Anyone who has used wave soldering to assemble PCBs knows about that chunky layer of metal that collects on the smooth surface of the molten solder. This is solder dross; it is composed of oxidized metals and impurities that collect as the molten solder contacts the air and manufacturing environment. This happens regardless of alloy and is a normal part of the process, often consuming up to 50% of the bar solder added to the solder pot. In the past, this dross was collected as waste and disposed of, but solder dross is more than 90% valuable metal. This value should be recovered.
Nowadays, typically, this dross is collected and returned to a metals supplier for recycling. Indium Corporation now offers two programs for recycling solder dross. The first program involves simply sending back dross waste in return for a portion of the metal value as a credit. The second option involves sending back dross, which is converted to bar solder (within the original spec) and returned, with you paying only a fee for processing. When dross arrives, regardless of which program is chosen, it is electrolytically refined and the pure metals are recovered and converted back into usable bar solder. Often, this reclaimed/recycled metal has a better purity than virgin metal.
Dross is not the only form of solder that can be recycled. For instance, when changing to a different alloy in a wave soldering process, the entire solder pot will need to be emptied. The old alloy can be collected and recycled, lowering the amount of capital necessary to switch alloys. Bar solder and wire that have not been used within the shelf life can also be recycled to get back some of their value.
There seems to be a growing trend to use a low-Ag or Ag-free solder alloy for Surface Mount Technology (SMT) electronics assembly, similar to what is commonly offered for bar solder, used in wave and selective soldering.
For through-hole performance, the strength and stability come from the entire barrel of solder, whereas it is usually the foot and heel fillets that give SMT solder joints their strength.
Lets talk about the other issue with using a eutectic solder alloy in SMT: tombstoning. One of the benefits of using the SAC (tin-silver-copper) alloy for SMT and solder paste, is that it has a built-in plastic range, similar to that of Sn62 (62Sn 36Pb 2Ag). It is this plastic range that prevents tombstoning, and takes into account the inconsistent heating of the solder across the part (which is the sole cause of tombstoning). Switching to a eutectic alloy eliminates the plastic range and opens the door for tombstoning.
Any powder manufacturing issues, such as the inconsistent distribution of dopants throughout the alloy and powder matrix, takes a back seat to the surface mount reliability concerns.
There are other alternatives, such as SAC0307 (99Sn 0.3Ag 0.7Cu)… But, with the price of Ag finally coming down, and a long history of SAC usage, we don’t think it’s going to be a major player.
Next time, we'll talk about the manufacturing and costs associated with low-Ag and Ag-free alloys.
I work for a company that manufactures solder, so I have a pretty keen eye for it. Even so, it’s still hard to tell a Sn/Pb ingot of bar solder from a Pb-free bar at a glance. The best way to keep these solder alloys separate at your facility is to have a good storage/tracking system and to have them made in different shapes. These pictures depict 2 lots of solder ingot, one is Sn/Pb and the other is Pb Free. Notice the physical difference of the two. This is a fail-safe, in case the ingots are misplaced or left untracked at your facility. Of course, this isn’t anything revolutionary – but perhaps an way to easily add an extra measure of safety into your wave solder process.
Many consumer electronics transitioned to Pb-Free 4-5 years ago. However, there are still a substantial number of electronics being built with Sn/Pb solder. As some of these products begin to transition to Pb-Free, the decision on a bar solder alloy for the wave soldering process is a challenging one. There are two widely recognized options to choose from:
use SAC305 and keep the alloy the same as the SMT process
choose a low silver (Ag)/Ag-free alloy which is substantially lower in cost
From a cost perspective the Ag-Free alloys are 40-50% cheaper than SAC305. This is a substantial savings when you consider that solder pots hold around 1,000 lbs of solder. In this arena, Indium offers our Sn995 which is a cobalt doped Sn/Cu alloy. In addition to being lower in cost, Sn995 produces shinier solder joints than SAC305.
It would be great if that were the whole story, but there are cases when SAC305 should still be considered as a bar solder for wave soldering processes. One advantage of SAC305 is that it has a faster wetting speed than any of the Ag-Free alternatives. This can help when you have a board or process that is challenged with poor hole fill. The chart below shows the difference between popular Pb-Free alloys for wave.
Another potential advantage of SAC305 is in thermal cycling reliability. Because the typical wave solder joint is large and very robust, reliability may not be a concern for many consumer products. However, as the reliability requirements increase for industries such as automotive, aerospace, and military, the stronger SAC305 solder joint may be what is needed to meet more stringent reliability requirements.
Both SAC305 and Ag-Free alloys, such as Sn995, have their place within the electronics manufacturing arena, but it is essential that the correct decision is made based on the product being assembled.
Anyone who has used wave soldering to assemble PCBs knows about that chunky layer of metal that collects on the smooth surface of the molten solder. This is solder dross; it is composed of oxidized metals and impurities that collect as the molten solder contacts the air and manufacturing environment. This happens regardless of alloy and is a normal part of the process, often consuming up to 50% of the bar solder added to the solder pot. In the past, this dross was collected as waste and disposed of, but solder dross is more than 90% valuable metal. This value should be recovered.
Lets talk about the other issue with using a eutectic solder alloy in SMT: tombstoning. One of the benefits of using the SAC (tin-silver-copper) alloy for SMT and solder paste, is that it has a built-in plastic range, similar to that of Sn62 (62Sn 36Pb 2Ag). It is this plastic range that prevents tombstoning, and takes into account the inconsistent heating of the solder across the part (which is the sole cause of tombstoning). Switching to a eutectic alloy eliminates the plastic range and opens the door for tombstoning.
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