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Most electronic solder has been made with the same formula since World War II (and before, in the case of some manufacturers like Kester). Scientists knew that a mixture of 63% tin and 37% lead was the "eutectic mixture", meaning it had the lowest melting point. Since it's difficult to mass-produce solder to such a close standard, most solder manufacturers simply get it as close as possible and call it "60-40" solder. The difference is not critical (claims of some crackpots aside). Tin and lead are the least-costly metals that can be used in a solder, and give very reliable results when used properly, so the 60-40 mixture became the standard for electronics manufacturing. Solders made of bismuth can have a lower melting point, but are more costly and can have problems with reliable "wetting" of a wire or PC-board pad. Indium solders are excellent, but indium is extremely expensive. Solders carrying a high percentage of silver are also costly and are prone to oxidation. So the 60-40 lead-tin solder has been the most reliable, cost-effective mixture for more than 90 years.
(Fair warning: audiophiles sometimes DIY electronics, and go around raving about hideously expensive, exotic solders. These solders, such as [http://www.cardas.com/content.php?area=oem&pagestring=Solder+and+Flux&content_id=19 Cardas solder], will not get you anything in the way of better reliability or connection quality. They are similar to the $8,000 power cables sold in high-end audio shops. If you really think this will make your synthesizer "sound better", feel free to waste your money. Similarly, people asking "which solder to use" in musician forums are often fed a line of crap by lunatics. No kidding: I have seen a few self-appointed experts, calmly telling people to use ''acid-core'' plumbing solder on their guitar amps. This will destroy the wiring. Very stupid.)
Lead does one thing for solder that is still poorly understood today: it helps prevent a bizarre effect called "[http://www.guardian.co.uk/technology/2008/apr/03/research.engineering tin whiskering]". For some specialized applications, such as medical equipment or space-rated electronics, manufacturers will attempt to use a pure tin or tin-silver solder, to avoid the toxicity or other problems of lead. Whiskers can sometimes grow between electrical connections, shorting them out.
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