GB2265102A - Soldering apparatus and method - Google Patents

Abstract

Apparatus (1) for applying solder to electrical components such as integrated circuit packages (2) has a chamber (4) which contains a body of solder. The chamber (4) has outlet openings (10) which are below the surface of the solder and from which a flow of solder is provided. Since dress (oxidised solder) floats on the surface of the solder, the flow of solder is relatively pure. The leads of the packages are dipped into the flow of solder so that they are coated with a protective layer of solder. <IMAGE>

Description

SOLDERING APPARATUS AND METHOD The present invention relates to an apparatus and method for applying solder to electrical or electronic components. The invention is particularly suitable for the application of solder to the leads of integrated circuit packages.

Integrated circuit packages now come in a variety of different forms. For example "dual-in-line packages" ("DIP's") have a generally rectangular body with leads projecting from two opposing sides. "Plastic leadless chip carriers" known as "PLCC's" and "small outline integrated circuits" known as "SOIC's" are two further examples of integrated circuit packages. The latter two packages are formed from "quad flat packs" which have a relatively flat body, rectangular in plan view and with leads projecting from all four sides generally in the same plane as the flat body. The leads, normally of copper, are bent to the required shape so as to form either a PLCC or a SOIC. The shape of the leads can be e.g. stepped or J-shaped.

Current quad flat packs available may have 20 to 200 or more leads along each side and are generally square or rectangular or similarly shaped in plan view.

The spacing between the leads, i.e. their pitch, can vary from 0.100 inches (2.5mm) down to as small as 0.010 inches (0.25mm) or even smaller.

The leads of electronic components and in particular the leads of integrated circuit packages are usually precoated with solder to provide them with a protective coating before they are mounted e.g. on a printed circuit board.

There are a number of methods known for achieving such precoating. Electroplating is one such method.

However, this method has the disadvantage that it is possible to plate on even those surfaces which have been contaminated by dirt, grease or oxidation. If a coated lead having a contaminated surface is connected to the printed circuit board, the resulting connection is weakened and can result in malfunction. To avoid this problem, solder coating is preferred by the end-user since solder will not attach to a contaminated surface so the risk of forming a weakened connection is reduced.

Moreover, during the trim and form process, the crosssection of the leads being trimmed will have exposed base matter. This will result in eventual oxidation and hence defeat the purpose of a protective coating.

One method of solder coating is known from European Patent Application No. 0171257 which discloses a wave soldering method. In this system, a carrier for the integrated circuit packages is passed over a bath of solder in which a standing wave is formed. The arrangement is such that the component portions to be coated pass through the peak of the standing wave of solder as the carrier is moved over the bath.

Another method, known as drag soldering, is disclosed in European Patent Application No. 0252771.

In drag soldering, the carrier which holds the circuit packages is moved down into the solder, through the solder and then upwardly to remove the packages from the solder. During the entire operation, the carrier is kept in a horizontal orientation.

However both of these methods suffer from the problem that dross, formed on top of the liquid solder as a result of oxidation, may be present on the coated leads and consequently may result in a protective coating of poor quality. A thicker layer of solder can be used to compensate for the poor quality caused by the presence of dross, but this in turn means that the packages need to be dipped for longer and thus the packages are subjected to a thermal experience for a longer time. This is disadvantageous in that electronic packages are particularly heat sensitive and in general it is desired to reduce their exposure to heat.

Moreover, thicker coatings impose a limit on the minimum spacings between leads.

It has now been realised that the quality of solder used to coat leads can be improved if advantage is taken of the fact that dross is less dense than solder and therefore will tend to float on the surface thereof.

According to one aspect of the invention there is provided apparatus for applying solder to an electrical or electronic component, comprising a chamber arranged to contain a body of solder with an upper surface, the chamber having an outlet opening arranged to be below said upper surface, wherein, in use, a flow of solder is discharged from the outlet opening to a cavity adjacent thereto where a component is dipped into the solder flow.

According to another aspect of the invention there is provided a method for applying solder to an electrical or electronic component, comprising supplying a chamber with solder to form a body of solder with an upper surface, discharging a flow of solder from an outlet opening of the chamber disposed below said upper surface, and dipping the component into the solder flow.

