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Leadfree dip soldering

Hannu

#34064

Leadfree dip soldering | 4 May, 2005

OK, granted, this is not exactly your hi-tech SMD production but pertains to lead-free so I'll post this anyway:

Can anybody share their experiences on lead-free dip soldering of transformer terminals? I have not done any testing myself but have seen one company do it and it looked ghastly to say the least, especially when dealing with litz wire. Those unfortunate souls amongst us (yours truly included) who will have to battle with this will be facing tough times indeed. Lead-free wave soldering will be kids' play compared to this. Am I the only one among this esteemed society who is faced by this challenge?

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Rob

#34069

Leadfree dip soldering | 4 May, 2005

Hi Hannu,

Just out of interest, are these big, heavy transformers, where the weight of lead in the solder will be less than 0.2% of the overall weight of the dipped part?

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RDR

#34071

Leadfree dip soldering | 4 May, 2005

What exactly what "Ghastly" about it?

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HOSS

#34074

Leadfree dip soldering | 4 May, 2005

Rob,

Are you hinting that the lead content of this component with dipped tin/lead solder would not be an issue with RoHS?

If so, our understanding is that the legislation requires that the 6 banned substances cannot exist at greater than X% in any one homogeneous material. Specifically in a transformer, some of the homogeneous materials would be: Lead frame material, lead plating, copper wire, wire coating, magnet, plastic housing. None of these materials, individually, can contain more than the X% specified in the legislation.

I bring this up because there seems to be a wide range of interpretation regarding what homogeneous means. Our company is proceeding down the path of most resistance as that is what our interpretation is. We are a CM and we have some customers that are hanging their hats on a component level measurement and were having to debate the issue.

Can I get a witness?

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Rob

#34076

Leadfree dip soldering | 4 May, 2005

Hi Hoss,

I was speculating - there are others out there with more experience in this field than me, who may know the definitive on this. I've heard rumours on transformers but nothing concrete.

But I was guessing that the transformer (as a whole) would be treated as a single component.

Cheers,

Rob.

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Hannu

#34089

Leadfree dip soldering | 5 May, 2005

I do not mean big, heavy transformers but rather small ferrite core transformers, with ferrites in the range of EF16 .. EF40. The weight of the lead and its percentage of the total weight is not the issue here. We simply do not want lead-containing through-hole transformer pins coming in contact with leadfree alloy on our wave soldering machine and contaminating the solder pot.

The problem with soldering seems to do with temperature here. So far we have been using temperatures of up to 500�C and dip times of no more than 2-3 seconds (with Sn/Pb alloy). If we take the SN100C3 or SN100C4 leadfree alloys (copper contents 3% and 4%), we cannot go much higher than 400�C. This simply is not enough to burn off the PU and silk/nylon insulation of the leads - within reasonable time at least. If we increase the dip time, the plastic material of the bobbin core starts to melt, the pins start to come off the bobbin or get all twisted and you have a ruined transformer.

Another problem is the alarming rate at which copper from the leads starts dissolving into the Sn/Cu alloy as we increase temperature. Within not very many seconds you will have lost something like 20% of the wire thickness so the individual strands become very brittle and break easily. Not to mention the increase in the copper content of the alloy as a result of this dissolving taking place. A typical dip soldering machine only has something like 3-4 kg of alloy so the alloy gets off balance very quickly.

Soldering could be done by pre-tinning the wires prior to wrapping them on the pins but unfortunately this is not an option in automatic production.

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#34092

Leadfree dip soldering | 5 May, 2005

Hi Hannu,

You just gave a very realistic and practical view of the lead-free dilemma. The main reason that your coil coating doesn't get removed is because of the inability of lead free alloys to wet the surface and conduct the heat. Higher temperatures will improve the wetting behavior but will also increase oxidation rate on the solder surface and again resulting in non-wetting and insufficient heat transfer. The ability of high tin content alloys to dissolve other metals should be of great concern in wave and dip soldering. I have not seen any studies about the limits and effects of various impurities in lead free alloys, but we all know the dissolution of Cu, Ag and Au will be at an alarming rate.

Now your warned about all of the above and if you want to go ahead with lead-free anyway the only solution in your application will be a re-designed solderpot with an ultrasonic transducer, this is the only way to promote sufficient wetting for lead-free alloys.

Good luck Pat

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Hannu

#34139

Leadfree dip soldering | 9 May, 2005

Hi Pat,

And thanks for the sympathy :)

This thing with ultrasonic transducer, is it something tried and trusted or is this a new idea for improving the wetting behaviour of leadfree alloys? This is completely new to me, can you please elaborate on that point?

regards Hannu

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#34145

Leadfree dip soldering | 9 May, 2005

Hannu,

First and for all I have a problem with your question "tried and trusted" for the simple reason that you are going to change too lead-free.

Ultrasonic soldering or welding is not new, as far as I know it is only used applying the ultrasonic energy directly to the metals. See example here http://www.amtechultrasonic.com/articles/theory/theoryarticle.shtml I have done work and study on ultrasonic energy in wave soldering 10 years ago, but since am a flux manufacturer I stopped the research because I came too close to the possibility of soldering without flux (I don't think this needs further explanation).

I am not aware of any solderpots commercially available with ultrasonic transducers, but it would function the same way as ultrasonic cleaning. The ultrasonic energy creates cavitation (vacuum bubbles) in the molten solder promoting wetting of a surface.

Maybe the ultrasonic experts here in the forum can further elaborate on the possibilities.

Pat

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