CA1202377A - Short-pulse soldering system - Google Patents

Abstract

ABSTRACT
A system for rapidly soldering wire to terminal pads of a printed circuit board. A soldering tool is heated to a high temperature above 1000 degrees F., and preferably between 1600 to 2000 degrees F., and has a predetermined effective mass for holding a quantum of heat just sufficient to make an effective solder joint over a broad range of terminal pad conditions.

Description

~2

2 Of: Ronald Morino For: SHOP~T PULSE SOLDERING SYSTEM

6 This invention relates to solderin~ ~nd, more 7 particularly to method and apparakus for short pulse soldering & suitable for automatic wiring equipment.
q Back~round of the Invention 11 The technology for production of circuit boards for 1~ interconnecting electronic components has advanced considerably 13 in recent times. The advance in integrated circuit technology i4 has brought about ever increasing densification of electronics which in turn has brought about an ever increasing demand for 16 denser and more reliable circuit boards for interconnecting the 17 Componen~s.
18 According to one highly successful technique, dense lg circuit boaràs are created by scribing or writing wires on the surface of the board using very fine insulated copper wire.
21 The wires zre deposited according to a computer generated 22 program. The wire pattern is thereafter encapsulated and the 23 end~ of the wires are connected to terminal pads on the board 24 surface. The technology is generally described in U.S. patents

3,674t602 and 3,674t914. ~ne of the significant advantages of 26 the wired circuit boards over conventional printed circuit 27 technology is that the lnsulated wires can cross one another 2~ and therefore very dense connection boards can be made in a single layer thereby eliminatins the need for interlayer connections.

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l In the past, the connection.s to terminal pads in the 2 ~ired circuit boards has usually been accomplished ~y plating.
3 After the wire pattern is deposited and encapsulated, ~oles are

4 drilled throuqh the board at appropr:iate locations and then plated. The hole plating is done in a manner that not only 6 plates the hole and forms the terminal pad, but also so that 7 the end of the insulated wire exposed ~y the drilled hole is 8 electrically connected to the pad.
9 ; Soldering techniques have, of course, long been used to connect wires to terminals. However, such soldering 11 techniques have generally not been regarded a use~ul in high 12 speed automatic production of circuit boards because o~ the 13 1 difficulty in keeping the solder joint localized, because of 4 the need to avoid heat damage to the plastic board substrates, 1~ and because of the danger af solder entering the holes 16 associated with the terminal pads.
17 There have been prior attempts at solving these 18 pro~lems, such as:
l9 ~ ~a) con~iguring the solder pads to provide a 20 l narrow heat transfer restr$ction, between the solder area 21 and the plated hole (Stranco US patent 3,573,~81);
22 (~) providing an electrically conductive, heat 23 1 resistive nickel layer under the solder regions 24 (Stranco U.S. patent 3,673,681);
25 ' (c) cooling the soldering area wi`tb the flo~ of 2~ air or lnert gas`~(Stranco U.S. Patent 3,673,681, ~arsen ~.S.
~7 patents 3;650,45D and ;3!812,581):
28 (d) prestripping the segment o wire to be 29 soldered so that the solder joint nee~ not be subjected to high temperatures required for insulation stripping (~icholas ~.S.

~2~3~7 patent 4,031,61~);
(e) using parallel gap soldering where the heat is generated at the solder surface by using thl' solder pad to comple-te an electrlc heat generatincl circuit (Mulchay U.S. pa-tent 3,~44,347); and (f~ controlling the heat generated by ns;rlg -temperature measurements -to con-trol the electric current generating the heat (Denney U.S. patent 3,77~,5~1).
Although by using a combination of the above teachings it is possible to produce satisfactory solder joints, these techniques do not provide a system cap~)le of tolerating the range of variations and conditions enconlltered ln commercial production. I~lso, some of the methods accordillg to the prior ~rt techrliques enulllerated above add considerable cost and complexity to the operation or require special board configurations which reduce -the board's surface area available for routing wires.
Sumrnary of -the Invention In accordance with this invention it has been found, surprisingly, that there is a combina-tion oE conditions and hardware capable of soldering very fine insulate~l wires to terminal pads under the wide range ofl conditions normally encountered in commercial production without darnaging the substrate or the plated holes, without requiring special circuit board configurations and without significantly increasing costs.
The invention relates to a high s~eed me-thod of operating in the presence of solder melting at about 450 F
for solderlng wire lying across a terminal pad or the like on a circuit board, using a soldering tool of prede-termined mass, comprising: (a) selectirlg the efEecl:ive mass (-~ lle soldering tool so tha-t -the quan-tum of heat energy stored kh/ r~

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Jl,.~'J ~ d~ il' t therein is only slightly in excess of that required for an effective sol.der joint of solder meltling at about 450 F and is swbstantially used ~Ip during formation oE
the solder joint; (b) heating the effective rnass to a preselected high ternperature below that which would cause rapid deterioration of the tool;(c) bring:ing the tool into thermal contact wi-th the wire to be soldered when lying across the terminal pad; (d) while in thermal contact (i) substantially imparting just enough heat to tlle W:i:l e and the terminal pad to complete a solder joint, and (ii.) permitting solidification oE the solder; (e) t-.he q~lantllm of heat energy being insufficient to permit signi.ficallt heat m:Lgration into the circu.it board beyond the t~rm:Lnal pad; and (.E) wlle:rein the solderillcl tool :ls raisecl l..o a temperature selected so that -the solder joint ~5 Eormed in less than 500 milliseconds.
In another aspect, the invention relates to a high speed method of soldering insulated wire lying across a terminal pad of a circuit board coated with solder melting at about 450 F using a heated soldering tool, comprisillg: (a) selecting the ef:Eect:ive IIIclSS o:E tlle soldering tool so that, when heated, the quantum of heat energy which can be imparted for making a solder connection is just sufficient :to vaporize insulation off the wire and -to liqueEy -the solder melting at about ~50 ~ to make an eEfec-tive solder joint; (b) hea-ting -the soldering tool -to a temperature at leas-t in excess of the vaporizing tempera-ture of -the insulation on the wire; (c) bringing the soldering tool into contact with -the insulated wire lying across the solder coated terminal pad; (d) the heating period for the soldering tool, and the temperatllre the:reof when heated, being selected to provide a temperature profile - 3a -kh/ ~