Thus, since the solder to be applied to the component flows from a region of the chamber below the upper surface of the solder, where the solder is not exposed to atmosphere, the solder is substantially free from dross and thus the quality of the coating is improved as compared with the known systems. Any dross will tend to float on the upper surface. The solder emerging from the outlet opening does not have time to be substantially oxidised, so that the component is dipped into relatively pure solder. Because of the improvement in quality, it may be possible to reduce the temperature of the solder and/or reduce the dwell time as compared with the known systems, so that the thermal stress experienced by the component is reduced.

A pump may be provided to supply the apparatus with solder. It is desirable to maintain the depth of solder above the outlet opening as constant as possible, so that the rate of discharge through the outlet opening remains substantially constant and gives consistent results. This can preferably be achieved by providing the chamber with an overflow above the outlet opening.

Solder then flows out of the chamber both via the outlet opening and via the overflow and can then be recycled.

Fluctuations in the supply of solder to the chamber can be compensated for by the overflow to keep the depth of solder reasonably constant.

Preferably the outlet opening faces substantially sideways. This will tend to produce a flow of solder initially sideways and then downwards under gravity into which flow the leads of the component may be dipped.

With this arrangement, the component can be lowered and lifted vertically into and out of the dipping position and this further assists the quality of soldering.

Furthermore, the apparatus can be arranged so that only the leads of the component are in contact with the flow of solder, so that the body of the package is not subjected to any unnecessary thermal experience.

In a simple form, the apparatus may have a single outlet opening producing a flow of solder which may, for example, be used to dip a single row of leads along the side of a component. The cavity may simply be a space adjacent to the outlet opening, for example at the edge of the apparatus. However, the cavity is preferably defined by being enclosed or partly enclosed. Preferred apparatus comprises a set of outlet openings with the cavity located therebetween, the outlet openings being arranged to produce a plurality of flows of solder into the cavity. Thus a respective flow of solder may be provided for the leads along each side of a component, so that all leads are simultaneously coated with solder.

For example, if the component is a "DIP" having leads on two opposing sides, two opposing outlet openings may be provided. If the component is a quad flat pack having leads on four sides, four outlet openings may be provided in a rectangular or square configuration. The or each outlet opening may be in the form of an elongate slot.

It is desirable to treat more than one component at a time for reasons of efficiency and preferably therefore the apparatus comprises an array of sets of outlet openings and an array of cavities. The components to be dipped using such apparatus may be mounted in a carrier which locates them in a corresponding array and which can be lowered and lifted vertically into and out of the dipping position. One type of carrier which may be suitable has a holder with an array of vacuum cups or clamps each for engaging the body of a respective component. A preferred carrier is arranged to be mechanically vibrated during dipping.

There are different ways in which the chamber and the cavity or cavities may be arranged. In two different embodiments of the apparatus, a hollow post is used. In a first embodiment, solder flows into the hollow post, whereas in a second embodiment the hollow post is part of the solder chamber and solder flows out of the post.

Thus according to the first embodiment, the or each cavity is defined in a hollow post which is formed with the outlet openings and into which, in use, the solder flows from the chamber. Thus the hollow post will normally have an open top into which a component is lowered, the post projecting above the upper surface of the body of solder so that solder flows into the cavity only via the outlet openings. The hollow post may have four sides each formed with a respective outlet opening.

If an overflow is provided for the chamber, this will be above the outlet openings but below the upper edge of the hollow post. Where an array of cavities is to be provided, there will be an array of hollow posts each defining a respective cavity. The solder in the chamber generally surrounds all the hollow posts and an overflow can then conveniently be provided at the side of the chamber. Such an overflow arrangement provides good regulation of the solder depth and therefore produces consistent results.

According to the second embodiment, a set of hollow posts is provided about the periphery of the or each cavity, each post being formed with a respective outlet opening via which, in use, the solder flows from the chamber into the cavity. Thus the set of hollow posts form part of the chamber. The body of solder thus has an upper surface defined within each hollow post, which will project above the level of the upper surfaces. If an overflow is provided for regulating the level of the solder, there will preferably be an overflow for each of the hollow posts. This may not be essential since in practice all the hollow posts form part of a common solder chamber, but it is envisaged that an overflow for each of the posts will improve the control of the level of each individual upper surface.Moreover, this arrangement ensures that dross does not accumulate in the hollow posts, but rather it flows away via the overflows. Each overflow is preferably provided at a location remote from the outlet opening, to avoid interference with the application of solder to the components. In a preferred arrangement, the hollow posts are four sided, e.g. square or rectangular, in plan view, the outlet opening being provided on one side of the post and the overflow being provided on another (e.g. perpendicular) side of the post. The overflow may be a cut-out portion.

