Patent Publication Number: US-8534533-B2

Title: Solder paste transfer process

Description:
BACKGROUND 
     The subject matter disclosed herein relates to a solder paste transfer process and, more particularly, to a solder paste transfer process for non-planar surface mount assemblies. 
     Standard circuit cards are flat and do not have heat sinks, components or connectors attached to them. This allows solder paste to be applied using an automated screen print process. When a circuit card is not flat, however, the automated process cannot be used and the standard alternate process is to dispense solder paste onto the circuit card locations. This process is often very time consuming and has a high potential for causing solder defects. 
     Dispensing improvement efforts have included the introduction of the auger positive displacement valve. Auger valves use rotating threads in close proximity to a cylinder wall to force paste downward through a needle tip. These valves rely on fluid pressure to supply a continuous feed of paste into the auger chamber during dispense. Although this can be more precise than time/pressure valves, auger valves usually are more complicated to operate and have a larger degree of variables controlling dispense volume. Dispensing solder paste is also a very labor dependant process increasing assembly cost. 
     SUMMARY 
     According to one aspect of the invention, a solder paste transfer process is provided and includes defining an arrangement of solder pad locations on a surface, applying solder paste onto a transfer tool in a pre-defined configuration reflective of the arrangement, disposing the transfer tool in an inverted orientation proximate to the surface and reflowing the solder paste to flow from the transfer tool to the solder pad locations. 
     According to another aspect of the invention, a solder paste transfer process is provided and includes defining an arrangement of solder pad locations on a portion of a non-planar surface, screening solder paste through a stencil onto a surface of a transfer tool formed of a material that is non-wettable with respect to the solder paste, disposing the transfer tool in an inverted orientation proximate to the portion of the non-planar surface such that the surface of the transfer tool faces the portion of the non-planar surface and reflowing the solder paste to flow from the transfer tool to the solder pad locations of the portion of the non-planar surface. 
     According to yet another aspect of the invention, a method of assembling a printed circuit board (PCB) is provided and includes at least one of forming solder pads on the PCB and attaching electrical components to the PCB to form a non-planar surface, defining, at a portion of the non-planar surface, an arrangement of solder pad locations where solder pads are to be formed, applying solder paste onto a transfer tool formed of a material that is non-wettable with respect to the solder paste in a pre-defined configuration reflective of the arrangement of the solder pad locations, disposing the transfer tool in an inverted orientation proximate to the portion of the non-planar surface and reflowing the solder paste to flow from the transfer tool to the solder pad locations of the portion of the non-planar surface 
     These and other advantages and features will become more apparent from the following description taken in conjunction with the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The subject matter, which is regarded as the invention, is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
         FIG. 1  is a side view of a printed circuit board (PCB) with a device layer; 
         FIG. 2  is a side view of solder paste and a transfer tool; 
         FIG. 3  is a top down view of the PCB of  FIG. 1 ; 
         FIG. 4  is a top down view of stencils from which a particular stencil is to be chosen for use; 
         FIG. 5  is a side view of a stencil mated with the transfer tool or  FIG. 2 ; 
         FIG. 6  is a side view of the transfer tool of  FIG. 5  with the stencil removed and solder paste mounds left; 
         FIG. 7  is a side view of the transfer tool being inverted and disposed proximate to the PCB of  FIG. 1 ; and 
         FIG. 8  is a side view of the PCB of  FIG. 1  with solder pads formed thereon. 
     
    
    