Z~377 such that (i) the temperature imparted to the insulation in the contact area exceeds the vaporization temperature thereof, (ii~ l:he temperature of the solder on the pre-sol.dered terminal pad exceecls the liquefaction temperature of about ~50 ~ for a minimum period of time sufficient for an effective solder joint and (iii~ the temperature of the circuit board in -the vicinity of the terminal pad does not rise above the temperature causing deterior-ation thereof.
In its appara-tus aspect the invention relates to a system operating 1n the presence oE solcler :Eor soldering wire to a terminal pad on a circuit boa:rcl, comprising: (a) a solder.lng tool havincJ a p.rodelor~l.rled eEEective mass; (b) means for applying the:rmal energy to the soldering tool (i) to raise the temperature of the soldering tool to a preselected high temperature below that which causes rapid deterioration thereof, and (ii) to store in the soldering -tool a quantum of heat energy only slightly in excess of that required for an effective solder jo:Lnt and which is substantially used up during formation of a solder joint; (c) means for br:Lnc3ing the soldering tool into thermal contact with the wire to be soldered while lying across the terminal pad (i) to imp~rt jus-t enough heat to complete a solder joint~ and (ii) to permit solidification of the solder; and (d) the effective mass and the application of thermal energy there-to being so selec-ted that the solder joint is formed in less than 500 milliseconds.
An essentia]. aspect of the inven-tion is -to control both the peak temperature of the soldering tool and the quantum of heat made available during a soldering operation. The - 3b -1 ; quantum 'Qf heat i~ a function of the temperat~re o~ the 2 soldering tool, the mass of the soldering tool, and the ~ duration of energy application. The quantum of heat is 4 adju~ted so that it is just sufficient to provide,a reliable

5 1 solder joint under the range of conditions expected to be

6 encountered. It is important that the quantum of heat not

7 exceed the re~uired value. The peak temperature employed in

8 the solderins tool during the operation is quite high, above

9 lOOO~F and preferably in the range 1600 to 2000F. These high temperatures are capable of vaporizing insulation o~ the wire 11 to expose the bare copper for soldering. Also, if the quantum 1 of heat and timing is properly controlled it is possible to 13 achieve a steep temperature gradient which confines the high i~ ! temperatures to the regions where th~y are useful in stripping insulation and melting solder while at the same time avoiding 16 high temperatures where they would be harmful. By controlling 17 the quantum of heat applied and the time of energy application, 18 the wire can be stripped and soldered before there is 19 sufficient heat migration to raise the temperature in the 20 ; sensitive regions to a point where damage to the circuit hoard, 21 insulated wire or terminal pad would occur.
22 With the technique accordin~ to the invention the 23 soldering time is very short, below 500 milliseconds and 24 preferably less than 50 milliseconds.
25i With proper conditions and apparatus it is possible to 26 complete a solder connection in less than 50 milliseconds 27 durin~ which the peak temperature for vaporizing insulation off 2~ the wire exceeds 750F, the peak temperature availahle ~or 29 soldering the wire to the termina~ pad exceeds 450~, but the ~0 termperature in the substrate adjaoent the terminal pad does 37~

not e~ce~d 550F. ~y controlling the quantum of heat applied and the timing of the operation, the high temperature insulation removal and the soldering are completed and the heat used up before the hea-t can migrate into areas where damage would occur.
BrieE Description of the Drawinqs Fig. 1 is a partial plan view and partial block I diagram illustrating the system according to the invention;
Fig. 2 is a detailed drawing illustrating the placement of wire on the circuit board and the soldering tool associated therewith;
i ~ig. 3 is a perspective view showing the wire, soldering tool, and terminal pad;
Figs. 4 and 5 are illustrations showing a completed solder joint when made according to the invention;
Fig. 6 is a diagram showing the temperature profile during a soldering operation;
Fig. 7 is a schematic diagram illustrating a single pulse type control system for energizing the soldering tool; and Fig. 8 is a schematic diagram illustrating a multiple pulse control system for energizing the soldering too~.
Detailed Description The soldering apparatus according to the invention can be incorporated in an automatic circuit board wiring machine as illustrated in Figs. 1 and 2.
Further details of the overall apparatus are disclosed in U.S. patents 3,674,602 and 3,674,914.

