer  23  on the first surface  31  of the leadframe  29 , as in  FIG.  2 E . The plurality of wires  34  create an electrical connection between the die  22  and the leadframe  29  and an adhesive layer  19  connects the backside of the die to the top of the leadframe  29 . With these elements in place, the encapsulant  38  is added over the die  22 , the plurality of solder balls  36 , the plurality of wires  34  and the leadframe  29 . The encapsulant  38  also fills the second plurality of recesses  48 . 
     The encapsulant  38  is applied to the system at a selected high temperature and appropriate pressure. For example, some molding compounds will flow and form around the various components between 160 and 180 degrees Celsius. Some solders and solder balls have a melting point below this temperature. However, recent developments have created solders with a melting point at or above 200 degrees Celsius and therefore the plurality of solder balls  36  will not melt and flow when the encapsulant  38  is applied if a high temperature solder is used. The encapsulant  38  can be an acceptable molding compound, polymer, epoxy or other acceptable encapsulant having the properties described herein relative to the solder. With the solder balls  36  present when the encapsulant  38  is applied, there is balancing and tradeoff in the selection of the two components, solder and encapsulant, to select materials that are compatible with each other in the same process. 
     In particular, an encapsulant  38  is selected that has an application temperature which is lower than the melting temperature of the fully formed solder balls  36  for the type of solder used. This can be accomplished by having either a high temperature melting point of the solder balls  36  or a low temperature melting point of the encapsulant  38  that creates encapsulant  38 . Thus, one option is to use a low temperature encapsulant  38  that will flow at a temperature lower than the melting temperature of standard solder balls  36 . In addition, since the encapsulant  38  flows differently with different pressures and temperatures, and may be used in a transfer mold, a compression mold, or other type of mold, the encapsulant  38  can be selected with the properties of the solder ball  36  already known to ensure that, based on the type of molding process used, the solder balls  36  remain in the same location and remain relatively stable when subjected to the various temperatures and pressures involved in the encapsulation process. It is acceptable if the solder is brought toward the melting temperature and, perhaps, partially reflows; however, the temperature and pressure for the encapsulation process should be selected to ensure that the solder balls  36  remain substantially in the same place and shape become even more tightly bonded o the leadframe  29 . 
     One benefit of forming the encapsulant  38  after the solder ball is present is that the additional temperature and pressure heating which the solder ball  36  undergoes will tend to more solidly attach the solder ball  36  to the leadframe  29 , as well as to the plating layer  23 , while at the same time solidly embedding the solder ball  36  into the encapsulant  38 . In most selections of the materials for the encapsulant  38  and the solder balls  36 , a combination will be selected that will cause the solder balls  36  to mechanically bond with, and be rigidly attached to, the encapsulant  38 . 
     After encapsulating, remaining portions  50  of the leadframe  29  opposite the plurality of solder balls  36  and the second plurality of recesses  48  are removed as shown in  FIG.  2 G . Removing these portions  50  isolates the die pad  24  from the plurality of leads  26  so that an improper electrical connection does not short the leads to each other. The removal of the backside of leadframe  29  can be carried out by any acceptable technique, such as etching with a chemical etch that is selective to etch the leadframe material and not the plating layer  23 , by laser cutting, a mechanical blade or other acceptable technique. 
     As shown in  FIG.  2 H , a mechanical blade or other dicing technique is used to separate the resulting package  20  at the locations  52 . As shown, the locations  52  correspond to approximately a midpoint of the plurality of solder balls  36 , although it is possible to make the cut such that a different proportion of the plurality of solder balls  36  remains. In addition, removing the portions  50  and separating the package  20  from the remaining material at locations  52  after the solder balls  36  are coupled to the leadframe  29  ensures that the solder balls  36  occupy the concave region  32  of each of the plurality of leads  26 . In this embodiment, the concave regions  32  are a top side recess filled with a solder ball  36 . The solder ball  36  has a portion remaining that allows the solder to flow down and cover the entire sidewall  62  of each of the plurality of leads  26  during reflow when the package  20  is attached to the substrate  21 . This helps to ensure complete solder coverage of the sidewall  62  and results in a stronger solder joint. 
     Next, as shown in  FIG.  2 I , the package  20  is coupled to the substrate  21  with solder  25 . When the solder  25  flows between the plurality of leads  26  and the substrate  21 , the solder  25  joins with the plurality of solder balls  36  to form the plurality of integral solder portions  54 . The integral solder portions  54  extend beyond the sidewall  62  and the concave region  32  of each of the plurality of leads  26 , which creates additional strength against the effects of different CTEs between the materials in the package  20  compared to packages without solder wettable sides. 