Where the second embodiment includes an array of cavities, there will be an array of hollow posts from which the solder flows into the cavities. It may then be convenient for each hollow post to provide a flow of solder into more than one cavity. Preferably, a foursided, e.g. square or rectangular hollow post has two outlet openings on opposite sides and it may then have two overflows on the other two sides.

Preferably means are provided to define a closed loop for recycling the solder so that it is only necessary to replace the used solder at periodic intervals. The surface of the solder may periodically be skimmed to remove accumulated dross. Skimming may take place every eight hours, for example. The chamber for solder may be in the form of a tank and part of the tank may be free of any obstacles, such as the hollow posts, so that the molten solder can be skimmed.

Alternatively, the apparatus may have two tanks for containing solder, the two tanks being in fluid communication. Thus coating of leads could take place in one tank whilst the surface of the solder in the other tank could be skimmed either continuously or at preselected intervals. In such a system, there is preferably a pump to cause movement of solder from the skimming tank to the coating tank. Solder may only be removed to the skimming tank at predetermined intervals, the fluid path between the two tanks being normally closed.

Apparatus embodying the present invention may be arranged such as to expose the upper surface of the solder and/or the flow of solder discharged from the outlet opening to an inert atmosphere comprising, for example, nitrogen. This may be achieved by enclosing the tank in a sealed vessel and pump an inert gas into the vessel. This measure enhances solderability and reduces dross formation to a minimum. Such precautions may be taken when exceptionally good quality of solder is required.

Certain preferred embodiments of the invention will now be described by way of example ar #ith reference to the accompanying figures in which: Figure 1 shows a schematic perspective view of a first dipping system for applying solder to quad flat packs; Figure 2 shows an enlarged perspective view of a hollow post of the first dipping system; Figure 3 shows a plan view of the hollow post of the first dipping system; Figure 4 shows a schematic perspective view of a second dipping system for applying solder to quad flat packs; Figure 5 shows an enlarged perspective view of a hollow post of the second dipping system; Figure 6 shows a schematic plan view of part of the second dipping system; Figure 7 shows a schematic sectional view of apparatus for causing vibratory motion; and Figures 8a to 8d show cross-sections of the apparatus of Figure 7 (taken along lines VIII-VIII) in four different positions.

The first dipping system 1 for applying solder to quad flat packs 2 will now be described with reference to Figures 1 to 3. A tank 3 provides a chamber 4 for a body of molten solder. Heating elements (not shown) are arranged in the bottom of the tank 3 and are appropriately controlled to keep the solder at the correct temperature. A number of hollow posts 5 are arranged in the tank 3, each post 5 having a cross sectional shape which is substantially square so as to receive a correspondingly shaped quad flat pack 2. The posts 5 are regularly arranged in parallel rows to form an array. Each post 5 is spaced apart from the adjacent post by a spacing 6.

Each hollow post 5, as can be clearly seen from Figures 2 and 3, has four upwardly extending walls 7 which form a square cavity 8 arranged to receive a quad flat pack 2 therein with the sides 9 thereof being parallel to each of the four walls 7. Each wall 7 is formed with a solder outlet opening in the form of an elongate slot 10. The tank 3 and the hollow posts 5 are preferably of titanium but can be of any suitable material.

Along one side 11, the tank is provided with an overflow 12 for regulating the level of solder contained therein.

A pump 13 is arranged at a location below the level of the hollow posts 5 and to one side 14 of the tank.

The pump 13 is arranged so as to be able to maintain the level of the solder at least at the level of the overflow 12 which is above the upper edges 15 of the outlet slots 10 but below the top 16 of the hollow posts 5. At the bottom of each hollow post there is a drain hole (not shown) through which solder in the cavity 8 is removed. The removed solder can then be repumped through the system via the pump 13.

A region 17 of the tank is arranged so as to be free from any hollow posts 5 so that the surface of the solder can be skimmed to remove any dross which accumulates on the surface of the solder.

The operation of the first dipping system will now be explained. The pump 13 supplies solder at a rate sufficient for it to pass over the overflow 12 so that the solder has an upper surface with a level below the top of the hollow posts 5 but above the elongate slots 10. Any dross floats on the upper surface. At the lower level of the elongate slots 10, solder surrounding each hollow post 5 flows into the cavity 8 within the post 5 through the slots 10. The flow of solder forms a cascade of dross free solder into the path of which the leads 18 of the quad flat pack 2 are dipped so as to be coated with solder. Since solder is removed from the cavities 8 via the drain hole, they do not fill up with solder. Periodically, the surface of the solder in the part 17 of the tank 3 which is free from the hollow posts 5 is skimmed so as to remove any dross accumulated on the surface.Additional solder need only be periodically added since unused solder is repumped through the system.

