Certainly, most often, the focus of pot analysis is directed at controlling impurities. Maintaining proper level of tin is important, also. When the portion of tin decreases: * Liquidous temperature increases. * Ability to wet decreases. * Solder connection looks dull. * Theoretically, it is a weaker connection along the intermetallic layer, since there is even less tin after depletion in creating the copper tin intermetallic compound.
We add this paper, because we can no longer find it on the Metcal site.
The Effect Of Metallic Impurities On The Wetting Properties Of Solder by: Dennis Bernier Vice President, Research & Development Kester Solder Company
SOURCE OF IMPURITIES
The solder used for this investigation was all from one batch with thefollowing analysis:
Element Weight % Element Weight % Sn (tin) 60.1 Fe (iron) 0.006 Sb (antimony) 0.02 Bi (bismuth) 0.006 Cu (copper) 0.004 As (arsenic) <0.01 Au (gold) <0.002 In (indium) 0.005 Cd (cadmium) 0.0003 Ni (nickel) <0.001 Zn (zinc) 0.0002 P (phosphorous) <0.001 Al (aluminum) <0.001 S (sulfur) <0.001 Ag (silver) 0.001 Pb (lead) balance
Though the purpose of this testing was to determine the effect of impurities dissolved during the soldering process, it is important to note that national specifications for solder are not strict enough to assure obtaining high purity metal. Secondary or refined metal could contain excessive impurities and shorten the usable life of the solder bath. Impurities such as copper, antimony, zinc and aluminum have an effect on soldering quality and should be kept to a minimum.
Assuming that high purity solder is being used, the impurities are introduce into the solder from parts being soldered, from holding fixtures and from the solder pot itself.
Copper - Nearly everything on a printed circuit assembly is made of or plated with copper which dissolves rather rapidly in solder. The circuit board itself, component leads and jumper wires all introduce copper into the solder in a wave soldering machine.
Gold - No longer used as an overall protective plating, gold is used on certain component leads such as nickel-iron alloy used to make transistors, diodes and integrated circuits.
Cadmium - Sheet metal chassis frames and other parts might be cadmium plated to prevent rusting and improve appearance and solderability.
Zinc - Brass is an alloy of zinc and copper; so brass terminals, lugs and bolts are sources of impurities.
Aluminum - Fixturing devices, bolts and fabricated metal parts might be made of aluminum. The tough oxide film on the aluminum will usually prevent solder wetting; but with multiple solder immersions or if abraded, aluminum can dissolve in the solder. It is doubtful that aluminum will remain in the solder under production conditions since it will dross out when combining with copper, gold or antimony.
Silver - Many parts are silver plated to preserve solderability. Like the other coinage metals, gold and copper, silver will dissolve in the solder.
Iron - Temperatures over 430 degrees C will cause the solder to dissolve iron from the solder pot itself. An improperly alloyed solder using too much heat could contain excessive iron. A new solder pot -- whether cast iron, cold-rolled steel or stainless steel -- will have exposed iron available for dissolution into the solder. Excessive cleaning of the pot walls with a wire brush can also introduce iron into the solder. The problem associated with iron contamination is excessive drossing which usually clears up as the iron compounds are removed with the dross.
Sulfur - It is very unlikely that sulfur would contaminate the solder bath during normal production. Sulfur might be present in secondary metals since it is used to remove copper during the refining process. Sulfur should be limited by national solder specifications to avoid its presence in solder.
Phosphorous - The main source of phosphorous is copper that has been deoxidized with phosphorous.
PROBLEMS ASSOCIATED WITH IMPURITIES
Phase 1 of the investigations involved a compilation of analyses performed over the last ten years for the specific purpose of solving soldering problems. The amount of impurities in the solder was related to observed defects or solder conditions. These defects, specifically cause by contaminated solder, are noted below with some discussion about the impurities which caused the problem. The table following this discussion shows the percentage range of impurities which seemingly caused the observed soldering defects.
Icicles, Shorts, Bridges Cadmium, zinc and aluminum in trace amounts increase the surface tension of the solder to cause this defect. Copper and gold increase the solder viscosity to cause the same problem.
Large Solder Fillets Copper, gold and antimony increase the melting point of the solder and the intermetallic compounds with tin or lead make the solder more sluggish. The result is larger fillets and more solder consumed to create the solder joint.
Unfilled Holes The speed of wetting is reduced by the presence of copper, gold, antimony and cadmium. Though no instance occurred with zinc and aluminum, these metals are likely to also affect wetting speed because of their ability to increase the surface tension of the solder.
Dull Solder, Gritty Solder Cadmium and zinc in trace amounts make the solder surface dull. Gold also dulls the surface but is quite often indicated by a sparkling, crystalline surface condition. Bismuth or antimony in large amounts above 2.5% also dull the surface. Copper and aluminum contamination result in a gritty-looking solder surface. Both phosphorous and sulfur have caused gritty solder though rarely are these two impurities found in solder samples.
Dross Inclusions Dross inclusions in the solder show up as visible particulate grit or hidden inside a bump or pimple in the otherwise shiny solder surface. Quite often the source of this problem is an unusual amount of iron in the solder.
Cracked Joints Inclusions in the solder such as intermetallics of tin or lead with copper, gold and antimony can provide the nucleus for crack propagation.
Dewetting Zinc, antimony and phosphorous can cause solder to dewet on copper.
By looking at the real world of wave soldering and the ten years of analytical records, we can summarize the impurity levels which traditionally caused problems.
Impurity % When Problems Occur Cu (copper) 0.250 - 0.500 Au (gold) 0.005 - 0.200 Cd (cadmium) 0.005 - 0.150 Zn (zinc) 0.001 - 0.010 Al (aluminum) 0.001 - 0.006 Fe (iron) 0.010 - 0.100 Sb (antimony) 0.100 - 1.000 Ag (silver) 0.200 - 2.000 Bi (bismuth) 0.250 - 1.000 As (arsenic) 0.030 - 0.100 In (indium) no data Ni (nickel) 0.010 - 0.030 P (phosphorous) 0.010 - 0.100 S (sulfur) 0.002 - 0.030
Immediately obvious by an examination of this list is the fact that the percentages established by experience are not precise numbers. The explanation for this is that the defects cause by the impurities may be acceptable at one company and cause for rejection at another company. Rigid inspection requirements for aerospace or military products might reject solder joints which are acceptable for consumer products. Difference between fluxes, soldering machines, circuit board density, component layout, hole sizes, solderability and amount of heat all contribute to the quality of soldering.
REFERENCES
1.Soldering Manual, 1959, New York, American Welding Society 2.C. L. Barber: Solder, 1965, Chicago, Kester Solder Company 3.H. Manko: Solders and Soldering, 1964, New York, McGraw-Hill 4.M. L. Ackroyd et al. : Tin Research Institute Publication No. 493, 1975, The Metals Society. 5.D. Mackay: Proceedings of Institute of Printed Circuits, Meeting, September, 1972, San Francisco
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We don't know if our solder pot is producing too droos or no...
- May 20, 2005
by
ING
We would need more info to answer.
How many boards get ru...
- May 21, 2005
by
jdengler
