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By Ray Prasad, Ray Prasad Consultancy Group
Despite the industry's switch to SMT from thru-hole for almost three decades now, thru-hole has not disappeared yet, and don't expect it to any time soon. The most common way to solder thru-hole components en masse is by wave soldering. For tin/lead, the wave pot temperature for soldering thru-hole components is generally maintained at 260°C.
When switching to lead-free, the wave pot temperature need not exceed 260°C, although some people use as low as 255°C and some may go as high as 270°C. However, there is no need to go over 260°C. In fact, there are various reasons for not going much higher than 260°C.
Higher pot temperature has the increased potential for more board warpage and increased dissolution of copper from component leads and PCB surfaces. Even at 260°C, due to essentially 100% concentration of tin in lead-free solder, copper dissolution is much higher in lead-free solder than in tin/lead alloys with tin concentration of only 63%.
Increased copper dissolution in lead-free pots has various adverse effects. The lead-free wave pot gets contaminated faster and requires much more frequent analysis and dumping than was the case with tin/lead. The pot also will require more frequent cleanup due to increased oxidation of tin in lead-free and sinking of heavier tin/copper intermetallic compound (IMC Cu6Sn5) to the bottom of the pot.
The densities of lead-free, IMC and tin/lead solder are 7.39, 8.25, and 8.8 respectively. So, in tin/lead solder, the IMC floats at the top, since it is lighter than solder. Hence, IMC gets skimmed off during dross cleanup. In lead-free solder, however, the IMC will tend to sink to the bottom, as it is heavier than lead-free solder. If the IMC is not frequently removed from the pot, it will get intermixed with solder and may clog the baffles and channels in the pot, causing serious problems in proper functioning of the wave.
A higher rate of copper dissolution in lead-free solder not only forces frequent pot maintenance, but also has serious implications on solder joint reliability when copper at the knee of the thru-hole pad is totally dissolved. Some of the copper is dissolved during the initial wave soldering step. The remaining solder can disappear completely during subsequent rework, potentially causing an open solder joint.
What causes the copper to disappear completely during rework and not during initial wave soldering? During initial soldering in the wave, the dwell time is less than 10 sec. in the wave pot (5 sec. is common), but during rework in a mini solder pot, the dwell time can be more than 60 sec. This is long enough to remove the copper completely, especially at the knee of pads, causing an open solder joint. A more accurate way to describe this problem is that copper removal in the plated thru-hole (PTH) knee in thru-hole barrels during mini pot rework is an erosion problem, not just a dissolution problem, since it is affected by the direction of wave flow in mini-wave rework pot. Erosion also has been found to be related to the quality of copper plating by some investigators, but there is lot of work to be done in this area.
There are many companies proposing different solder pot chemistries to alleviate these problems. However, there is no industry consensus about solder composition to be used for wave soldering. It should be noted that there seems to be an industry consensus on SAC 305 for solder paste for reflow soldering, although some people are experimenting with other chemistries as well.
Most solder suppliers appear to claim that their proprietary formulation with traces of nickel or germanium or cobalt or some other secret element does not dissolve copper as much as SAC 305 solder does, and costs less since it doesn't contain the expensive silver. They also seem to be focusing on the cosmetic advantage of their chemistries without silver, which can provide shinier solder joints. These claims are certainly true to some extent for initial wave soldering, but when it comes to opens at PTH knees during rework or solder pot contamination with IMC, none of the chemistries can claim an advantage. Why? You cannot reduce the dwell time too much during rework or change the density of IMC, which will sink in the pot regardless of the solder composition used from any supplier.
What is also worth noting is that alternative lead-free chemistries have approximately 10°C higher melting point than SAC 305 and may require higher than 260°C pot temperature for good soldering results. Higher temperature will increase the potential for oxidation and board warpage.
ConclusionSo where do we go from here? There are no easy answers. You need to do a detailed pro/con evaluation of any solder chemistry you plan to use. Do not expect any recommendations from industry organizations any time soon. Rely on your own research and expertise or the salesmanship of your supplier.
As for solving pot contamination and increased frequency of pot maintenance, the only way to get there is to switch to a selective soldering system. There are many suppliers of various selecting soldering systems on the market. But switching to selective soldering solves only the pot contamination issue. We still do not have a good solution for rework of multi-leaded thru-hole components, such as sockets and connectors.
So what is really needed is an alternative rework system for thru-hole that does not exist today. Anyone who can come up with a rework system that can solder and remove thru-hole components quickly without causing disappearance of copper at the PTH knee or causing internal barrel or trace cracking truly can claim an advantage. Remember you got this tip from me go out and invent this system and retire.