Machine Design

Switch Tips: Lead-free solders and electromechanical switches

North America, the EU, and Japan are trying to eliminate lead from the printed-circuit-board (PCB) assembly process. Specifically, eliminating lead from soldering will and is having a huge impact on the manufacture of PCBs and components.

The two leading candidates for a lead-free solder are tin-silver-copper (SnAgCu) and tin-copper (SnCu). However, both these alloys have melt temperatures considerably higher than tin-lead (SnPb). The reflow temperature profile for SnPb can reach 210 to 225°C. The reflow profile for SnAgCu will reach 260°C, and rework temperatures are even higher.

No commercial products in the U.S. currently use lead-free solders. However, major solder manufacturers such as the Kester unit of Northrop Grumman Corp. and Cookson Electronics Assembly Materials have been testing various alloys for a couple of years.

A move to lead-free solders could force switchmakers to change the material they use for some switch parts. The housings and the noncurrent-carrying internal parts of most switches are molded from thermoplastic resins. One advantage of these materials is that they can be recycled just by reheating and remolding. But the injection-mold temperature and the reflow temperature of SnAgCu are about the same. The higher reflow temperature profile required by the lead-free alloys can cause the switch housing material to soften and lose shape.

Thus switchmakers will be forced to change housing material if the industry adopts a lead-free solder alloy that requires a peak profile temperature exceeding about 240 to 250°C. Switchmakers say there may be huge costs associated with moving to a higher temperature thermoplastic. One reason is that the new material would need all new mold tooling. Additionally, any switches that currently have UL, CSA, or other agency approvals will have to be retested when they incorporate new material. The cost of UL testing for a single switch currently runs about $7,000 to $10,000.

Tin-lead plating can also be found on the PC mounting brackets of switches, but seldom on the switches themselves. Switchmakers are moving away from using lead on such components. For example, switchmaker NKK says it has been changing all its tin-lead plating to either pure tin or SnCu. It also says the changeover is about 90% completed.

There are some long-term unresolved problems with the two leading candidates for lead-free alloys (SnAgCu and SnCu). Whiskers associated with the tin can be one. Another is the long-term reliability of lead-free solder joints caused by temperature cycling. Agencies, such as the National Electronics Manufacturing Initiative (NEMI), American National Standards (ANSI), Electronic Industries Alliance (EIA), and the Association of Connecting Electronics Industries (IPC) have been testing these materials and making recommendations but as yet there are no firm commitments to any specific solder alloy.

Finally, switchmakers point out that less than one-half of 1% of the total lead consumed in the world goes into solder. The main industrial use is in batteries, which account for 80% of all consumption.

Technical material for this column was supplied by NKK Switches,

TAGS: Fasteners
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