~ kh/~ 3~ ) 1 A circuit board ~ is mounted for movement by an X- Y
2 transport 40 and is moved from point-l:o-point according to a ~ ; computer control 42. A wire guide unit 10, scribing stylus 2 4 and soldering tool 30 are mounted above the circuit board so 5 , they can rotate as a unit. Insulated copper wire 11 is fed 6 1 tllrough the wire guide toward stylus 20 which presses the wire 7 into the tacky surface coating 6 on the circuit board as is 8 best seen in Fig. 2. The rotational position of the scri~ing q unit (including wire guide 10, stylus 20l and soldering tool 30) is determined in accordance wi~h the direction of the table 11 movemènt so that the wire is laid down on the board surface as 1~ the board moves away from the scribing unit.
13 l, The soldering tool is pivotally mounted with respect 14 ~ to a pivot 31 so that it can be raised and lowered by a 15 ~ suitable solenoid or pneumatic actuator 48. When lowered into 16 the soldering position (shown in solid lines in Fig. 1) the 17 soldering tip 32 straddles the wire being laid down. A
18 soldering operation is normally performed while the table is at , .
19 ` rest at a point where the wire overlays a terminal pad area to 20 ,, which the wire is to be connected. The terminal pad is 21 preferably pretinned and thus, when the appropriate heat is 22 applied the insulation is stripped and a solder joint is 23 completed.
2~,~ A position sensor 44 is attached to the X-Y transport 40 to sense the table position and determine when it is in the i 26 proper posltion for the soldering operation. Wken in the proper ~7 position compu~er control unit 42 provides activation sign~ls 28 for a solenoid control unit '49 and a timer control unit 45.
29 Solenoid control unit 49 is connected to solenoid 48 and raises and lowers soldering tool 30. A high current power supply 52 3~7 1 is connected to soldering tool 30 via a switching circuit 5~, 2 the switching circuit in turn being controlled by timer control 3 unit 46. The timer control causes one or more high current 4 pulses of predetermined enersy content to be applied to the S soldering tool when called for by somputer control 42.
6 Details of soldering tip 32 o~ the soldering tool 7 stradling wire 11 an~ located over a terminal pad is as shown 8 in Fig. 3. The terminal pad can be formed by pressing a 9 pretinned hollQw copper ter~inal into circuit board 5 at the proper location or by using printed circuit techniques to ll copper plate a drilled hole and by subsequently tinning the 1' plated surface. ~he completed terminal pad 40 include~s a 13 ~! cylindrical body portion 42 passing through the hole and a 14 surface ~lange portion 41.
lS Tip 32 of the soldering tool has a generally ~-shaped i6 cross section with the bridge portion o~ the "~ being the 17 effective mass of the tool. Tip 32 is preferably made of 18 tungsten. allo~s which are only slightly oxidized at high l9 temperatures. In some cases it ma.y be desirable to carry out 20 l the soldering operation in an inert atmosphere to avoid 21 oxidation of the soldering tool. The bridge portion of the 22 soldering tool preferably has a groove on the lower surface 23 dimensioned to partially accommodate the wire ll being soldered 24 to maintain good thermal contact. The dimer.sions of the 2~ effective mass are "W", the width between tbe leg portions 34 26 and 35 o~ the tip, "Ln, the length in the direction of the 27 wire, and ffH~ the height of the bridge port.ion. The legs 34 and 35 of the bridge po~tion are integral with ~upport arms 3 29 and 37 secured in a suitable mounting str~cture 38 (shown in Fig. l).

_ 7 _ ~P~i377 1 ` A typical solder joint appears as shown in Figures 4 2 1 and 5~ The hole through body portion 42 of terminal pad ~0 has 3 a diameter of .04 inch, the radial dimension of flange 41 is --4 j .015 inch and the outside flange diam~ter is .07 inch. The 5 I copper flange is .002 inch thick. The insulated wire ll 6 1l includes the copper wire with a ~004 inch diameter surrounded 7 , by insulation which is .0005 inch thick.
8 1l The solder layer is about .0015 inch thick. The q 1I solder ~`illet 50 which joins the stripped wire to the terminal

10 `` pad is about .015 inch wide and .040 inch long.

11 Preferably the solder for the solder joint is supplied 17, by a solder coating on the terminal pad which can be a 13 ` pretinned coati~g as mentioned above, or a plated 4-j (non-eutectic) coating._ Alternatively, the wire can be solder -lS coated to supply solder for the joint. Other techniques for 16 supplying the solder can also be employed. For example, solder 17 , in a powder or paste form can be ejected into the solder joint 18 ~ area, preformed solder in the shape of washers, disks, or l9' ribbons can be placed in the solder joint area, or solder in the ~orm of microdots or microspheres can be used.
21`
22 he Method According to the Preferred Embodiment l !
In accordance with the invention the quantum of heat 24 utilized in a soldering operation is carefully controlled. The ,~ !
25 !, quantum o~ heat depends on the effective mass o~ the soldering 26 tool, the temperature of the soldering t~ol, the energy applied 27 during the so1dering operatiQnl and the mass of the wire and 28 copper foil forming the terminal pad on the circuit board.
29 The soldering tip tamperature is also con~rolled to achieve a desired temperature profile so that maximum hea' .

l energy is available for completing the solder joint and so that 2 there is minimum heat migration into the areas of the board ~ sensitive to heat.
4 Conditions-are selected to achieve rapid soldering since, when constructing a circuit board by point-to-point 6 wiring, the time efficiency can be very important to the 7 realization of an effective wiring machine.
8 The selected conditions should permit the apparatus to 4 make good solder connections resardless of the surrounding ln structure on the board. In commercial circuit board operations l1 the size of the terminal pads may vary and ~n many cases are

