# Patent

## Title
Solder paste transfer process

## Abstract

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.

## Background
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, P1 and P2, above a plane, P3, 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.

## Claims

1. A solder paste transfer process, comprising:
defining an arrangement of solder pad locations on a portion of a non-planar surface;
attaching electrical components to the non-planar surface, the electrical components protruding above a plane of the portion of the non-planar surface;
applying solder paste onto a transfer tool in a pre-defined configuration reflective of the arrangement;
disposing the transfer tool aside the electrical components in an inverted orientation proximate to the portion of the non-planar surface such that a displacement between the transfer tool and the portion of the non-planar surface is greater than a depth of the solder paste below the transfer tool such that the solder paste does not contact the solder pad locations; and
with the transfer tool disposed with the displacement, reflowing the solder paste to flow from the transfer tool to the solder pad locations.

2. The process according to claim 1, further comprising providing the transfer tool as a glass surface.

3. The process according to claim 1, further comprising machining the transfer tool from an aluminum body.

4. The process according to claim 1, wherein the applying comprises screening the solder paste through a stencil, the process further comprising choosing the stencil from a plurality of different stencils.

5. The process according to claim 4, further comprising mating the stencil with the transfer tool during at least the screening.

6. The process according to claim 5, further comprising removing the stencil from the transfer tool prior to the disposing.

7. The process according to claim 1, wherein the disposing comprises:
calculating the displacement between the transfer tool and the portion of the non-planar surface; and
disposing the transfer tool with the calculated displacement from the portion of the non-planar surface.

8. The process according to claim 1, further comprising cooling the solder paste to form solder joints at the solder pad locations.

9. A solder paste transfer process, comprising:
defining an arrangement of solder pad locations on a portion of a non-planar surface;
attaching electrical components to the non-planar surface, the electrical components protruding above a plane of the portion of the 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 aside the electrical components 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 with a displacement between the surface of the transfer tool and the portion of the non-planar surface that is greater than a depth of the solder paste below the surface of the transfer tool such that the solder paste does not contact the solder pad locations; and
with the transfer tool disposed with the displacement, reflowing the solder paste to flow from the transfer tool to the solder pad locations of the portion of the non-planar surface.

10. The solder paste transfer process according to claim 9, further comprising at least one or more of providing the surface of the transfer tool as a glass surface and machining the transfer tool from an aluminum body.

11. A method of assembling a printed circuit board (PCB), comprising:
attaching electrical components to the PCB such that the electrical components protrude above a plane of the PCB to form the PCB as a non-planar surface;
defining, at a portion of the non-planar surface, an arrangement of solder pad locations where solder joints 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 aside the electrical components in an inverted orientation proximate to the portion of the non-planar surface such that a displacement between the transfer tool and the portion of the non-planar surface is greater than a depth of the solder paste below the transfer tool such that the solder paste does not contact the solder pad locations; and
with the transfer tool disposed with the displacement, reflowing the solder paste to flow from the transfer tool to the solder pad locations of the portion of the non-planar surface.

12. The method according to claim 11, further comprising at least one of providing the transfer tool as a glass surface and machining the transfer tool from an aluminum body.

13. The method according to claim 11, wherein the applying comprises screening the solder paste through a stencil and the method further comprises choosing the stencil from a plurality of different stencils.

14. The method according to claim 13, further comprising choosing the stencil from a plurality of stencils each having differing solder paste aperture properties.

15. The method according to claim 13, further comprising mating the stencil with the transfer tool during at least the screening.

16. The method according to claim 15, further comprising removing the stencil from the transfer tool prior to the disposing.

17. The method according to claim 11, wherein the disposing comprises:
calculating the displacement between the transfer tool and the portion of the non-planar surface; and
disposing the transfer tool with the calculated displacement from the portion of the non-planar surface.

18. The method according to claim 11, further comprising cooling the solder paste.