Old solders never die . . .
A look at lead free solder
By John Prentice

There have been some troubling comments in the model press on the fact that we might have a problem in the future with lead free solder requiring unacceptable high temperatures, harsh fluxes and producing bad joints. My immediate reaction was to check how much Tin/Lead solder I had in stock and to lay in a lifetime supply. To my consternation I found that all my usual suppliers had already changed to the lead free stuff. After my initial dismay, something started to tell me, "Don't Panic Mr Mainwaring". The electronics industry world-wide is bigger than governments. Semiconductor devices cannot tolerate high temperature. Printed circuit boards (PCBs) cannot be subjected to aggressive flux. There must be a solution. So I thought time for a little more research and testing.

First I looked up an early reference work to see what solder used to be fifty or so years ago. I found Fine Solder (1.5 parts tin, 1 part lead - our modern 60/40), Tinman's Solder (1 part tin, 1 part lead), and Plumber's Solder (1 part tin, 2 parts lead) with melting points increasing in that order. It was also noted that the proportions for Fine Solder give the lowest possible melting point for these two metals. In addition there was Pewterer's Solder (1 part tin, 1 part lead, 2 parts bismuth) and Wood's Metal (1 part tin, 2 parts lead, 4 parts bismuth, 1 part cadmium) both of which had lower melting points.

Looking at a modern reference issued just before the lead free issue occured, I found 60/40 tin-lead, used for fine electrical work and fully molten at 188°C, 40/60 tin-lead used for more structural electrical work and fully molten at 234°C, Low Melting Point solder (62% tin, 36% lead, 2% silver) and fully molten at 179°C, and High Melting Point solder (5% tin, 93.5% lead, 1.5% silver) being used for applications such as electric furnaces or within motors and fully molten at 301°C. Apart for the latter they can all be used with normal soldering irons which typically have approximate maximum bit temperatures of 370°C for 15 watt irons and 420°C for 25 watt irons.

Next let's take a look at the regulations. At the back of the WEEE (waste electrical and electronic equipment) and RoHS rules is a European Union Directive - Oh why am I not surprised. If you are interested it is 2002/95/EC of 27 January 2003 and is "on the restriction of the use of certain hazardous substances in electrical and electronic equipment". As well as lead it covers other heavy metals, I quote "Member States shall ensure that, from 1 July 2006, new electrical and electronic equipment put on the market does not contain lead, mercury, cadmium, hexavalent chromium, polybrominated biphenyls (PBB) or polybrominated diphenyl ethers (PBDE)".

Now this legislation does not prevent the manufacture or sale of lead based solder, just its use in electrical and electronic equipment that is for sale. I did wonder why these health and safety mandarins were trying to protect me in this way. I was not about to eat my computer. But in fact the purpose is quite sensible in that it is to protect the environment and operatives when the mountains of old electrical equipment is disposed of or recycled. It is also reasonable in its requirements. It does not make draconian demands. It only requires that more friendly metals are substituted where this is technically and scientifically possible. Clearly this has been found to be the case with solder. There are also a number of exemptions. Relevant to us are "in high melting temperature type solders (i.e. tin-lead solder alloys containing more than 85% lead), lead in solders for servers, storage and storage array systems (exemption granted until 2010), and lead in solders for network infrastructure equipment for switching, signalling, transmission as well as network management for telecommunication (a specific time limit for this exemption will be determined later)".

In practice the new regulations mean that the manufacturers of solder have already switched to lead free, as the stuff we buy in electrical and DIY shops etc. is just a spin off from the massive requirements of industry. The exemptions mentioned above and the other non-electrical applications are sufficient to ensure that the lead based solders will still be available, but no doubt only from specialist suppliers and at a premium price.

So what of the lead free substitutes. Well obviously industry are happy with these or there would have been a colossal row and demands for more exemptions. The reported experiences about high temperatures and joint failures were very worrying, so again I enquired of the scientific world. There are basically two new products on offer. The first is standard Lead Free Solder. This is for general purpose electrical work. It is 99.3% tin, 0.7% copper and melts at 227°C. The second is Lead Free Silver Solder. This is best for fine electrical work. It is 95.5% tin, 0.5% copper, 4% silver and melts at 217°C. (Individual manufacturers' specifications very a little). The working temperatures of these solders are well within the range of the previous lead based products, are suitable for all soldering irons and as far as I can tell the flux contained in them is similar to that used in the lead versions. The main claims for the Lead Free Silver version are that it has a lower melting point (not that much higher than 60/40 tin-lead), has a lower electrical resistance and it flows better. The downside is that it is about 40% more expensive than the ordinary Lead Free version, which itself is a similar price to the last quoted figure for tin-lead solder.

Anyway time for a test. In the light of quoted troubles with Lead Free and the manufacturers' claims about Lead Free Silver, I decided to buy a small quantity of the silver product. I tested it with a range of soldering irons, both old and new, in a direct comparison with some of my stock 60/40 tin-lead product. I tried soldering nickel-silver rail. I did some light electrical wiring. Finally I put together a tiny etched brass kit. I had no problems at all with the Lead Free Silver solder. It melted easily enough, flowed well and produced a strong joint. There was no apparent overheating of either iron or work. In fact I quite liked the stuff and went out and bought a 250 gram reel for future use, saving my leaded solder for the time being for any special project where it might seem more suitable to have a slightly lower melting point.

One thing I think is worthy of mention. The form of crystalline cracked joint that has been reported, sounds like what you get when either the solder has not fully melted or when the work moves while it is still setting. One of the special features of 60/40 lead-tin, apart from its low melting point, is the fact that it starts to melt at 183°C and is fully molten at 188°C, a very narrow range indeed. This means that if it appears to be fully molten it probably is and on cooling it passes through the tricky intermediate stage fairly quickly. With say 40/60 the figures are also 183°C for the start of the melt but it is fully molten at 234°C, a wide range and plenty of chance of a bad joint. I do not have the full figures for the lead free solder, but I expect that the range may be quite wide on the non-silver version and this accounts the unfortunate failures.

My conclusions. Well if you like the leaded solder and can get some, then lay in a stock. Otherwise definitely go for the Lead Free Silver Solder. The extra cost is not really that great considering the relatively small quantities that we use and I am sure you will not be disappointed with the results.

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© Copyright John R. Prentice Software 2005