Abstract:
An injection molded soldering head includes a substrate that is flexible (compliant) and stable at high temperature. The substrate includes an aperture therethrough for holding and dispensing solder onto a mold and a low friction coating on the bottom side of the substrate to provide a lower friction surface for the head.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    Not Applicable. 
       STATEMENT REGARDING FEDERALLY SPONSORED-RESEARCH OR DEVELOPMENT 
       [0002]    Not Applicable. 
       INCORPORATION BY REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISC 
       [0003]    Not Applicable. 
       FIELD OF THE INVENTION 
       [0004]    The invention disclosed broadly relates to the field of injection molded soldering (IMS), and more particularly relates to the field of injection molded soldering heads for high temperature application. 
       BACKGROUND OF THE INVENTION 
       [0005]    Injection molded soldering (IMS), also known as C4NP (C4 New Process) when used for C4 (controlled collapse chip connections bumping, is increasingly used in the semiconductor industry as a method of plating C4 solder bumps onto wafers or modules. The IMS technology was invented and developed at IBM Research. IMS works by injecting molten solder into molds with the desired C4 dimensions, then transferring those C4 onto the wafers or modules. Most of the technology involves the scanning head. The scanning head consists of the reservoir containing the molten solder and the solder injection aperture. The part with the solder injection aperture requires careful engineering for optimal solder filling into the C4 cavities without any solder leakage or solder defects. 
         [0006]    The main advantage of IMS is that there is no need to develop individual recipes for plating as you change the components of the solder material. The material which forms the solder injection aperture for IMS used to fill molten solder need to have characteristics of thermal stability, compliance, and low friction. Typically, for eutectic solder or other low temp solders, compliant materials such as a fluorocarbon (such as that sold under the trademark Rulon®) are used. Also, a material such as an elastomer or fluoropolymer elastomer, or simply fluoroelastomer, (commonly sold under the trademark Viton® used for O-rings) is used on certain applications. The problem with most polymers or elastomers is that they are not thermally stable at high temperature. Typically, above degrees 260 C these materials either melt or decompose. Material such as polyimide or polyimide film are stable up to 400 degrees C. but the coefficient of friction is high (typically 0.6) which makes it difficult to scan smoothly over flat surfaces such as silicon or glass. 
         [0007]    Graphite has an extremely low coefficient of friction (0.1) but is not compliant so any protrusion or non-flatness on the scanning surface might cause leakage of solder while dispensing the solder. The challenge is to discover and use a material that satisfies the above criteria and is also economical to manufacture. Therefore, there is a need for a solution that overcomes the above shortcomings of the prior art. 
       SUMMARY OF THE INVENTION 
       [0008]    Briefly, according to an embodiment of the invention, an injection molded soldering head includes a substrate that is flexible (compliant) and stable at high temperature. The substrate includes an aperture (such as a slot) therethrough, for holding and dispensing solder onto a mold, and a low friction coating on the substrate to provide a lower friction surface for the head. 
         [0009]    According to another embodiment of the invention, a method includes steps of coating a flexible layer with a thin coating of a low friction material to provide a lower surface for the head. The low friction material may be a Diamond like carbon or amorphous carbon. 
         [0010]    In another embodiment, polyimide film may be used as the substrate because it offers good thermal stability and compliance and because it is easily laminated to head base plates. Diamond-like-carbon (DLC) or amorphous carbon is a material known to have low friction. 
         [0011]    In another embodiment of the invention the low friction coating is applied to any compliant material which requires a lower friction. The low friction coating is thin enough so that it does not interfere with the polymer&#39;s original properties such as compliance, thermal stability or vacuum sealing ability. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a side view of an amorphous carbon coated polyimide film layer used in an IMS head. 
           [0013]      FIG. 2A  is a side view of an IMS head with amorphous carbon coated polyimide film used as the solder injection aperture assembly. 
           [0014]      FIG. 2B  is a bottom view of an IMS head with amorphous carbon coated polyimide film used as the solder injection aperture assembly. 
           [0015]      FIG. 3  is a schematic of an IMS head with amorphous carbon coated polyimide laminated on a thin flexible metal sheet used as the solder injection aperture assembly. 
           [0016]      FIG. 4A  is a schematic of a bottom view of an O-ring sealed IMS head. 