     If the package is attached to a substrate where the solder must be heated to about 200 degrees Celsius, then there might be a concern that the encapsulant  38 , which is of a thermoset type in this embodiment, would degrade or decompose. However, once the encapsulant  38  is cured by heat and pressure during the encapsulation process, then the thermoset encapsulant  38 , does not degrade and cannot be reshaped by melting and reprocessing at standard solder melting temperatures. Instead, if the encapsulant  38  is a thermoset type, it does not experience any significant degrading at temperatures below 400 degrees Celsius. Thus, only if the temperature is raised above 400 degrees Celsius after curing does degradation or decomposition of the encapsulant  38  become an issue of concern, which is well above any solder application or reflow temperatures. 
     In alternative embodiments, it is possible to use an encapsulant comprised of thermoplastic, which can be reshaped by melting and reprocessing after curing. In this case, the concern is that the encapsulant would liquefy during the solder application and reflow process. However, a thermoplastic material may be selected, based on its properties, to be compatible with the solder as mentioned above. In other words, curing the thermoplastic encapsulant can raise the melting point of the encapsulant to above 300 degrees Celsius, which is above that of the melting point of the solder to be used. 
     Thus, soldering the package to the substrate will not raise the temperature of the encapsulant to a point where it will begin to melt. Having the solder balls partially embedded in the encapsulant can therefore be safely used with both a thermoset and a thermoplastic encapsulant. 
     The solder balls  36  and the encapsulant  38 , whether of a thermoset or a thermoplastic type, can be used in combination in a final package, while, of course, there will be time and temperature monitoring in the final soldering of the package to the substrate  21 . Namely, the encapsulant  38 , after it is cured, whether by UV curing, additional heat after the first mold flow, a hardener which becomes more rigid over time as it cures, or other curing technique, will remain rigid at extremely high temperatures. For example, an encapsulant  38  of a thermoset type is selected which, during the encapsulation process may flow at a temperature range between 140 to 180 degrees Celsius and then, after full curing, will remain solid and and will not degrade or decompose even though the temperature may exceed 400 degrees Celsius. Such encapsulant  38 , which prior to being cured has good flow properties below 180 degrees Celsius, can after curing remain a solid at temperatures above 400 degrees Celsius, are well known in the art and commercial available from a number of suppliers. One of ordinary skill in the art can use such an encapsulant  38  having these properties with a solder having complementary properties as disclosed herein. 
     In an alternative embodiment shown in  FIG.  3   , separating the package with a mechanical blade or other dicing techniques creates enough force to smear or otherwise spread the solder ball  36 . This smearing or spreading of the solder ball  36  can be accomplished by a number of techniques. For example, the mechanical motion of a moving blade may cause pressure against the exposed portion of the solder ball  36 , as shown in  FIG.  2 H , that is above the concave portion and may press it somewhat into the concave region  62 . Thus, the mechanical pressure of a blade contacting the solder ball  36  will press some of the solder ball material, some from the upper portion of the solder ball, to be depressed into and fill some of the concave region  62  in the leadframe  29 . This causes a portion  56  of the first solder ball to press down into this region and create a stronger contact to the lead  26 . Alternatively, the singulation of the package, whether by cutting with a blade or laser cutting, may raise the temperature of the solder balls  36  at the cut interface to a point slightly melting the solder material that comprises the plurality of solder balls  36  so that a spreading or smearing occurs. In this case, the heat causes a portion  56  of a first solder ball to flow down, or “smear,” onto a lead portion  58  of a first sidewall  68  of the package  43 . Similarly, a portion  56  of a second solder ball is smeared onto the lead portion  58  of a second sidewall  70  of the package  43 . This result is advantageous because it decreases the separation distance between the plurality of solder balls  36  of the package  43  and a substrate. As such, solder that is added when coupling the package  43  to the substrate will not have to flow as far in order to form a connection with the plurality of solder balls  36 . This decrease in flow distance helps to ensure that any solder added when connecting the package  43  to the substrate will form a complete connection, or in other words, will flow completely to the plurality of solder balls  36  in order to form the plurality of integrated solder pieces described above. 
       FIG.  4 A  shows an alternative embodiment of a process for forming a leadframe package  45  with a side solder ball contact. In some cases, a stronger connection is desired between the plurality of solder balls  36  and the leadframe  29 . In these circumstances, the plurality of solder balls  36  can be reflowed after deposition beyond the peripheral edge  35  so that the solder balls  36  form an intermetallic connection with the metal plating layer  23  on the first surface  31  of the leadframe  29 . A larger amount of solder for the balls  36  can be added and then a solder reflow heat treatment step performed to cause the solder ball  36  to spread out into the layer  31 , as shown in  FIG.  4 A . Then, the process continues as described above with respect to  FIGS.  2 A-I . The end product, shown in  FIG.  4 B , has a plurality of integral solder portions  60  where a portion of the solder ball  36  extends beyond the peripheral edge  35  and contacts the metal plating layer  23  on each of the plurality of leads  26 . Increasing the contact area between the plurality of integral solder portions  60  and the metal layer  23  further increases the strength of the solder joints formed between the leadframe package  45  and the substrate  21 . In addition, forming the bond between the metal layer  23  and the plurality of solder balls  36  when the solder balls  36  are placed ensures that the solder balls  36  remain in their proper location during the various processing steps. 
