Lead-free for repair of old leaded stuff

[Renate]

I use 99 Sn/Cu for building new stuff (0.020", 0.5mm, not no-clean as I don't mind cleaning).
I've used that to repair older stuff in the past without a thought or a problem.
Recently I repaired a 30 year old blond (light tan, bakelite?) single-sided board.
Even though I tried to remove all the old solder, the new lead-free kind of made a slushy/crunchy/dull joint.
I guess that I ought to pick up a bit of lead solder for these rare situations?

[wraper]

You would need to reapply a new solder and wick it off again to remove a significant presence of lead, probably even twice. Also I would at least use stuff like SAC305 due to lower melting temperature than Sn99Cu. Older circuit boards may not like higher temperature which may cause the pads coming off, especially due to increased stress while you remove the old solder.

[SilverSolder]

It is easiest and best to use leaded solder on boards that were made with it.  Don't forget the lower melting point of leaded vs. unleaded solder...  some older boards may not like the high unleaded temperatures.

[Renate]

I thought for sure that you would advocate silver solder!

I used to fix old Tektronix stuff with the ceramic tie points and the little spools of special solder tucked in the chassis.

[SilverSolder]

I thought for sure that you would advocate silver solder!

I used to fix old Tektronix stuff with the ceramic tie points and the little spools of special solder tucked in the chassis.

I do use silver solder privately, as it gives very shiny solder joints! 

That is safe for use on older boards too.

[jonpaul]

Hello all

The physics of solder shows a liquid><plastic><solid phase transition at temperatures depending on the % mix of metals. The exception is the Eutectic 63% Sn 37% Pb which has no plastic phase.


Silver and most lead free demand much higher solder temps.

The Eutectic 63/37 has the lowest melting point.

The older PCB especially the paper /brown of cheap consumer cannot stand the high MP of Au or Pb free and tens to burn or delaminate.


Finally the mix of old Pb solder and then Pb Free means the reult is NOT Pb free.

We maintain a larger stock of 63/37 Kester for use on old equipment.

We us SN100C AIM bar solder on SMD gull wing transformers.

We have small amounbts of Ag solder for old valve TEK scopes.

Best regards,

Jon

[Ian.M]

...
The exception is the Eutectic 63% Sn 37% Pb which has no plastic phase.
...
The Eutectic 63/37 has the lowest melting point.
...

Other eutectic alloys exist, and here's a list of eutectic solders with lower melting points

Alloy         Melting point
Sn63Pb37      183°C
Sn62Pb36Ag2   179°C
In97Ag3       143°C
Bi58Sn42      138°C
In52Sn48      118°C

The problem is: the Pb free low melting point eutectic solders typically have inferior mechanical properties for PCB assembly or, in the case of alloys with a significant proportion of Indium, are *far* more expensive.

[SilverSolder]

Sn62Pb36Ag2 is my go-to for leaded solder work.  Shiny metal + slightly cooler iron.