Method and means for positioning surface mounted electronic components on a printed wiring board

An SMT electronic component is mounted to a solder-bearing floatation plate by fusible or other heat-responsive releasable mounting means which suspend the component above the floatation plate. The bottom of the floatation plate is effectively substantially the mirror image of a component-positioning pad formed on the board surface adjacent the solder-bearing contact pads corresponding to the electrical contacts on the component. In the assembly process, the floatation plate is placed on the positioning pad. The solder on the bottom of the floatation plate has a melting point lower than the release temperature of the mounting means and the melting point of the solder on the contact pads. With the floatation plate on the component-positioning pad, on heating the solder on the floatation plate liquifies first, wetting the component-positioning pad and floating the floatation plate and component on a thin film of molten solder. Surface tension forces bring the floatation plate into registry with the component-positioning pad. On further heating, the solder on the contact pads liquifies, and the heat-responsive mounting means allows the component to fall freely onto the contact pads. Guide means are provided to prevent rotational and lateral displacement of the component during the fall. The disclosure includes alternative floatation plate constructions and component mounting arrangements.

BACKGROUND 
1.Field of the Invention 
This invention relates to methods and means for positioning surface mounted 
electronic components, especially fine pitch components, on the surface of 
a printed wiring board. More particularly, it is concerned with methods 
and means which employ surface tension effects for precisely positioning 
electronic components on a printed wiring board during the assembly 
process. 
2. Prior Art 
Fine pitch surface mounted electronic components with lead spacing of 
0.040" or less offer a number of advantages over conventional 50 
mil-spaced components in addition to small size. These include, among 
others, increased input and output handling capacity, enhanced throughput 
speed, relatively low cost, and the potential for substantial reductions 
in the cost of integrated systems adapted to utilize fine pitch 
technology. 
Unfortunately, the characteristics that provide these advantages, small 
size, high lead count and close lead spacing, are the source of a number 
of significant disadvantages as well. By way of example, lead-to-pad 
misregistration, solder bridging, and slumping, all relatively minor, 
fairly easily avoided annoyances with 50-mil on center components, become 
major problems when pad spacing gets down to 40-mils or less. 
To some extent these problems can be overcome or alleviated by conventional 
means, such as taking pains to improve the quality of the circuit artwork, 
providing for greater consistency in solder plating on the contact pads, 
paying special attention to circuit and pad geometry, exercising more 
effective control over the solder reflow process, and the like. But 
greater precision, special handling and enhanced quality control are 
costly. Ultimately, the conflicting demands of miniaturization and 
cost-effectiveness can be satisfied only by improving the accuracy with 
which the components are positioned on the board surface. 
Heretofore, the usual approach to improving placement accuracy has been to 
design ever more sophisticated pick-and-place machines. Computer 
controlled and provided with remote monitors and elaborate robotics, these 
high output precision machines are so costly to manufacture, complicated 
to use and difficult to maintain and repair they are for all practical 
intents and purposes out of the reach of many potential users. The subject 
invention represents a totally different approach. 
One of the objects of the invention is to provide a novel method and a 
family of unique devices for accurately positioning surface mounted 
components, particularly fine-pitch components, on the board surface 
without need for elaborate high precision pick-and-place equipment. 
Another object is to provide a method and means for accurately positioning 
components, which employ surface tension principles in place of customary 
prior art pick-and-place techniques. 
Yet another object is to provide a method and means for positioning surface 
mounted components, that require a minimum of human intervention. 
Still another object is to furnish a family of high-precision positioning 
devices that are readily adaptable for use with a variety of surface 
mounted components. 
A further object is to provide a line of such devices that are inexpensive 
to manufacture, simple to use and substantially free from maintenance and 
repair needs. 
Other objects will become apparent from the following summary of the 
invention and detailed description of several of its preferred 
embodiments. 
SUMMARY OF THE INVENTION 
On a conventional printed wiring board the location or "footprint" of an 
electronic component on the board surface is defined by metallic contact 
pads formed on the surface in a pattern corresponding to the layout of the 
terminal leads or other electrical contacts on the component. In 
production, the pads are chemically cleaned and tinned or otherwise 
conventionally treated to make them "solderable," that is, capable of 
being wetted by molten solder. Then solder is applied to the pads by 
conventional means, for example, as a stencilled and the components are 
mounted to the board with their leads or contacts resting on their 
associated pads in readiness for solder bonding by reflow, infrared 
heating, or other conventional means. 
