Method and apparatus for aligning and attaching a surface mount component

A method and an apparatus align and attach a leadless surface mount component (402) including a termination at each end of the component (402). The termination has bottom (704) and end (702) portions for attaching to a corresponding pad on a substrate (102) by a reflow solder process (1200). A pad arrangement (100) is formed including two opposite pads (108), each pad (108) occupying a tri-oval-shaped area. The tri-oval-shaped area includes an elliptical area (110) substantially centered under the bottom portion (704) of the corresponding termination of the component (402) when the component (402) is aligned with the pad arrangement (100), and an arcuate area (112) contiguous with the elliptical area (110) and extending towards the opposite pad (108) in a central lengthwise direction. A solder paste (202) is applied to the elliptical area (110), and thereafter reflowed, whereby solder (302) in the solder paste (202) liquefies and flows onto the arcuate area (112), thereby facilitating alignment of the component (402) with the pad arrangement (100).

FIELD OF THE INVENTION 
This invention relates in general to a method and apparatus for mounting 
electronic components to a substrate, and more specifically to a method 
and apparatus for aligning and attaching surface mount components to 
substrate mounting pads. 
BACKGROUND OF THE INVENTION 
There are many well-known methods of mounting electronic components to a 
substrate. One method is the conventional "reflow soldering" process used 
for attaching terminations of surface mount components. In the 
conventional reflow soldering process the terminations of the surface 
mount components have a thin pre-tin solder coating and are attached 
during a manufacturing process to rectangular mounting pads etched onto a 
substrate. The process comprises printing a solder paste through a stencil 
having apertures matching the size and location of the mounting pads, 
placing the surface mount component terminations on top of the solder 
paste in alignment with the mounting pads therefor, and passing the 
substrate and surface mount components through a reflow solder oven for 
heating the pre-tin solder coating and solder paste to a liquefied state 
to attach the terminations to the mounting pads. 
Disadvantageously, during the conventional reflow soldering process, 
termination attachment defects occur because the terminations of the 
surface mount components do not always remain aligned with the mounting 
pads. Errors in initial placement of the surface mount components, 
vibrations from equipment used to move the substrate through a 
manufacturing area, and general handling also can cause misalignment. 
Unfortunately, the mounting pads used in the conventional process are only 
partially effective in correcting any misalignment that occurs. 
The trend of electronic devices, such as selective call receivers, towards 
smaller sizes requiring micro-miniature components tends to increase 
defect rates even further in the conventional reflow soldering process. 
This is because the defect rate due to misalignment increases as the 
terminations, mounting pads, and alignment tolerances become more 
critical. 
Thus, what is needed is a better way of mounting surface mount components 
to corresponding mounting pads on a substrate. A method and apparatus that 
can minimize component misalignment and reduce attachment defects on 
microminiature components is highly desired. 
SUMMARY OF THE INVENTION 
One aspect of the present invention is a method of aligning and attaching a 
leadless surface mount component comprising a termination at each end of 
the component. The termination has bottom and end portions for attaching 
to a corresponding pad on a substrate by a reflow process of liquefying 
and subsequently solidifying a conductive material positioned between the 
termination and the pad. The method comprises the step of forming a pad 
arrangement comprising two opposite pads, each pad occupying a 
tri-oval-shaped area. The tri-oval-shaped area comprises an elliptical 
area substantially centered under the bottom portion of the corresponding 
termination of the component when the component is aligned with the pad 
arrangement, and an arcuate area contiguous with the elliptical area and 
extending towards the opposite pad in a central lengthwise direction. The 
method further comprises the step of applying the conductive material to 
the elliptical area, and thereafter performing the reflow process, whereby 
the conductive material liquefies and flows onto the arcuate area, thereby 
facilitating alignment of the component with the pad arrangement. 
