Reprinted from the ACJ - September, 1999
Quick! What's better for you, apples or oranges? Depends on whether you are trying to keep the doctor away, or if you have scurvy, right? Quick! What's more toxic to our health, Tin\Lead or Lead Free solder? I had someone ask me this question the other day. Actually the question went more like this. "Hey I just got some @#$!!&%$ Lead Free solder from you guys because I wanted to move away from the dangers of #$@%!# Lead. Now, I read the ##$@** MSDS for the lead free and it sounds worse than tin/lead. S'up?" The answer lies in our ability to interpret and compare the information given on the MSDS.
The Health Hazard section of a well put together MSDS is written to convey all the hazards, both acute and chronic, presented by that material. This includes hazards presented by abuse of the product, not just normal use. So it could be called a `worst case scenario'. If we compare the health concerns associated with tin/lead and lead free solders, we find these similarities listed: skin, eye and respiratory tract irritation; excessive inhalation may cause `metal fume fever'; excessive ingestion may be toxic; chronic and/or repeated exposures can cause adverse liver, kidney and blood effects. Both types of solder sound about the same as far as worst case hazards are concerned. How come lead free solders are considered safer? The answer lies in tempering those health concerns with the reality of the products use. All of the health problems listed are a concern when we exceed the exposure limits of the material. There in lies the key; `exposure limits'.
Looking at the most probable route by which we would be exposed, inhalation, we find that for Tin the OSHA permissible exposure limit (PEL) over an eight hour work day is 40 times higher than the PEL for Lead. Antimony at ten times the airborne concentration, or PEL, of Lead is just becoming hazardous. Copper is considered safe to be exposed to, at 4 times the limit for Lead. So, even though the possible worst case health hazards are the same, it takes a lot more exposure to the lead free components to poison yourself. Thus Lead is said to be more toxic.
Another consideration involves the actual percentage of each element in the product. Most leaded solders range in Lead concentration from 30 to 70 percent lead. While the nasty components of lead free solder, Copper and Silver, normally range between 1 and 10 percent. Less hazardous stuff equals less chance of detrimental exposure.
One more angle to look at is the ease with which the elements can get into the air and be available for breathing. Speaking generally, vapor pressure is a good indicator of a substance's volatility, and as a rule of thumb, the lower the boiling point the higher the vapor pressure at a given temperature. Lead has a boiling point approximately 800F lower than the boiling point of Tin, Silver or Copper. The result is that more Lead goes into the air at a lower temperature, raising our potential inhalation of Lead. This raises our chances of getting to those `worst case' health effects sooner.
Higher concentration in the product, lower exposure threshold, and greater volatility all add up to make tin/lead solders more hazardous to work with than lead free solders. Now the disclaimer; lead free solders are not mother's milk, they are hazardous if abused, and a health concern wherever used. All precautions taken for tin/lead should be assessed and implemented for lead free solders. We just get a greater margin of safety with lead free.
A little knowledge is a dangerous thing, I am proof of that. But if we read our MSDS with a little practical understanding of exposure limits, concentrations and use conditions, we can more easily differentiate between apples and oranges. Fruit salad anyone?
The above article was written by David M. Brown, Chief Engineer of Johnson Manufacturing Company, Inc. and is published by JOHNSON with the expressed approval of the National Automotive Radiator Service Association and the Automotive Cooling Journal. Other reproduction or distribution of this information is forbidden without the written consent of JOHNSON and NARSA/ACJ. All rights reserved.
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