In a preferred embodiment of the invention, the wiring board is 
substantially indistinguishable from a conventional board, except for the 
provision of a one or more novel component-positioning pads in conjunction 
with the footprint of each component. In many instances, these positioning 
pads can be retrofitted into an existing printed wiring layout with little 
or no design modification. With new wiring designs, the 
component-positioning pads are easily integrated into the board layout at 
its inception. 
In the subject invention, a surface mounted electronic component is mounted 
to what I call a "floatation plate." As its name suggests, the floatation 
plate is intended to serve as a kind of specialized raft for the 
component. 
The generally flat bottom of the floatation plate is effectively 
substantially the mirror image of the plan shape of the 
component-positioning pad for that component on the board surface. By this 
I mean, the overall peripheral shape of the pad or pads on the board 
surface and the overall peripheral shape of the floatation plates are 
substantially similar, even though each is made up of individual parts or 
segments of dissimilar shapes or sizes, respectively. Preferably, though 
not necessarily, the floatation plate is more or less the same size as the 
positioning pad. The bottom of the plate is solderable, and bears a layer 
or shim of solder in solid or paste form. 
Fusible, or other heat-responsive mounting means on the floatation plate 
support or suspend the component at a height above the bottom surface of 
the plate, that is, at a height above the surface of the board. The 
mounting means is adapted to release the component when the temperature of 
the mounting means, ("release temperature") reaches the melting point of 
the solder on the contact pad or pads. The solder on the bottom of the 
floatation plate has a melting point lower than the release temperature of 
the mounting means and lower than the melting point of the solder on the 
contact pads associated with the component. 
In production, the floatation plate with the component mounted to it is 
placed on the prepared board by convenient assembly means, the principal 
requirement being that the plate be located roughly on its associated 
component-positioning pad. Since great precision is not required in 
placing the plates, the board can be populated using relatively 
unsophisticated pick-and-place equipment, or even by hand, if 
circumstances warrant. 
The assembled board is processed in much the same manner as a board 
assembled by conventional means, the next step being reflow, infrared, or 
some other method of heating. On heating, the solder on the floatation 
plate liquifies first, wetting the component-positioning pad. The 
floatation plate with its component cargo is buoyantly supported on the 
thin film of liquid solder. The floating plate is free to move virtually 
frictionlessly on the positioning pad. Asymmetrical surface tension forces 
created in the film by any rotational or translational displacement of the 
floatation plate with respect to the positioning pad quickly reposition 
and reorient the plate to establish a state of equilibrium. The effect is 
to bring the floatation plate into precise symmetrical registry with the 
component-positioning pad. To take advantage of this effect, in designing 
the board layout care is taken to locate and orient the 
component-positioning pad so that when the floatation plate is in 
symmetrical registry with the positioning pad, the component on the 
floatation plate is in registry with the footprint of the component on the 
surface. 
On further heating, the solder on the contact pads liquifies and the 
heat-responsive mounting means releases the component, allowing it to fall 
freely onto the board surface. As the floatation plate is oriented, the 
component leads or contacts should land precisely on the contact pads. In 
some instances, however, external influences, such as unavoidable 
vibration or irregular motion in the processing line may effect the 
trajectory or orientation of the falling component. To prevent this, in 
several alternative embodiments of the invention guide means are provided 
to prevent rotational and lateral displacement of the component during the 
fall. 
To compensate for and accommodate variations in component structures and 
surface mounting characteristics and requirements, the invention 
encompasses alternative floatation plate constructions and component 
mounting arrangements. By way of example, as indicated earlier the 
invention can be adapted as readily for use with biaxial devices having 
metallized termination areas as to components, such as gullwing-lead 
devices or quad packages having terminal lead contacts. It will be 
understood, therefore, that when I use the term "contact" with reference 
to a surface mounted device, I intend to embrace all types of electrical 
contact structures, including but not limited to conductive surfaces, 
plates, and leads. 
For a fuller understanding of the invention, reference is made to the 
accompanying drawings, in which:

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIGS. 1-1c, a typical leaded fine pitch surface mounted 
electronic component 11 comprises a non-conductive housing 12 
encapsulating the device itself, and a plurality of conductive terminal 
leads 13 extending outwardly and downwardly from the housing 12 to make 
electrical connection with the conductive traces and pads 14 which give 
the printed wiring board 15 its name. In other types of components, such 
as biaxial devices (not shown) contact is made with the traces and pads on 
the board through conductive areas on the surface of the device or through 
other structural terminals, rather than through leads. For convenience, I 
shall describe the invention as it is exemplified by the leaded type of 
component, but the reader will understand that it is applicable to other 
types of devices as well. 
In designing the artwork for the printed wiring board 15, provision is made 
for a component-positioning pad 16 to accommodate the floatation plate 17. 
