<<Actually I like Silver (stocks) as much, if not more than Gold here... not too much profitable production globally in this price environment.... just as the cure for $10 Oil - was $10 Oil; the cure for $4 Silver - is $4 Silver... >>
--------------------------------------------------------------------------------
A Study of Lead Free Solder
By: Karl F. Seelig
INTRODUCTION:
The study of lead-free solder has become a hot subject over the past few years. Government regulations are becoming more strict, and handling of waste materials are becoming more regulated. Now is the time to take a serious look at alternative materials for making electrical interconnections. The history of government regulation suggests that targeted materials often become the subject of a ban within a certain timeframe; lead in paint, lead in plumbing, and lead in gasoline have all been eliminated thus far. There is no reason to believe that lead in solder will not meet the same fate.
Figure 1 Due to potential litigation, liability risk is the primary reason for economic concern. If a worker is not properly monitored and is exposed to an elevated lead level, the lead could be carried home on his clothing, leading to contamination in the home, which is an area of concern and is a potential problem.
The next concern is waste treatment in manufacturing. There are all types of wastes generated from a soldering operation: solder, solder dross, wipes and packaging containers. Some have a recycling value and others have to be disposed of as hazardous waste. On the process side, there are effluent wastes during cleaning, where solder balls and some heavy metal salts are washed off. In general, electronic manufacturing is a clean and safe environment to work in; however, governments still are targeting the removal of lead from solder, due to lead pollutants generated in other industries. The reason for the recent concern is that a great number of products are being disposed of in landfills, products such as televisions, radios, games and other products available to the consumer, and potential solder from these products is leaching into municipal water supplies. In response to the new wave of regulations, the following list of desirable attributes has been compiled for lead-free solder.
Due to potential litigation, liability risk is the primary reason for economic concern. If a worker is not properly monitored and is exposed to an elevated lead level, the lead could be carried home on his clothing, leading to contamination in the home, which is an area of concern and is a potential problem.
The next concern is waste treatment in manufacturing. There are all types of wastes generated from a soldering operation: solder, solder dross, wipes and packaging containers. Some have a recycling value and others have to be disposed of as hazardous waste. On the process side, there are effluent wastes during cleaning, where solder balls and some heavy metal salts are washed off. In general, electronic manufacturing is a clean and safe environment to work in; however, governments still are targeting the removal of lead from solder, due to lead pollutants generated in other industries. The reason for the recent concern is that a great number of products are being disposed of in landfills, products such as televisions, radios, games and other products available to the consumer, and potential solder from these products is leaching into municipal water supplies. In response to the new wave of regulations, the following list of desirable attributes has been compiled for lead-free solder.
Figure 2 1. The selected element will have no negative environmental impact now or in the future.
2. Sufficient quantities of base materials must be available now and in the future.
3. Melting temperatures similar to 63/37 tin/lead, preferrably below 200°C.
4. Equal or similar thermal and electrical conductivity.
5. Adequate joint strength and thermal fatigue resistance.
6. Easy repairability.
7. Low cost.
8. Compatibility with existing processes.
EXPERIMENT:
Starting with the above list, we began to search for the best alloy. Looking at the periodic table of elements, the choices of acceptable elements dwindles rapidly (see figures 3 & 3A).
Silver is in adequate supply but has a cost disadvantage. Bismuth poses a potential supply problem since it is a by?product of lead mining, and also has embrittlement problems. Bismuth is also a poor conductor, both thermally and electrically. Concerning cadmium, toxicity is the leading reason not to use this element.
Figure 3
Figure 3A
With copper, there is plenty of supply, and it is soluble in tin. In low percentages, copper works well. There is a long history of tin alloys containing copper. Gallium supply and costs are the main reason not to use it, as well as its brittleness. The cost, inadequate supply, poor resistance to corrosion and rapid oxide formation during melting all eliminate the element indium.
Figure 4A Figure 4A shows a solder fillet formed with an indium eutectic, aged at 90°C and 93% relative humidity. The corrosion of indium can be seen as the dark areas on the solder joint. The remaining area is the tin. The reaction that has occurred is seen in figure 4B. For the most part, indium is safe if kept in low humidity conditions, or if it is conformally coated. Antimony has an adequate history and supply to be a viable solder additive. Tin is the base of solders, toxicity is low and supply is adequate. Zinc is in adequate supply, but has oxidation problems and also causes solder to become brittle. This oxidation problem creates an issue with existing automatic soldering equipment.
Figure 4B After review of acceptable elements, and a good deal of testing, the CASTIN alloy was developed. CASTIN is a combination of copper, antimony, silver and tin. The nominal composition is 96.2% tin, 2.5% silver, .8% copper and .5% antimony. The grain structure is much coarser than the existing 63/37 alloy. This would lead one to believe that the alloy would be more brittle; however, this is not the case. The melting point of CASTIN is 216°C. The alloy was tested to compare with the eutectic 63/37. The initial test showed that CASTIN was superior to the 63/37 (figure 5). ...
aimsolder.com |
