GB2175019A

GB2175019A - Flux preparation

Info

Publication number: GB2175019A
Application number: GB08610779A
Authority: GB
Inventors: Keith Thornbury Harrison; Dr Clive Frederick Knights
Original assignee: UK Atomic Energy Authority
Current assignee: UK Atomic Energy Authority
Priority date: 1985-05-13
Filing date: 1986-05-02
Publication date: 1986-11-19
Also published as: GB8512059D0; GB8610779D0

Abstract

The invention relates to the preparation of flux suitable for use in soldering. Thus flux components (e.g. chlorides of zinc, lithium potassium and ammonium chloride) are prefused together and finely ground to produce a flux powder. The flux powder may be used in soldering for example copper, copper alloys or mild steel with zinc and zinc-based alloys.

Description

SPECIFICATION Flux preparation The present invention relates to the preparation of flux in the form of a flux powder suitable for use in soldering.

According to one aspect of the present invention there is provided a process for the preparation of a flux in the form of a flux powder which comprises prefusing flux components (as hereinafter defined) together and finely grinding the resulting material to give a flux powder.

The term "flux components" as used in this specification embraces any suitable substance useful in soldering operations examples of which are halide compounds such as zinc chloride, lithium chloride, potassium chloride, tin chloride (SnCl2), sodium fluoride, ammonium chloride and nickel chloride. It is to be understood that all flux components of a chosen group may be prefused together at the same time. However, in some circumstances it may be preferable to prefuse certain combinations of flux components separately and then combine them. Thus, for example, a first batch of flux components from a group can be prefused, a second batch of flux components, comprising the remainder of the group, can be prefused separately and the separately prefused first and second batches subsequently brought together.

The prefused first and second batches may be brought together in any suitable way. Thus, for example, the first and second batches may be separately finely ground and the resulting two powders mixed. Alternatively, by way of example, the prefused first and second batches can be fused together and the resulting material finely ground.

In one particular embodiment flux components are selected such that the flux powder is suitable for use in soldering with zinc and zinc-based alloys.

It has been found that it is difficult to grind flux components such as zinc chloride, lithium chloride, potassium chloride, ammonium chloride, tin chloride (SnCI2), sodium fluoride and nickel chloride to fine powders without prefusing due to moisture content; mere physical mixing of unground flux components (e.g. zinc chloride, lithium chloride, potassium chloride and ammonium chloride) gives rise to a flux which does not give good capillary filling and joint strength during soldering with zinc and zinc-based alloys.

However, in accordance with the present invention prefusing together flux components (e.g.

zinc chloride, lithium chloride, potassium chloride and ammonium chloride) and then finely grinding the resulting material gives a flux powder which can be used in soldering with zinc and zincbased alloys to give good capillary filling and joint strength.

For example, good joint formation and capillary filling was obtained when soldering copper and a-brass at 4500C with pure zinc. Copper, a-brass and mild steel have also been satisfactorily soldered at temperatures not greater than 435 C.

In one embodiment the prefusing of chloride flux components may be carried out under a flow of argon/HCI to remove water and oxide contamination.

It has been found in accordance with the present invention that prefusing of flux components reduces the rapidity of moisture sorption from the atmosphere and thus facilitates fine grinding.

Hygroscopic flux components used in soldering of zinc and zinc-based solders (melting point > 388 C) tend not to lose water readily but rather react to give oxide/hydroxide charged fluxes which reduce the rate of sorption or reaction with oxides of the solder and substrate. It is believed that the present invention may be used to reduce or substantially avoid difficulties arising from excessive moisture or oxide in flux powders.

Optionally after finely grinding the material prepared by prefusing, a further finely divided powder may be added to give a flux powder.

The addition of the finely divided powder (e.g. zinc oxide) may be used to inhibit the tendency of the powder particles to aggregate or cake, or to increase resistance to moisture adsorption. It is thus possible to help to maintain free flowing properties in the powder. The finely divided powder may have a particle size of for example about 0.11 ,um and may be added, for example, at low concentration (e.g. 0.25 to 0.5 weight per cent).

Optionally other substances may be added to vary the composition of the flux powder (e.g.

SnCI2 powder at about 1 weight percent may be added to improve soldering). By way of further example NaF optionally may be added.

In addition to soldering with zinc using flux powders in accordance with the present invention, flux powders in accordance with the present invention have been used for soldering with various zinc based alloys as shown in Table TABLE I

Alloy Zn Al Cu Ni Ti I Mg | Cr Zamak-3* bal 4 -'#0.03 0.03 Zamak-2* bal 4 3 0.1 Zinc-Cu bal 0.7 bal 1.4 bal 3.6 Zinc-Ni bal 0.5 bal 1.0 ILZRO-16+ bal 1.3 0.2 0.1 * Die casting alloys + High strength die casting alloy Figures are weight percent compositions Examples of two flux powders prepared in accordance with the present invention are (a) ZnC12 53%, NH4CI 17%, KCI 17% and LiCl 13% and (b) ZnCI2 57% NH4CI 18%, KCI 14% and LiCI 11%.

