Method and apparatus for selective soldering

Disclosed is a method and apparatus for applying flux and solder in order to attach connectors to printed circuit boards (PCBs). Flux is applied to the connector tails and the PCB pads by means of a roller mounted above a flux reservoir. The piece parts are then exposed to a solder wave such that the solder sticks only to the selected area to which the flux was applied.

BACKGROUND OF THE INVENTION 
This invention relates to selective soldering and, in particular, to 
electrical and mechanical attachment of connectors to printed circuit 
boards (PCBs). 
In the fabrication of circuit packs, electrical connectors are usually 
mounted on one edge of a printed circuit board. The connectors typically 
include either pins or pin-receiving receptacles within a housing and tail 
portions extending therefrom which must be electrically connected to pads 
on the circuit board. This connection is usually achieved by soldering the 
tails to the pads. (See, e.g., U.S. Pat. No. 5,080,722 issued to Englert 
et al.) 
One method of soldering components in general involves applying a flux and 
solder to the piece part and, subsequently, cleaning off the excess 
material. (See, e.g., U.S. Pat. No. 4,720,324 issued to Hayward and U.S. 
Pat. No. 4,306,674 issued to Charles et al.) The cleaning, however, 
generally requires the application of chlorofluorocarbon (CFC), such as 
FREON.RTM., which is harmful to the environment. While alternative 
cleaning techniques have been proposed (see Patent to Englert et al. cited 
above), it is also desirable to eliminate the cleaning step altogether. 
One approach to no-clean soldering is to apply the flux in a carefully 
controlled manner by spraying the entire board and then apply solder by 
wave soldering. Since only the amount required to ensure a good solder 
connection is applied, no cleaning is required. (See L. A. Guth and J. R. 
Morris, "No-Clean Soldering Processes," AT&T Technical Journal, pp. 37-44 
(March/April 1992). 
While such an approach is adequate for soldering components in general to 
PCBs, attachment of connectors to PCBs present special problems since the 
PCB pads are usually close to the connector housing and it is generally 
required to keep flux out of the housing. 
SUMMARY OF THE INVENTION 
The invention, in one aspect, is a method of soldering a component to pads 
on a printed circuit board. A flux is applied selectively to the pads by 
means of a roller which is rotatably mounted above a flux reservoir so 
that the roller picks up flux from the reservoir and applies it by 
contacting the pads as the board moves over the roller. Solder is then 
selectively applied to the pads by directing a wave of solder thereto such 
that the solder sticks essentially only to the pads. 
In accordance with another aspect, the invention is an apparatus for 
soldering a component to pads on a printed circuit board. The apparatus 
comprises a roller rotatably mounted above a flux reservoir and positioned 
such that the roller picks up flux from the reservoir and deposits it on 
said pads as the board moves over the roller. The apparatus further 
comprises a solder reservoir including means for producing a solder wave 
in a narrow area with a width essentially equal to the width of the pads.

