Method and apparatus for soldering electrical components to circuit boards

An apparatus and method for soldering miniaturized electrical components to the surface of PC boards. A layer of solder paste is provided, into which the contact portions of electrical component leads are pressed. The component with solder paste adhering to the leads is moved to its placement station on the board and placed thereon with the leads contacting their respective pads.

BACKGROUND OF THE INVENTION 
The present invention relates to a new and improved method and apparatus 
for soldering miniaturized electrical components to printed circuit (PC) 
boards and more particularly to soldering components having depending 
leads to the mounting surface of a printed circuit board. 
A type of printed circuit board construction that has found extensive 
utility and popularity in recent years is direct surface mounting of the 
components on one or both surfaces of the board. This is in distinction to 
the prior through-the-hole mounting wherein the component leads would be 
inserted in holes in the PC and soldered to the circuit on the opposite 
side. Surface mounting provides particular advantages over 
through-the-hole mounting that are well known in the art. 
In conventional surface mountings solder is screened onto one side of the 
board in selective locations and the components are then placed upon the 
board with the solder pads on the board matched to the footprints made by 
the components package. The solder must then be reflowed by heating and 
melting to complete the operation. An alternative to reflowing is by wave 
soldering. Reference is made to the publication Electronics, Feb. 9, 1984, 
pages 113-124 for a description of prior art techniques for surface 
mounting components with leads to PC boards. 
As seen from the cited publication various commercially available machines 
provide automatic placement and mounting of the components. 
These machines provide both automatic and semiautomatic surface mounting of 
components on PC boards. The machines will pick a component from a supply 
station by a vacuum pick-up head, move it to its location over the PC 
board and place it down on the previously solder screened PC. Thereafter 
by reflow soldering the components are secured in position. 
In the prior art surfacing mounting methods and apparatus, the PC board 
must first be solder screened in the reflow method or wave soldered. The 
present invention contemplates an entirely new technique for suface 
mounting leaded components. In particular, a leaded surface mounted device 
(SMD) is picked up and solder paste is applied to the leads prior to 
placing the component on the PC. Thereafter the board is heated in order 
to reflow the solder paste by one of several methods known in the art, for 
example convection, conduction and vapor phase in order to secure the 
components in place on the PC board. The primary advantage of this method 
is that it eliminates the need for solder screening the board as in prior 
art methods. 
SUMMARY OF THE INVENTION 
Therefore, with the foregoing in mind it is a primary object of the present 
invention to provide a new and improved apparatus and method for soldering 
leaded components for surface mounting to printed circuit boards. 
A further object of the invention is to provide an improved method for 
soldering leaded surface mounted devices to printed circuit boards in 
which solder paste is applied to the ends of the leads of the SMD prior to 
its placement on the PC board. 
A further object of the invention is to provide a method for surface 
mounting leaded components to a PC board without the need for first solder 
screening the PC board. 
A still further object of the invention is to provide apparatus for surface 
mounting components to a PC board in which the ends of the component leads 
have solder paste applied thereto prior to their placement on the board. 
In accordance with these and other objects of the invention there is 
provided a surface mounted device (SMD) assembly system having multiple 
stations at which the various steps in the mounting procedure are 
performed. A supply station serves as a source of SMD's which may be of 
various types. These are selectively and individually picked up and moved 
to a solder application station of unique design where an appropriate 
amount of solder paste is applied to the leads while avoiding any solder 
contact with the body of the device. 
The selected SMD with solder paste on the ends of the leads is then moved 
to a placement station where the device is set down with its leads 
contacting the terminal pads on the board. Thereafter the PC board with 
the components thereon are moved to an oven for reflow soldering. 
Other objects of the invention will become apparent from the following more 
detailed description taken with the accompanying drawings.

DETAILED DESCRIPTION 
Referring now to the drawings and more particularly to FIG. 1 numeral 10 
indicates a portion of a PC board of any well known construction as 
epoxy-glass or paper-laminate. Mounted to the upper surface of the PC are 
two SMD 11 and 12 mounted thereon. 
Component 11 is shown as a small outline integrated circuit (SOIC) which 
may be of a size 0.25.times.0.34 inches and 0.055 inches high. The unit 
shown is a 12 pin SOIC with six leads on each opposite side. Other SOIC's 
have 8, 14, 16, 20 etc. leads. The leads 13 are of the well known 
gull-wing type in which the end of the lead is bent outward with the outer 
flat end portion of each resting upon its respective pad 14 to which it is 
soldered by the novel technique hereinafter described. Component 12 is of 
the type known as a plastic leaded chip carrier having leads 15 on all 
four sides which have J-bends with the outer bottom of the bends resting 
upon and soldered to their respective terminal pads 16. 