12 associated with, or are near to, sizable ground planes. The

13 size of the copper area in the soldering r~egion affects heat

14 dissipation and heat migration. According to this invention, condltions can be se~ected which will provide a good solder 16 joint for the broad range of conditions normally encountered in 17 a circuit board. In order to arrive at the appropria~e set of 18 conditions a series of experimental runs were made utilizing 19 soldering tools o different dimensions and energizing these 20 I tools at different ener~y levels and different periods of 21 time. The results of these tests are summarized in Table I
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1 The tool dimensions are in terms of the width, height 2 and length ~W x H x L) for the effective mas~ on the soldering 3 ` tool as indicated in Fig. 3. Thus, for example, the dimensions 4 ; in the first line o~ Table I indicate a width of .022 inch, a 5 1 height of .012 inch and a length of .32 inch for a total volume 6 of effective mass of 5.28 x 10 6 in3. The voltage applied 7 to the tool is 4.25 volts which results in a current of 175 8 amperes.
g T3 test the operation over the range of conditions 10 ; likely to be encountered in commercial circuit board 11 operations, tests were conducted on standard board 12 ; configurations using a relatively small terminal area in the 13 form of a 0030" wide strip and relatively large terminal area 14 ~ in the form of a 1 inch wide strip. Tests on these str1ps represented approximately a thirty to one ratio of width and, 16 ~ hence, simulated the broad range of conditions encountered in 17 commerical operations. Tests were then run using different 18 ` soldering times for the various conditions and thereafter 19 inspecting and visually judging the solder joint.
20 ~ The conditions that resulted in acceptable solder 21 ' joints are set forth in Table I. For example looking at the ~2 second line of the t~ble, a soldering tool with an effective 23 ma~s volume of 5.28 x 10 6 in3 is energized with 6.4 volts 24 resulting in a 250 ampere current flow through the tool. On the smaller strip (.030 inch width) an acceptable solder joint 26 was achieved using energization periods between 11 and 14 27 milliseconds~ For shorter energization periods ~in this case 2~ less then ll milliseconds) there was insufficient heat for 29 abl~tion of the insulation or insufficient heat to form a good 3~ solder joint. Longer energization periods ~in this case ~1'2~37~

1 greater then 14 milliseconds~ resulted in damage to the 2 substrate or dama~e to the insulation on the wire outside the ~ ~ solder area. Tests run with resp~ct to the larger strip (one 4 l¦ inch width) determined that acceptable solder joints were S ¦i achieved using an energization period between 12 milliseconds 6 1 and 23 milliseconds. Thus, in the range between 12 7 ~ milliseconds and J4 milliseconds, using this particular size 8 ,ll tool, acceptable solder joints are achieved regardless of q ~ terminal pad size, i.e. in the range from ~030" to 1".
-The temperature of the tool was determined visually by 11 l observing the incandescent color and estimating the 12 ll corresponding temperature. For the second line of Table I the 13 ~l tool temperature was estimated to be 1830F from the bright 14 cherry-red color.
This series of experiments es~ablish that there are 16 conditions which can be selected for makin~ satisfactory solder 17 joints over the range o~ conditions encountered in normal com-18 mercial operations. As indicated on lines 2 and 3 of the 19 table, if the soldering tool mass is relatively small 20 l' (corresponding to an e~fec~ive mass volume of 5.28 x 10-6 21 ll in ) and the tool is energized with a potential of 6.4 or 8.6 22¦~ volts with corresponding current 10ws of 25G and 275 amperes, 23 1I then energization periods exist which will sati~y the full 24l range of board conditions. At the lower potential of 6.4 volts 25~ (corresponding current of 250 amperes~ an energization period 26 of 12-14 milliseconds resulting in a tip temperature of 1830F
27 produces acceptable solder joints over the range of terminal 28 pad sizes. hikewise, at the somewhat higher potential of 8.6 29 volts (corresponding current of Z75 amperes) an energiæakion period in the range of 6-8 milliseconds at a corresponding 37~

1 soldering tool temperature of 2000-2200F also achieved 2 acceptable solder joints over the range of terminal pad sizes.
3 j As can be seen from Table I, where the ran~e of ' terminal pad sizes is narrower or where all terminal pads are S ll, of a uniform siæe, o~her conditions can be found that produce 6 ,,, satisfactory solder joints. It should be noted that in all ,, :
7 ; cases the energization period is less than 500 millisecunds and 8 thus provides a rapid soldering technique useful in automatic 4 il machinery. In mos~ cases satisfactory solder joints are ln achieved using energization periods less than 150 milliseconds 11 and in many cases beIow 50 millisecônds. I
12 Although in a few cases acceptable solder joints have 13 ll been made at low soldering tool temperatures, in general, the 1~ l tool temperature should be above 1000~ and preferably in the

15 I range of 1600 to 2000~F. Lower solderin~ tool temperatures

16 require longer soldering times and ener~i~ation perjods and

17 result in greater heat migration into the substrate of the 1. .

18 circuit ~oard. Higher temperatures generally provide a steeper 1~ temperature gradient with less heat migration and shorter energization periods and contact times. Temperatures above 21 2000F, however, are undesirable because of the more rapid 22,, deterioration of the soldering tool at such temperatures.
23 Fig. 6 of the drawings shows a typical temperature 24 profile estimated for soldering tool temperatuxes above 100~F
25 ~ at approximately 1400~F. The tip 62 of soldering tool 58 26 presses insulated wire 68 down on copper foil terminal pad 64 27 ~located on plastic circuit board 65. The insulated wire is 28 shown after ablation of the insulation in the solder joint 29 area. The wire is shown soldered to the copper foil pad bv ~ shown in Fi~ures 4 and 5 solder fillet 5~~ A solderin~ temperature of 1400F is ~ ~ p! ~ ~