DETAILED DESCRIPTION 
FIGS. 1 and 2 illustrate a typical circuit pack which may be soldered by 
the present invention. The pack includes a printed circuit board, 10, and 
a pair of electrical connectors, 11 and 12, one on each major surface of 
the board at one edge. The connectors in the example are of the socket 
type which include tynes (not shown) within the connector housing for 
receiving conductive pins therein. Electrically coupled to each tyne is a 
conductive tail, e.g., 13, which extends out of the connector housing. 
Each tail needs to be electrically connected to one of a plurality of 
pads, e.g., 14, formed on the major surfaces of the board, 10. Each pad, 
in turn, is electrically connected to some other portion of the board by a 
deposited conductive line, e.g., 15. In this example, the pads are shown 
as connected to via holes, e.g., 16, in order to electrically connect the 
two sides. However, it will be appreciated that a wide variety of 
components can be mounted on the board and coupled to the pads. 
The pads and conductive lines are typically made of copper with a thickness 
of approximately 0.035 millimeter. The pads typically measure 2.0 by 3.0 
millimeters and are usually spaced approximately 1.8 to 2.0 millimeters 
from the connector housing. 
The tails, e.g., 13, are initially unattached to their corresponding pads, 
e.g., 14. The tails are then fluxed and subsequently soldered to the pad 
in a manner which keeps flux away from the connector housing and requires 
no cleaning after the soldering operation. 
FIG. 3 illustrates schematically the apparatus and method of the invention. 
Each component shown in FIGS. 1 and 2 is snapped into a holder (e.g., 30 
of FIGS. 4 and 5) on a conveyor belt, 20. The conveyor carries the 
components over a first station, 21, which applies flux to the pads and 
will be described in more detail below. The components then pass over 
station 22 where they are heated. This station typically includes a 12 
element preheater. The components are usually heated to a temperature 
within the range 93.3 to 104.4 degrees C. 
The components then pass over station 23 where solder is deposited by wave 
soldering to the areas where flux was previously applied. This portion 
will also be described in more detail below. The solder employed is 
typically 60 percent tin and 40 percent lead. 
The conveyor then passes wheels 24 and 25 which causes the components to 
flip over to allow processing of the opposite surfaces of the components. 
Thus, the components pass over stations 26, 27 and 28 which are 
essentially equivalent to stations 21, 22 and 23, respectively, so that 
when the components reach the end of the line, pads and tails on both 
major surfaces of the boards will be soldered. 
FIGS. 4 and 5 illustrate in more detail the station, 21 or 26, for applying 
flux to the components. The components themselves have been omitted from 
FIG. 4 to illustrate the apparatus. A roller, 31, is mounted above a 
reservoir, 32, which includes the liquid flux material. The roller in this 
embodiment is an open cell rubber disc, 33, sandwiched between two 
stainless steel press plates, 34 and 35 of FIG. 5. The disc and plates are 
mounted to a shaft, 36, which is held at one end by a support, 37, and at 
the other end is driven by a motor, 38. The shaft and the roller are 
mounted so as to be free to rotate in either a clockwise (station 21) or 
counterclockwise (station 26) direction by the action of the motor 38. 
The width of the rubber disc is essentially the width of the pads (e.g., 14 
of FIG. 1) of the components but can be adjusted, e.g., by stacking discs, 
to accommodate varying pad widths. The roller is mounted so that a minimum 
(approximately 3.0 millimeters) portion of the disc is submerged in the 
flux in the reservoir 32. 
Thus, in operation, the roller is driven by the motor at a speed which 
matches the speed of the conveyor. That is, the RPM of the motor is chosen 
to give a tangential speed of the disc which essentially equals the speed 
of the conveyor. In a typical example, an RPM of approximately 4 RPM would 
be appropriate for a conveyor speed of 364 millimeters per minute. As the 
components of FIGS. 1 and 2 move over the roller, the disc will make 
contact with the pads and with the tails attached thereto, thus depositing 
the flux from the reservoir onto the row of pads (e.g., 14) of each 
component. The flux will be confined to the pads and to the spaces 
therebetween since, as previously noted, the width of the disc 33 is 
essentially equal to the width of the pads. 
In a typical example, a low solids flux comprising approximately 98 percent 
isopropyl alcohol and approximately 2 percent activator, such as sold 
under the name LONCO 27F, is deposited onto the pads and tails to a 
thickness within the range 0.05 to 0.08 millimeter by means of the roller. 
It may be possible to dispense with the motor and cause the roller to 
rotate solely by the friction of the components contacting the disc. 
Alternatively, a clutch arrangement (not shown) would also be supplied in 
combination with the shaft and motor so that the roller could turn slowly 
to keep the surface of the disc wet until such time as the disc was 
contacted by the components. Then, the clutch could disengage the motor 
and the roller would be rotated by the components. It will also be 
appreciated that several rollers could be mounted on a single shaft to 
apply flux to parts with several rows of pads. In such cases, the rollers 
could have varying widths to accommodate varying pad widths in different 
rows. 
FIGS. 6 and 7 illustrate in more detail the soldering stations, 23 and 28 
of FIG. 3. Liquid solder, 40, such as 60 percent tin and 40 percent lead, 
is provided within a standard solder pot or reservoir, 41. The pot 
includes a standard pump (not shown) which creates solder waves which will 
contact the components as they pass over the pot. The top of the pot is 
covered by a plate element and a narrow columnar element, 42, (weir). The 
element 42 has a narrow opening, 43 of FIG. 7, which is essentially the 
same width as the pads (e.g., 14 of FIG. 1) of the components to be 
soldered. The element 42 is mounted so that its length dimension is 
essentially parallel to the conveyor and the opening 43 is aligned with 
the component pads. 
Thus, in operation, as each component passes over the solder station, waves 
of solder will be applied essentially only to the pads (e.g., 14 of FIG. 
1) of each component through the narrow opening, 43, in the element 42. 
The solder will stick to the solder pads coated at the flux station (21 or 
26 of FIG. 1) so that solder is selectively applied to the pads and tails. 
(It will be appreciated that, although flux is also applied to spaces 
between the pads, these spaces are covered by a solder mask (not shown) to 
which the solder will not stick.) The solder will burn off all the 
deposited flux. Thus, the need for post-solder cleaning is avoided. The 
thickness of the solder layer is typically in the range 0.1 to 0.4 
millimeter. 
As before, a plurality of elements, 42, may be provided over the solder pot 
41 to apply solder to a plurality of rows of pads on the components. 
The invention is most useful where the pads have a narrow width, usually in 
the range 2.0 to 3.0 millimeters, and where the pads are close to the 
connector housing (11, 12), usually a distance of no greater than 2 
millimeters so that flux needs to be kept out of the housing. 
Various modifications of the invention will become apparent to those 
skilled in the art. All such variations which basically rely on the 
teachings through which the invention has advanced the art are properly 
considered within the scope of the invention.