Coating that enables soldering to non-solderable surfaces

A composite coating provides a solderable surface on materials that cannot otherwise be soldered. The solderable coating is comprised of a composite layer of two components made of metals or metal alloys that function as a solderable material and as a material that enhances solder flow. A thin layer of at least one of the two components can also be incorporated along with the composite layer. A solderable coating can be deposited on a non-solderable surface by any of a variety of thermal spray techniques, including plasma spraying or wire arc spraying. These solderable coatings are particularly useful in the manufacturing of high power electronic components.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to compositions coated onto a base material 
to provide a surface for soldering and to methods for applying a 
solderable coating to a non-solderable surface. 
2. Related Art 
The ability to solder parts that have non-solderable surfaces is of 
particular importance to the electronic manufacturing industry. This 
problem often arises, for example, in the bonding of high power electronic 
devices to heat sinks. Although a copper heat sink can be soldered to 
another component, copper heat sinks have several disadvantages, including 
a high density and a high thermal expansion coefficient which greatly 
differs from the typically low thermal expansion coefficient of electronic 
components. This mismatch in thermal expansion coefficients of an 
electronic device and a copper heat sink can lead to stress in the 
components, as well as failure when the device heats during operation. 
Therefore, it is desirable to use a material other than copper for the 
heat sink. 
Heat sinks can also be composed of materials such as aluminum, graphite, 
and composite materials. These non-copper heat sinks can be bonded to an 
electronic device using approaches such as thermally conductive adhesives. 
Soldering is generally preferred because it provides a very strong bond 
and excellent thermal conductivity. However, soldering requires the 
deposit of a solderable coating on the surface of most heat sink materials 
other than copper. For example, materials such as gold, nickel, silver and 
copper can be coated on such non-copper heat sinks by a plating process to 
provide a solderable surface. Plating processes, however, can be expensive 
and environmentally unsafe. 
The processes of thermal spraying are capable of producing coatings on a 
large variety of materials. Thermal spraying can be performed in an 
environmentally safe manner. The process of thermal spraying involves 
injecting powder, wire or rod materials into a high temperature heat 
source such that the material is melted and atomized, and then sprayed 
onto a prepared surface, where the material solidifies to create a 
coating. Materials commonly deposited with thermal spraying include 
metals, ceramics, carbides, and plastics. The coatings produced are used 
to provide properties such as wear resistance, electrical insulation, 
corrosion protection and protection from high temperatures. The common 
processes of thermal spraying include plasma spraying, flame spraying, 
wire arc spraying, and high velocity oxy-fuel (HVOF) spraying. U.S. Pat. 
No. 4,341,816 to Lauterback, et al. describes the use of plasma spraying 
processes to apply a first, adhesive, layer to a generally non-solderable 
target body for a sputtering system, and to apply a second, solderable, 
layer over the adhesive layer. The coated surface of the target is then 
soldered to a metal cooling plate. The need to apply at least two separate 
layers using separate plasma spraying steps makes such a process more 
costly for larger scale production than a single layer process, such as 
for the attachment of electronic components to heat sinks. 
SUMMARY OF THE INVENTION 
The present invention provides a solderable coating that can be applied to 
a surface in a single layer by thermal spraying. The invention also 
provides methods for depositing a solderable coating on a non-solderable 
surface using a single thermal spraying step. 
The coating in accordance with the present invention may be applied in a 
single layer utilizing a thermal spraying process. The two components that 
will comprise the layer are provided to the thermal spraying equipment and 
are reduced to a mixture of small discrete particles, which may be fully 
liquid or solid particles with liquified or softened surfaces. The mixture 
of particles, in which each of the components is retained in the form of 
individual discrete particles rather than being alloyed or reacted with 
the other component, is deposited in a layer onto the surface of the 
normally non-solderable base material. The base may be a heat sink formed 
of a material which normally cannot be readily or feasibly soldered to 
electronic components. When the individual discrete particles are 
deposited, they retain their discrete characteristics, with each particle 
being comprised either of the first component or the second component, but 
generally not both. However, the particles contact each other in the layer 
and, as the particles cool and their surfaces solidify, the particles are 
firmly adhered together in a layer and are adhered to the surface of the 
base material. Consequently, a very strong, cohesive, and strongly 
adhering coating layer is provided. 
In the present invention, the first component may be selected from metal 
such as copper, copper alloys, and nickel, which are materials that are 
readily solderable, and the second component may be a metal, such as tin 
or tin alloy, which serves to facilitate flow of solder. 