It will be understood that a wide range of flux powder compositions may be prepared.

For example KCI and LiCl may be prefused together and ZnCI2 and NH4CI may be prefused separately to give two prefused materials; these materials can then be brought together in various ratios to give flux powders of a variety of different compositions.

Flux powders in accordance with the present invention may be used as dry powders, or alternatively may be formed into a "cream", for application to workpieces, by mixing with a suitable liquid medium (e.g. propanol or water).

Flux powders prepared in accordance with the present invention have been used to obtain good capillary filling and joint strength when soldering with a-brass, copper and mild steel. For example good capillary filling and joint strength were obtained when soldering a-brass with pure zinc at 450 C. Thus with a ring and plug test arrangement (with 0.12 and 0.22mm gap width) shear strengths of 73 MPa (25 C) have been obtained; the soldering to obtain these shear strengths was conducted using the solder in rod form and flame heating without special handling of flux powder or the work pieces to be soldered.

According to another aspect of the present invention there is provided a flux powder comprising prefused and finely ground flux components.

According to a further aspect of the present invention there is provided a method of soldering which includes the use of a flux powder comprising prefused and finely ground flux components.

Flux powders in accordance with the present invention suitable for use in soldering operations may also find application in brazing operations (i.e. operations at higher temperature than temperatures normally used in soldering operations).

The invention will now be further described by way of example only as follows: Example 1 A composition close to the KCl/LiCl eutectic composition was made by melting 112 gram of potassium chloride (melting point 771 C, BDH Analar grade) in a silica beaker under air by heating in an electric furnace to 8200C and adding lithium chloride with stirring (2 batches of 44 gram) (melting point, 610 C, BDH, General Purposes Reagent (GPR)).

The fully molten mixture (melting point, 354 CI was tipped onto a clean copper plate, was broken into smaller piece and the resulting material stored in a sealed bottle.

Example 2 A composition close to the ZnCl2/(NH4Cl eutectic was made by mixing 150 gram of zinc chloride (melting point, 317 C, BDH, GPR) and 50 gram of ammonium chloride (melting point, 520 C; M and B Pronalys AR grade (#99.5% NH4Cl)). This mixture was placed in a pyrex flask with attachments to allow an argon-hydrochloric acid mixture to flow onto the mixture and to exit to a ventilator. The flask and contents were heated gently in a laboratory heating mantle until melting commenced. The flask was maintained at this temperature until all the mixture was molten. Water vapor condensing in the upper part of the flask head was evaporated by heating it with hot air.

The fully. molten mixture (melting point, 180 C) was tipped onto a copper plate to chill it swiftly, broken-up and the resulting material stored in a Kilner jar. [The hydrochloric acid was present to suppress the reaction between zinc chloride and water of hydration associated with the zinc chloride: ZnCl2+H2O(gas)=ZnO+2 HCl(gas).

For water vapour at atmospheric pressure, ZnO formation can be suppressed at 4300 C by at.33 vol % HCI in atmospheric pressure argon. This suppression also serves to remove oxide from the melt].

Example 3 The two prefused materials prepared in Examples 1 and 2 were fused together (in proportions of 70 weight % ZnCl2/NH4Cl material and 30 weight % KCl/LiCl material such that the composition of the flux powder produced would be ZnCI2 53 weight %, NH4CI 17 weight %, KCI 17 weight %, and LiCI 13 weight %) under conditions similar to those described for the preparation of the ZnCl2/NH4Cl material of Example 1. Thus the materials were heated until melt appeared (about 180 C) and the temperature raised slowly to 4360 C until all the KCl/LiCl material had melted into the ZnCl2/NH4Cl material. The melt was cast onto a copper block, broken up and the resulting material stored in a Kilner jar.The material was ground to give a flux powder of 4150 ,am particle size under an argon atmosphere in a glove bag.

Without prefusion as described in Example 1 and 2, it was not possible to grind the flux components into fine powders because of their water content. The water present caused the material to agglomerate. Prefusion also reduced the rate of moisture adsorption from laboratory air compared with the rate of absorption by the individual components.

Example 4 The procedure of Example 3 was followed with the exception that the ZnCl2/NH4Cl material was 75 weight per cent and the KCl/LiCl material was 25 weight per cent such that the flux powder produced would be ZnCI2 57 weight %, NH4CI 18 weight %, KCI 14 weight %, LiCl 11 weight %.