Referring now to FIG. 2 there is diagramatically shown in plan an apparatus 
for carrying out the method of the present invention. Numeral 20 
represents the overall SMD assembly system having a supply station 21, 
solder application station 22 and placement station 23. Extending somewhat 
above and across these stations is a stationary bar or rod 24 mounted upon 
end supports 25, 26. A pick up head 27 is mounted to the bar 24 and 
through any suitable motor arrangement travels back and forth along the 
bar to the three operation stations. The general type of mechanism for 
selectively moving the pick up head from station to station is well known 
in the art and is shown for example in U.S. Pat. No. 3,958,740. 
A supply of SMD's are loaded at the supply station. As shown there are six 
tubes 28 each containing a number of SMD's of a particular type for the 
particular board being assembled. Other types of SMD supply arrangements 
could also be used. The traveling head 27 contains depending vacuum 
pick-up tubes 29, these being three such tubes shown in FIG. 2. 
Placement station 23 includes an X-Y table 31 that can be moved in either 
coordinate direction, either automatically under computer control or 
manually or a combination of both. In operation a PC board 32 is located 
on the table and appropriately placed by movement of the table to receive 
the SMD from the vacuum pick-up tube. 
The solder application station 22 is shown in detail in FIGS. 9 and 10. A 
stationary base plate 33 is secured in place at the application station 
and has an upstanding bracket 34 secured thereto. An upper mount 35 is 
secured to the rear bracket as by machine screws 36 and an intermediate 
bracket 37 extension parallel to the base plate 33 and upper mount 35. 
A block 38 secured to the upper surface has a hole 39 bored therethrough to 
receive a solder paste plunger 41 which terminates in a tubular end 42 
through which solder paste is expelled onto a flat solder paste plate 43 
which is selectively reciprocated back and forth as indicated by arrows 
44. 
Positioned above the plate 43 are two blade-like shutters 45 and 46 which 
will be described in detail hereinafter. The shutters 45 and 46 are 
mounted at the inner ends of flexure brackets 47 and 48 respectively, the 
outer ends thereof being received in a slot 49 in a friction fit in 
bracket 34. Thus numbers 47, 48 may be flexed up and down whereby the 
shutters are selectively lowered into close proximity to the solder paste 
plate 43. 
As seen in FIG. 9, flexure 48 is U-shaped with the two arms of the U 
designated 48A and 48B. The outside shutter 45 is secured to the ends of 
the U arms 48A and 48B by machine screws 50. Thus shutter 45 may be raised 
and lowered with the flexing of flexure 48. Inside shutter 47 is a 
straight parallel sided arm of a width to permit it to fit in the space 
between the arms 48A, 48B of flexure 48 and thus either inside shutter 46 
or outside shutter 45 may be lowered into the proximity of plate 43 
without interfering with the other. 
As shown in FIG. 10, the shutters 45 and 46 secured to the ends of flexures 
47 and 48 respectively may also secured to the lower ends of activiating 
plungers 51 and 52 respectively, the upper portions of which are received 
within hollow tubes 53, 54. The latter are suitably connected to a 
pneumatic system that selectively applies pressure to plungers 51, 52 to 
depress either inside shutter 46 or outside shutter 45. 
The sequence of operational steps at the application station is that with 
the mechanism in the position shown in FIG. 10, a predetermined quantity 
of solder paste is expelled from the dispenser tube 42 onto the plate 43. 
Through the pneumatic system either shutter 45 or both shutters 45, 46 are 
depressed. If the former, it will be lowered until its outer feet 45A and 
45B contact plate 43 at which point the blade portion 45C of the shutter 
will be about 0.010 inch above the plate. The latter will-then reciprocate 
outward or to the right as viewed in FIG. 10 to cause the blade portion 
45C of the outside shutter (assuming that it is the depressed shutter) to 
smear the paste in a thin layer 55 as shown in FIG. 4. If both shutters 
are depressed rather than just outside shutter 45, the blade portion 46A 
will be in contact with plate 43 and a solder paste smear as shown at 56 
in FIG. 7 will result. The latter consists of two paste tracks 56A and 56B 
with a clean strip between them where the inner shutter 46 was in contact 
with table 43. Thus it is seen that there are two modes of operation i.e. 
the single broad smear resulting from the depression of outer shutter 45 
during reciprocation of the table and the two track smear resulting when 
both shutters are depressed. 