     The detailed description explains embodiments of the invention, together with advantages and features, by way of example with reference to the drawings. 
     DETAILED DESCRIPTION 
     Unique constraints inherent to the designs of circuit card assemblies, such as thermal requirements for heat sinks and precise connector locations due to mating systems, often make circuit cards non-producible with standard assembly processes. However, in accordance with aspects, inverting precision controlled amounts of solder paste at a controlled height allows for correct solder volumes to be applied to each solder joint. The calculated height allows the solder paste to make contact with the area to be soldered and allows for a proper wetting path during the melting of the solder. The calculated amounts of solder are applied to the transfer tool by the use of a laser cut stencil. This controlled amount of solder ensures that quality requirements are met. 
     With reference to  FIGS. 1-8 , a solder paste transfer process is provided. The solder paste transfer process may be employed in various applications and technologies, such as, but not limited to, the assembly of a printed circuit (or wiring) board (PCB). For purposes of clarity and brevity, the following description will apply to the exemplary assembly of the PCB although it is to be understood that the description is not intended to otherwise limit the scope of this application. 
     With reference to  FIG. 1  and, in accordance with embodiments, the solder paste transfer process is prepared. During this stage, a PCB  10  is initially assembled with a device layer  11  defined at a top-most surface thereof. The device layer  11  may have a set of solder pads  12  formed thereon by conventional processes, electrical components  13  electrically coupled to or otherwise attached to the device layer  11  and solder pad locations  14  where solder joints are required to be formed but are not and cannot be formed until other processes can be completed. The device layer  11  may be non-planar as a result of design or due to warping or other types of deformations. In other cases, the device layer  11  may be substantially planar with the solder pads  12  and/or the electrical components  13  defining multiple additional planes, P 1  and P 2 , above a plane, P 3 , of the device layer  11 . In either of these cases, the device layer  11  is effectively non-planar and, thus, conventional solder paste transfer processes cannot be employed easily or efficiently with respect to the solder pad locations  14  due to the fact that the conventional solder transfer tools are prevented from being positioned sufficiently proximate to the solder pad locations  14 . 
     In addition, with reference to  FIG. 2 , the preparation stage further includes the provision or formation of a solder paste  20  and a solder paste transfer tool  30 . The solder paste  20  includes some combination of solder material and flux material and has a consistency similar to that of toothpaste. The transfer tool  30  may be provided in any form, shape or size such that it can be manipulated by an operator and/or an automatic system. In general, the transfer tool  30  has a body  31  and a surface  32 . The body  31  and the surface  32  may be formed of the same or different materials. In either case, the surface  32  is formed of a material that is non-wettable under certain conditions with respect to the materials of the solder paste  20 . In accordance with embodiments, the surface  32  may be formed of glass or some other similar material and/or may be machined from a body made of aluminum or some other similar material. 
     With reference to  FIGS. 3 and 4 , the device layer  11  is shown in top down perspective such that the arrangement or configuration of the solder pad locations  14  can be observed along with at least the solder pads  12  and the electrical components  13  as shown in  FIG. 3 . In  FIG. 4 , it is seen that a stencil  40  is chosen from a plurality of stencils  400 . Each of the plurality of stencils  400  may have different properties, such as, but not limited to stencil thickness, aperture configuration and aperture size. As shown in  FIG. 4 , the particular stencil  40  is formed with a predefined configuration to define apertures  41  and is chosen to have a stencil thickness, an aperture configuration and aperture sizes that correspond to the arrangement or configuration of at least the solder pad locations  14 . That is, each aperture  41  of the stencil has a position, size and shape that correspond to an associated solder pad location  14 . 
     In addition, the thickness of the stencil  40  is chosen to determine a total amount of solder paste that will be applied to the solder pad locations  14  and is related to the types of solder joints to be formed and their proximity to one another. In particular, it is to be understood that if an excess of solder paste is applied to a solder pad location  14 , it is possible that the solder paste will form bridges with solder paste of adjacent solder pad locations  14  and cause an electrical failure or short circuit. This can be avoided by employing the stencil  40  of appropriate thickness. 
     At the conclusion of the preparation stage, the main solder paste transfer process is begun. As shown in  FIG. 5 , the main solder paste transfer process initially includes an operation of mating the stencil  40  to the surface  32  of the transfer tool  30 . This mating may be achieved by simply pressing the stencil  40  onto the surface  32  or temporarily adhering the stencil  40  onto the surface  32 . Once the mating is complete, the solder paste  20  is screened through the stencil  40  onto the surface  32  of the transfer tool  30 . The screening may be conducted by applying the solder paste  20  to the stencil  40  and passing a squeegee  42  over the stencil  40  such that a portion of the solder paste  20  is forced into the apertures  41 . 
     With reference to  FIG. 6 , the stencil  40  may then be removed from the surface  32  of the transfer tool  30 . In accordance with embodiments, the removal of the stencil  40  is not necessary and may be skipped in some cases. For the purposes of clarity and brevity, however, it will be hereinafter assumed that the stencil  40  is removed to thereby leave solder paste mounds  50 . The solder paste mounds  50  are thus reflective of the configuration and arrangement of the apertures  41  and by extension the solder pad locations  14 . The solder paste mounds  50  adhere to the surface  32  under certain conditions and, as such, the transfer tool  30  can be manipulated without risk of the solder paste mounds  50  substantially deforming. 
     With reference to  FIG. 7 , the transfer tool  30  is inverted such that the surface  32  with the solder paste mounds  50  applied thereto faces downwardly and moved to a position above the portion of the device layer  11  where the solder pad locations  14  are defined. In accordance with some embodiments, the transfer tool  30  may be secured in this position by being coupled with a fixture that is itself anchored on the PCB  10 , although this is not required. The transfer tool  30  is then disposed in this inverted orientation proximate to the portion of the device layer  11  such that the solder paste mounds  50  are proximate to corresponding solder pad locations  14 . The solder paste  20  of the solder paste mounds  50  is then heated and reflowed such that the solder paste  20  flows from the surface  32  of the transfer tool  30  to the solder paste locations  14  of the portion of the device layer  11 . 
     The disposition of the transfer tool  30  in the inverted orientation proximate to the device layer  11  may include calculating a displacement, D, between the surface  32  of the transfer tool  30  and the portion of the device layer  11  whereby the reflowing of the solder paste  20  from the surface  32  to the device layer  11  can be reliably conducted. Here, it is to be understood that the reflow cannot or should not result in misplaced solder joints and/or bridging between adjacent solder joints among other types of failures. To this end, the amount of solder paste  20  in each solder paste mound  50  is controlled by selection of the stencil  40  having appropriately sized and shaped apertures  41 . In addition, the displacement, D, is calculated to permit reflow from the surface  32  to the device layer  11  while preventing misplaced solder joints and bridging. Once the calculation is complete, the disposition further includes disposing the transfer tool  30  with the calculated displacement from the device layer  11 . 
     With reference to  FIG. 8 , once the solder paste  20  is reflowed to the device layer  11  and cooled, solder joints  60  are formed. 
     While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.