GB2201917A - Improvements in solder joints - Google Patents

Abstract

A method of soldering an assembly of components to be joined together, wherein the assembly of components is first dipped in a bath of molten high melting point (HMP) solder to produce an HMP joint 24 and then dipped in a bath of a molten low melting point (LMP) solder to form a coating 28, 30 of LMP solder over the HMP solder. <IMAGE>

Description

DESCRIPTION IMPROVEMENTS IN SOLDER JOINTS.

The present invention is concerned with solder joints.

Solder joints are used in the manufacture of electronic/electrical components to provide electrical connection and although conventionally it is considered bad working practice to use the joint as a physical retention means, increasingly, because of the constraints of miniaturisation, a solder joint often per brims the two functions of electrical and physical connection.

Where this approach is adopted, when the solder joint is near to the termination of the component, a problem can occur when the terminations are themselves soldered onto, for example, printed circuit board.

Because of the close proximity to the soldering iron and the conduction of heat along the termination, the internal joint can melt and damage the internal construction of the component.

Other methods of soldering, such as hot ovens, infra-red conveyors and vapour phase soldering systems apply heat to the whole component and inevitably in these cases the soldering process can melt and damage the internal joints.

In order to mitigate this problem, it has been conventional practice for internal solder joints to have utilised solder alloys which melt at higher temperatures than the external joints. Correct selection of the soldering temperature then prevents re-flowing of the internal joints. Typically, the external joint to the termination would utilise standard low melting point WIMP) 60/40 solder which melts at 180 C and the internal joints would use a higher melting point solder (HMP), such as 96% lead, 4% silver, which melts at 3050C.

However, with miniature components, it is often difficult to apply HMP solder consistently, principally because of the need to use conventional non-corrosive fluxes which tend to dissociate at the higher soldering temperatures and do not, therefore, assist in the soldering operation (by removing surface oxides) as they do effectively with lower melting point solders such as 60/40 alloys. As a consequence of this, special techniques are often adopted for HMP soldering which attempt to minimise exposure to oxidising atmospheres, either by pre-heating at below oxidation temperatures and then rapidly heating to the solder melting point (RF and oven soldering) or by the use of reducing atmosphere furnaces such as hydrogen.

Vapour phase soldering of HMP solders would be a useful alternative but, unfortunately, no suitable boiling point liquids are currently available for temperatures above 2120C and this may well be a fundamental limitation to the use of vapour phase soldering of HMP solders.

A soldering technique which is used extensively with 60/40 solder is "dipping", whereby the joint to be soldered is designed to retain solder by surface tension after it has been dipped into a molten bath of solder. The joint is then formed surrounded by molten solder and thus oxidation problems are minimised with consequent improved quality of the solder joint. Such dipping techniques have substantial potential for mass production.

It would be very advantageous if this technique could be extended to HMP solders. There is, however, a problem in that when solder dipping is used to form an internal joint, parts of the component which are not part of the joint are also coated with solder.

The accompanying yawing shows a typical example of this where the termination is coated in the solder dipping operation to effect the solder joint at 'A'.

With 60/40 solder this is not a problem, indeed it can be considered desirable as the whole of the exposed part of the termination is then coated with a good solderable finish (the 60/40 solder coating).

On the other hand, when a molten bath of HMP is used then the whole of the exposed part of the termination is coated with HMP solder, which is not a good solderable finish for subsequent 60/40 solder joints.

It is an object of the present invention to provide a method of soldering by which the aforegoing problem can be eliminated.

In accordance with the present invention, an assembly of components to be joined is first dipped in a bath of molten HMP solder to produce an HMP joint and then dipped in a bath of molten LMP solder to form a coating of LMP solder over the HMP solder.

The invention is described further hereinafter, by way of example only, with reference to the accompanying drawing, which is a longitudinal section through a typical ceramic lead-through filter (pi-section) employing soldered joints.

The illustrated device, which is shown purely by way of illustration of a typical component assembly which involves soldered joints, comprises essentially an inner metal rod-like termination 10, a cylindrical ferrite core 12, a ceramic tube 14, and an outer tubular termination collar 16. The device also includes metallisation areas 18, 20 and 22. It is required to apply solder to the regions A and B in order to mechanically locate the internal components and to provide a coating on the energised portions of the inner termination 10 for enabling subsequent soldered connections thereto.

In accordance with the present method, a double dipping technique is used whereby the two end regions A and B are first dipped in a bath of molten HMP solder to produce HMP joints at 24 and 26, and then dipped in a bath of molten LMP 60/40 solder to apply coatings 28 and 30 of 60/40 solder on top of the previously applied HMP solder coatings, respectively.

As mentioned above, high melting point (HMP) solders form solder joints less easily than 60/40 because of the dissociation of the fluxes at the higher temperatures and also because the alloy composition in the joint, for various reasons, is often not a true eutectic and inevitably therefore; the joints are of poor visual appearance, often grainy and with poor wetting characteristics. An advantage of coating HMP joints with 60/40 as described above is that the normal characteristics of a good solder joint can be achieved, giving the joint a shiny appearance and a meniscus with the correct wetting angle.

In addition to providing a solderable finish, the second 60/40 coating also considerably improves the quality of the HMP joint.

Furthermore, when the HMP joint is used as a hermetic seal, there is also a considerable advantage to the double coating technique in that the second 60/40 coating also improves the sealing integrity of the HMP joint.

In addition to combinations of HMP (305us) and 60/40 (180"C) solder, the present technique can be used for other combinations of two solders characterised by the fact that their melting points differ by more than 400C.

In applying the second, lower melting point solder coating to the HMP solder joint, inevitably some alloying between the two types of solder will occur.

Depending on the relative solubilities of the one in the other, the time allowed for alloying to occur and the amount of other metals which may have dissolved in the HMP joint, an intermediate region can form between the outer lower melting point solder and the HMP joint. The melting point of this region may then also be intermediate to the original two solders but in general the intermediate region does not significantly affect the characteristics of the outer solder coating or the original HMP joint.

Claims (5)

1. A method of soldering an assembly of components to be joined together, wherein the assembly of components is first dipped in a bath of a molten high melting point (HMP) solder to produce an HMP joint and then dipped in a bath of a molten low melting point (LMP) solder to form a coating of LMP solder over the HMP solder.

2. A method of soldering an assembly of components to be joined together, wherein the assembly of components is first dipped in a bath of a first solder to produce a basic joint and then dipped in a bath of a second solder to form a coating of the second solder on the first solder, the first and second solders being selected such that the first solder has a melting point which is at least 40#C higher than that of the second solder.

3. A method as claimed in claim 1, wherein the low melting point LMP solder is a conventional 60/40 alloy solder which melts at approximately 180 C.

4. A method as claimed in claim 1 or claim 3, wherein the HMP solder contains approximately 96% lead and 4% silver and melts at approximately 305QC.

5. A method of soldering an assembly of components to be joined together, substantially as hereinbefore described with reference to the accompanying drawings.