Referring now to FIGS. 3 and 6 there are shown two types of components. A 
PLCC and SOIC respectively. These are the same as the corresponding units 
shown in FIG. 1 mounted on the PC board 10 and carry the same reference 
numerals as in FIG. 1. 
In operation when it is desired to apply solder to the leads of a PLCC, a 
single solder paste smear as 55 is produced by the outside shutter 45. The 
component 12 is lowered and pressed against the smear and then raised with 
the solder paste adhering to the ends of the J-bend leads. 
When applying solder to the gull-wing leads of an SOIC a two track smear as 
56 is made by the depression of both shutters during reciprocation of the 
solder paste table 43. Thus when the SOIC 11 is lowered, each line of 
leads contacts one of the tracks 56A, 56B of solder paste. This insures 
that solder is not brought into contact with the underside of the SOIC. 
The distance between the undersurface of PLCC 12 and the bottom bent 
portion of the J-bend is approximately 0.030 inches which is sufficient so 
that the solder paste will not reach the underside surface when the PLCC 
is lowered into the solder smear which may be 0.010 inches thick. On the 
other hand, the distance from the underside of an SOIC 11 to the bottom 14 
of the gull-wing leads is approximately 0.005 inches. Thus it is desirable 
to have a clear solder paste-free strip 56C to insure against paste coming 
into contact with the underside of the SOIC. The width of the strip 56C 
may be for example 0.150 inches, generally corresponding to the distance 
between the two rows of parallel leads. 
The sequence of operation of the system will now be described. 
Supply Station 
Initially components to be mounted are placed in tubes at the supply 
station. In the specific embodiment herein shown, the components are 
PLCC's and SOIC's. It is of course understood that the invention is also 
applicable to other surface-mounted devices having extending leads. Also, 
the tube or magazine supply is shown by way of example. The traveling head 
27 will be automatically moved along the bar 24 to bring one or more of 
the three vacuum pick-up tubes over the SMD supply tubes at the supply 
station. The pick-up tube is lowered and the SMD is raised into traveling 
position. 
Solder Application Station 
The head 27 then carries the selected SMD to the station 22. At this time, 
an amount of solder paste is expelled out of the solder paste supply tube 
end 42 onto the solder paste table which is in the position shown in FIG. 
10. If the particular SMD is of the SOIC type, both shutters 45, 46 are 
depressed by pneumatic operation through tubes 53, 54 and the table 43 is 
reciprocated to the right as viewed in FIG. 10, resulting in a paste smear 
as 56 in FIG. 7. It is not necessary that an amount of solder paste be 
expelled in each cycle for each component. A single amount of solder paste 
will often suffice for a number of cycles and a number of components. Also 
the volume of solder paste that is placed on the table 43 may be different 
for different applications and components. It should be noted that the 
inward end of bracket 37 is U-shaped with its bottom surface bearing 
against the upper surface of table 43. This serves to maintain the solder 
paste in place on table 43 with repeated reciprocations thereof. That is, 
reciprocations of 43 would tend to move the solder further to the right on 
the table 43 as viewed in FIG. 10. Thus the U-shaped end prevents this 
tendency and maintains the paste on table 43. The vacuum pick-up tube 
holding the SOIC is lowered to press the component into the paste as shown 
in FIG. 8 where the bottom surfaces 14 of the leads 13 receive an 
appropriate amount of solder paste. The vacuum tube is then raised and the 
SOIC is returned to the hover position, ready to be conveyed to the 
placement station. If the component SMD to receive solder paste is a PLCC, 
then only the outer shutter would have been depressed to provide a single 
paste smear as 55 in FIG. 4. 
Placement Station 
At the placement station a PC board 32 is located on the X-Y table 31 which 
is automatically or semi-automatically moved to bring the proper position 
of the board into place, The head has moved the SMD with the solder on the 
leads from the applicator station to the placement station where the 
vacuum tube 29 is lowered to place the SMD on its pads. Thereafter, the 
assembled PC board is moved manually or automatically to a suitable oven 
or heater where the SMD's are reflow soldered. 
The various movements and operations of the system may be computer 
controlled being self programming and using diskette program storage. The 
specific controlling mechanism may be of any convenient design well known 
in the art and is shown in block diagram in FIG. 11. The various 
mechanical movements are designated in blocks 61, 62, 63, 64 under control 
of a central control unit and may be programmed for the users needs. 
While the invention has been particularly shown and described with 
reference to a preferred embodiment thereof, it is understood that various 
changes and modifications thereof may be made without departing from the 
scope and spirit of the invention as defined in the appended claims.