mixing lead solder with lead free solder

[8bitformat]

I everyone new here and hope you can help me!
I have searched google and have not found a straight answer with regard to mixing lead solder with lead free solder. A few people have told me that it could lead to cracking etc over time so my not be a good idea as they are very different in there make up/melting points and such.
So does anybody on here now for sure. I am restoring a radio from the 1960s and need to touch up some joints on the board. I must admit that It never crossed my mind until one of my students asked me about it when he came into my class early the other day and was looking at it. So I told him the first to find out gets a free lunch!

[jeroen74]

Just use leaded sodder and be done with it

The answer to your question is of course interesting, and I don't know either.

[SeanB]

No problem mixing them, as the lead will alloy nicely with the extra tin and will still form an alloy that is somewhere on the curve. It will mely at a different temperature, and will be smooth unlike lead free.

[T4P]

Unless you're mass-producing to comply with the RoHS of course, lead-free started the disposable age and led to poor design on everything
But i doubt anything will go wrong only issue i know is that SAC Lead-free is a pain in the butt if you don't know yet!

EDIT: Yes i was right in the start, lead-free solder is NOT an eutectic alloy and when combined with SnPb which is a eutectic alloy will crack microscopically when soldered and that SAC is to be avoided like a pest

@free_electron
Yes yes i know  I've just seen Sn42Bi58 Lead-free mixture, seems to melt around the 138C, fantastic. Gonna give that one a try

[alanb]

The question is why do you want to mix leaded with lead free?

Traditional lead/tin solder is still available. You can still buy solder similar to that used in the 60's so why not use that?

[free_electron]

No problem mixing them, as the lead will alloy nicely with the extra tin and will still form an alloy that is somewhere on the curve. It will mely at a different temperature, and will be smooth unlike lead free.

NO!

Never ever mix leadfree with leaded solder. solder is eutectic . meaning it has 1 and only one point where it goes from liquid to solid. to get this eutecticity the mix ratio between tin and lead is important. only 63/37 is eutectic . any other mixture is NOT. even the 60/40 is not eutectic . the band is about 4 degrees C. in this band the solder is brittle. Any mixtures that go even further away from 63/37  are even worse and this may lead to cracking , whisker growing and all other kinds of instability over time ( tin pest , blooming etc ).

So : either REMOVE all the old in/lead using a vacuum sucker and solderwick , or solder with 63/37.

the same goes for those special kits being sold to easily remove tqfp's. that 'special' metal is actually bismuth . this melts at ver ylow temperature ( 140 degree i believe ) and stays liquid very long before it solidifies. that's why you can easily desolder stuff. before placing a new part you must remove all trace of the bismuth using solderiwck , re-wetting using regular solder , taking that waya using wick and then placing the new part. any bismuth in your solder allow will mak it so soft that vibrations o the board may shear things off the pads ...

2T4p. SAC305 is the biggest pile of tripe out there ... it is intended for automated, controlled , mass production. NOT for rework. Never ! Jamais. there are tons o better alloys out there with MCM ( Mitsubishi Chemicals ) having the SN99c and SN100c alloys. Those work perfectly, have been used for at least 10 years in mass production with excellent results.

[Kremmen]

No problem mixing them, as the lead will alloy nicely with the extra tin and will still form an alloy that is somewhere on the curve. It will mely at a different temperature, and will be smooth unlike lead free.

NO!

Never ever mix leadfree with leaded solder. solder is eutectic . meaning it has 1 and only one point where it goes from liquid to solid. to get this eutecticity the mix ratio between tin and lead is important. only 63/37 is eutectic . any other mixture is NOT. even the 60/40 is not eutectic . the band is about 4 degrees C. in this band the solder is brittle. Any mixtures that go even further away from 63/37  are even worse and this may lead to cracking , whisker growing and all other kinds of instability over time ( tin pest , blooming etc ).

[...]

Beg to differ on a couple of points. Firstly, the eutectic point of tin/lead alloy is at 63/37 % as you state. But this just means that there is a single temperature where the phase change between liquid and solid states takes place. Other ratios such as the also common 60/40 will have solid and liquid mixed over some range of temperatures as you also state. But that's it. Tin whiskers don't enter the picture until there is less than some 3% lead in the mix (accordingly to some sources). So that will not be a real risk here. Cracking - don't really think so. Spontaneous cracking in anything like a 60/40 mix does not happen or i would be up to my ears in cracks. Due to the range of solidification temp it is theoretically possible to mess the joint by moving it while the solder solidifies.
To my knowledge the curses you list apply to leadless solder mixes. As soon as you introduce an appreciable amount of lead, all of that basically just stops from happening.
Regarding the other alloys such as bismuth etc - no argument.

