Method of improved oven reflow soldering

Method of efficiently refiow soldering one or more devices to a printed circuit board with a convective heating oven, each device having a plurality of plate supported electronic connecting pins that are to be soldered within respective mating openings of the printed circuit board for electronic connection, the method comprising: (i) placing a solder paste in each of the openings; (ii) assembling the device to the circuit board by positioning the plate of the device along, but spaced in relation to, the printed circuit board to define a planar space therebetween, with the pins extending into such openings and into the soldered paste to thereby form an assembly; and (iii) passing the assembly through the convective heating oven for a time period to melt the solder paste and form a solid connection upon removal of the convective heat, said passing being carried out by use of convective heat focusing features that guide the convective heat flow between the plate and board to enhance convective heating of the solder paste from both sides of the board.

BACKGROUND OF THE INVENTION 
Technical Field 
This invention relates to the technology of mounting and securing 
electronic connectors and components to a printed circuit board and more 
particularly to reflow soldering to obtain such securement. 
Discussion of the Prior Art 
Large mass and area devices, such as pin-in-hole connectors, present a 
difficult problem to achieve cost effective circuit board assembly by a 
single pass reflow soldering operation, in a convective oven operation. 
This problem results from the imperviousness and high thermal mass of the 
device, such as the supporting plate of the pin-in-hole connector which 
isolates one side of the circuit board from efficient convective heat 
flow. The plate supporting the pins is positioned paralleled but spaced 
from the circuit board, which spacing does not readily receive convective 
heat flow and therefore inhibits reflow soldering. 
Prior art attempts to improve uniformity of heating of circuit boards in 
convective ovens for reflow soldering have included use of a plurality of 
cross-flow blowers with diverging output nozzles stationed perpendicular 
to and along the path of the oven conveyer (see U.S. Pat. No. 5,163,599). 
Unfortunately, such dedicated apparatus is not sufficient to improve the 
heating of solder material about the pin-in-hole connectors since they are 
shielded on one side usually by a closely spaced large mass/area 
supporting plate. 
Summary of the Invention 
It is an object of this invention to provide a method of oven reflow 
soldering that more effectively ensures high quality reflow soldered 
joints of pin-in-hole connectors to printed circuit boards when heated in 
such convective ovens. 
The invention is a method of efficiently refiow soldering one or more 
devices to a printed circuit board within a convective heating oven, each 
device having a plurality of plate supported connecting pins that are to 
be soldered within respective mating openings of the printed circuit board 
for electrical connection, the method comprising: (i) placing solder paste 
in or over each of the openings; (ii) assembling the device to the circuit 
board by positioning the plate of the device along, but spaced in relation 
to, the printed circuit board to define a planar space therebetween, with 
the pins extending into such openings and into the solder paste to thereby 
form an assembly; and (iii) passing the assembly through the convective 
heating oven for a time period to melt the solder paste and form a solid 
connection upon removal of the convective heat, said passing being carried 
out by use of one or more convective heat focusing features that guide the 
convective heat flow between the plate and board to enhance convective 
heating of the solder paste from both sides of the board.

DETAILED DESCRIPTION AND BEST MODE 
As shown in FIG. 1, reflow soldering of an assembly 10, comprised of a pin 
connector device 11 and a printed circuit board 12, is accomplished in a 
convective heating oven 13. The assembly 10 is conveyed through such oven 
13 in a time period so that the solder material is heated above its 
melting temperature for a given period of time and forms a solder fillet 
14 (see FIG. 3) about each pin 15 and forms a solid electronic connection 
to the edge 16 of the opening 17 (the opening in this case being a plated 
through hole) within which each pin is aligned to complete an electronic 
path 18. 
