RoHS Lead (Pb) Weight Calculation on paper before we begin costly testing

[David Hartman]

BTW: If I understand your original question, it appears that you are attempting to justify the amount of Pb by expressing its weight as a fraction of the overall weight of the flex. If this is the case, I believe that your premise is incorrect in that the RoHS requirement is for less than 1% by weight of the "homogeneous" materials. The European Commission’s own documented guidelines defines homogeneous as the individual components that make up a product, where if they were to be separated they would no longer exhibit their individual characteristics (i.e. solder, copper wire, enamel, etc.).

With this in mind the fact that 60/40 solder/tinning is being used already tells you that it is non-compliant in that the solder (a homogeneous material) contains greater than 1% Pb.

 

[Steve McQuality]

BTW: If I understand your original question, it appears that you are attempting to justify the amount of Pb by expressing its weight as a fraction of the overall weight of the flex. If this is the case, I believe that your premise is incorrect in that the RoHS requirement is for less than 1% by weight of the "homogeneous" materials. The European Commission’s own documented guidelines defines homogeneous as the individual components that make up a product, where if they were to be separated they would no longer exhibit their individual characteristics (i.e. solder, copper wire, enamel, etc.).

With this in mind the fact that 60/40 solder/tinning is being used already tells you that it is non-compliant in that the solder (a homogeneous material) contains greater than 1% Pb.

Thanks David. This has been a "point of confusion" for us here since we've been trying to understand the RoHS requirements - i.e. "What is a homogeneous material?"... If the Sn/Pb is electrodeposited on a flex, is it still a "seperate" material? If our flex weighs 1 gram, the transducer weighs 100 grams, then you're saying (or the EU is saying...) it doesn't matter that this is a coating or how it's applied? In the extreme, this would imply that any single solder joint in any size device, it it's 60/40 Sn/Pb, would disqualify the entire device! THAT'S INSANE!!

We haven't done any calculations yet on mere Sn/Pb weight, but if what you're saying is correct, then it doesn't matter what calculations we do - we're non-compliant right out of the gate. I guess I now know why there are so many exemptions and why the electronics industry was so "up in arms" when this was first proposed!

 

[David Hartman]

If the Sn/Pb is electrodeposited on a flex, is it still a "seperate" material? If our flex weighs 1 gram, the transducer weighs 100 grams, then you're saying (or the EU is saying...) it doesn't matter that this is a coating or how it's applied? In the extreme, this would imply that any single solder joint in any size device, it it's 60/40 Sn/Pb, would disqualify the entire device! THAT'S INSANE!!

We haven't done any calculations yet on mere Sn/Pb weight, but if what you're saying is correct, then it doesn't matter what calculations we do - we're non-compliant right out of the gate. I guess I now know why there are so many exemptions and why the electronics industry was so "up in arms" when this was first proposed!

This is exactly the reason that the electronics industry has been striving to move towards an acceptable replacement for Pb/Sn solder (of which there are many variaties today).

Let's look at the calculations in the other extreme (BTW: the is an actual scenario that played out with one of our suppliers). If as a supplier, I provide you with a choke that in total weighs 4 lbs. (most of which is the copper wire wrapped around a small ferrite core) and I tin approximately 1/4 of an inch of each of the resulting 6 leads of this wire - the percentage of Pb in this tinning compared to the overall weight of the wire is miniscule. With this understanding in order to be compliant I really don't have to reduce the amount of Pb I'm putting into our environment, all I have to do is increase the amount of wire/weight that I'm using.

If the purpose of the EC Directive is to eliminate/reduce hazardous materials from our environment, which of these practices makes the most sense?

 

[Steve McQuality]

OK - Let's bag the whole association w/ RoHS and all it's political and business implications...

