Abstract:
A surface-mount device attach method for attaching solder ball-grid array or solder column-grid array surface-mount devices to a printed circuit board where the conventional solder mask structure is replaced with a layer of plated nickel to prevent printed circuit board warping caused by the mismatch in the coefficients of thermal expansion between the conventional solder mask and the printed circuit board. A native nickel-oxide layer that forms on the surface of the exposed portions of the plated nickel layer functions as a solder dam.

Description:
This application claims the benefit of U.S. Provisional Application No. 60/236,496, filed Sep. 29, 2000. 
    
    
     BRIEF DESCRIPTION OF THE INVENTION 
     This invention.relates to an improved method for mounting surface-mount technology (SMT) devices such as solder ball-grid array (BGA) or solder column grid array (CGA) devices onto a printed circuit board. 
     BACKGROUND OF THE INVENTION 
     Among the variety of SMT attach methods available in the electronic packaging industry, BGA and CGA devices are often used for attaching devices with high I/O requirements. In these SMT device attach systems, an array of solder balls or solder columns form both the electrical and structural interconnections between the SMT device and the printed circuit (PC) board. 
     In a BGA attach system, the PC board is provided with one or more BGA device bonding sites and each site is provided with an array of bonding pads that matches the array of solder ball interconnections on the BGA device. To attach the BGA device onto the PC board, low-temperature solder paste, whose reflow temperature is lower than that of the BGA solder balls, is deposited onto each of the bonding pads on the PC board. Solder paste made from eutectic Pb—Sn solder is typically used for this application. The BGA device is then aligned and placed on a device bonding site so that the array of BGA solder balls is aligned with the array of bonding pads on the PC board. The whole assembly is then heated to a temperature sufficiently high to melt the low temperature solder paste and form metallurgical bonds between the BGA solder balls and the bonding pads. Because the solder paste has a lower melting point than the BGA solder balls, only the solder paste melts during the BGA bonding process step. 
     In addition to the bonding pads, a PC board typically has surface-wiring structures that provide electrical connections between various points on the surface of the PC board. Many of the bonding pads are connected to other bonding pads within the same surface-mount device bonding site or to various other points on the surface of the PC board through these surface-wiring structures. For example, FIG. 1 illustrates a conventional BGA device assembly system. In FIG. 1, a prior art PC board  20  is shown with a BGA device bonding site  25 . BGA device bonding site  25  has an array of bonding pads  23  configured to match the array of solder balls  24  on BGA device  26 . And as discussed above, some of the bonding pads are shown as being connected to surface-wiring structures  22   a  and  22   b . Two types of surface-wiring structures  22   a  and  22   b  are shown for illustrative purposes. Surface-wiring structure  22   a  connects those bonding pads to other parts of the PC board and surface-wiring structure  22   b  connects two bonding pads. 
     Bonding pads  23  and surface-wiring structures  22   a  and  22   b  are typically formed from the PC board&#39;s surface metallurgy generally comprising a layer of copper (Cu) base conductor layer deposited on the PC board&#39;s surface. The Cu base conductor layer is deposited in patterns forming bonding pads  23  and surface-wiring structures  22   a  and  22   b . Then very thin layers of nickel (Ni) and gold (Au) are deposited over the Cu at bonding pads  23 . The remaining Cu base conductor layer portions without any additional metal layers form surface-wiring structures  22   a  and  22   b . The metal layers are typically deposited using processes such as electrolytic plating or electro-less plating that are well-known in the electronics industry. 
     The layer of Ni plated on top of the Cu layer at bonding pads  23  functions as an adhesion promoting layer between the Cu and Au layers and also as a diffusion barrier between the Cu and Au layers. The Au layer provides corrosion protection. 
     The BGA device bonding site is then coated with a layer of solder mask  28 . Solder mask  28  is typically a layer of organic material, such as polyimide and covers the whole BGA device bonding site but has apertures exposing the Au surface of the bonding pads  23  to allow the BGA solder balls to bond to them. Because solder does not wet to the solder mask material, the solder mask functions as a dam around each bonding pad and prevents the molten solder from bleeding out along the surface-wiring structures during the BGA bonding process. And because of its electrically insulating property, solder mask  28  also functions as a passivation layer protecting the surface-wiring structures  22   a  and  22   b.    
     FIGS. 2 and 3 illustrate the details of a conventional BGA assembly structure using a solder mask. FIG. 2 illustrates a cross-sectional view of BGA bonding site  25  from FIG. 1 before the BGA device is bonded to the PC board. The Cu/Ni/Au multi-layer structure of bonding pads  23  is shown by the reference numbers  34 ,  32 ,  30  respectively. As discussed above, surface-wiring structure  22  consists of the Cu layer only and does not have Ni or Au plated over it. 