The convective to flow oven 13 usually comprises an elongated heating 
chamber 19 throughout which a flow of heated gas 20 is generally 
vertically introduced from a top plenum 21 as well as from a bottom plenum 
22. Each of the plenums have a plurality of openings or ports 23 through 
which the air flow is emitted. Although the oven gas or air flow 20 is 
generally vertical, some plenums can provide a slight pitch to the 
direction of such flow. The ovens will vary depending upon the air flow 
rate through such plenum ports, as well as the pitch and quantity of the 
plenum openings. The heated gas will flow through the openings or ports 23 
of the plenums and intersect the top and bottom sides 33, 34 of the 
printed circuit hoard if not inhibited by some other plate or member. A 
conveyer 24, such as one comprised of a stainless steel mesh belt or edge 
conveyer, is moved through such chamber; the assemblies are placed on the 
conveyer at an entrance 25 of the oven and retrieved at an exit 26 within 
the prescribed heating time period. Typically, the furnace is operated at 
a temperature around 210.degree.-220.degree. C. for solder materials that 
are normally designed to melt around 180.degree.-190.degree. C. The solder 
material typically used for pin connectors is comprised of tin (Sn) and 
lead (Pb) solder alloys (i.e. 605.cndot.40 Pb, 635.cndot.37 Pb). the oven 
can also be programmed to operate at lower or higher temperature using 
other solder alloys. 
Attaching pin connector 11, by refiow soldering within the convective 
refiow oven, is difficult because the pins 15 are aligned and supported on 
a continuous plate 28 which must be spaced from the board in a parallel 
plane 29 leaving a gap or space 30 therebetween which is open only at the 
edges of the gap and therefore not readily penetrated by the natural 
convective flow of the oven. As a result, the solder paste 32, positioned 
in the openings 17 to secure the pins, is heated unevenly and primarily 
from the top 33 of the printed circuit board. Very little heating takes 
place at the underside 34 as inhibited by the presence of the spaced 
connector plate; the soldered assembly will possibly contain weak joints 
due to improper fillet formation. A vertical flow oven fails to migrate 
sufficient heating flow through spacing 30 because the flow must be turned 
90.degree.. 
To overcome this problem the method steps of this invention comprise: (a) 
placing or stenciling a soldered paste 32 in each of the openings 23 of 
the printed circuit board 12 adapted to receive the respective electronic 
connecting pins 15; (b) assembling the inn connector device 11 to the 
circuit board 12 by positioning the plate 28 of the device along and in 
spaced relation to the printed circuit board carrying the pins to define a 
planar space 30 therebetween while having the pins extending into the 
openings of the circuit board and into such soldered paste, thereby 
forming an assembly; (c) passing the assembly through the convective 
heating oven 13 for a time period to melt the soldered paste 32 and form a 
solid connection upon removal of the convective heat, the passing being 
carried on by use of one or more convective heat focusing features 35 
which guide convective heat flow between the plate 28 and board 12 to 
enhance convective heating of the soldered paste from both sides 33, 34 of 
the board. 
The solder paste 32 is spread onto the printed circuit board prior to 
assembly of the pin connector device thereto by automatic stenciling 
across one of the flat surfaces of the printed circuit board leaving a 
predetermined deposit of solder material of each opening as desired. 
The convective heat focusing features comprise at least one of the 
following: (i) a directional flow converging tool 36 and (ii) perforations 
or slottings in the supporting plate 28. Turning first to the tool 36, it 
reflects the vertical flow 20 of the oven in a manner to converge and turn 
at 31 into and be aligned with the plane 29 of the gap or spacing 30 for 
ready entrance thereinto. The tool has side upright walls 38 welded to an 
upright back wall 39 to form a frame 40, between the frame is secured flow 
baffles 41 extending thereacross to define deflection flow channels. Top 
baffles 41a and 41b converge from a top opening 42 to a throat opening 43, 
thereby turning the flow approximately 90.degree. from its entrance. 
Similarly, bottom baffles 41c and 41d converge from a bottom opening 44 to 
the same throat opening 43 to deflect upwardly rising convective flow to 
be turned again approximately 90.degree.. The frame and flow baffles can 
be constructed of stainless steel sheet metal (0.010 inches thick). 
The tool 36 can be suspended on the printed circuit board 12 by use of 
slotted fingers 45, as shown in FIG. 2, or by use of extended slots (not 
shown) in the side upright walls 38 which need to be extended forward of 
the baffles. Use of the fingers 45 or slots require placement of the tool 
on the printed circuit board prior to or at the same time as placement of 
the assembly onto the conveyer. Alternatively, the tool may be hinged to 
the conveyer 24 so that it is easily aligned with the assembly 10 when the 
assembly is placed in abutting relation to the tool on the conveyer. 
Alternatively or additionally, the convective heat focusing feature may 
comprise perforations, slots or openings 37 (see FIGS. 1, 4, 6, and 7B) 
provided through the supporting plate 28 of the pin connector device. 