1 / provided by a tool having dimensions .022 x .016 ~ .02 (7.04 x 2 ~ 10 6 in3) energized by 6.5 volts (current of 250 amperes).
An energization period of 22 milliseconds provides an acceptable solder joint to a pad with a diameter of Ø07 inch.
I The soldering tool first vapori~es the insulation off 6 ~ the wire. The unstripped insulation outside the solder area 7 reaches a maximum temperature of about 700~. The copper wire 8 reaches a temperature above 750~F and approximately 900~F, 9 , i.e., a drop of about 500~F from the peak tool temperature.
ln l, The solder reaches a temperature above the melting point of 50F and normally about 530F which is a drop of about 370F
1' ' below the temperature of the copper wire. The temperature of 13 1, the copper terminal pad is close to that of the solder. The pad temperature would normally reach & peak of about 500~, 15 ' approximately 30~F below the solder temperature/ and remains 16 ~ safely below 550F. The plastic in the circuit board would 17 normally reach a maximum temperatu~e of about 480F, about 20~
18 ,' below the pad temperature and, hence, would remain safely below 19l~ the temperature of 55CF where damage to the substrate occurs.
20 l Most of the solder inside the hollow portion of the terminal 21 pad stays below a temperature of about 350~ and, hence, stays 22 below the solder melting point of 450F.
23 l The temperature profile is important since it provides 24,l hi~h temperatures and heat energy where required for stripping 25' and soldering without providing excessive heat inside the 26 hollow portion of the terminal pad or in the circuit board 27 where damage could occurO ~igher tool temperatures (and 28 shorter energizaLion periods) tend to provide steeper 2g~ temperature gradients and, hence, more heat is available at the soldering area without significantly increasing temperature in 2;377 1 areas where damage would occur. Of particular significance ls the 2 selection of conditions so that the quantum of heat supplied is onl~ slightly in excess of that required for an effective solder joint so that the heat is used up in making the solder '! i joint and does not migrate to areas where damage would occur.
6 In operation when the heated soldering soldering tool 7 first comes in contact with the insulated wire there is 8 relatively poor heat conduction. After the insulation breaks q down, the soldering tool comes in contact with the copper wire and the thermal resistance drops considerably. At this point 11 the wire in contact with the soldering tool is heated so that 1' the portion of the insulation between the wire and the terminal 13 pad can be removed. The heat remaining after removing the 1~ insulation must be sufficient to melt the solder and form the lS solder joint. ~emperatures in excess of 7Q0F are required in 16 the ablation of the insulation whereas temperatures in excess 1 7 OI 450F are required to melt the solder.
18 The energy supplied to the tool should be DC or high

19 frequency AC and should be in the form of high current pulses.

20' Where o~ly a single electrical pulse is used in a soldering

21 operation, the current should be in the range of 50-500 amperes 22l and the pulse duration in the range of 500 to 5 milliseconas.
23 As previously indicated the two primary factors which 24 are controlled in accordance with the invention are the tool temperature and the quantum of heat delivered. Generally the 26 too~ temperature ~hould be as high as possible but below the 27 temperature causing significant deterioration of the soldering 28 tool. The quantum of heat delivered to the soldering arear 29 which is a function of the tool temperature, the effective mass of the tool and the energization period, should be only 7'7 1 slightly in excess of the amount of heat required for stripping 2 the insulation and completing the soldering joint. This , objective can be achieved by constructing the soldering tool having a certain mass and controlling the current and 5 ~¦ energizing the tool for a predetermined period while the tool 6 ~l is in contact with the wire being soldered. Proper control of 7 the ~uantum of heat applied can also be achieved by preheating 8 a preselected tool mass to store the correct quantum of heat q and by then bringing the tool into contact with the solder area. Also, appropriate control can be achieved by 11 combinations of prestored heat and energy supplied during 1~ contac~.
13 1, Preferably the soldering tool applies pressure to 1~ li maintain the wire in contact with pad area and stays in contact 15 1, until the solder solidifies. In this fashion the risk of wire 16 movement during solidification of the solder is minimized. In 17 , cases where the required quantum of heat is p~estored in the 18 i tool, the tool contact period is selected to achieve the heat 19 ~I transfer and also to include an appropriate cooldown period.
20 1~ Where energy is supplied during contact, the contact period 21 ' after completion of the energization period is controlled to

22 provide an appropriate cooldown period. Preferably the

23 , soldering tool is designed so that after the correct quantum of

24 , heat has been delivered to the solder joint the soldering tool, while still in ~ontact, dissipates heat from the solder joint.
26 This can be achieved, for example, by including thermally 27 conductive support arms 36 and 37 (Fig. 3~ in thermal contact 28 with the effective mass 32 of the soldering tool.
2~ In some cases multiple energization has advantages.
For example, a first energization pulse may be supplied for ~;2377 1 stripping the insulation and a separate second pulse may 2 thereafter be supplied for soldering. A suitable two pulse ~ program sequence could include a first pulse at 6 volts for 8 4 miliseconds to strip insulation follo~wed by a second pulse at 4 S 1 volts for 25 milliseconds to complete the soldering joint.
6 With this arrangement there is a higher temperature (above 7 700) for stripping for a short interval followed by a lower 8 temperature (above ~50F) for a longer period ~or solderins.