     An alternative embodiment of a process for manufacturing a leadframe package with a side solder ball contact is shown in  FIG.  5 A . In this embodiment, the first plurality of recesses  46  and the second plurality of recesses  48  are formed in the body of the leadframe  29  from the first surface  31  before the plating layer  23  is added. Once the recesses are formed, the metal plating layer  23  is formed on the leadframe  29  as in  FIG.  5 B . Notably, the metal plating layer  23  covers the first plurality of recesses  46  and the second plurality of recesses  48 . This allows the plurality of solder balls  36  to form a strong intermetallic connection with the metal plating layer  23  when the solder balls  36  are coupled to the leadframe  29 , as in  FIG.  5 C . This increased bond reduces the likelihood that the different CTE between the various metals in the final package will result in separation, or delamination, of the solder balls  36  from the leadframe  29 . In addition, this provides additional mechanical strength to assist the solder balls  36  to remain in their intended positions during the rest of the processing steps. 
     After coupling the solder balls  36  to the metal layer  23 , the process continues as in other embodiments until a leadframe package  37  is formed, as in  FIG.  5 D . The package  37  has a portion of each solder ball  36  exposed on a first sidewall  74  and a second sidewall  72  of the package  37 . The leadframe package  37  differs from other embodiments because the metal layer  23  covers the concave region  32  of each of the plurality of leads  26 . As such, a strong bond forms between the plurality of solder balls  36  and the concave region  32 , which further increases the integrity of the solder joints that are formed between the plurality of leads  26  and the substrate  21 . However, this embodiment does not include having the metal layer  23  positioned on a sidewall  62  of each of the plurality of leads. In order to achieve such an arrangement, a different process may be utilized. 
     An alternative embodiment of a leadframe package  39  is shown in  FIG.  6   . In this embodiment, the metal layer  23  is formed on the sidewall  62  of each of the plurality of leads  26 . As such, the sidewall  62  and the concave region  32  of each of the plurality of leads  26  can form a strong bond with the solder  25  and the plurality of solder balls  36 . To create a package  39  with these characteristics, the manufacturing process begins similar to the process described with respect to  FIGS.  5 A-C . However, once the leadframe  29  is etched from the bottom side singulation, as in  FIG.  2 G , the remaining portion of the sidewall is plated with the metal layer  23 . Namely, after step  2 G is performed, the underside of the leadframe  29  has a plating layer  23  applied across the entire underside. This may serve to somewhat thicken the lower layer while at the same time placing a plating layer  23  on the concave portions of the leadframe  29  and the die pad  24 . This additional plating layer  23  can be formed by electroplating, simple blanket deposition, or other technique on the backside of the leadframe assembly  29  shown in  FIG.  2 G . 
     While this requires an additional manufacturing step, positioning the metal layer on the sidewall  62  of each of the leads  26  allows solder to flow onto the metal layer  23  covering the sidewall  62  when the leadframe package  39  is attached to the substrate  21 . This maximizes the contact area between a plurality of integral solder portions  54  and each of the plurality of leads  26 , which, in turn, maximizes the solder joint strength between the plurality of leads  26  and the substrate  21 . 
     As will be appreciated, only a single leadframe  29  is shown in  FIGS.  2 A- 2 H ; however, the leadframe  29  will typically be one in an array several hundred or, perhaps, several thousand leadframes that are connected in a single contiguous strip in a manner well known in the art. The die are placed at the appropriate place on the leadframe array in which many hundreds or thousands of die are present with the plating, wire bonding, soldering, applying the encapsulant  38 , and completing the encapsulation occurs on the large array with many thousands being encapsulated at the same time. After this process, shown in  FIG.  2 H , the individual packages are singulated to obtain the final package as shown in the respective figures, such as  FIGS.  2 I,  4 B,  5  and  6   . 
     The various embodiments described above can be combined to provide further embodiments. All of the U.S. patents, U.S. patent application publications, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in this specification and/or listed in the Application Data Sheet are incorporated herein by reference, in their entirety. Aspects of the embodiments can be modified, if necessary to employ concepts of the various patents, applications and publications to provide yet further embodiments. 
     These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.