Advantageously, plate 17 is made of sheet metal, preferably a lightweight 
metal, such as aluminum, which can be formed by precision-stamping or 
photochemical etching. The pad design and precise location with respect to 
the component footprint must take into account the size, shape, weight and 
other pertinent characteristics of the component 11, as well as the layout 
of the footprint and the surrounding circuit features. 
As far as I can ascertain, the shape of the pad 16 is a matter of choice. 
For single pads, I prefer to use simple regular geometric figures, 
principally squares, rectangles and equilateral triangles. Clearly, since 
the purpose of the pad 16 is to provide angular as well as lateral 
orientation to the floatation plate 17, an individual circular positioning 
pad 16 or plate 17 would have limited utility. As will be shown shortly, 
in some instances it is useful or necessary to provide a positioning pad 
or floatation plate of more complex design. Whatever the design, 
preferably the pad 16 and plate 17 are adapted to place the center of mass 
of the component 11 as close to the center of the footprint as possible. 
Preferably, the bottom of floatation plate 17 is formed as the substantial 
mirror image of the component positioning pad 16. A small disparity in 
size between plate 17 and pad 16 will not seriously denigrate the 
operation of the invention. To the contrary, I believe, but I am by no 
means certain, that an overlap of about 10 mils by one or the other of the 
two elements may be advantageous. As I shall explain below, in some 
instances it is even possible to use pads and plates of differing sizes 
and shapes to achieve the same result. 
In the embodiment of FIG. 1-1c, component 11 is releasably mounted to 
floatation plate 17 by corner posts 19 or a connecting shim (not shown) of 
solder, fusible epoxy or other suitable heat-responsive material, secured 
by conventional means, for example adhesive bonding, to the top of plate 
17 and the underside of component 11. 
In mounting the component 11 to the floatation plate 17, conveniently, 
plate 17 is chrome, tin, or tin-lead plated for solderability and a shim 
of solder 18 is attached to its plated bottom surface. The corner posts 19 
or connecting shim which will mount the component 11 to plate 17 are then 
bonded to the upper side of plate 17. A layer of adhesive, preferably a 
high temperature-resisting epoxy, is applied to the tops of posts 19 or 
the upper surface of the connecting shim. At this point, a conventional 
vision assisted alignment machine is employed to align component 11 
precisely with plate 17 and bring the posts 19 or shim and the component 
11 into bonding contact. When the epoxy has hardened and cured, the plate 
17 and component 11 form a unitary package with the component's leads 13 
in perfect alignment with plate 17. 
Conventionally, the electrical contact pads 14 on a printed wiring board 15 
have a melting point of about 216.degree. C. To insure compatibility of 
the subject invention with existing production equipment, and especially 
with existing reflow facilities and furnaces, this temperature is targeted 
as the "release temperature" of the posts 19 and connecting shim mounting 
the component 11 to floatation plate 17. Preferably, the solder 18 on the 
bottom of plate 17 has a melting point of about 140.degree. C. 
After assembly, with plate 17 roughly positioned on pad 16, heating the 
package to 140.degree. C., for example in an infrared furnace, liquifies 
the shim 18. The molten solder wets pad 16 and floats plate 17 and 
component 11. Surface tension centers and orients plate 17 on pad 16. 
Further heating to a temperature of about 216.degree. C. fuses the posts 
19 or the connecting shim supporting the component 11 and allows the 
component to fall freely onto board 15, each of the leads 13 landing 
precisely on its respective contact pad 14. 
Aside from the provision of positioning pads on the board surface, the only 
change in existing practice my invention will call for is a slight 
modification in the design and construction of the electronic components 
used with my self-aligning floatation devices. To allow space for the 
floatation plate 17 after the component 11 lands on the board 15, the 
leads 13 will have to be lengthened or reformed so as to elevate the 
component an additional approximately 20 mils. above the board surface. 
The embodiment of FIGS. 1-1c is quite adequate for applications which do 
not call for a high degree of precision or repeatability in component 
placement. Where precise placement and reliability are needed, however, 
pains must be taken to insure that the orientation and trajectory of the 
component are not disturbed while it is falling to the board surface. The 
embodiments of the invention shown in FIGS. 2-2e provide a pair of 
illustrative guidance mechanisms or landing devices for achieving that 
result. 
Outwardly, the only evidence of the presence of the landing device of FIG. 
2d visible on examination of component 21 are the access ports in the four 
corners of the housing 22. The device of FIG. 2e is virtually 
undetectable. Neither of the landing devices shown requires a major 
modification of the basic design of any existing electronic component to 
which these devices can be applied. 