q .
The Preferred Control Apparatus 11 ~s previously mentioned with regard to Fig. 1 the 1' control apparatus for the soldering tool generally includes a 13 power supply, a switching circuit and a timer control. A
1~ specific preferred system for single pulse soldering operations is shown schematically in ~i~. 7.
16 As indicated in Table ~, short, high current pulses in 17 the range of 150 to 400 amperes are required. Although any 18 high current source can be used, a storage battery 70 including 19 up to four Gates BC cells manufactured by Gates Energy Products, Inc., Denver, Colorado, provides a convenient low 21l, voltage, high current source. Each cell is rated at 2.0 volts 22 l and 25 ampere-hours. A conventional charging circuit 72 is 23 ' connected across the battery to maintain a f~ll state of charge.
2~ ~he switching circuit includes six NPN power switching transistors 100-105, three NPN drive transistors 92-94 and an 26 initial transistor gO, also of the NPN type. The timer control 27 includes a controllable monostable multivibrator 82 and an 28 associated flip-flop circuit 80~
29 In the illustration, a switch 78 is shown for 3C initiating an energizing cycle. In an actual system switch 78 37~

1 may be the contacts of a relay in the control computer. The 2 normally closed contact of switch 78 is connected to the reset input R of flip-flop circuit 80 and the normally open contact is connected to the set input S. One of the outputs of flip-S flop circuit 80 is connected to the trigger of monostable 6 flip-flop circuit 82. A variable resistor 83 and a capacitor 7 84 are connected ~etween the ~12 volt supply and the monostable 8 circuit to provide the timin~ control. The variable resistor 9 and capacitor have values selected to provide time intervals between 5 and 500 milliseconds.
11 Each time switch 78 moves to the alternate position 1 , from that shown in Fig. 7, flip-~lop circuit 80 changes state 13 and produces a transient change at the out~ut. Monostable 14 circuit 82 responds to the transient change and produces a 15 ! positive pulse at its output having a duration determined by ,. ;
16 the se~ting of variable resistor 83.
17 The output of the monostable circuit is connected to 18 the base of transistor 90. The collector of transistor 90 is 19 , connected to the ~12 volt supply via a resistor 89 and the emitter is connected to the ground. A bias resistor 88 is 21 connected between the base of transistor 90 and the ~12 volt 22 supply.
23,~' The collector of transistor 90 is connected to the 24 i base terminals of drive transistors 92-~4 and to ground via a

25 I resistor 91. The collectors of transistors 92-94 are connected

26 to the posltive terminal of battery 70 via variable resistors

27 96-98, respectively. The emitter of transistor g2 is connected 2~ to the base terminals of power transistors 100 and 101; the 29 emitter of transistor 93 is conne~ted to the base terminals of power transistors 102 and 103; and the emitter o~ transistor 94 3~7~

1 is connected to the base terminals of power transistors 104 and 2 105. The positive terminal of battery 70 is connected to one 3 arm 36 of soldering tool 30 and the other arm 37 is connected 4 to the common collector connection of transistors 100-105. The emitters of transistors 100-105 are connected to the negative 6 terminal of battery 70 via fuses 110-115, respectively.
7 Variable resistors 96-98 are used to balance the drive and 8 power transistors circuits for a proper sharin~ of the load.
q A positive pulse at the output of monostable circuit 82 renders transistor 90 conductive which renders drive tran-11 sistors 92-94 conductive which in turn render power transistors 1,' 100-105 conductive. When the power transistors are conductive 13 current flows from the positive terminal of battery 70 through ~ soldering tool 30 and then through the parallel paths of the collector-emitter circuits sf power transistors 109-105 back to 16 the negative terminal of the battery.
17 Thus, actuation of switch 78 produces a high current 18 pulse through soldering tool 30 having a duration in accordance 19 with the setting of variable resistor 83. The amount of current that ~lows through the soldering tool depends on the 21 dimensions and composition of the tool. Two cells in batte~y 22 70 provide a voltage somewhat above 4 volts and, with ~ools 23 dimensioned as indicated in Table I, current pulses in the 24 range o~ 160 to 250 amperes are produced~ With three cells the potential is somewhat above 6 volts and current pulses in the 26 range of 250-325 amperes are produced. With f'our''cells the 27 potential is somewhat above 8 volts and the current pulses are

28 in the range of 275 to 425 amperes.