     The BGA bonding site is coated with a solder mask layer  28  that has apertures  37 . Apertures  37  expose Au layer  30  of the bonding pads for bonding to BGA solder balls. To bond a BGA device to BGA bonding site  25 , low-temperature solder paste  29  is first deposited onto the bonding pads using typical solder paste deposition techniques used in the industry. Some examples are an off-contact printing process, such as screen printing, or an on-contact printing process, such as stenciling. 
     After the solder paste has been deposited, a BGA device is aligned and placed onto the bonding site so that BGA solder balls  24  align with corresponding bonding pads. The whole assembly is then heated to an elevated temperature sufficiently high to melt the low temperature solder paste. Upon cooling, the low temperature solder forms metallurgical bonds with BGA solder balls  24  and the bonding pads. PC board  20  is also shown with a multi-layer internal structure with internal wiring metallurgy  27  such as is found in many PC boards. 
     FIG. 3 illustrates the cross-sectional view of PC board  20  after BGA device  26  has been bonded to BGA bonding site  25 . Solder balls  24  are bonded to bonding pads  23  and form the interconnections between BGA device  26  and PC board  20 . 
     A problem with this conventional BGA attach system is that because the solder masks are formed from materials with relatively high coefficients of thermal expansion (CTE) compared to the CTE of the PC board, the mismatch of CTEs between the solder mask and the PC board causes the PC board to warp after the assembly is exposed to elevated temperature during the BGA bonding process. For example, the CTE of a typical polyimide solder mask is about 500×10 −7 /° C. compared to the CTE of Fr-4, a typical PC board material, which is about 158×10 −7 /° C. Because the solder mask has higher CTE, it expands more than the PC board. But, because the BGA bonding process temperature is above the glass transition temperature (Tg) of the solder mask, there is no stress transmitted to the PC board during the high temperature stage of the bonding process. However, upon cooling down to the room temperature, below the Tg of the solder mask, the solder mask and the PC board shrink at different rates, causing the PC board to warp. 
     This poses a significant reliability concern. In many electronic devices, the component assemblies undergo many thermal cycles as the devices are turned on and off during their use. These thermal cycles strain the solder interconnections because the solder mask and the PC board expand and contract different amounts and eventually may result in metal fatigue failure of the solder interconnections. 
     An additional problem associated with the use of a conventional solder mask is that the interface between the solder mask and the Au plated surface-wiring is subject to degradation. The degradation may be in the form of de-lamination or peeling. 
     SUMMARY OF THE INVENTION 
     The invention provides a PC board with at least one SMT device bonding site for mounting SMT devices comprising: at least one surface for mounting one or more SMT devices; at least one base conductor layer plated on a portion of the surface; at least one plated Ni outer layer deposited on the base conductor layer; and an array of bonding pads formed by selectively plating at least one layer of Au on a portion of the plated Ni outer layer, and leaving a remainder of the plated Ni outer layer exposed to form a layer of native nickel-oxide. This structure, where a plated Ni outer layer covers the base conductor layer, eliminates the need for the conventional solder mask. And because solder does not wet to the layer of native nickel-oxide on the surface of the plated Ni outer layer it serves as a solder dam preventing the molten solder balls from wetting and bleeding out beyond the bonding pads. 
     A typical conductor material, Cu, may be used for the base conductor layer. Alternatively the base conductor layer may have a multi-layer structure having Cu/Ni/Au layers where Au is the top layer. 
     The invention also includes a method of forming a PC board for mounting surface-mount devices without utilizing a conventional solder mask. The process includes providing a printed circuit board having at least one surface having at least one surface-mount device bonding site; plating at least one base conductor layer on a portion of said surface; plating at least one plated nickel outer layer on said base conductor layer; and selectively plating at least one layer of gold on a portion of said plated nickel outer layer to form an array of gold bonding pads at said surface-mount device bonding site and leaving a remainder of said plated nickel outer layer exposed to form a layer of native nickel-oxide. 
     It would be appreciated by one of ordinary skill in the art that the metal layers described above, such as the base conductor layer, the Ni outer layer, and the Au layer may be deposited using electrolytic plating as well as electro-less plating processes. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a better understanding of the invention, reference should be made to the following detailed description taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 illustrates a prior art BGA attach assembly system. 
     FIG. 2 illustrates a cross-sectional view of the PC board of the prior art BGA attach assembly of FIG.  1 . 
     FIG. 3 illustrates a BGA device bonded to a PC board according to the prior art process. 
     FIG. 4 illustrates a top-down view of bonding pads on a PC board constructed in accordance with the invention. 
     FIG. 5 illustrates a cross-sectional view of the PC board from FIG.  3  through the cross-sectional plane A—A. 
     FIG. 6 illustrates the cross-sectional view of the PC board from FIG. 4 with a BGA device bonded to the PC board. 
     FIG. 7 illustrates a cross-sectional view of another embodiment of a PC board constructed according to the invention. 
    
