Abstract:
A method of packaging a semiconductor component with a printed wiring board is disclosed. The method includes determining a first distance, applying a thin film onto a surface of the semiconductor component such that the thin film is spaced apart from a support of the semiconductor, applying a solder pad onto the printed wiring board, placing the semiconductor component with the thin film onto the printed wiring board, and positioning the thin film adjacent the solder pad.

Description:
RELATED APPLICATION 
   This is a Divisional of U.S. patent application Ser. No. 11/032,526 filed Jan. 10, 2005, which is incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   The invention relates to reliability enhancement processes, and more particularly, to a reliability enhancement process for packages having an integrated circuit (“IC”) mounted on a printed wiring board (“PWB”). 
   Moderate to high power original-equipment-manufacturer (“OEM”) perimeter pattern IC&#39;s such as perimeter pattern ball grid array (“BGA”) and thin small-outline packages (“TSOP”) generate a large amount of heat during operation. Typically these OEM IC&#39;s are mounted on a PWB at several solder joints. The large amount of heat typically leads to substantial thermal gradients or differences between the IC&#39;s substrate portion and the PWB. As a result, the PWB is also used as a primary heat sink. However, not only do high junction temperatures degrade the reliability of the package, but the differences in the thermal coefficient of expansion also reduce the life of the solder joints. For example, the difference in thermal expansion between the semiconductor component and the PWB is usually at least a factor of 2. That is, for every degree of temperature change, the PWB expands twice as much as the semiconductor component, which then creates mechanical stress. 
   SUMMARY OF THE INVENTION 
   In one form, the invention provides a method of packaging a semiconductor component with a printed wiring board. The semiconductor component has a first surface, and a support that extends from the first surface. The support also has a distal end. The method includes determining a first distance between the first surface and the distal end of the support, and applying a thin film onto the first surface of the semiconductor component. The thin film typically has a first thickness that is less than the first distance, and preferably has a plurality of thin film planar dimensions (length and width). The thin film also has a plurality of thermally conductive particles therein. Furthermore, the thin film is spaced apart from the support of the semiconductor. 
   Thereafter, the method includes applying a solder pad onto the printed wiring board. The solder pad has a solder pad size that matches the thin film&#39;s planar dimensions or size. The method then includes placing the semiconductor component with the thin film onto the printed wiring board, and creating a thermal path between the thermally conductive particles of the thin film and the solder pad, that is, from the semiconductor component to the printed wiring board. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
       FIG. 1  shows an original equipment manufacturer (“OEM”) perimeter pattern ball grid array (“BGA”); 
       FIG. 2  shows a sectional view of the BGA of  FIG. 1  mounted onto a printed wiring board (“PWB”); and 
       FIG. 3  shows a detailed sectional view of a portion of the BGA of  FIG. 2 . 
   

   DETAILED DESCRIPTION 
   Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. Unless limited otherwise, the terms “connected,” “coupled,” and “mounted”and variations thereof herein are used broadly and encompass direct and indirect connections, couplings, and mountings. In addition, the terms “connected” and “coupled” and variations thereof are not restricted to physical or mechanical connections or couplings. 
     FIG. 1  shows an original equipment manufacturer (“OEM”) perimeter pattern ball grid array (“BGA”)  100 . Although  FIG. 1  illustrates a BGA, other semiconductor components with outlying supports or contacts or electrical interconnects such as thin small-outline packages (“TSOP”) can also be used. The BGA  100  includes a plurality of supports such as solder balls  104  distributed along the perimeter  108  of a substrate  112 . The solder balls  104  can include metals such as tin, silver, gold, copper, nickel, tin Bismuth, tin lead, and the like. The solder balls are typically arranged directly on a surface  114  beneath the substrate  112 . As a result, any heat generated in the substrate  112  travels straight through the solder balls  104  into any connected components. For example, if the BGA  100  is mounted on a printed wiring board (“PWB”), the heat will be transferred to the PWB. In some embodiments, the BGA  100  can have between 100 and 150 solder balls  104  around the perimeter  108 . In such cases, the heat generated travels into the substrate  112 , into the center of the substrate  112 , then propagates to the perimeter  108  of the substrate  112 , and distributes through the solder balls  104  to any attachments. The substrate  112  can include any printed wiring board or materials such as ceramic, alumina, plastic, silicon, metallic elements (such as copper, Kovar®, aluminum silicon composites (“AlSiCp”), and carbon composites), and the like. 
