Heater Bar Assembly

This invention is directed to a heater bar for soldering closely spaced components to printed wiring boards, and particularly apply heat to leads to melt solder already in place to cause reflow soldering.

Printed wiring boards have pads thereon which are coated with solder. Electrical components are positioned so their electrical leads lie on the pads. The solder is heated to melting so that the leads are connected to the pads by means of a solder joint. There are a number of ways for heating the solder. One of the ways is to apply a heater bar to the lead, for heating the lead and solder. An example of this is Joseph M. Jacos and Robert F. Hubbard U.S. Pat. No. 4,654,507 entitled "Solder Reflow Heater Bar Assembly." The structure described in that patent is eminently successful.

The present trend in the technology where components are mounted on the surface of the printed wiring board is toward closer spacing between the components. As a result, it is difficult to gain access to the leads to apply heat to the leads in these closely spaced structures. In addition, the surface of the printed wiring board may not be absolutely level, and rocking of the heater bar assembly is necessary to accommodate for this out-of-level configuration. Rocking of the heater bar around a high pivot point causes lateral displacement of the leads. When the pads are smaller, due to attempts to increase the number of components in the printed wiring board, the lateral shifting due to rocking of the heater bar may move a lead off of its pad. Thus, a compact structure which minimizes lateral shifting upon rocking is required in order to meet the close spacing of components and small pads found on modern printed wiring boards.

In order to aid in the understanding of this invention, it can be stated in essentially summary form that it is directed to a heater bar assembly wherein the heater bar is U-shaped with the soldering surface close to the U and the pivot point close to the U to place the pivot point closely adjacent the soldering surface.

It is thus a purpose and advantage of this invention to provide a heater bar assembly which is able to reach and apply heat to leads on printed wiring board solder pads where the components are closely spaced.

It is another purpose and advantage of this invention to provide a pivot for the heater bar which is forward close to the soldering surface to cause less misalignment between the component leads and the printed wiring board pads.

It is another purpose and advantage to provide a heater bar assembly which has a small and restorable face so as to permit it to be dressed without removal of essential heater bar material.

Other purposes and advantages of this invention will become apparent from a study of the following portion of this specification, the claims and the attached drawings wherein:

FIG. 1 is a rear elevational view of the heater bar assembly of this invention.

FIG. 2 is a substantially central section as seen generally along line 2--2 of FIG. 1.

FIG. 3 is an upwardly looking transverse section, as seen generally along line 3--3 of FIG. 1.

FIG. 4 is a downwardly looking transverse section, as seen generally along the line 4--4 of FIG. 1.

FIG. 5 is an enlarged side-elevational view of the heater bar, shown in relationship to component leads being heated.

FIG. 6 is a front-elevational view of the heater bar showing it coming into contact with leads which are on an out-of-level group of pads.

FIG. 7 illustrates the lateral displacement caused by rocking motion about a high pivot as a result of out-of-level pads.

FIG. 8 is a diagram showing the amount of displacement due to rocking motion around a low pivot as a result of out-of-level pads.

FIG. 9 shows the lead-to-pad displacement as a result of the rotation illustrated in FIG. 7.

FIG. 10 illustrates the lead-to-place displacement as a result of the rotation illustrated in FIG. 8.

The heater bar assembly of this invention is generally indicated at 10 in FIGS. 1 and 2. Body 12 is rectangular and preferably metallic and serves as the mounting structure for the heater bar assembly. Body 12 extends upward from the broken-away line in FIGS. 1 and 2 and has fastener means thereon for mounting upon the soldering machine. The soldering machine preferably automatically positions the heater bar assembly and brings it down in to soldering position. In addition, the soldering machine supplies the correct current for the correct time to provide sufficient heat for proper soldering. Body 12 extends downward from its mounting location and is slotted across to provide guide section 14. Below the guide section, the body is narrowed to define rectangular finger 16. Pressed into a drilled opening in the lower end of the finger is pivot pin 18. The lower end of pivot pin 18 carries pivot point 20 thereon, see FIGS. 2 and 5. This provides the rigid structure which defines the precise location of the pivot point with respect to the body 12 and thus with respect to the soldering machine.

FIG. 1 is a back view of the assembly, and in this orientation, it is seen that right and left busses 22 and 24 are positioned adjacent the sides of finger 18 and are spaced therefrom and extend downward therealong. The buss bars 22 and 24 extend upward to the back of the guide section 14 where they respectively lie on pads 26 and 28. Right pad 26 is also seen in FIG. 2 to show that the buss bars are spaced from the metallic guide section. The pads 26 and 28 are of insulative, low-friction material to electrically separate the buss bars from the guide section and to permit lateral sliding motion of the buss bars with respect to the guide section. Opposite the upper end of the buss bars is thermocouple connector body 30. Body 30 is made of dielectric material and carries thereon thermocouple connectors 32 and 34. Thermocouple leads 36 extend downward from the thermocouple connectors. Right and left lugs 38 and 39 are positioned between the thermocouple connector body and the two busses, respectively. Power leads from the right and left lugs are connected to the soldering machine, which supplies the heater current therethrough. Clamp screws 40 and 41 have their heads engaging on the front of the thermocouple connector body, extend through the thermocouple connector body, through lugs 38 and 39, and threadedly engage in the busses 22 and 24. The screw 40 is shown in FIG. 2 in dashed lines to illustrate this connection function. FIG. 1 illustrates that the lugs 38 and 39 are slotted so that they may be pulled out simply by loosening the clamp screws. When the clamp screws are tightened, the busses 22 and 24 on the back and the thermocouple connector body on the front are not clamped tight to the guide section 14, but lateral motion is provided. The fit is close so that rotation around the center line, as defined by the axis of pivot pin 18, is minimized.