A second dipping system 101 for applying solder to quad flat packs 2 will now be described with reference to Figures 4 to 6. As with the first system, the apparatus has a tank 103 which provides a chamber 104 for a body of molten solder. Heating elements (not shown) are arranged at the bottom of the tank and are controlled to keep the solder at the correct temperature. In this dipping system, the cavity 108 for a quad flat pack 2 is located between a set of four hollow posts 105. The hollow posts 105 are substantially oblong shaped and have a solder outlet opening in the form of an elongate slot 110 on each of their longer sides 118, thereby providing a solder outlet for two adjacent cavities 108. The hollow posts 105 are arranged in an array to form an array of cavities 108. The posts 105 at the periphery of the array have only one elongate slot 110 facing inwardly.

An overflow 112 is provided along one side 111 of the tank for removing solder therefrom. In this system the overflow 112 is below the elongate slots 110 of the hollow posts.

As with the first dipping system, the level of the elongate slots 110 is arranged so as to be parallel to and slightly above the sides 9 of the quad flat packs 2 when they are lowered into the cavities 108 defined by the sets of four hollow posts 105. The bottom portions of the hollow posts 105 are interconnected, for example by a manifold (not shown), such that the posts 105 all form part of a common chamber for solder which is supplied by a pump 113 arranged at a location below the hollow posts 105. The pump 113 is arranged to maintain the solder level in the posts 105 above the upper edges 115 of the elongate slots 110 but below the top of the hollow posts 105.

At the top of each of the two shorter walls 119 of the hollow posts 105, there are cutout portions 120 which define an overflow for regulating the level of solder in the hollow posts 105. Thus solder at the top of the posts 105, where dross will tend to accumulate, can overflow out of the hollow posts 105 in a direction away from the leads 18 of the quad flat packs 2, via the cutout portions 120.

The operation of the second dipping system will now be explained. Solder is supplied by the pump 113 to the hollow posts 105 and flows therefrom through the slots 110 so as to form cascades. The leads 18 of the quad flat packs 2 are dipped in the cascades so as to be coated with solder. The level of solder in the posts 105 is regulated by the cutout portions 120 at each end thereof, via which solder overflows. This also ensures that dross is removed from the hollow posts. The solder flowing out of the posts 105 via both the elongate slots 110 and the cutout portions 120 collects in the region of the tank surrounding the posts 105 and is then returned to the pump 113 by overflowing over the overflow 112 at the side of the tank 111.

Reference will now be made to Figures 7 and 8A to 8D which schematically show apparatus 20 for subjecting the quad flat packs 2 to vibratory motion during dipping, suitable for use with the apparatus of Figures 1 to 3 or that of Figures 4 to 6. The apparatus 20 has a number of holders 21, each of which is capable of holding a quad flat pack 2. Only three of these holders 21 are shown in Figure 7. The holders comprise vacuum cups 22 which are coupled to a vacuum source 23 which when switched on causes the quad flat packs 2 to be held against the cups 22 by a sucking action. The cups 22 are of titanium which is a particularly advantageous metal in that it does not pick up solder, is thermally stable and is inert. Alternatively, the vacuum cups 22 may be made of rubber or thermoplastic.The holders 21 are each attached to a horizontal rigid mounting plate 24 through which vacuum lines 25 extend between each of the cups 22 and the vacuum source 23.

A vertical shaft 26 is fixedly connected to the mounting plate 24. The shaft 26 is arranged so as to be vibrated such that the mounting plate 24 and the holders 21 vibrate in a horizontal plane. The shaft 26 is fixedly attached, at its end remote from the mounting plate 24, to a cuboidally shaped box 27. The box is open topped and has a substantially square cross section. Four upright posts 28 are arranged in the interior of the box 27, each post 28 being located midway along each of the sides of the box. The posts 28 protrude a small distance into the interior of the box as can be seen from Figure 8.

The box 27 is itself movably received on a ledge 29 in a substantially square recess 30 of a casing 31. The recess 30 and the ledge 29 have dimensions slightly larger in both directions than the external dimensions of the box 27. The box 27 is free to vibrate within the recess 30 of the casing 31.