           [0017]      FIG. 4B  is a schematic of a side view of an O-ring coated with amorphous carbon film. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]      FIG. 1  is a side view of a part of an IMS head according to an embodiment of the invention. A polyimide film layer (such as Kapton® polyimide film)  102  is coated with an amorphous carbon layer  104 . The amorphous carbon layer  104  may be deposited by plasma sputter deposition, laser ablation, ion beam assisted deposition, direct ion beam deposition, ion beam sputter deposition, plasma enhanced CVD (chemical vapor deposition), or microwave ECR (Electron Cyclotron Resonance) CVD or other suitable processes. The thickness of the amorphous carbon layer  104  can be in the range of 50 to 2000 Angstroms. The optimal thickness of the amorphous carbon layer  104  is thick enough to provide the carbon characteristics of low friction, but not so thick as to cause cracking or delaminating due to thin film stress. The use of the above materials provides low friction sliding of the bottom surface of the head, while remaining stable at temperatures up to 400 degrees C., and providing compliance during the scanning of a surface which may have protrusions. 
         [0019]      FIG. 2A  is a side view of an IMS head  200  with a solder injection aperture  212 . The IMS head  200  comprises a base plate  202 , a solder reservoir  204 , a polyimide film layer  206  (similar or identical to layer  102 ), and an amorphous carbon layer  208  (similar or identical to layer  104 ). The aperture  212  (shown in broken lines) leads to a slot  210  (also shown in broken lines) for holding solder to be dispensed on a circuit board surface.  FIG. 2B  is a bottom view of the IMS head  200 . The part of the IMS head  200  with the aperture  212  is shown with the slot  210 . The slot is formed through the polyimide film layer  206  and the amorphous carbon layer  208 , exposing a part of the base plate  202  to the bottom of the head  200 . This IMS head  200  is typically operated by the following method. The IMS head  200  sits on a mold with cavities which is to be filled with solder. The molten solder containing the IMS head  200  is scanned over the mold surface while pressure is applied to the solder reservoir. The solder is forced to flow through the small aperture  212  due to applied pressure from above and it fills the aperture area  210 . The solder cannot escape from the aperture  210  until it encounters a cavity on the surface of the mold. As the aperture  210  of the IMS head  200  passes over the cavity in the mold, the solder is dispensed and fills the cavity. The friction during scanning is lowered by the low friction coating  208 . In one embodiment, the amorphous carbon  208  is deposited by a RF (radio frequency) sputter deposition method. The polyimide film surface  206  is pre-treated by standard brush cleaning with a detergent followed by an oven bake at 140 degrees C. for two hours to bake out any water absorbed in the polyimide, then O2 plasma ash is applied to promote good adhesion. The Carbon deposition system is operated at 300 Watts for thirty minutes to degas the chamber. The RF power during deposition for the actual deposition is at 200 Watts. For this application, the system is operated for four hours to give an amorphous carbon thickness of about 500 Angstroms. 
         [0020]      FIG. 3  is a schematic of an IMS head  300  similar to the one shown in  FIG. 2 , but with an additional feature of a thin flexible metal sheet  302  supporting the low friction thin film coated polyimide film sheet  206 . The head  300  includes a set of springs  304  disposed between the base plate  202  and the thin flexible metal  304 . The thin flexible metal sheet  304  provides additional compliance over a large area. The polyimide film layer  206  can be easily laminated onto the metal sheet. Details of a similar scheme is described in previous U.S. Pat. Nos. 6,056,191 and 6,527,158, which are hereby incorporated by reference. 
         [0021]      FIG. 4A  is a schematic of a bottom view of a fluoroelastomer O-ring  402  sealed IMS head  400 . The solder is dispensed from an aperture  404  (in the IMS head bottom plate assembly  406 ) and is contained within the O-ring  402 . As the IMS head  400  scans over the mold, solder is injected into ariy cavities it passes over.  FIG. 4B  shows the O-ring  402  with a thin layer of amorphous carbon  406 . The coating performed on the side where the O-ring  402  makes contact with the mold. Only one side is being deposited as that side is the sliding surface. In principle, the low friction coating  408  can be applied to the surface opposite of the O-ring surface, in this case, the mold surface. The situation (e.g., economics, durability etc) will dictate whether the O-ring  402  or mold gets coated. However, it is possible both sides receive the coating for minimizing the friction and wear. While fluoroelastomer is used for the O-ring in this embodiment, any compliant material with thermal stability can be used. 
         [0022]    Therefore, while there has been described what is presently considered to be preferred embodiments, it will be understood by those skilled in the art that other modifications can be made within the spirit of the invention.