Another aspect of the present invention is a method of increasing and 
better directing surface tension utilized for aligning a leadless surface 
mount component comprising a termination at each end of the component. The 
termination has bottom and end portions for attaching to a corresponding 
pad on a substrate by a reflow process of liquefying and subsequently 
solidifying a conductive material positioned between the termination and 
the pad. The method comprises the step of forming a pad arrangement having 
a center and comprising two opposite pads positioned on either side of the 
center. Each pad occupies a tri-oval-shaped area comprising an elliptical 
area having a center substantially centered under the bottom portion of 
the corresponding termination of the component when the component is 
aligned with the pad arrangement, and an arcuate area contiguous with the 
elliptical area and extending towards the opposite pad in a central 
lengthwise direction. The method further comprises the step of applying 
the conductive material to the elliptical area, and thereafter performing 
the reflow process, whereby the conductive material liquefies and flows 
onto the arcuate area, losing thickness and thereby increasing the surface 
tension of the liquefied conductive material. 
Another aspect of the present invention is a pad arrangement for aligning 
and attaching a leadless surface mount component comprising a termination 
at each end of the component. The termination has bottom and end portions. 
The pad arrangement is for interconnecting the surface mount component 
with other circuitry and comprises a substrate for supporting and 
interconnecting the surface mount component with the other circuitry, and 
two opposite pads formed on the substrate and interconnected with the 
other circuitry. Each pad occupies a tri-oval-shaped area comprising an 
elliptical area substantially centered under the bottom portion of the 
corresponding termination of the component when the component is aligned 
with the pad arrangement, and an arcuate area contiguous with the 
elliptical area and extending towards the opposite pad in a central 
lengthwise direction. The pad arrangement further comprises a conductive 
material applied to the elliptical area and thereafter reflowed onto the 
arcuate area, thereby facilitating alignment of the component with the pad 
arrangement.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, a top orthographic view depicts a pad arrangement 100 
in accordance with the preferred embodiment of the present invention 
comprising a substrate 102 and two tri-oval pads 108. The substrate 102 
has a protective coating 104 (solder resist), which has been removed from 
the tri-oval pads 108 to allow further processing of the pads 108. The 
protective coating 104 is also removed from between the pads 108 and from 
a pad surround area 106 to allow free movement of a surface mount 
component 402 (FIG. 4) for realignment during reflow soldering, as will be 
described herein below. Each tri-oval pad 108 comprises an elliptical area 
110 having a first centroid 111 and an arcuate area 112 contiguous with 
the elliptical area 110. Each tri-oval pad 108 has a second centroid 113, 
the second centroid being positioned closer to the center of the pad 
arrangement 100 than is the first centroid 111. 
Preferably, the substrate 102 comprises a glass filled epoxy material, such 
as FR4 filled epoxy, and the protective coating 104 comprises a thermally 
cured wet-film resist, such as Probimer.TM., manufactured by Ciba-Geigy 
Corporation of Terry Town, N.Y. Preferably, the tri-oval pads are formed 
of copper coated with a tin-lead alloy. It will be appreciated that 
similar alternative materials may be utilized for the substrate 102, the 
protective coating 104, and the tri-oval pads 108. 
Referring to FIG. 2, a top orthographic view depicts the pad arrangement 
100 after a solder paste 202 is applied using a stencil to limit the 
solder paste 202 to only to the elliptical areas 110 of the tri-oval pads 
108 in accordance with the preferred embodiment of the present invention. 
The solder paste 202 is applied evenly, such that the centroid 204 of the 
solder paste is substantially centered over the first centroid 111 of the 
elliptical area 110 after the application. Preferably, the solder paste 
202 is a fine pitch solder paste, such as Kester 247B, manufactured by 
Kester Solder Division, Litton Systems, Inc., of Des Plaines, Ill., 
applied to the elliptical areas 110 by printing through an six-mil (0.15 
mm) stainless steel stencil (not shown) having elliptical openings 
corresponding to the elliptical areas 110. It will be appreciated that 
other materials may be used for the solder paste 202 and the stencil, and 
that other thicknesses of stencil also may be used, depending upon the 
component size and component termination size, and the pad sizes. 