29 As previously mentioned, in some cases a multiple pulse sequence is preferable such as 6 volts ~or B milliseconds 1 for high temperature insulation stripping followed by 4 volts 2 for 25 milliseconds for completing the solder joint. A
suitable circuit for such a pulse sequence is illustrated in Figure 8. In this case the energy for the soldering tool is provi~ed by a three cell battery includin~ a pair of cells 121 6 in series with another cell 120. A charging circuit 122 is 7 connected across the battery to maintain the battery at a full 8 state of charge.
~ ~ Two power transistors lB0 and 181, in parallel, are used to connect the four volt source to soldering tool 30 and 11 transistors 170-172 form the drive circuit therefor. Three 12 power transistors 160-162, in parallel, are used to connect the 13 ; six volt source to the soldering tool and transistors 150-152 ~ form the associated drive circuit. A monostable flip-flop 130 forms timer ~ for controlling the ~ volt energization period 16 and a monostable ~ultivibrator 140 forms timer B for 17 controlling the 4 volt energization period.
18 Monostable circuits 130 and 140 have variable 19 resistors 131 and 141, respectively connected between the circuit and the ~2 volt source. The variable resistors and 21l; associated capacitors 132 and 142 form the timing circuits for 22 the monostable multivibra~ors 23 An input terminal is connected to the trigger input of 24 circuit 130 and the output of circuit 130 is connected to the 25, trigger input of circuit 140. If resistors 131 and 141 are set 26 for 8 milliseconds and 25 milliseconds, respectively, then a 27 transient trigger signal on terminal 135 produces a positive 8 28 millisecond pulse at the output of circuit 130 followed by a 29 positive 25 millisecond pulse at the output o~ circuit 140.
The output of circuit 130 is connected to the base of 2~377 1 NPN transistor 150 via resistor 153. The collector of 2 transistor 150 is connected to the *12 volt source via resistor ~ 154 and to the base or NPN transistor 151. The collector of ~ transistor 151 is connected to the +12 volt source via series ~ resistors 155 and 156 and the junction of the resistors is 6 connected to ~he base of NPN ~ransistor 152. The emitters of 7 transistors 150 and 151 are connected to ground whereas the 8 ' emitter of transistor 152 is conne~ted to the +L2 volt source.
q The collector of transistox 152 is connected to the ba~e terminals Q~ NPN power transistors 160-162. The positive 11 terminal of battery 120 is connected to the common collector ~, 1~ ; circuit of parallel transistors 160-162, the emitters thereof 13 being re~urned to the negative battery terminal through fuses 1~ . 163-165, respe,ctively, and soldering tool 3Ø.
15 . The output o~ circuit 140 is connected to ground 16 ' through resistors 173 and 174 and the junction of the resistors 17 is connected to the base of NPN transistor 170. The emitter of 18,, transistor 170 is connected to the base of NPN transistor 171 19 and the emitter thereof is connected to ground. The collectors or transistors 170 and 171 are connected to the ~12 volt source 21 through series resistors 175 and 176 and the junction of the 22 resistors i connected to PNP transistor 172. The emitter of 23 ' transistor 172 is connected to the +12 volt source and the 24 ~ collector thereo~ is connected to the base terminals of NPN 1, power transistors 180 and 181. The positive terminal of 26 battery 121 is connected to the common collector circuit of 27 parallel transistors 180 and 181 and the emit~ers thereof are 28 connected to the negative battery terminal through fuses 182 29 and 1~3, respectively, and soldering tool 30.
A trigger pulse at terminal 135 causes monostable 1 circuit 133 to pr~duce an output pulse which renders 2 transistors 150-152 in the drive circuit conauctive which in 3 turn renders parallel power transistors 160-162 conductive. As 4 a result, a high urrent pulse o~ a duration determined by the 5 ,` setting of variable resistor 131 is supplied to the soldering 6 tool from the 6 volt battery source 120-121. Termination of 7 the pulse at the output of clrcuit 130 triggers operation of 8 monostable circuit 140 which then produces an output pulse q , which renders drive transistors 170-172 conductive which, in turn, render parallel power transistors 180-181 conductive.
11 This results in a high current pulse being supplied to the 1' soldering tool via transistors 1~0-181 from the 4 volt source 13 ,l 121 for a period of ~ime determined by the setting of variable 14 I resistor 141.
While only a few illustrative embodiments have been 16 ~ described in detail, it should be obvious that there are 17 numerous variatlons within the scope of this invention. The 18 inven~ion is more particularly defined in the appended claims.

20, ~4 j 25'

Claims (35)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A high speed method of operating in the presence of solder melting at about 450° F for soldering a wire lying across a terminal pad or the like on a circuit board, using a soldering tool of predetermined mass, comprising:
(a) selecting the effective mass of said soldering tool so that the quantum of heat energy stored therein is only slightly in excess of that required for an effective solder joint of solder melting at about 450°
F and is substantially used up during formation of the solder joint;
(b) heating said effective mass to a preselected high temperature below that which would cause rapid deterioration of the tool;
(c) bringing said tool into thermal contact with the wire to be soldered when lying across the terminal pad;
(d) while in thermal contact;
(i) substantially imparting just enough heat to said wire and said terminal pad to complete a solder joint, and (ii) permitting solidification of said solder;
(e) said quantum of heat energy being in-sufficient to permit significant heat migration into the circuit board beyond said terminal pad; and (f) wherein said soldering tool is raised to a temperature selected so that the solder joint is formed in less than 500 milliseconds.

2. The method in accordance with claim 1 wherein said total contact time is less than 50 milli-seconds.

3. The method in accordance with claim 1 wherein said soldering tool is heated to a temperature above 1000 degrees F.

4. The method in accordance with claim 3 wherein said soldering tool is heated to a temperature between 1600 degrees F and 2000 degrees F.

5. The method in accordance with claim 1 wherein the soldering tool is heated to said preselected high temperature prior to contact with the wire to be soldered.

6. The method in accordance with claim 1 wherein the soldering tool is heated to said preselected high temperature water contact with the wire to be soldered.

7. The method according to claim 1 wherein said terminal pads are solder coated prior to contact with said soldering tool.

8. The method according to claim 7 wherein said terminal pads are plated with solder in a non-eutectic state.

9. The method according to claim 1 wherein said terminal pads are pre-tinned.

10. The method according to claim 1 wherein said wire is pre-coated with solder prior to contact with said soldering tool.

11. The method according to claim 1 wherein solder used for forming said solder joint is in a preformed configuration.

12. The method according to claim 1 wherein solder used for forming said solder joint is in a fluid form.

13. A high speed method of soldering insulated wire lying across a terminal pad of a circuit board coated with solder melting at about 450° F using a soldering tool of a predetermined effective mass comprising:
(a) selecting the effective mass of said soldering tool so that the quantum of heat energy stored therein when heated to a preselected high temperature is only slightly in excess of that required to vaporize insulation off the wire and to create an effective solder joint of solder melting at about 450° F;
(b) heating the soldering tool of said selected mass to a preselected high temperature close to, but below, the temperature which would cause rapid deterioration of the tool;
(c) bringing the soldering tool into contact with the insulated wire while lying across the solder coated terminal pad;
(d) said quantum of heat energy in said tool being just sufficient (i) to vaporize insulation on the wire in the area of the contact point, (ii) to then vaporize the insulation on the side of the wire opposite to said contact point, and (iii) to then melt the solder on the solder coated terminal pad;
(e) said quantum of heat energy being insufficient to permit significant heat migration into the circuit board or other components beyond said terminal pad; and (f) the total contact time for making the solder joint being less than 500 milliseconds.