   A portion  116  of the BGA  100  is generally covered with a thin film  120 . The thin film  120  is applied onto the surface  114 . As best seen in  FIGS. 2 and 3 , which show a sectional view of the BGA  100  and a detailed sectional view of a portion the BGA  100  mounted on a PWB  124 , respectively, each of the solder balls  104  has a diameter  128 . As a result, a gap distance  132  approximately equal to the diameter  128  exists between the surface  114  of the BGA  100  and the PWB  124 . The thin film  120  has a thin film thickness  136  that can be less than the distance  132 . In addition to the thin film thickness  136 , the thin film  120  also a plurality of planar dimensions such as length and width. The thin film  120  can include epoxy, glass, adhesive, and the like. The thin film  120  has also thermally conducting materials, or metallic particles  152  therein, which may include without limitation, gold, nickel, copper, tin, silver, and the like. Furthermore, the thin film  120  can be applied to the portion  116  of the BGA  100  by a variety of thin film deposition techniques. Exemplary techniques include spraying the thin film  120  such as a thin layer of epoxy on the surface  114  of the substrate  112 , brushing the thin film  120  or the thin layer of epoxy on the surface  114  of the substrate  112 , attaching the thin film  120  or the thin film of epoxy on the surface  114  of the substrate  112 , and the like. Furthermore, the thin film  120  is also positioned such that when the thin film  120  is applied, the thin film  120  is spaced apart from the solder balls  104 . In some embodiments, the thin film  120  has a random pattern of thermally conductive particles  152 . In some other embodiments, the thin film  120  can have some oriented pattern of thermally conductive particles  152  to ensure some thermal bonding or connection between the thermally conductive particles  152  and the solder pad  140 . 
   To dissipate the generated heat, a solder pad  140  is applied to the PWB  124 . The solder pad  140  has planar dimensions (length and width) that generally match with the planar dimensions of the thin film  120 . The solder pad  140  can have any typical solder metal alloys such as tin lead (“SnPb”), tin bismuth (“SnBi”), and the like. The solder pad  140  can also have a solder mask thereon to facilitate the soldering process, which is discussed hereinafter.  FIG. 3  also shows some material of the solder pad  140  aligned as a result from being heated. In some embodiments, some materials of the solder pad  140  are lined up with the thermally conductive particles  152 , while other particles may not develop any physical contacts. Furthermore, also shown in  FIG. 3 , a thermal path can be established by having direct physical contact between the material of the solder pad  140  and thermally conductive particles  152 . Other thermal paths can also be established by arranging the solder pad  140  close to the thermally conductive particles  152  such that heat generated by the substrate  112  can still be distributed to the solder pad  140 . 
   Particularly, after the gap distance  132  and the thin film thickness  136  have been determined between the surface  114  of the substrate  112  and the PWD  124 , a thickness  144  of the solder pad  140  is also determined. The thickness of the solder pad  140  is generally the difference between the gap distance  132  and the thin film thickness  136 . For example, if the gap distance  132  between the substrate  112  and the PWB  124  is 20 mils, and a thin film of epoxy has a thickness of 12 mils with several coated copper particles, the solder pad thickness is generally about 8 mils. 
   In general, the thin film  120 , the solder pad  140  and some metallization (not shown) on the substrate  112  are collectively referred to as a stack. In some embodiments, in addition to thermally expanding in planar directions as discussed, the stack can also expand in the z-direction, which is transverse to the x-y plane of thin film  120 . As the stack expands in the z-direction, the semiconductor component like BGA  100  can push away from the PWB  124 , and therefore disconnect the solder balls  104  from the substrate  112 . As a result, the thickness of the stack including the thin film thickness  136  and the solder pad thickness  144  can also be considered during design. For example, the thin film thickness  136  can be small relative to the solder pad thickness  144  such that when the thin film  120  expands due to thermal conditions, the expansion of thin film thickness  136  can be relatively negligible. In this way, the expansion of the thin film  120  in the z-direction is relatively small even when the coefficient of thermal expansion of the thin film  120  is generally higher than the coefficient of thermal expansion of the solder pad  140 . In some embodiments, the thin film thickness  136  is about 2 percent of the stack whereas the solder pad thickness  144  is about 95 percent of the stack. Furthermore, the solder pad  140  can also be chosen such that the coefficient of thermal expansion of the solder pad  140  is comparable to the coefficient of expansion of the solder balls  104 . In some other embodiments, the material for solder pad  140  can also be chosen such that the coefficient of thermal expansion of the solder pad  140  is less than the coefficient of thermal expansion of the solder balls  104 . In yet some other embodiments, the material for solder pad  140  can be chosen such that the coefficient of thermal expansion of the solder pad  140  matches exactly the coefficient of expansion of the solder balls  104 . 