Preferably, the slots will divide the pins into groupings 47 that limit 
the thermal mass and thereby enhance rapid heating by the redirected heat 
flow 31. As shown in FIGS. 4 and 5, larger slots 50 extending transversely 
of the supporting plate, divide the pins into groups that have about six 
pins in a longitndinal row 48. Slots 49, which are slightly narrower and 
extend longitudinally, divide the pins into groups containing up to three 
rows, (preferably only two rows). The vertically directed convective heat 
flow 20 will easily reach the solder paste from the top side 33 of the 
printed circuit board, as shown in FIG. 4, but more importantly, the 
convective flow from the bottom which normally would be inhibited can now 
migrate through the transverse and longitudinal openings 49, 50 to reach 
the underside 51 of the solder paste and promote effective rapid uniform 
heating. Such heating may now approximate without the presence of a pin 
supporting plate. Preferably, the openings have an opening area at least 
10 mm.sup.2 or greater. 
To facilitate uniform heating, auxiliary structure aids 52 may be used in 
the form of spacing guides 53 (shoulders protruding from the side 54 of 
the supporting plate 28 which faces the circuit board 12) that limit the 
extent the pins 15 protrude through the openings and also fixes the 
dimension of spacing 30 between the printed circuit board and plate (see 
FIGS. 6 and 7A). Preferably, such spacing shouht be in the range of 0.025 
to 0.250 inches. The plate itself may also be limited in a thickness 55 to 
the range of 0.025 to 0.50 inches, to thereby limit its insulative effect 
and facilitate more rapid uniform heating of the solder paste. It is 
desirable that the cycle time, measured from the moment of inserting the 
assembly into the heating oven to the time it is extracted from the 
heating oven, less than 12-18 minutes. 
In the ordinary use of printed circuit boards possessing pin connectors, it 
is desirable to facilitate insertion of mating connectors to the pins by 
use of guide pockets or walls. Such guide walls have been heretofore 
integrally formed as part of the pin support plate and have thus inhibited 
uniform heating of the solder paste when it is passed through the 
convective heat flow oven. The support plate 28 of this invention is 
preferably flat, possessing a thickness that is thinner than normal, and 
is a single monolithic substrate comprised usually of a thermoplastic or 
thermoset polymer, such as PBT (Polybutylene terephthalate) and Novalac 
epoxy, respectively; it is devoid of any projecting guide walls. However, 
integral guides, if needed, can be formed as winged guide posts 57 located 
as each corner 58 of the pin groupings (such as grouping 59 separated by 
transfers slots 50) as shown in FIGS. 7A and 7B. Such guide posts are 
integral to the plate 28 and will not significantly affect the uniform 
heating of the solder paste as the assembly is passed through the 
convective heating oven. 
If, in some cases. it is desirable to provide greater rigidity and strength 
for the support plate during use, a guide frame 60 can be added as a post 
soldering procedure (see FIGS. 9A-9B). A separate insulative frame 60 is 
designed to fit onto the bottom side of the plate 28 and surround the pins 
15 extending therethrough. The frame carries a series of annular closed 
guide cups or pockets and has openings 62 aligned with the pin grouping 
59; guide walls 63 extend from the plane (54 of the frame to surround the 
inn grouping (such as grouping 59 which are divided by transfers slots 
50). The frame 60, in addition, may also carry sealing element or pad 65 
that, upon placement of the frame in the position onto the support plate, 
will snugly close off the openings 49, 50 in the connector plate 28 and 
restore the plate structurally to a continuous non-interrupted plate. If a 
sealing element or pad is not adequate, an adhesive can be used to bond 
the frame 60 directly to the base plate 28, creating a hermetic (water 
tight) seal and proving added mechanical integrity to the assembled 
connector. 
By reducing tim cycle time at which refiow soldering of pin connectors 
takes place with this invention, printed circuit boards may now also 
reflow solder regular electronic components at the same time. Electronic 
components as well as pin connector devices may be soldered all in the 
same oven pass and cycle time. This is a significant increase in 
productivity capability without sacrificing the reliability and integrity 
of the solder joints. 
While particular embodiments of the invention have been illustrated and 
descried, it will be obvious to those skilled in the art that various 
changes and modifications may be made without departing from the 
invention, and it is intended to cover in the appended claims all such 
modifications and equivalents as fall within the true spirit and scope of 
this invention.