    
     The drawings are only schematic and are not to scale. Like reference numerals refer to corresponding parts throughout the drawings. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention addresses the PC board warping concerns associated with the use of solder masks in the prior art BGA attach systems by eliminating the solder mask. Although the present invention is equally applicable to BGA devices and other SMT devices such as CGA devices, for ease of discussion, BGA application will be discussed as an example. But those skilled in the art will readily recognize the applicability of the present invention to other SMT devices. 
     FIG. 4 illustrates a top-down view of a BGA device bonding site  48  on a PC board  40  constructed in accordance with an embodiment of the invention. This exemplary BGA device bonding site has a 3×3 array of bonding pads  23  and surface-wiring structures  60  that are formed from at least one plated Ni outer layer deposited over a base conductor layer of Cu. 
     Each bonding pad  23  is a continuous layer  50  of Au that has been selectively plated on a portion of the plated Ni outer layer. The Au layer provides a bonding surface having an excellent solder-wetting characteristic and corrosion resistance. The remaining portion of the plated Ni outer layer is left exposed, allowing its surface to oxidize and form a layer of native nickel-oxide. Thus, the surfaces of peripheral portions  51  of bonding pads  23  and surface-wiring structures  60  are covered with a layer of native nickel-oxide. 
     Because solder does not wet to the layer of native nickel-oxide, the plated nickel outer layer and its native nickel-oxide surface functions like the conventional solder mask and prevent molten BGA solder balls from wetting along the surface-wiring structures  60  and bleeding out beyond the bonding pad  23 . 
     The plated metal layers may be plated using electrolytic or electro-less plating processes. The typical thicknesses of the Cu base conductor layer and the selectively plated Au layer are about 0.25-0.5 mils and 0.02-0.05 mils respectively. A preferred range of thickness for the plated Ni outer layer is about 0.075-0.3 mils. FIG. 5 illustrates a cross-sectional view of PC board  40  of FIG. 4 along the plane A—A. Each bonding pad  23  consists of a base conductor layer  54  of Cu, at least one plated Ni outer layer  52 , and at least one selectively plated Au layer  50 . A layer of native nickel-oxide  56  is formed on the exposed surface of plated Ni outer layer  52 . Au layer  50  provides the bonding surface for the BGA solder balls while layer of native nickel-oxide  56  passivates surface-wiring structures  60  and also functions as a solder dam at each of bonding pads  23  preventing the molten solder from bleeding out onto surface-wiring structures  60  during the BGA bonding process step. 
     Thus in accordance with the invention, native nickel-oxide layer  56  replaces the conventional solder mask of the prior art BGA attach systems. And because the conventional solder mask, whose CTE is substantially greater than that of the PC board, is eliminated and the plated Ni outer layer  52  and Cu base conductor layer  54  have CTEs that more closely match the CTE of the PC board, the PC board warping concern associated with the prior art solder mask systems is reduced. A comparison of the CTEs for a typical PC board material Fr-4, Cu, Ni, and conventional solder mask material is shown below: 
     