Referring to FIG. 3, a top orthographic view depicts the pad arrangement 
100 after reflow soldering in accordance with the preferred embodiment of 
the present invention. In this view, solder 302 in the solder paste 202 
has liquefied and has flowed onto the arcuate area 112, moving the 
centroid 306 of the solder 302 in the direction of the arrows 304 on each 
of the pads 108, to a position substantially over the second centroid 113 
of the tri-oval pad 108. It will be appreciated that at this point that 
alternative reflow processes, such as solid solder deposition can be 
applied. The solid solder deposition reflow process utilizes solid solder, 
predeposited onto the tri-oval pads 108, flux deposited on each reflowable 
joint on the substrate 102, with subsequent surface mount component 
attachment during reflow. However, as will be explained herein below in 
accordance with the preferred reflow process, the movement of the solder 
302 onto the arcuate area 112 is beneficial to the alignment of a surface 
mount component 402 (FIG. 4) being attached to the pad arrangement 100 
during the reflow soldering process. 
Referring to FIGS. 4, 5 and 6, top orthographic views depict the pad 
arrangement 100 and the surface mount component 402 before, during, and 
after reflow soldering, respectively, in accordance with the preferred 
embodiment of the present invention. In FIG. 4, terminations 404 of the 
surface mount component 402 are depicted as grossly misaligned with 
respect to the two tri-oval pads 108 of the pad arrangement 100. In FIG. 
5, during the reflow soldering process, the solder 302 is liquefied by 
heat, and flows onto the terminations 404 and onto the arcuate area 112, 
thereby generating movement and increasing surface tension forces 502, 
which tend to rotate the surface mount component 402 in the direction 
indicated by the curved arrow 504. In FIG. 6, after the reflow soldering 
process, the surface mount component 402 has been pulled into alignment 
with the pad arrangement 100 by the surface tension forces 502 and 
movement of the solder 302 when the solder 302 was liquefied and 
resolidified during the reflow solder process. 
Referring to FIGS. 7, 8, and 9, side orthographic longitudinal 
cross-section views depict the pad arrangement 100 and the surface mount 
component 402 before, during, and after reflow soldering, respectively, in 
accordance with the preferred embodiment of the present invention. In FIG. 
7, ends 702 and bottoms 704 of the terminations 404 are shown resting on 
the solder paste 202, which preferably has been printed onto only the 
elliptical areas 110 of the tri-oval pads 108. In FIG. 8 the solder paste 
has melted, forming the solder 302, which is in a liquefied state. The 
solder 302 is shown having moved onto the arcuate areas 112 of the 
tri-oval pads 108, and also partially adhering to the terminations 404 of 
the surface mount component 402. The movement of the solder 302 onto the 
arcuate areas 112 reduces the thickness of the solder 302, thereby 
increasing the surface tension of the solder, and thus increasing the 
forces tending to align the surface mount component 402 with the pad 
arrangement 100. In FIG. 9, the solder 302 substantially adheres to the 
terminations 404 and the tri-oval pads 108. The resolidified upper 
surfaces of the solder 302 proximate the tri-oval pads 108 are concave in 
shape--evidence that an increased level of surface tension has occurred 
for aligning the surface mount component 402 during the reflow soldering 
process. 
A side advantage provided by the arcuate areas 112 is an increase in the 
tolerance window for placing the surface mount component without 
deleterious effects. This result is due to the inherently higher accuracy 
of the process for forming the pad arrangement 100 compared to the 
accuracy of printing the solder paste 202. In other words, the arcuate 
areas 112 can be placed closer to one another without causing an 
electrical short than would be possible if the entire pad arrangement were 
to be printed with the solder paste 202. The closer placement of the 
arcuate areas provides a larger area of liquid solder for contacting and 
aligning a component 402 that has become misaligned prior to the reflow 
soldering process. 
