How to tell leaded solder from lead-free solder on a PCB?

[cincin]

Hi,

I can't believe this hasn't been answered before, but I'm not finding a conclusive elaborate answer:

How can I tell leaded from lead-free solder on an existing PCB?

I'd like to replace some components on a board made in the early 2000's, and I'd like to use the right kind of solder for the job.

Here's what I've done so far:

- Visual inspection: the soldering points on my board are shiny like leaded solder.

- Melting test: I've set my soldering station to 600F and confirmed that this was hot enough to melt a contact on a board that's from the early 90's (leaded). Then I tried melting a contact on my questionable 2000's board and it melted in similar fashion. It was shiny even after being melted a bit a couple times.

- Mechanical test: I've tried scoring some solder on the board from the early 90's with needle nose pliers, and it's pliable like you would expect lead to be. The solder on the 2000's board is a little less pliable, but it's hard to say because there's less solder to squeeze.

Anyone got an easy test to conclusively tell wether a PCB is sone with leaded or lead-free solder?

Thanks!

[ataradov]

Just use whatever solder you have. It does not matter. If you are really worried, remove as much of the old solder as you can from the affected joints and apply the new solder.

There are so many solder formulations that it is virtually impossible to visually tell leaded from lead free. The only sure way is to do chemical composition tests, which is an overkill here.

[jpanhalt]

There are lead test kits made by 3M and possibly others.  I wanted to test some plumbing for non-potable water (heating system) and bought one.  Two tests for about $8 at Menards and other home stores.  Trouble is, the first kit failed miserably, even with the positive control.  The second kit was completely dried out and couldn't be used either. 3M cannot be contacted meaningfully by consumers.

So, that won't work and don't waste your money.  Just go with cleaning and replacing with you favorite solder.

[BrokenYugo]

There's no harm in wicking it off and replacing with leaded. As mentioned by ataradov there's no easy way of knowing the exact alloy used, so you may still run into mystery alloy issues even if you do identify lead or lead free.

If you want to do some crude melt point tests out of curiosity turn the iron down to 200C (400F), typically encountered leaded alloys melt below that, most unleaded above.

[Whales]

Just use whatever solder you have. It does not matter. If you are really worried, remove as much of the old solder as you can from the affected joints and apply the new solder.

I'm only a single sample point here but I have a similar opinion.  I've always repaired joints with the "wrong solder" (I use lead free, *spits over shoulder and tosses salt*).  I suspect it's more important to make sure the final joint looks well shaped and tidy; that _probably_ affects joint performance/lifetime more than using an alloy mix between two different common solder types.  I don't think common soldering alloys are unstable-quantity compositions (ie changing composition slightly shouldn't dramatically change performance outside what you can handle, for the most part).

Remember that the alloy in your roll is not the alloy that ends up on your board.  Your solder dissolves the pads, pad plating and component leads.  Every leg maker plates/coats/dips in a different alloy.  HASL PCB makers will probably use whatever solder leftovers they have, up to a certain % of contamination from recovered solder from previous HASLing.  There are a lot of acceptable alloys and alloy gradients (I saw a roll of something like 80% lead once?).  People who use leaded solder are soldering to unleaded platings on most modern components, some thin some thick, so every component will end up with a different alloy within several %.

There will be edge cases.  If you have any doubt: clean away the old solder completely and replace it.  Especially if it isn't flowing or does something else weird.

I think I recall some HP scopes coming with a special roll of solder clipped in the back for the repairman to use?  I think they did have very specific alloy requirements in certain sections.  I don't think this is very common today, everyone wants to make components & PCBs that "work with anything".

[cincin]

Thanks! Not having to deal with the 2 kinds of solder is a great money/time saver.

Cheers!

[gek]

I think I recall some HP scopes coming with a special roll of solder clipped in the back for the repairman to use?  I think they did have very specific alloy requirements in certain sections.  I don't think this is very common today, everyone wants to make components & PCBs that "work with anything".

I think you're referring to the older Tektronix scopes, which came with a small length of 3% sliver bearing solder.  It was apparently critical to prevent the sintered contact strips from separating from the Ceramic strips