Heater 42 is a U-shaped structure of metal of intermediate resistance so that, when electric current is passed therethrough, heating of the heater results. Heater 42 is U-shaped with a front leg 44 and a back leg 46. The front leg is attached to right buss 22 and is spaced from left buss 24. Similarly, the back leg 46 is connected to left buss 24 and is spaced from right buss 22, see FIG. 4. In this way, current passing through the busses passes through the heater, from one leg to the other. As is seen in FIG. 5, heater 42 has a first bend 48 of about 90 degrees which produces a platform 50 between the front and back legs of the heater. Beyond the platform, second bend 52 is of more than 90 degrees and carries the front 54 of the U-shaped heater past the platform 50. Pivot opening 56 in the platform receives pivot point 20. At the front of the heater, heated tip 58 is attached by any convenient means which is of high thermal conductivity and is strong at heater temperatures. Heated tip 58 has a soldering face 60 which applies the heat to the required location for soldering.

The movable structure comprised of heated tip 58, busses 22 and 24, lugs 38 and 39, thermocouple connector body 30 and the screws 40 and 41 which hold it together thus pivot around pivot opening 56 engaging upon pivot point 20. The small dimension of these parts is such that conical binding does not occur, although spherical surfaces could be employed if the structure was larger. The closeness of the busses and thermocouple connector body to the guide section 14 constrains the pivoting motion to a plane parallel to the paper in FIG. 1 with its axis normal to the plane of the paper in FIG. 1 with the axis passing through the engagement of the pivot pin in the pivot opening.

As thus far described, nothing retains the movable assembly upon the body 12. This retention is by tension spring 62, see FIGS. 1 and 2. The upper part of the tension spring lies in spring slot 64. Cross pin 66 lies in a cross pin opening through the body adjacent the top of slot 64 with the spring engaged upon the cross pin. The lower end of the spring is engaged upon cross pin 60, see FIG. 2. Dielectric insulating cups 70 and 72 lie on the ends of cross pin 68. These cups engage in slots 74 and 76, respectively, in busses 22 and 24. The tension of spring 64 not only holds up the movable structure, but also urges it toward the centered position illustrated in FIG. 1. Any rotation of the movable assembly stretches the spring. The thermocouple leads 36 extend down into heater 42 and terminate in a thermocouple 78 within the U of the heater, directly above the heated tip 58. In this way, heater temperature is signaled.

The heated tip 58 is thin enough and long enough to reach between components, as shown in FIG. 5, and is as wide as is necessary to reach across the desired number of leads and pads. An additional advantage of having this extended heated tip is that the face 60 can be dressed should it become other than flat along the face. Repeated dressings of the face to make it planar in a direction parallel to its rotational axis B give long life to the heater. Such dressing does not wear away the current-carrying U-portion of the heater, but merely reduces the height of heated tip 58. Thus, many dressings can be obtained without reducing the utility of the heater.

FIGS. 6, 7, 8, 9 and 10 illustrate the significance of having a pivot point close to the soldering point at the face of the heated tip. FIG. 6 illustrates the heater 42 moving downward so that the face 60 of the heated tip 58 engages on a plurality of component leads on contacts on a printed wiring board 80. One of the pads on the printed wiring board is indicated at 82, and the component lead 84 is shown in association with that pad. In FIG. 6, it is seen that component 86 has five such component leads in association with a corresponding number of pads. The component 86 may be a flat pack with the leads forming a lead frame therefrom. In the case where the surface and pads of the printed wiring board are not absolutely level, the heater 42 must pivot so that its face 60 contacts all of the leads. The angle Theta at the lower right of FIG. 6 illustrates this lack of level condition of that portion of the printed wiring board. Such may occur from board warping. If the pivot point of the heater 42 is high, for example at point A, far away from the face 60, then the rotation about the angle Theta produces a large displacement 88, as seen in FIG. 7. FIG. 7 is on the same scale as FIG. 6, and FIG. 9 is on an enlarged scale to illustrate how this displacement moves the component lead 84 at least partially off of the pad 82. This results in unsatisfactory solder conditions so that reliable soldering is difficult to achieve.

When the pivot point is low close to the face 60, for example at point B substantially at the lower second bend 54, the same rotation causes lesser displacement. When the heater 42 comes and pivots at point B in FIG. 6, the same scale displacement 90 is illustrated in FIG. 8. On an enlarged scale, as seen in FIG. 10, this displacement 90 does not offset the lead from its pad. Thus, reliable soldering can be achieved. Actual angles are quite small, but alignment of leads to pads is very critical, and even small displacement reduces reliability. Acceptable soldering joints for all of the leads of a multi-leaded component to their corresponding soldering pads on a printed wiring board cannot be achieved unless equal force is applied to all the leads of the component. The limited rocking of the heater and its face 60 provide this equal force, and the low pivot minimizes the offset which occurs. Since the spring 62 centers the movable structure, including the heated tip 58, the face 60 starts level and rotates only to provide equal force on all of the leads. Thus, translational forces are not produced when the board is level. The rocking limit of the moving structure is the striking of one or the other buss bars against the center finger 16, as seen in FIG. 1. However, this is a mechanical limit which prevents damage to the heater bar assembly and is not reached during normal soldering operation.

FIG. 5 illustrates how two closely adjacent components 86 and 92 can have their leads reached by the heater bar assembly of this invention due to the narrow front-to-back dimension and due to the fact that heated tip 58 is thin and extends down from the U-shaped heater to keep the U-shape of the heater above and away from the component. In this way, reliable soldering is achieved.

This invention has been described in its presently contemplated best mode, and it is clear that it is susceptible to numerous modifications, modes and embodiments within the ability of those skilled in the art and without the exercise of the inventive faculty. Accordingly, the scope of this invention is defined by the scope of the following claims.