       
         
               
               
               
             
           
               
                   
                   
               
               
                   
                   
                 Coefficient of Thermal Expansion 
               
               
                   
                 Materials 
                 10 −7 /° C. 
               
               
                   
                   
               
             
             
               
                   
                 Fr-4 (typical PC board) 
                 158 
               
               
                   
                 Cu 
                 170 
               
               
                   
                 Ni 
                 133 
               
               
                   
                 Polyimide (typical solder 
                 500 
               
               
                   
                 mask material) 
               
               
                   
                   
               
             
          
         
       
     
     FIG. 6 illustrates a cross-sectional view of PC board  40  of FIG. 5 where a BGA device  26  has been bonded to PC board  40 . Low-temperature solder paste is first deposited onto the bonding pads by screen printing or on-contact printing process. The BGA device is then placed onto the BGA bonding site and heated to a temperature sufficiently high to melt the low temperature solder. Upon cooling, the low-temperature solder forms metallurgical bonds between BGA solder balls  24  and bonding pads  23 . Plated Ni outer layer  52  with its native nickel-oxide layer  56  functions as a solder dam at bonding pads  23  and contains solder balls  24  on the bonding pads while the low temperature solder is in its molten sate. 
     In another embodiment of the invention illustrated in the cross-sectional view of FIG. 7, a plated Ni outer layer is deposited over a base conductor layer having a multi-layered structure of Cu/Ni/Au layers  34 ,  32  and  30 . Then a layer  50  of Au is selectively plated over plated Ni outer layer  52  to form bonding pads as described in the embodiment illustrated in FIG.  5 . The resulting surface metallurgy has a Cu/Ni/Au/Ni structure with additional Au selectively plated on portions of the plated Ni outer layer to form the bonding pads. And as with the embodiment of the invention illustrated in FIGS. 5 and 6, native nickel-oxide layer  56  formed on the plated Ni outer layer  52  functions as a solder dam at bonding pads  23  preventing the molten solder from bleeding out onto surface-wiring structures  60  during the BGA bonding process. 
     Unlike the conventional system where the interface between the solder mask and the Au layer of the surface metallurgy is susceptible to degradation, such as de-lamination or peeling, the interface between the plated Ni outer layer and the underlying metal layer is less susceptible to such problems because of the excellent adhesion between Ni and the underlying metal layer which may be Cu or Au as discussed above. This property is well known especially in the electronics industry where Ni is used as an adhesion promoting interface layer between Au and a third metal as seen in the Cu/Ni/Au structure of the surface metallurgy on conventional PC boards. 
     The foregoing description, for purposes of explanation, used specific references to a BGA device. However, it would be apparent to one of ordinary skill in the art that the invention is applicable to other surface-mount devices where an organic solder mask is utilized in a similar manner as in the conventional BGA devices. 
     The illustrative discussions above are not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many modifications and variations are possible in view of the above teachings. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.