Just a basic chemistry question - If I know the composition of a flex circuit trace of given size, can I use that information to calculate the amount of lead by weight? I know;

• Length and Width of trace.
• The fact that it's 1-oz. copper (35.4 um thick)
• Tin/Lead plating thickness (in this case 200u-in)
• Tin/Lead percentage

Is there a formula into which I can plug in this known info and calculate the amount of lead by weight (or tin or copper for that matter)? I'll worry about the regulations later - I'm just looking for a method, formula, etc.

 

[Al Rosen]

OK - Let's bag the whole association w/ RoHS and all it's political and business implications...

Just a basic chemistry question - If I know the composition of a flex circuit trace of given size, can I use that information to calculate the amount of lead by weight? I know;

• Length and Width of trace.
• The fact that it's 1-oz. copper (35.4 um thick)
• Tin/Lead plating thickness (in this case 200u-in)
• Tin/Lead percentage

Is there a formula into which I can plug in this known info and calculate the amount of lead by weight (or tin or copper for that matter)? I'll worry about the regulations later - I'm just looking for a method, formula, etc.

Steve, since you know the length, width, thickness of the plating and the % of lead. (l)(w)(t)(.4)=the volume of lead Now you can go here to calculate the mass or weight. Then convert it to % weight of the total assembly.

 

[Steve McQuality]

Thanks Al!!! I should have guessed there was a web site somewhere that would help me out without having to resort to digging back into my old Chemistry books! This is great.

Of course, now I have the dilema presented by David...

I would hope that the EC on RoHS would come up with a reasonable ruling when considering the solder connections to PZT... but time will tell.

Thanks again Al.

-Steve

 

[David Hartman]

Steve, since you know the length, width, thickness of the plating and the % of lead. (l)(w)(t)(.4)=the volume of lead Now you can go here to calculate the mass or weight. Then convert it to % weight of the total assembly.

You'll probably be close enough with this formula, but keep in mind that solder tinning is NOT "plating". Applying solder is a chemical reaction that takes place at a molecular level, it is not the simple bonding of a solder to the copper, but is the creation of several layers of intermetallic bonds that are comprised of various percentages of copper, tin, lead, and their various compounds.

My point is that the "pure" copper lead diameter is actually reduced, while the "tinning" changes the structure of the outer layers - so the "thickness" of the Pb (lead) content may actually be a fraction greater than specified.

All of which will more than likely be immeasurable as a percentage of the overall weight.

 

[Steve McQuality]

You'll probably be close enough with this formula, but keep in mind that solder tinning is NOT "plating"...

But our flex circuits ARE plated - I believe I'm using the correct terminology. The sheets of etched copper are placed in a bath, electrically charged and are wet plated to a given thickness on both sides of the trace (for the purposes of discussion, 200 micro-inches on each side). We verify this thickness using XRF when the flexes arrive and, within reason, the plating thickness is fairly close to nominal. We then reflow this solder onto our PZT ceramic elements.

There are other places within the transducer where we do "tin" using standard 60/40 solder - but in the case I was asking about, it is a plated flex. Am I missing something, or was this just a misunderstanding early on? I know there are different way of plating (sputtering, wet plating, etc.) and these flexes are wet plated.

-Steve

 

[Jim Wynne]

But our flex circuits ARE plated - I believe I'm using the correct terminology. The sheets of etched copper are placed in a bath, electrically charged and are wet plated to a given thickness on both sides of the trace (for the purposes of discussion, 200 micro-inches on each side). We verify this thickness using XRF when the flexes arrive and, within reason, the plating thickness is fairly close to nominal. We then reflow this solder onto our PZT ceramic elements.

There are other places within the transducer where we do "tin" using standard 60/40 solder - but in the case I was asking about, it is a plated flex. Am I missing something, or was this just a misunderstanding early on? I know there are different way of plating (sputtering, wet plating, etc.) and these flexes are wet plated.

-Steve

You are correct--the circuits are being electroplated with Sn/Pb. It's a common practice in PCB and connector manufacturing, and different from (for example) wave soldering.