Making solder.

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It is a silly idea, I cant understand why you would want to do it...
Over 30 years involved with PCB design and assembly its the first time anyone has proposed such an idea. Two different solders with different wetting and reflow temperatures should NOT BE MIXED, your solder joints will suffer. They are not designed to be mixed together... a solder joint is a complex beast with different layers of intermetalics that are critical to the solder joint, people spend millions investigating solder joints and solder joint reliability, it isn't just mixing the two solders together... why do you think they have such specific formulas...
In fact as we speak I have the IPC-610 document in front of me...
 
The advantage of 60/40 tin-lead solder (or actually 62/38 - no idea why they always round it to 60/40) is that it is (almost) a eutectic alloy, which means that it becomes solid at one temperature rather than gradually over a temperature range. This makes it easier to keep the parts you are soldering still while the solder is becoming solid. The disadvantage is that the lead is poisonous.

The advantage of lead-free solder is that it is not as poisonous as leaded solder. The disadvantage is that it is not eutectic.

Mix the two and you combine the disadvantages of both: poisonous solder that is not eutectic.
 
No eutectic is 63/37, they don't round it up it is 60/40...the mix is 60/40 when indicated. The lead in solder is not going to do any harm to anyone it is mixed with the tin and will not leach out, infact banning leaded solder has caused many problems over the years, not the least in an increase in greenhouse gasses due to the higher reflow temperatures.
So no tin lead solder is not poisonous so you don't have to worry on that count. The majority of the designs I work on are still soldered with tin/lead 63/37 solder, the assembly houses have no special handling requirements, a bigger concern is actually flux inhalation.
 
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