A rotatable drive shaft 32 has a portion 33 of pentagonal cross sectional shape arranged in the box 27 and a narrower portion 34 of substantially circular cross section arranged above the pentagonal portion 33 and the open top of box 27. The narrower portion 34 is coupled to a drive motor 35. The drive shaft 32 is fixed in position relative to the casing 31 and movement of the drive shaft 32 causes the box 27 to move within the recess 30. Retaining members 36 extend from the casing 31 to a position above the box 27 to prevent the box 27 from moving upwardly out of the recess 30. The box 27 is thus retained between the ledge 29 and the retaining members 36 so as to be movable in the recess 30 in a horizontal plane.

In operation, the quad flat packs 2 are held against the holders 21 by the vacuum cups 22 and are lowered so that each quad flat pack 2 is received in a cavity such as described in relation to Figures 1 to 3.

As the cups 22 are lowered, the drive shaft 32 is rotated by the motor 35. As the drive shaft 32 rotates, successive corners 37 of the pentagon strike in succession the four upright posts 28 in the box 27, thereby causing the box 27 to be moved in the recess 30 to follow a generally square shaped path of movement.

Successive positions of the box 27 during rotation of the drive shaft 32 can be seen from Figures 8a to 8d.

For ease of reference, the five corners 37 of the pentagonal cross-section of the drive shaft 32 have been numbered I to V and the four posts 28 of the box 27 have been referenced by the letters a to d. In Figure 8a, corner V is striking post a to cause the box 27 to move to the extreme right of the recess 30. As can be seen from Figure 8b, when the drive shaft 32 has rotated by 18 , corner I strikes post b causing the box 27 to move in the "downward" direction of the figures as far as is permitted by the recess 30. After a further 180 of rotation, corner II strikes post c as can be seen from Figure 8c to move the box 27 to the extreme left of the recess 30.Finally, after another 180 of rotation of the drive shaft, corner III strikes post d, to thus move the box to the extent permitted by the recess in the "upward" direction of the figures. This can be seen from Figure 8d. Thus only one fifth of a complete rotation ie. 720 of rotation of the drive shaft 32 is sufficient to cause the box 27 to complete a cycle consisting of a square shaped path of movement.

This vibrational movement may be In the range of 0.25mm to lOmm, more preferably 0.25mm to 2mm e.g. lmm in each direction. The frequency of vibration is preferably in the range of 50 to 500 Hz, more preferably 50 to 150 Hz. However, it will be appreciated that any suitable frequency and amplitude of movement are all possible within the scope of the present invention.

The vibratory apparatus can be used during all stages of the processing of quad flat packs during insertion, dipping and removal from the tank.

Furthermore, this apparatus can be used during the washing and cleaning steps as the vibration movement provides a scrubbing effect.

Claims (10)

Claims

1. Apparatus for applying solder to an electrical or electronic component, comprising a chamber arranged to contain a body of solder with an upper surface, the chamber having an outlet opening arranged to be below said upper surface, wherein, in use, a flow of solder is discharged from the outlet opening to a cavity adjacent thereto where a component is dipped into the solder flow.

2. Apparatus as claimed in claim 1, wherein the chamber has an overflow above the outlet opening.

3. Apparatus as claimed in claim 1 or 2, wherein the outlet opening faces substantially sideways.

4. Apparatus as claimed in claim 1, 2 or 3, wherein the outlet opening is an elongate slot.

5. Apparatus as claimed in any preceding claim, comprising a set of outlet openings with the cavity located therebetween, the outlet openings being arranged to produce a plurality of flows of solder into the cavity.

6. Apparatus as claimed in claim 5, comprising an array of sets of outlet openings and an array of cavities.

7. Apparatus as claimed in claim 5 or 6, wherein the or each cavity is defined in a hollow post which is formed with the set of outlet openings and into which, in use, the solder flows from the chamber.

8. Apparatus as claimed in claim 7, wherein the hollow post has four sides each formed with a respective outlet opening.

9. Apparatus as claimed in claim 5 or 6, wherein a set of hollow posts is provided about the periphery of the or each cavity, each post being formed with a respective outlet opening via which, in use, the solder flows from the chamber into the cavity.

10. Apparatus as claimed in claim 9 when dependent on claim 6, wherein the hollow posts have four sides and two outlet openings on opposite sides.