Solder is a combination of metals that form that are created in this process are not fully an alloy with a melting point lower than any purged. Chemicals such as sulfur (S), of the individual combined elements. In the chlorine (Cl), phosphorous (P) and sodium process of alloying, the metals are added and salts are often used in this process, and if not melted together and then cooled to a properly removed, these will remain in the predetermined point above the melting point alloy through the bar manufacturing process. of the alloy. In the case of electronic-grade These chemicals then can inhibit dross tin-lead (Sn63/Pb37) bar, this would be a separation rates and cause the dross to be point above 183°C (361°F) and for a lead- marbled into the bar during casting. This free alloy such as SAC305 will yield a high dross content bar. Some bar (Sn96.5/Ag3.0/Cu0.5) it would be a point manufacturers will add an electroplating above 217-219°C (423-426°F). When an step in this recycling process. Although this alloy is melted, the surface of the alloy is process will lower impurity levels, as stated exposed to air. This interaction of the air on above this does not guarantee a low dross the alloy surface forms an oxide layer called bar. dross. The density of the dross and the alloy are very similar, which causes a slow Contrarily, some bar solder manufacturers separation of the two. Typically, dross is not will use only “virgin” metals in their bar related to impurities but is more related to solder production. In the case of Sn63/Pb37, oxidation rates (although some impurities this means buying pure tin and pure lead such as aluminum (Al) and zinc (Zn) do (and pure antimony if this addition is increase dross rates due to their rapid required) and then alloying these together. oxidation). Most impurities that are within For a lead-free alloy like SAC305, pure the limits specified by IPC-J-STD-006, silver and pure copper would be combined however, are found to be acceptable for with pure tin to create the final alloy. In normal soldering and do not result in dross theory this should yield a high-quality bar. formation. Then why doesn’t this process always yield a low dross bar? As stated above, dross What Causes Some Bar Solders To Dross comes from metal oxides, not impurities. More Than Others? Sometimes manufacturers of tin and lead It is common for some companies to use may produce “virgin” materials that contain recycled metals in the production of their high oxide levels. These oxides have to be bar solder. This does not necessarily mean removed in the alloying process. If not that this bar is of poor quality. However, properly removed, these will yield dross due to poor refining practices in the past characteristics equally poor to those of there is a common belief that the use of poorly handled recycled alloy. recycled metals in bar solder is not acceptable. A common problem stemming Dross Reduction Techniques from the use of recycled metals is that after There are two distinct approaches that solder the process of chemically removing manufacturers take to reduce the dross impurities, the chemicals or oxidized metal content of their bar. The first method is to

take the final alloy after manufacturing and 2. Competitor A’s Virgin Bar Produced add “oxygen getters”; the most commonly With Phosphorous used is phosphorous (P), which will oxidize 3. Competitor B’s Virgin Bar Produced and give the appearance of a low dross bar. with Dross-Reducing Process Unfortunately, there are some problems 4. Competitor C’s Virgin Bar with associated with phosphorous. If the levels Dross-Reducing Process of phosphorous are too high, it will cause 5. Competitor D’s Virgin Bar with dross to cling to iron parts and often will Dross-Reducing Process cause pumps or wave baffles to plug. Also, 6. Competitor E’s Virgin Bar with if the solder is not replenished often, the Dross-Reducing Process phosphorous may be removed from the pot 7. Competitor F’s Electrolytically during dross removal, minimizing the Refined Recycled Bar Produced intended dross-reduction effects. A more With Phosphorous serious problem results from the fact that phosphorous is not easily analyzed for and is As detailed above, drossing rates do not readily oxidized out of the solder pot. This necessarily correlate with phosphorous can yield an uncontrollable limit over time additions or electrolytic refining. as related to the chemistry of the solder joint being formed. Ackroyd and MacKay1 have Nitrogen in Wave Soldering shown that solder pots with phosphorous Atmospheric modification is the process of levels above .01% can cause dewetting and flooding the process area with an inerting cracked solder joints, which can lead to substance such as nitrogen to reduce the solder joint failure. oxygen level. This reduction in oxygen results in a decreased formation of dross. The preferred method to reduce the dross Those who have implemented a nitrogen content of bar is to utilize a system that wave solder process have achieved a alloys bar without oxide entrapment. The reduction of dross costs up to 95%.2 In Electropure™ system developed by AIM is addition, the nitrogen changes the surface a process that eliminates elements that will tension of the solder, which ultimately potentially increase dross formation while improves wetting. Typically, the oxygen reducing suspended oxides in the final bar. level in an inerted system is maintained at around 50 to 100 ppm. Drossing Rates Comparison Several electronics manufacturers have The main benefits of inerting a wave performed comparative studies on the soldering process stem from the reduction of drossing rates of competitive bar solders. dross production. Lowering macrodross Below is one such comparison that was production saves money and lessens performed by a leading multinational maintenance requirements. Reducing telecommunications corporation. The microdross on the surface of flowing waves results listed are from the lowest dross improves wetting to the solderable surfaces. generated (best) to the highest (worst): Inerting at the site of soldering can be accomplished by diffusing the gas on each 1. AIM’s Virgin Bar Produced With the side of the flowing waves in an open system, Electropure™ Process or by installing a hood over the solder pot,