Referring to FIG. 2d, an upstanding alignment pin 29b is welded or brazed 
to the floatation plate 27. In mounting the component 21 to plate 27, the 
pins 29b are inserted into alignment holes 29d provided in some convenient 
part 29c of the structure of the component. No additional alignment will 
be needed during the mounting process. 
Preferably the part 29c is of sufficient thickness to serve as a sleeve for 
guiding the pins 29b when the component 21 is in free-fall. 
In this embodiment of the invention, the component 21 is mounted to plate 
27 by means of a fusible sleeve 29a bonded to plate 27 and component 21. 
The fuse point or release temperature of sleeve 29a is well above the 
melting point of the solder 28 on the bottom of plate 27. Preferably it is 
the same as the melting point of contact pads 24, 216.degree. C., and the 
melting point of the solder 28 is about 140.degree. C., as in the previous 
embodiment. 
In the embodiment of FIG. 2e, the component 21 is mounted to the plate 27 
by means of an upstanding pin 29f secured to plate 27. Pin 29f passes 
through a small opening in a washer 29g bonded by an appropriate adhesive 
to the underside of component 21. A bead 29h of solder, wax, adhesive, or 
the like, having a relatively high melting point supports plate 27 on 
washer 29g. An inverted cup 29i of fusible material secured to the top of 
washer 29g acts as a spacer to support component 21 on plate 27. The 
melting point of cup 29i is relatively high, preferably on the order of 
about 216.degree. C. Again, as in the two previous examples, the melting 
point of the solder 28 on the bottom of plate 27 is well below that of the 
releasable mounting means 29i, and preferably about 140.degree. C. 
The package defined by plate 27 and component 21 is provided with an array 
of floatation pads 26. Plate 27 is stamped out of a sheet of metal as a 
unitary element. It will be noted that the presence of relatively small 
features, such as webs 27a and tabs 27b, in addition to the principal 
design features of the positioning pad or pads is believed to have little 
if any adverse effect on the self-aligning ability of the device. 
Positioned on its floatation pads 26, on initial heating to the fusing 
point of solder 28, plate 27 and component 21 are floated on a film of 
solder and brought into registration with the pad array. On further 
heating to the melting temperature of the cup 29i, component 21 is allowed 
to fall onto the surface of board 25, its leads 23 in perfect registry 
with pads 24. 
The embodiment of FIGS. 3-3b illustrates two additional features of the 
invention. The first is its ability to employ an array of positioning pads 
36 and corresponding floatation plates 37 which extend laterally well 
beyond the walls of the component 31. The second is its use of two further 
alternative heat-sensitive mounting means, an expansion ring 39a, and a 
high temperature fusible or combustible adhesive, such as the well known 
cyano acrylic "Crazy Glue," for attaching a component 31 to the plates 37. 
FIGS. 4-4b illustrate two further embodiments of the invention. As shown in 
FIG. 4, an array of floatation plates 47 with attached solder shims 48 on 
their low surfaces are connected through a superstructure 43, 47a which 
overarches the plates 47 and provides means for supporting a component 41 
from above, rather than from below as in the previously described 
embodiments. In the version of FIG. 4a, a guidance device, inverted pin 
49b extending through a guidance hole 49d in structural element 49c, and a 
high temperature fusible material 49a, serve as the attachment means. In 
the embodiment of FIG. 4b, the component 41 is mounted to the 
superstructure 43 by means of a layer of adhesive 49e such as "Crazy 
Glue," which either fuses or oxidizes completely at a temperature close to 
the 216.degree. C. melting point of the solder on contact pads 44. 
FIGS. 5, 5a, 6 and 6a illustrate the application of the principles of my 
invention to self-aligning devices which employ floatation plates and 
positioning pads whose individual components are dissimilar in sizes and 
shapes. 
In FIG. 5a, to conserve wiring board surface 55, instead of one pad, an 
array of individual spaced square positioning pads 56 are provided. A 
single floatation plate 57 designed to conform in plan to the overall 
periphery of the array of pads 56 supports the electronic component. In 
FIG. 6a, a, rectangular plate 67 is adapted to support a component on two 
triangular positioning pads 66. In each instance, the plates 57, 67 are 
effectively substantially the mirror image of the plan shape defined by 
pads 56, 66 respectively. Surface tension urges the floatation plate 57,67 
into a predetermined position in which the forces are in equilibrium. 
While I have described my invention in terms of several preferred 
embodiments, it is not to be construed as limited to those embodiments, 
and they are to be regarded as illustrative rather than restrictive. It is 
my intention by this specification to encompass any and all variations of 
the examples I have chosen for purposes of the disclosure, which do not 
depart from the spirit and scope of the following claims.