The present invention thus yields a coating on a normally non-solderable 
material that can be applied in a single thermal spraying step, rather 
than requiring multiple steps or multiple layers, thereby providing 
efficient and cost effective production. Because the coating retains 
discrete particles of the individual components, rather than alloys of the 
individual components, each individual component retains its desirable 
functional characteristics for subsequent soldering steps. For example, 
the first component, which may be formed of copper or other solderable 
metals, has a much higher melting point than and is wettable by the 
solder, allowing the solder to form a uniform layer over the coating, 
while the second discrete component, which preferably has a melting point 
higher than but close to that of the solder, facilitates the flow of such 
solder and can melt and mix with conventional solders to further enhance 
the bond between the solder itself and the coating. 
After the coating has been applied to the substrate base material, a solder 
paste or solder preform may then be applied to the coated surface and, if 
required, a solder flux may also be supplied. The component or the surface 
with the solder formed thereon may be heated to the melting point of the 
solder, which melts and flows over the surface and wets the surface to 
create a metallurgical bond to the coating. In one process, electronic 
components may be directly engaged to the solder as it is initially 
liquified, with subsequent cooling of the solder then resulting in a bond 
between the electronic components and the solder coated base substrate. 
Further objects, features and advantages of the invention will be apparent 
from the following detailed description when taken in conjunction with the 
accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION 
As described herein, a solderable coating is produced from a solderable 
material and a material that improves solder flow. These materials are 
referred to as the first component and second component, respectively, of 
the solderable coating. The function of the first component in the coating 
is to provide a high melting point material in the coating which the 
solder preform or paste will wet. The function of the second component is 
to enable the solder preform or paste to flow over the coating surface 
while the preform or paste is molten. 
The solderable material, or first component, is a metal or metal alloy. 
Suitable solderable materials include copper, nickel, and copper alloys. 
Copper and nickel are preferred solderable materials. The solder flow 
enhancing material, or second component, also a metal or metal alloy, has 
a low melting point temperature compared with the first component. 
Preferably, the second component will have a melting point that is 
slightly higher than that of the solder to be used (e.g., within about 
30.degree. C.). Suitable materials for the second component include tin 
and a babbitt metal. Such babbitt metals include any of several tin- or 
lead-based alloys, such as: (1) a high-tin alloy with small quantities of 
antimony and copper, (2) a high-lead alloy containing antimony, arsenic 
and tin, and (3) an intermediate tin-alloy with antimony and copper. Tin 
and tin alloys are preferred materials for the solder flow enhancing 
component. 
As described below, the two components of the solderable coating are 
deposited on a non-solderable (or poorly solderable) surface of a 
substrate to create a coating that, on a microstructural level, consists 
of the two separately identifiable components. That is, there is little or 
no chemical reaction or metallurgical mixing (alloying) of the two 
components in the coating structure. Both materials can be identified in 
the same layer of the coating structure by metallurgical analysis. Such a 
layer that contains the two identifiable components of the solderable 
coating is referred to as a "composite layer." 
FIG. 1 shows a microstructural cross-section of a solderable coating 38, 
which is comprised of the distinct components as discrete particles 39 and 
40 deposited on the base material 36. As the discrete particles 39 and 40 
are deposited, they contact and adhere to adjacent particles as the 
particles harden, but without significant alloying of individual 
particles. As used herein, "particles" is intended to refer to individual 
small drops of fully liquified metal and to small pieces of solid metal 
that are sufficiently heated so that the surfaces of the pieces, at least, 
may be softened or liquified so that the individual particles will adhere 
together in the coating and to the substrate surface as the deposited 
particles cool. As shown in the figure, the solderable coating is 
comprised of a distinct composite layer of the two components. 
Identification of the composite layer can be accomplished by 
cross-sectioning the coating, polishing the coating, and examining the 
coating with a light microscope. 
Typically, the composite layer of a solderable coating in accordance with 
the invention contains 10-80% by volume of solderable material and 20-90% 
by volume of the solder flow enhancing component. Moreover, the composite 
layer will typically have a thickness of about 0.001 to 0.005 inches, 
although thicker layers also provide a suitable solderable coating. The 
particular composition of a solderable coating will depend on the desired 
conditions of the soldering operation. 
In addition to a composite layer, the solderable coating of the invention 
also can contain one or two distinct layers of either a solderable 
material or a solder flow enhancing material. Such a layer of either 
component can be deposited on the surface of the base material before or 
after deposition of the composite layer. Moreover, a solderable coating 
can comprise a layer of either component adhered to and residing above and 
below the composite layer. Preferably, the solderable coating comprises a 
layer of a tin alloy (babbitt metal) at the coating surface. Although a 
thin layer of each component can also be identifiable in the coating 
structure, a composite layer of both components must be present in the 
coating structure of the present invention. 