Referring to FIG. 10, a side orthographic longitudinal cross-section view 
depicts a "tombstoned," i.e., up-ended, surface mount component 402 after 
reflow soldering utilizing a conventional pad arrangement 1000. The 
conventional pad arrangement 1000 comprises a substrate 1002 and pads 
1004, which extend little or no distance beyond the bottoms 704 of the 
terminations 404 of the component 402. Due to processing variations, the 
component 402 sometimes can become cantilevered during reflow soldering by 
the solder 1006 surrounding one of the terminations 404. In such a 
cantilevered position the opposite termination 404 may not be able to 
contact the opposite solder 1008, and the attachment of the component 402 
is rendered defective. 
Referring to FIG. 11, a side orthographic longitudinal cross-section view 
of the surface mount component 402 and the pad arrangement 100 in 
accordance with the preferred embodiment of the present invention depicts 
an anti-tombstoning force generated during reflow soldering. As solder 
moves onto the arcuate area 112 at the start of the reflow soldering 
process, a downward force 1102 between the termination 704 and the arcuate 
area 112 is generated by the movement and the resultant increased surface 
tension of the solder 302. The downward force 1102 tends to rotate the 
component 402 in the direction shown by the curved arrow 1110, thereby 
bringing the elevated termination bottom 704 downward and into contact 
with the solder 302. Once the bottoms 704 of both terminations 404 are in 
contact with the liquefied solder 302, alignment and attachment of the 
component to the pad arrangement 100 can proceed normally, as described 
herein above in reference to FIGS. 4-9. 
Referring to FIG. 12, a block diagram of the preferred manufacturing 
process 1200 for mounting the surface mount component 402 in accordance 
with the preferred embodiment of the present invention shows a photo 
lithographic processor 1202. The photo lithographic processor 1202 is used 
to deposit patterns of a photo-imageable etch resist, such as Dupont 
Vacrel, onto the substrate 102. The substrate 102 is plated with a metal, 
such as one-half ounce copper, covered with, for example, a 
hot-air-solder-leveled tin-lead alloy, for use in forming circuit paths 
and the tri-oval pads 108. Next, the substrate 102 passes to a chemical 
etcher 1204 comprising an etchant such as ferric chloride for etching the 
metal plating not protected by the etch resist, thereby removing all the 
metal except for desired circuit paths and the tri-oval pads 108 to 
comprise a printed wiring board. 
In a similar manner, the photo lithographic processor 1202 and the chemical 
etcher 1204 are used to make a stencil from a material such as stainless 
steel, the stencil having apertures matching the location, size, and shape 
of the elliptical areas 110 of the tri-oval pads 108. Then the substrate 
102 moves to a resist applicator 1205, where the protective resist 
material is screened over the substrate, thermally cured, and then 
selectively removed from the tri-oval pads 108 and the pad surround areas 
106. 
Next, the substrate 102 and the stencil move to a solder printer 1206 where 
the apertures of the stencil are aligned with the elliptical areas 110 on 
the substrate 102, and the solder paste 202 is applied to the elliptical 
areas 110 through the stencil. After receiving the solder paste 202, the 
substrate 102 passes to an automated placement unit 1208 that places the 
surface mount components 402 on top of the solder paste 202 with the 
surface mount component terminations 404 approximately aligned over the 
elliptical areas 110 of the tri-oval pads 108, including the arcuate areas 
112. 
Next, the substrate 102 and surface mount components 402 pass into a reflow 
oven 1210 for liquefying the solder 302 in the solder paste 202. 