1 “Contamination of Solder Baths: Influence on Wetting Properties of Some Impurities”, Ackroyd and MacKay, Tin Research Institute, Greenford, Middlesex, UK 2 Air Products and Chemicals, Inc. website cost of ownership study: http://www.airproducts.com/electronics/microelectronics/matrox.htm

which effectively closes off the soldering temperature range should be 480° to 490°F, environment. Some wave solder machines with a maximum temperature of 500°F for inert the tunnel leading to the solder pot as Sn63/Pb37. A similar temperature range well, to prevent oxidation from forming on applies for a lead-free alloy such as the solderable surfaces during preheating. SAC305, however in some applications pot temperatures as high as 520°F have been Changing Bar Solders used to improve topside hole fill. Above When changing solder over from one brand these points drossing will often increase to another after a pot dump there are a few rapidly; however, by reducing the important steps that should be followed to temperature too much, soldering results ensure low dross formation. After the solder (top-side fill, bridging, etc.) may be has been refilled into the pot, it is important compromised. to run the pot for at least 8 hours to make sure the thin skin of oxidized solder that has Conclusion been left on the pot wall, baffles and pump Although utilizing a low impurity level bar impeller has been removed by the solder. solder is important for a wave soldering Turn off the pump and any other device that operation, this in itself does not ensure a low will cause the solder to be turbulent. Let the drossing process. In addition, electrolytic pot sit at 370°F for Sn63/Pb37 or 423°F for refining is a costly manufacturing process SAC305 overnight in an undisturbed state. that does not necessarily improve soldering After an 8 to 16 hour time period, remove results or reduce drossing. Drossing, rather, the dross from the material that has been is the result of suspended oxides in bar mixed into the solder from the changeover. solder that come in contact with air during Following this, restart the process and the wave soldering process. Bar solder continue normal soldering procedures. oxide levels may be reduced either by the addition of “oxygen getters” such as Dross Troubleshooting phosphorous or through the use of advanced The quantity of dross is affected by several production processes. Due to the negative process variables. Excessive dross will be and inconsistent results of phosphorous in a formed any time excessive surface wave soldering operation, the most turbulence occurs. This can result from a consistent wave soldering results and lowest bolt missing from a baffle, worn pump drossing rates are achieved with bar solder shafts, excessive wave heights, etc. produced with a system that alloys and Furthermore, the use of the chip wave often manufactures bar without oxide entrapment. results in excessive drossing. In order to minimize dross formation, molten solder Solder Pot Contamination should be maintained in a placid state and If a solder pot is out of specification on turbidity should be minimized whenever elements some corrective action generally possible. In addition, make certain that the needs to be taken in order to avoid soldering wave is running in standby mode if so problems and potential defects. To maintain equipped (wherein the wave is running at solder purity, a program of regularly full velocity only as more assemblies scheduled pot analyses is recommended. approach and pass over the pot). The following list of elements is listed with proper limits, corrective action and possible The higher your solder pot temperature, the source of contaminant. higher your dross level. In general, the pot

Proposed Maximum Limits of Solder Bath Contaminant per IPC J-STD-001E for Sn/Pb and Lead-Free

Contaminant Preconditioning Assembly Maximum Preconditioning and Maximum Contaminant Weight Assembly Maximum Contaminant Weight % % Limit Sn/Pb Contaminant Weight % Limit Sn/Pb Alloys Alloys Limit Lead-Free Alloys Cu 0.75 0.3 1.1 Au 0.5 0.2 0.2 Cd 0.01 0.005 0.005 Zn 0.008 0.005 0.005 Al 0.008 0.006 0.006 Sb 0.5 0.5 0.2 Fe 0.02 0.02 0.02 As 0.03 0.03 0.03 Bi 0.25 0.25 0.25 Ag 0.75 0.1 4.0 Ni 0.025 0.01 0.05 Pb N/A N/A 0.1 Total of Copper, Gold, Cadmium, Zinc, N/A 0.4 N/A Aluminum Contaminates

*When precipitating copper from a pot make sure the pot is stagnant and is undisturbed for at least 8 hours.

Tin levels will typically drop over time. ±1.5% of the nominal tin content is acceptable. To raise this level, calculate the weight in your pot times the analyzed percentage. This gives the actual weight of tin. Take the weight of solder held by the pot and multiply it by .63; this is the amount of tin that should be in the pot. Subtract the analyzed weight from the theoretical weight. This is the amount of tin needed in the pot. Remove 15% more solder than this and add the calculated amount of tin. Top off the pot with bar solder.

Proposed TAL Contamination Levels per SPVC for SAC305 Contaminant Normal (%) Increase Pot Adjust Pot Monitoring (%) Ag < 3.2 > 3.5 > 4.25 Al ≤ 0.001 > 0.002 ≥ 0.006 As < 0.02 > 0.02 > 0.03 Au 0-0.03 0.08 0.02 Bi ≤ 0.03 > 0.03 0.25 Cd ≤ 0.001 > 0.002 > 0.005 Cu < 0.6 > 0.8 1.2 Fe 0.01 > 0.01 > 0.02 In < 0.01 > 0.01 > 0.1 Ni < 0.01 > 0.025 > 0.05 Pb ≥ 0.10 Sb < 0.05 > 0.05 > 0.2 Sn Balance > 97.25 > 97.5 Zn 0.003 > 0.003 > 0.005

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