Although solderable coatings can be deposited on a non-solderable surface 
of a substrate using various standard methods, a preferred method for 
applying a composite solderable coating in the present invention is 
thermal spraying. In the thermal spraying process, a heat source in the 
thermal spray device melts powder, wire or rod material comprising the 
coating components. The coating components are fed through injection ports 
into the heat source of the thermal spray system, which may be a plasma 
arc, electric arc, or flame. The coating components are then melted and 
sprayed onto the non-solderable surface where the materials solidify to 
create the solderable coating. Standard thermal spray processes include 
plasma spraying, wire arc spraying, flame spraying, and high velocity 
oxygen fuel spraying. 
As an illustration, FIG. 2 shows a device suitable for production of a 
solderable coating by plasma spraying. An example of a suitable thermal 
spray apparatus and process is set forth in the U.S. patent to Lenling, et 
al., U.S. Pat. No. 5,217,746 (1993), incorporated herein by reference. The 
plasma gun 10 comprises a cathode 12, an anode 14, an arc gas inlet 16, 
and a powder injection port 18. These components are arranged within an 
insulating housing 20. Typically, a pointed or conical cathode 12 is 
positioned in the rear of the plasma gun 10, and the cathode is connected 
to a negative electrical connection 22. The cathode is maintained at a 
negative electrical potential during operation of the plasma gun. The 
anode 14 is usually positioned at the front of the plasma gun, and it is 
shaped to define an open-ended chamber 24. The anode 14 is connected to a 
positive electrical connection 26 and is maintained at a positive 
electrical potential during operation of the plasma gun. 
The arc gas inlet 16 is in communication with the open-ended chamber 24. A 
suitable pressurized arc gas is introduced into the arc gas inlet 16, 
resulting in the formation of an arc from the electrical current crossing 
the gap between the cathode 12 and the anode 14. A plasma 28 is formed 
with a plasma axis running from the cathode 12 to the surface to be coated 
36. The cathode 12 represents the upstream direction along the plasma axis 
and the surface 36 represents the downstream direction. Generally, 
temperatures within the plasma decrease farther downstream along the 
plasma axis. 
The plasma 28 results in a zone of intense heat that begins at the tip of 
the cathode 12 and extends through and emanates from the open-ended 
chamber 24. The magnitude of the heat in the plasma flame 28 is dependent 
on the current applied between the cathode 12 and the anode 14 and the 
choice of arc gas. Because of the intense heat generated by the plasma gun 
10, the parts typically are water-cooled. Water enters through a water 
inlet 30, flows through a passageway 32 in the anode 14, is routed through 
the housing 20, and exits at a water outlet. 
The coating 38 is formed from a material that is, for example, in powder 
form. The powder is metered by a powder feeder or hopper and is introduced 
into a suitable carrier gas that suspends and feeds the material to the 
plasma 28 through the powder injection port 18. The plasma 28 heats the 
powder into a molten or semi-molten state and the powder is propelled to 
impinge upon a base material part 36 to form the coating 38 thereon. The 
open-ended chamber 24 is shaped as a nozzle through which the plasma 28 
exits and the molten material contained therein is projected onto the base 
material part 36. A bond is produced at the interface between the base 
metal part 36 and the coating 38 as the particles cool and fully solidify. 
FIG. 1 illustrates the deposition of the coating on the base material 
part. 
The heat of the plasma 28 is adjusted, depending on the powder used to form 
the coating 38, so that the heat is sufficient to melt the powder into an 
appropriately molten or plastic state. In producing a coating 38 upon a 
base material part 36, operating conditions are controlled by regulating 
the power level between the cathode and anode, the pressure and flow rate 
of the arc gas, the flow rate of the carrier gas, the powder feed rate 
(i.e., the quantity of powder introduced into the arc per unit time), and 
the cooling water flow. 
In the method of the present invention, a second injection port 40 can be 
employed, which is located downstream of the first port 18. The first 
injection port 18 is used to introduce a solderable material, such as 
powdered copper 41, which is carried by a carrier gas into the plasma 28. 
In the second injection port 40, a lower melting point material, such as a 
tin alloy, can be injected in powder form and carried by a carrier gas 
into the plasma 28. Injecting the two components at different locations 
allows optimization of the heat input for each component to provide proper 
melting with minimal over-heating. 
The concentrations of the two components injected at the first and second 
ports 18 and 40 may be altered by changing the powder feed rates. In the 
above example, the second injection port is located outside the housing 
20, although this does not necessarily have to be the case. The second 
injection port 40 can also be located within the housing, or both 
injection ports may be located outside the housing. 