   After applying the thin film  120  to the surface  114  of the substrate  112 , a pick and place machine and process is then used to populating the PWB  124 . In this way, the substrate  112  having the thin film  120  can have a corresponding solder pad  140  with a solder mask on the PWB  124 . Generally, the PWB  124  include materials such as epoxy glass, FR-4 epoxy, G-10 glass, polymide, multifunctional epoxy on THERMOUNT® reinforcement, Arlon 55NT which is a combination of multifunctional epoxy (Tg 180° C.) on DuPont Type E-200 Series on-woven aramid reinforcement with a resin content of 49, non-MDA polymide on THERMOUNT® reinforcement, Arlon 85NT which is a combination of non-MDA pure polyimide resin coated on DuPont Type E-200 Series non-woven aramid reinforcement, flex tape such as printhead wiring, ceramic, silicon, and liquid crystal display (“LCD”) glass. 
   The populated PWB  124  with the semiconductor components will then undergo a heat process to form a strong mechanical connection or bond and thus a thermal path between the thermally conductive particles of the thin film  120  and the solder pad  140 . Generally, the heating process can include reflowing the solder balls  104 . In reflowing the solder balls  104 , the thermally conductive particles of the thin film  120  form a strong mechanical bond, a thermal connection or thermal path with the solder pad  140 . In this way, not only can the reflowed solder pad  140  provide mechanical bonding between the semiconductor component and the PWB  124 , but the reflowed solder pad  140  also provides a thermal path to cool the semiconductor component  100 . Specifically, once the thermal path has been established, the heat generated in the semiconductor  100  during operation can be dissipated toward the PWB  124  via the thermal path. 
   Furthermore, since the thermal coefficients of expansion of the substrate  112  and of the PWB  124  are typically different, the substrate  112  and the PWB  124  will expand at different rates. In such a case, if the substrate  112  and the PWB  124  are held together only by the solder balls  104 , the difference in the expansion rates between the substrate  112  and the PWB  124  can create mechanical stress at the solder balls  104 . Having a thermal path, therefore, will cause the substrate  112  or the semiconductor  100  to expand less, which results in less mechanical stress at the solder balls  104 . 
   The solder pad  140  can be physically positioned adjacent the thermally conductive particles of the thin film  120  by processes such as heating the solder pad  140  and the thin film  120 , thermally coupling the solder pad  140  to the thin film  120 , and inductively heating the solder pad  140  and the thin film  120 . Although  FIG. 3  shows the thermally conductive particles  152  in the thin film  120  distributed randomly, the thermally conductive particles  152  can also be arranged in a pattern depending on the applications on hand. Furthermore, some of the thermally conductive particles  152  are also shown not to be in direct contact with the solder pad  140 . Rather, these thermally particles  152  are positioned adjacent to the solder pad  140  such that heat generated by the semiconductor component  100  can be dissipated to the solder pad  140  via the thermally conductive particles  152  due to their close proximity to the solder pad  140 . 
   In some embodiments, the thin film  120 , the solder pad  140 , and the PWB  124  can have different melting points and can be temperature sensitive. For example, when the melting point of the thin film  120  is less than the melting point of the solder pad  140 , the thin film  120  will melt before the solder pad  140  melts when heated. A premature melting of the thin film  120  can lead to several issues. In some embodiments, the premature melting will cause displacement of the thin film  120  from the substrate  112 . In such a thin film displacement, the thin film  120  and the solder pad  140  are only partially bonded at some locations. In some other embodiments, the premature melting can cause the thin film  120  to flow to the solder balls  104 . In such cases, the solder balls  104  can be inadvertently joined. In some cases, the inadvertent joining of solder balls  104  can short-circuit the semiconductor  100 . As a result, the melting point of the thin film  120  is typically selected to be greater than the melting point of the solder pad  140 . That is, a higher thin film melting point ensures the thin film  120  will not melt and thus the thin film  120  will not contact any solder balls  104 . In some other embodiments, the compatibility of thermally conductive particles used in the thin film  120  with the solder is also taken into consideration when the materials of the thin film  120  and the PWB  124  are chosen. For example, any thermally conducting materials that form a mechanical bond between the thin film  120  and the solder pad  140  can be used. 
   Various features and advantages of the invention are set forth in the following claims.