Lead-Free Solder

The soldering process for most industrial applications has been ignored or taken for granted for years. Since the process worked, there was no reason to expand engineering talent for improvements. Now, with increasing legislation both on a state and federal level restricting the use of lead in the workplace, more companies must find safe and cost effective alternatives.

In the wake of legislation many quote "patented" alternative tin-based alloys have appeared on the scene. The consumer, however, is having limited success with these alloys. This is due to a lack of understanding of the metallurgy of a tin-lead joint versus a tin joint. Changing to just any "lead-free" alloy without considering all of the metallurgical factors is a sure road to failure. Each industry, each application, must be studied to ensure that the appropriate lead-free alloy is chosen. Many factors must be considered such as temperature, flux, process changes, cost, joint strength, and corrosion factors and costs.

The increasing lead restrictions should be viewed as an opportunity to review and improve existing soldering operations. The right lead-free alternative alloy will produce a superior, cost-effective, and environmentally safe product.

Factors To Consider
An effective lead-free process is possible. The following considerations are important guidelines to ensure a successful change.

Alloy Melting Temperature:
One of the fastest methods of changing to lead-free is to use an alloy with a melting temperature and/or pasty range similar to the currently used lead-based alloy. Manufacturers product lists provide a selection of alloys by temperature. These lists do not include all alloys, however, nor do they include patented alloys. Choosing a lead-free alloy by melting temperature alone may be the fastest way, but problems can arise if it is the only factor considered. Cost, corrosion behavior, and availability are also important.

Service Temperature:
It is difficult to find a lead-free alloy with exactly the same melting temperature as the original lead-based alloy. Using alloys with 'approximately' the same melting temperature may cause problems. Metals start to lose their mechanical properties at approximately 3/4 of their solidus temperature, therefore the service temperature must be determined and a safety factor included. The required service temperature should not be too close to the alloys melting temperature. This factor is also related to solder joint configuration. The safety factor, for example, can be less for a Lapp seam solder joint than for a Butt joint.

Flux:
A change of solder alloy may require a flux change. This was a major problem in the plumbing industry because traditional fluxes used for 50Sn/50Pb would burn and char before the new lead-free alloys would melt. This happened with as little as 50°F difference in solidus between alloys. A working relationship between user and solder/flux manufacturer is an important consideration as more companies change to lead-free alloys.

Process Changes
Heat Source
One of the most common problems is an inadequate heat source for higher melting temperature tin-based alloys. An equipment change may be required for better temperature control.

Product Availability
Not all lead-free alloys are available in wire form, particularly small diameters. Small alloys are impossible to extrude and draw into wire. There are manufacturing limitations that cannot be overcome. A user may have to purchase pin-transfer or wave soldering equipment to use some alloys.

Lead Contamination
Changing to a lead-free alloy cannot be accomplished without removing all traces of lead from the process. Small amounts of lead will form a low melting eutectic (i.e. 63Sn/37Pb) when mixed with a lead-free tin-based solder. This will cause the solder joint to melt at 361°F and perhaps fail prematurely. It also may not meet the definition of lead-free. Public Law 99-399, June 19, 1986 (Safe Drinking Water Act) defines a lead-free solder or flux as one containing not more than 0.2% lead.

Cost
In the past, the only consideration when choosing a solder alloy were joint strength and cost. Lead-based solders were used because they offered the best metallurgical joint at the most economical cost. Now concern over the environmental hazards of lead are creating more and more regulations and controls. Lead training and monitoring programs, hazardous waste permits, and special handling and disposal costs are adding up. Previous cost decisions in favor of a lead-based alloy, therefore, may be reversed even though a lead-free alloy costs more per pound.

Density
There is a difference between lead (.410 lbs. per cubic inch) and tin (.254 lbs. per cubic inch). The same volume of tin-based solder, therefore, weighs approximately 1/4 less than a lead-based material. This adds up to more joints per pound of solder. Cost calculations should include the effect of density and the advantages of a lower weight product.

Joint Strength
The mechanical strength of a tin/lead solder joint has rarely been a problem for engineers as long as the temperature is not too high. It is the low temperature characteristics of a tin/lead joint that are beneficial. Most metals and joint filler alloys (i.e. brazing, welding) have a ductile to brittle transition temperature. This is the temperature at which a metal alloy loses its pliability and becomes very brittle. At this temperature, the metal is very crack sensitive and easily shatters. Tin/lead alloys do not have a ductile to brittle transition. The tin-based lead-free alloys are inherently stronger than lead-based alloys as it relates to joint strength, fatigue, and thermal cycling properties.

Corrosion
Corrosion of solder joints has seldom been a problem because of the relatively stable lead oxide that is formed on a tin/lead alloy. Most tin-based lead-free alloys form a tin oxide that is easily eroded or mechanically damaged. Testing needs to be done to understand the long-term effects of galvanic coupling, environmental exposure, and the effects of specific chemicals.

As legislative concern over the hazards of lead in the workplace increase, more and more companies will seek lead-free alternatives. Fry Technology offers an extensive line of lead-free products and has the metallurgical expertise to help companies find the right lead-free for their application.