Although the solderable coating can be produced by injecting the two 
components at separate locations in a thermal spray device, those of skill 
in the art will recognize various options. For example, the two components 
can be premixed to a desired coating composition before introducing the 
components into the device through a single injection port. The blended 
materials are then melted by a thermal sprayer device, mixed in the spray 
stream, and co-deposited to create the composite layer of the solderable 
coating. 
The coating of the present invention can be applied to various 
non-solderable surfaces. For example, a coating can be deposited on a 
substrate such as a heat sink made of aluminum, graphite, plastic or other 
normally non-solderable materials. Such a heat sink having a coating as 
described herein can then be readily soldered to an electronic device. 
A part having a solderable coating is soldered to another surface using 
standard methods of soldering. For example, a solder paste or solder 
preform is applied to the solderable coating and if required, a solder 
flux is also applied. The component having the solderable surface is then 
heated to the melting point of the solder. When the solder melts, it flows 
over the surface and wets the coating to create a metallurgical bond to 
the coating. 
Typically, the second component will have a melting point slightly higher 
than that of the solder. For example, a common babbit metal that can be 
used has a solidus temperature of 230.degree. C. (liquidus about 
354.degree. C.), whereas 60% tin, 40% lead solders typically have a 
melting point in the range of 180.degree. to 225.degree. C. 
The present invention, thus generally described, will be understood more 
readily by reference to the following examples, which are provided by way 
of illustration and are not intended to be limiting of the present 
invention. 
EXAMPLES 
A powder of solderable material containing 99.5% copper, and having a 
particle size ranging from 10 microns in diameter to 44 microns in 
diameter, was internally injected into a Miller Thermal SG-100 plasma 
spray gun equipped with a Mach II anode, cathode, and gas injector such as 
the device shown in FIG. 2. Two Miller Thermal powder hoppers were used, 
one containing copper and the other a tin alloy. The copper powder was 
injected at a rate of 36 grams/minute and sprayed at 29.9 kW of DC power. 
Simultaneously, a solder flow enhancing material in powder form was 
externally injected into the plasma. This second component, a tin alloy, 
contained 88% tin, 8% antimony, and 4% copper, and the powder had a 
particle size distribution of 10 microns in diameter to 90 microns in 
diameter. The tin alloy powder was injected at 54 grams/minute. The 
resultant composite layer of the solderable coating contained about 39% 
copper by volume and 61% tin alloy by volume. 
Testing of the coating is accomplished by heating the coated element to 
220-240.degree. C. A mildly reactive (RMA) flux is applied to the coating 
where soldering is to take place. A solder preform comprised of 60% tin 
and 40% lead is placed on the coating and is melted. While the solder is 
in the molten state, it flows over the surface and wets the coating. The 
absence of solder lumping along with good solder flow indicates that the 
coating has been soldered properly. 
A 60/40 lead/tin preform was placed on a coated part that was treated with 
an RMA flux and heated to 180-225.degree. C. The solder melted and flowed 
across the surface, demonstrating good wetting to the coating. 
In another example, similar equipment and materials are used. The equipment 
used to produce the solderable coating includes: A Miller Thermal SG-100 
plasma spray gun that is set up with a standard Mach II set-up, including 
a Mach II anode, cathode and gas injector. Two Miller Thermal powder 
hoppers are used, one containing copper and the other containing a tin 
alloy (88% Sn, 8% Sb, 4% Cu). The particle size distribution of the copper 
alloy is -44+10 (maximum 44 and minimum 10) microns, and the tin alloy is 
-75+38 microns. The copper powder is internally injected into the plasma 
and the tin alloy is externally injected into the plasma. The feed rates 
of the powders are 29 g/min. copper, and 54 g/min. tin alloy. The powder 
feed gas pressures are 80 psi copper and 80 psi tin alloy. The net energy 
the gun operates at is 31.2 kW. This energy is produced by mixing argon 
and helium with gas pressures of 90 psi argon and 60 psi helium. A surface 
is coated with the plasma gun traversing across the surface at 1200 
in./min. and a step increment of 0.25 in. A total of one sweep across the 
surface is required. 
After the part is coated, it can then be soldered to using a 60/40 Sn/Pb 
solder with a mildly reactive flux. The solder surface should be heated to 
225.degree. C. to 275.degree. C. for good soldering to occur. 
Although the foregoing refers to particular preferred embodiments, it will 
be understood that the present invention is not so limited. It will occur 
to those of ordinary skill in the art that various modifications may be 
made to the disclosed embodiments and that such modifications are intended 
to be within the scope of the present invention, which is defined by the 
following claims.