[xygor]

Beg to differ on a couple of points. Firstly, the eutectic point of tin/lead alloy is at 63/37 % as you state. But this just means that there is a single temperature where the phase change between liquid and solid states takes place. Other ratios such as the also common 60/40 will have solid and liquid mixed over some range of temperatures as you also state. But that's it. Tin whiskers don't enter the picture until there is less than some 3% lead in the mix (accordingly to some sources). So that will not be a real risk here. Cracking - don't really think so. Spontaneous cracking in anything like a 60/40 mix does not happen or i would be up to my ears in cracks. Due to the range of solidification temp it is theoretically possible to mess the joint by moving it while the solder solidifies.
To my knowledge the curses you list apply to leadless solder mixes. As soon as you introduce an appreciable amount of lead, all of that basically just stops from happening.
Regarding the other alloys such as bismuth etc - no argument.

YES.

... in this band the solder is brittle. Any mixtures that go even further away from 63/37  are even worse and this may lead to cracking , whisker growing and all other kinds of instability over time ( tin pest , blooming etc ).
...

I will add that 63/37 (eutectic) is *more* brittle than 60/40, not the other way around.  More brittle is more likely to crack.

[8bitformat]

Wow thanks guys for all the replies. It seems to be the consensus that it would be bad to mix solder.
To the person who said “why would you want to”. Well it is not about wanting to, it is something that never crossed my mind until a student asked me. I have been using lead free for years now so never gave it a thought when repairing old equipment. Doing a bit of research and from the late 80s onwards some repair manuals / schematics have put warnings on them to use only lead free when doing any repairs. Don’t know if it is a warning not to mix them or that using lead would invalidate its environmental rating.
Hope this has helped others as well as me.   :-+off to order some lead tin solder 

[G7PSK]

There is only one lead tin alloy that is eutectic. That is 63/37 any other alloy of lead tin is not eutectic. A eutectic alloy is one that has only one solidus state. That is why 63/37 every rapidly and the other alloys have a stage of setting where they are sludge and can therefore be moved while setting, plumbers solder used to be especially made for this so that joints could be "wiped".
So you can mix lead free and lead solder the thing is is to make sure that all the solder is melted and stirred together in order to get a uniform alloy and to make sure that what has become a slow setting solder is not disturbed while hardening or you will get a dry joint that cracks.

[free_electron]

So you can mix lead free and lead solder the thing is is to make sure that all the solder is melted and stirred together in order to get a uniform alloy and to make sure that what has become a slow setting solder is not disturbed while hardening or you will get a dry joint that cracks.

@kremmen &Xygor :

well there is your problem... how do you mix it ? and it will indeed be slow setting yielding brittle joints.

another problem is that the elements will come out of suspension. you will get a layer that is almost pure tin and then gradations of mixes with lead. the intermetallic zones become shear points.

Now, if this is those big rivets used in old vacuum tube radios i coudln't care less what will happen, thos things should be ground down and recycled.

But on surface mounted boards it is problematic. There are plenty of studies and articles in magazines like PCB manufacturing that show you all the crap that can happen when you take a SN/PB HASL and solder lead free or take a lead free HASL and solder SN/PB...  it is completely unreliable. Especially when you throw packages in the mix that have ENEPIG plated electrodes. Even mixing 'lead-free' alloys is a no-go . if the board was HASL'd with sac305 you can squirt sn99c paste on there and reflow it. it will be a mess.

http://nepp.nasa.gov/whisker/reference/tech_papers/2011-kostic-pb-free.pdf
http://www.calce.umd.edu/lead-free/LTMixedSolderProposal.pdf

http://thor.inemi.org/webdownload/newsroom/Presentations/APEX08/Lead-Free/Pb-Free_BGAs.pdf

in the last one slide 10 is of importance. as the ball INCREASES in size it mixes LESS... you can clearly see that the top portion of the ball is
smooth (SAC305) and the bottom of the ball is crackled (SNPB) these are defects waiting to happen ... a few slides further they show the disasters ....

[ptricks]

But on surface mounted boards it is problematic. There are plenty of studies and articles in magazines like PCB manufacturing that show you all the crap that can happen when you take a SN/PB HASL and solder lead free or take a lead free HASL and solder SN/PB...  it is completely unreliable. Especially when you throw packages in the mix that have ENEPIG plated electrodes. Even mixing 'lead-free' alloys is a no-go . if the board was HASL'd with sac305 you can squirt sn99c paste on there and reflow it. it will be a mess.

Agree 100%.
I have seen repairs that were done because of thermal cracking of lead free solder. The repairs were done with lead solder or something unlike the original solder applied to the cracks and the boards are back for the same repair. I wish they had left lead solder alone, while it is hazardous, it wasn't like people were licking the PCB.