14. The method in accordance with claim 13 wherein the soldering tool remains in contact with the wire until the solder has solidified.

15. The method in accordance with claim 14 wherein the total contact time is less than 50 milli-seconds.

16. The method in accordance with claim 13 wherein:
(a) said preselected high temperature is between 1600 degrees F and 2000 degrees F; and (b) the volume of the effective mass of the soldering tool is on the order of 5X10-6in3.

17. The method in accordance with claim 13 wherein the soldering tool is heated to said preselected high temperature by passing an electrical current in the range of 50 to 500 amperes for a period of 5 to 100 milliseconds.

18. The method in accordance with claim 13 wherein the soldering tool is heated prior to contact with the insulated wire.

19. The method in accordance with claim 13 wherein the soldering tool is heated after contact with the insulated wire.

20. The method in accordance with claim 13 wherein the soldering tool maintains the wire in contact with the terminal pad with a contact force in the range of 100 to 800 grams.

21. A high speed method of soldering insulated wire lying across a terminal pad of a circuit board coated with solder melting at about 450° F using a heated soldering tool, comprising:

(a) selecting the effective mass of the soldering tool so that, when heated, the quantum of heat energy which can be imparted for making a solder connection is just sufficient to vaporize insulation off the wire and to liquefy the solder melting at about 450° F to make an effective solder joint;
(b) heating the soldering tool to a temperature at least in excess of the vaporizing temperature of the insulation on the wire;
(c) bringing the soldering tool into contact with the insulated wire while lying across the solder coated terminal pad;
(d) the heating period for the soldering tool, and the temperature thereof when heated, being selected to provide a temperature profile such that (i) the temperature imparted to the insulation in the contact area exceeds the vaporization temperature thereof, (ii) the temperature of the solder on the presoldered terminal pad exceeds the liquefaction tem-perature of about 450° F for a minimum period of time sufficient for an effective solder joint, and (iii) the temperature of the circuit board in the vicinity of the terminal pad does not rise above the temperature causing deterioration thereof.

22. The method in accordance with claim 21 wherein the soldering tool is heated to a temperature close to, but not exceeding, that temperature which causes rapid deterioration of the tool to obtain a fast temperature rise time upon contact and minimum heat migration beyond the terminal pad during contact while the solder joint is being formed.

23. The method in accordance with claim 21 wherein the soldering tool is heated to a temperature between 1600 degrees F and 2000 degrees F.

24. The method in accordance with claim 21 wherein said effective mass has a volume on the order of 5X20-6in3.

25. The method in accordance with claim 21 wherein said temperature profile includes (a) a soldering tool temperature greater than 1000 degrees F, (b) a temperature for vaporizing insulation off the wire greater than 750 degrees F, (c) a temperature at the terminal pad for melting the solder greater than 450 degrees F, and (d) a temperature at the substrate adjacent the terminal pad of less than 550 degrees F.

26. The method according to claim 21 wherein the soldering tool is constructed including a small effective mass used to store heat for soldering which is thermally coupled to larger mass so that heat is dissipated into said larger mass to cool the solder upon completion of the solder joint.

27. A system operating in the presence of solder for soldering wire to a terminal pad on a circuit board, comprising:
(a) a soldering tool having a predetermined effective mass;
(b) means for applying thermal energy to said soldering tool (i) to raise the temperature of said soldering tool to a preselected high temperature below that which causes rapid deterioration thereof, and (ii) to store in said soldering tool a quantum of heat energy only slightly in excess of that required for an effective solder joint and which is substantially used up during formation of a solder joint;
(c) means for bringing said soldering tool into thermal contact with the wire to be soldered while lying across the terminal pad (i) to impart just enough heat to complete a solder joint, and (ii) to permit solidification of the solder;
and (d) said effective mass and said application of thermal energy thereto being so selected that the solder joint is formed in less than 500 milliseconds.

28. The system according to claim 27 wherein said effective mass and said application of thermal energy thereto are selected so that the solder joint is formed in less than 50 milliseconds.

29. The system according to claim 27 wherein said means for applying thermal energy to said soldering tool raises the temperature thereof to above 1000 degrees F.

30. The system according to claim 27 wherein said means for applying thermal energy to said soldering tool raises the temperature thereof to between 1600 degrees F
and 2000 degrees F.

31. The system according to claim 27 further comprising means coupled to said means for applying thermal energy for imparting a relative motion between said soldering tool and the circuit board to bring said solder-ing tool into thermal contact with the wire and terminal pad subsequent to storing said predetermined quantum of heat energy in said soldering tool.

32. The system according to claim 27 further comprising means coupled to said means for applying thermal energy for imparting a relative motion between said soldering tool and the circuit board to bring said soldering tool into thermal contact with the wire and terminal pad prior to storing said predetermined quantum of energy in said soldering tool.

33. A system according to claim 32 wherein said soldering tool includes a tip and a heat reservoir thermally coupled to said tip for dissipating thermal energy after formation of the solder joint.

34. The system according to claim 27 wherein said means for applying thermal energy to said soldering tool applies energy in a plurality of pulses.

35. The system according to claim 27 wherein said pulses include a short, relatively high energy pulse for stripping insulation from the wire followed by a longer, lower energy pulse for effecting the solder joint.