The reason for the copper is the intermetallic layer that is need for a bond to be created.
The bond in regular leaded solder is copper To tin. An top of the copper pad a layer of CuSn is grown ( dont remember exactly how many of each. Believe it's cu6sn4 , i'm no chemist or metallurgist)
Depending on temperature there may be a layer fo a different CuSn allow between Cu and Cu6Sn4. This layer is brittle and cracks easy under mechanical stress.
As the solder joint ages this layer becomes thicker. After half a year a few extra micron of CuSn has formed. The higher ambient temperature the faster this goes. That is why hot solder joints fail over time. The CuSn keeps growing and breaks off under mechanical stress.
Now, on top of the CuSn layer there is now a shortage of tin ( it has bonded to copper. ) so there we get an almost pure lead layer that then diffuses back to SnPb.
The more iron is thrown in the mix the faster this CuSn is formed ( were talking fractions of a second during solder being liquid. )
The more tin a solder alloy contains the faster it steals iron.In classical tin lead soldering the tin strips part of the iron plating of the soldertip ( this is necessary otherwise no bonding to the pins of parts). This is why lead free soldertips have a thicker iron plating. So they last the same amount of time.
lead free solders run at higher temperature. The higher the temperature the faster you steal copper and the thicker the CuSn layer would be. So in SAC 305 they pre saturate the alloy with copper and the silver takes the place of lead to form the crossoover from CuSn to Sn ( just like you had a pure Pb layer in leaded solder you have a pure Ag layer in SAC soldering.
Now it gets more complicated. If you have surface that have nickel , old or palladium ( enig or enepig plating ) then additional intermetallic layers are formed. Nickel bonds to tin, and the layer does not grow post soldering..
Tin bonds better to nickel than to copper , but nickel corredes like the pest so we need to throw gold on it to protect it , which lower shearing strenght so we add palladium in the mix ...
The reason for all this creazy metal in the mix stuff is to increase tensile strenght and shearing resistance of the joint.
Now, we only looked at copper to solder bonding. What about solder to pin of the component ? Most ic pins are an alloy called kovar and thats where the iron from the solder tip comes into play.
Its' all very complicated stuff...
SAC305 is now widely considered the 'garbage' of the lead free solders.. Snib , Snag , Sn99c Sn100c are other alloys that work better. Especially Sn99c and sn100 are really good . Less microcracks because the CuSn layer is thinner.
Snxxc alloys are good for wave soldering as there is almost no dross formation ( dross is a kind of scum that forms on top of the wave and doesnt flow and causes brittle joints. Its like bad solder.
Sac305 is horrible for that.
Picking solder is a science in its own. You need to know what is the board surface , is it wave where the wave is hot for hours and we dont want solder to go bad , or reflow
What flux is in the mix ? What is final application ? Hot ? Cold. .some solders work better than others for certain post production ambient operation.
I got some documents on all that stuff. Pretty complicated, lots of research and a bunch of contradictions as well..