The considerations required in evaluating whether to replace traditional tin/lead solder include, but are not limited to:
Melting temperature
63/37 SnPb alloy melts at roughly 183°C. But the lead–free alloys suitable for use in electronics melt at roughly 40° or more above that temperature. Some components such as larger package passive chips delaminate at these temperatures. The issue primarily relates to reflow oven profiles where peak temperatures must be set higher. In wave soldering, because of poorly considered specifications (260°C +5, –0) set decades ago by the U.S. military and telecom companies, most factories are already operating at such high solder temperature that a lead–free alloy can be applied at the same temperature.
Ductility and tensility, part one
Endless experiments have been conducted to determine whether lead–free solder connections will be as durable as the standard SnPb alloy. The experiments wasted resources because the results were easy to predict. The alloys are very hard, so their solder connections are more durable (with the exception of tin pest described below here) than SnPb connections.
Ductility and tensility, part two
Harder solder is not necessarily better solder. When heated, components — including the PCB — expand. If the coefficients of expansion are not identical for the components and the PCB, compressive forces may be applied to the components. The soft lead in SnPb alloys allows the solder to act as a pillow to relieve the stresses. The hard lead–free alloys do not relax. The effect is like building a bridge without expansion joints — the compressive force causes the bridge to fracture. So lead–free solder is a risky choice for products like automotive that will be subjected to high environmental temperatures.
Tin pest
In its pure state or mixed with a metal that reacts with tin, tin crumbles at temperatures below -30°C. The condition is known as tin pest. It is not an issue with SnPb because the lead does react with the tin (it goes into solution) and prevents the degradation. Copper and silver, the two metals substituted for lead in the mainstream lead–free alloys do react with tin and cannot prevent tin pest. An alloy does exist that can prevent the low temperature degradation.
Cost
Replacing lead with tin, copper and, especially, silver raises the cost of materials significantly. Higher operating temperature increases the electric bill.
Wetting
Taking the lead out dramatically increases the surface tension. So lead–free solders do not spread to the same extent as SnPb. In some forms of soldering, the higher surface tension reduces defects; in other soldering applications, process management becomes more difficult. (Some sources recommend selecting a more acidic flux to increase the wetting, but this creates higher risk of ionic contamination field failures and should be avoided.)
Flux
At higher soldering temperatures, less of the flux survives. Picking a flux capable of withstanding the high temperatures is important in some forms of soldering.
Equipment
Higher solder temperature causes machinery to wear out faster. More maintenance is needed. The tin/silver and tin/copper dendrites in lead–free solder abrade the surfaces over which the solder flows.
If you need to go lead-free or just need more information to determine the practicality of going lead-free, we can help. Please write or call Jim Smith at (727)866–6502, extension 21.