Example 5 The flux powder of Example 3 was mechanically mixed with 0.25 weight % ZnO powder (Durham Chemicals, Zinc Oxide Activox B) of about 0. 11 ,um particle size and 1 weight % SnCI2 and formed into a flux cream by mixing with propanol (2.59 per 109 of powder). (The SnCI2 was prepared by drying SnCl2.2H2O over magnesium perchlorate to give SnCI2 which was finely ground).

Examples 6 to 13 Ring and plug arrangements made of a-brass were used to perform soldering operations using a flux cream prepared as in Example 5 and using respectively zinc and the various alloys shown in Table II as solders. For each ring and plug used an annular gap of about 0.22mm existed between the ring and the plug when the plug was inserted into the aperture of the ring; this was achieved by using 3 Nickrome spacing wires (0.22mm dia). A shallow annular gutter was provided on the upper surface of each ring immediately adjacent the aperture so as to provide accommodation for flux cream. A butane air flame was used to heat each plug and ring arrangement.

In each case when the flux material became molten a solder rod (of Zn or Zn alloy according to the Example) was held in the molten flux and moved around the gutter in contact with the gutter and the plug whilst the temperature was raised to 450 C. The temperature of 4500C was maintained for 1 minute after appearance of zinc or zinc alloy at the base of the plug in each case and the assembly then allowed to cool naturally. Subsequently each assembly was washed in warm water and dried with tissues and warm air.

A thermocouple was positioned in the plug of each assembly to assist in maintaining similar procedures for each soldering operation. Results of tests on the joints obtained between rings and plugs using zinc rod and rods of various alloys used are shown in Table II.

TABLE II

Example Rod % filling Shear leak rate No. Composi- of strength mbar tion capillary (MPa) l.s~l 6 Zn pure 100 73 NDL* 7 Zamak-2 22 22 1.5x10-6 8 Zamak-3 70 95 NDL - .~ 9 Zn-0.7% 70 65 NDL Cu b 10 Zn-1.4% 50 51 Cu 11 Zn-0.5% 74 71 NDL Ni ~ 12 Zn-1%Ni 80 56 ...

13 ILZRO16 60 58 5x10-8 * NDL=no detectable leak.

Claims

1. A process for the preparation of a flux in the form of a flux powder which comprises prefusing flux components (as hereinbefore defined) together and finely grinding the resulting material to give a flux powder.

2. A process as claimed in Claim 1 wherein a flux component is a halide compound;

3. A process as claimed in Claim 2 wherein the halide compound is zinc chloride, lithium chloride, potassium chloride, tin chloride, sodium fluoride, ammonium chloride or nickel chloride.

4. A process as claimed in any one of the preceding Claims wherein all flux components of a chosen group are prefused together at the same time.

5. A process as claimed in any one of Claims 1, 2 or 3 wherein a first batch of flux components from a chosen group are prefused, a second batch of flux components are prefused, and the separately prefused first and second batches are subsequently brought together.

6. A process as claimed in Claim 5 wherein the first and second batches are separately finely ground and the resulting two powders mixed.

7. A process as claimed in Claim 5 wherein the prefused first and second batches are fused together and the resulting material finely ground.

8. A process as claimed in any one of the preceding Claims wherein the flux components are selected such that the flux powder is suitable for use in soldering with zinc and zinc-based alloys.

9. -A process as claimed in Claim 8 wherein the flux components are zinc chloride, lithium chloride, potassium chloride and ammonium chloride.

10. A process as claimed in any one of the preceding Claims wherein the prefusing of chloride flux components is carried out under a flow of argon/HCI.

11. A process as claimed in any one of the preceding Claims wherein prefused flux components are ground to a flux powder of G150 ,am particle size.

12. A process as claimed in any one of the preceding Claims wherein a further finely divided powder is added to give a flux powder.

13. A process as claimed in Claim 12 wherein the finely divided powder is zinc oxide.

14. A process as claimed in Claim 12 or Claim 13 wherein the finely divided powder has a particle size of about 0.11 im.

15. A process as claimed in any one of Claims 1 to 14 wherein the flux powder is formed into a cream with a suitable liquid medium for application to workpieces.

16. A flux powder comprising prefused and finely ground flux components.

17. A flux powder as claimed in Claim 16 comprising ZnCl2 53%, NH4CI 17%, KCI 17% and LiCI 13%.

18. A flux powder as claimed in Claim 16 comprising ZnCI2 57%, NH4CI 18%, KCI 14% and LiCI 11%.

19. A method of soldering which includes the use of a flux powder comprising prefused and finely ground flux components.

20. A process for the preparation of a flux powder substantially as hereinbefore described with reference to any one of Examples 3, 4 or 5.

21. A flux powder substantially as hereinbefore described with reference to any one of Examples 3, 4 or 5.

22. A method of soldering substantially as hereinbefore described with reference to any one of Examples 6 to 13.