Preferably, the reflow oven has an inert atmosphere to control oxidation 
of materials while in the reflow oven. Once the solder 302 has become 
liquefied, the solder 302 "wets," i.e., covers uniformly and adheres to, 
the surface mount component terminations 404 and the tri-oval pads 108 
(FIG. 1). While in the reflow oven 1210 the surface mount components 402 
are allowed to move freely in response to forces generated by surface 
tension of the liquefied solder. A characteristic of the surface tension 
of a liquid is that the surface tension is directed towards minimizing the 
surface area of the liquid. This characteristic causes the tri-oval pads 
108 and the surface mount component terminations 404 to interact with the 
surface tension of the liquefied solder 302 to pull the surface mount 
component terminations 404 into aligned positions with respect to the 
tri-oval pads 108 therefor, as is depicted in FIG. 6. 
When the substrate 102 emerges from the reflow oven 1210, the solder 302 
resolidifies, thus attaching the surface mount component terminations 404 
to the tri-oval pads 108 in the aligned position achieved while in the 
reflow oven 1210. The alignment automatically provided during the 
preferred manufacturing process in accordance with the present invention 
is responsible for a reduction in attachment defect rate compared to 
conventional processes. It will be appreciated that similar processes and 
materials can be substituted for the processes and materials described 
above for the preferred embodiment of the present invention. 
Referring to FIG. 13, an isometric view of a selective call receiver 1300 
constructed in accordance with the preferred embodiment of the present 
invention depicts a printed wiring board 1302 and surface mount components 
1304. The surface mount components are attached to tri-oval pads on the 
printed wiring board 1302 similar to the tri-oval pads 108 in accordance 
with the preferred embodiment of the present invention. The selective call 
receiver 1300 further comprises a housing 1306 for protecting circuitry 
contained therein, and user controls 1308 for control of the operation of 
the selective call receiver 1300. 
Referring to FIG. 14, an electrical block diagram of the selective call 
receiver 1300 in accordance with the preferred embodiment of the present 
invention comprises an antenna 1402 for intercepting RF signals. The 
antenna 1402 is coupled to a receiver 1404 for receiving and demodulating 
the RF signals intercepted. A decoder 1406 is coupled to the receiver 1404 
for decoding a demodulated address transmitted in any of a number of 
wellknown signaling protocols, such as POCSAG or GSC selective call 
signaling. A microprocessor 1408, e.g., the MC68HC05C8 or C11 series 
microcomputers manufactured by Motorola, Inc. of Schaumburg, Ill., is also 
coupled to the receiver 1404 for processing the demodulated information. 
The microprocessor 1408 is responsive to the decoder 1406 and is coupled 
to a random access memory (RAM) 1410 for storing recovered information 
having an address assigned to the selective call receiver 1300. An alert 
generator 1412 is coupled to the microprocessor 1408 for providing an 
audible or tactile alert to a user when the microprocessor 1408 has a 
message ready for presentation. 
An output device 1414 comprises a visual display or an audio transducer or 
both, the output device 1414 also being controlled by the microprocessor 
1408. A control section 1416 comprises user accessible controls for 
allowing the user to command the microprocessor 1408 to perform the 
selective call receiver operations well known to one of ordinary skill in 
the art, and typically includes control switches such as an on/off control 
button, a function control, etc. 
The microprocessor 1408 is coupled to a read-only memory (ROM) 1421 
comprising operating system software for controlling the selective call 
receiver 1300. It will be appreciated that the functions of the decoder 
1406, the RAM 1410, and the ROM 1421 may be incorporated into the 
microprocessor 1408 as well, as contiguous components thereof. It will be 
further appreciated that other types of non-volatile memory, e.g., 
programmable read-only memory (PROM) and electrically-erasable 
programmable read-only memory (EEPROM), may be used as well for the ROM 
1421. 
Thus, the present invention advantageously provides a better method and 
apparatus for aligning and attaching a surface mount component to a 
corresponding mounting pad arrangement. The present invention minimizes 
component misalignment and tombstoning, thereby reducing attachment 
defects compared to conventional methods and apparatus.