ITRI

The Partitioning of Impurities in the Solder-Dross Recovery Process

Report No. TE 47418

Author:
Dr Paul; Harris, Dr Ariela Samuel-Lewis, Mr Kaldev Chaggar

1. Introduction
One of the drawbacks of the wavesoldering technique is that substantial quantities of dross are built up, and this must be removed at intervals and the bath replenished with expensive virgin solder. Analysis of the dross reveals that it consists of a mixture of oxides and metallic solder, with normally more than 90% being the latter. The unoxidised metal is effectively prevented from rejoining the solder in the bath by the presence of the oxide with which it is intimately mixed.

EVS International have recently developed a machine to mechanically separate the unreacted metal so that it can be returned to the bath, and thus dramatically reduce the quantities of virgin solder that must be purchased. The machine operates by remelting the dross followed by mechanical compression against a perforated wall, so as to squeeze the liquid metal out from the solid oxide component. Potential customers for this machine have, however, expressed a concern that impurities in the dross may be returned to the bath, and this might necessitate more frequent bath changes.

The objective of the present work is to conduct preliminary studies of the partitioning of impurities in the solder-dross recovery process.

2. Experimental
For the purposes of the study a solder recovery system (EVS) instrument was brought to ITRI together with a batch of dross from a commercial bath. Two batches of dross were processed and the recovered solder and residues collected.

In order to minimise errors due to inhomogeneity of the specimens, the materials were sampled by drilling numerous cores randomly all over each specimen and combining the swarf generated.

The fractions were analysed using atomic absorption spectrometry, using a Varian Spectra 10 instrument. The quantity of oxide present in the dross was estimated by remelting a known quantity under an excess of zinc chloride/ammonium chloride flux, and stirring until all of the oxide had been dissolved, before cleaning and reweighing.

Scanning electron microscopy was used to study the dross and residues, using a Jeol 5400 instrument fitted with a Link Pentafet energy dispersive X-ray (EDX) microanalysis system. X-ray photoelectron spectroscopy (XPS) measurements were made using a Kratos XSAM 800 instrument.

3. Results and Discussion
The dross contained approximately 5% oxide, with the remainder being metallic. The Separation Process recovered approximately 50% (by weight) of the material as solder, This would suggest that the residue would contain ca. 10% oxide with remainder being metallic tin-lead. On this occasion the dross was added cold. It is understood that higher recoveries can be obtained by putting the dross in while still hot (presumably because more complete remelting is achieved).

Preliminary examination of the starting material detected low levels of copper and iron to be present, and for the purposes of this study analyses for these two elements were carried out on all of the specimens.

Table 1 shows the results of the analyses. The quantity of copper present was not excessive, only slightly above the maximum level that is specified by many solder standards (e.g. BS 219 Grade KP Cu 0.08% max), although a number of manufacturers market solders which contain much lower levels than this. Unfortunately no specimen was available of the solder bath itself, and so it is not possible to comment on how the purity of the dross relates to the solder from which it was derived. It appears that little or no partitioning between the various fractions had occurred.

XPS analysis of the black powdery oxide material indicated that it was essentially tin oxide, and there no evidence of any significant preferential oxidation of copper and indeed, given that tin oxide is more thermodynamically stable than those of lead or copper, none would be expected. At these concentrations the copper would be in solid solution within the solder, with discrete intermetallic crystals appearing only at higher concentrations. Hence given that metallic solder is the major component of all three fractions it is perhaps not too surprising that all three have similar compositions. This situation might change if the copper concentration were to reach 0.3-0.4% at which point copper-tin intermetallic crystals could start to appear (depending on bath temperature), and these might be preferentially trapped in the residue.

Interestingly, the iron impurities tended to report in the residues rather than in the recovered solder. The reason for this discrepancy is not yet clear: it may be due to the lower solubility of iron in solder (and hence that it was present as discrete crystals of FeSn2 rather than in solution). Alternatively it may have been present as some other form of particulate material such as oxide (either due to oxidation in the bath or due to contamination by rust particles). Earlier work at the Institute¹ indicated that reactive contaminants (e.g. zinc, aluminium etc) tended to segregate preferentially into the dross. Given that they would be present as solids (oxides) it seems likely that such materials would tend to remain with the residues rather than the recovered solder. The same would also be true of adventitious particulate contaminants, provided they were insoluble in the bath. More noble metals on the other hand, such as copper, gold or silver (which dissolve readily into the molten solder), would tend to remain in solution and report in both fractions.

Under equilibrium conditions the following relationship should hold:

Wa = Wrj + Wrd

where Wa is the weight or impurity added to the bath in unit time, Wrj is the weight of impurity removed in the joints of product passing through the bath and Wrd the weight removed in the dross. Wrj and Wrd will tend to increase with time as the concentration of impurities builds up (assuming a constant volume of solder is being removed in unit time). Wa will tend to decrease with time, because as impurity concentrations build up the rate of dissolution from the board will tend to decrease. Eventually, both sides of the above equation will balance and equilibrium will be established (provided, of course, that product throughput/dross removal rates etc. are kept constant).

The effect of the solder recovery process would be to reduce Wrd and thus give rise to higher equilibrium levels of contaminants. The key issue is whether equilibrium will be reached at tolerable levels of impurity. If the answer to this question is yes, then solder recovery offers an attractive way of minimising costs. If the answer is to this no, then the next question is how long can a bath be run before intervention is necessary, and will the reduction in bath life nullify any cost advantage of the solder recovery system. Unfortunately without information regarding the magnitude of the variables under typical operating conditions, it is not possible to make predictions on this matter.

References