Adding Copper to Solder Paste

[Fletch781]

If I were to add copper powder to tin/lead solder paste, could I increase the conductivity without adversely affecting the solder? I realize that the copper particles would not melt at the 183C reflow temp of the solder, but the solder would surround the copper particles. And the flux in the paste should prevent oxidation of the copper the same as it does for the solder particles. Also tin/lead solder generally bonds well to copper.

My application is very unique and it involves solder having to span distances of as much as ~1.75 inches while maintaining high conductivity. But I need a paste that I can spread, can't use copper wires.

The powder I have is very small particle size (8 micron).

My first post here. Thanks for any help.

 

[Resqueline]

I guess it'll help considerably on conductivity to saturate the solder paste with copper but I've never seen any data on it.
I don't know how fast it happens but be aware that such small particles may quite quickly be "assimilated" by - and form an alloy with the solder.
If the copper is a major constituent then of course not all of it will form an alloy. Adding some silver (1-2%) might help reduce the assimilation problem.

 

[rob_croxford]

Interesting question - i have not heard of this before however there may be issues with contamination... perhaps due to oxidation, this could weaken the solder and cause cracked joints or other issues. You may also have flow issues....

 

[(*steve*)]

I did a little research on solder after reading this. My first thought was "use silver solder", but there's insufficient silver to markedly affect the resistance.

One problem is that solder is going to have a higher resistance than copper, and adding small amounts of copper is not really going to make it that much better.

I looked at lists of various solders and their properties and none (other than those containing a high proportion of gold) list low electrical resistance as one of their properties.

Naturally, a thick layer of solder will have a lower resistance than a thinner one, but it may be problematic to get an even spread.

It is interesting to note that copper does dissolve to some extent in lead/tin solder. Indeed some types of solder had minute amounts of copper in them to prevent the erosion of copper soldering iron tips.

I didn't discover anything that would be helpful I'm afraid.

I did note though that aside from the poisonous nature of lead, tin/lead as a solder is quite hard to beat as a general purpose solder.

 

[Fletch781]

Thanks for the reply. I'm not an electronics or metallurgy expert but I'm not sure if you're saying assimilation as a technical term or general description. Generally speaking I do want the copper powder to mix in and assimilate.

It would even be fine if it formed an alloy, But I thought that the copper and solder could not form a true alloy unless the melting point of all metals involved was reached. And the melting point of copper is way too hot for my application. So I thought the copper would stay separate, but very well bonded and still protected from oxidation by the paste flux.

 

[(*steve*)]

But I thought that the copper and solder could not form a true alloy unless the melting point of all metals involved was reached

Yet tin/lead solder has a melting point lower than that of either tin or lead.

normal tin/lead solder with 1% copper has a melting point not significantly different from tin/lead solder (we're talking mixes close to the eutectic point with a mp of around 183C)

 

[Fletch781]

Steve, thanks. I think I understand the problem a little better now. It may or may not work but I'm going to try it today or tomorrow so it will be interesting to see how it turns out. I'm going to do a control unit and then one mixed with copper and compare.

 

[Resqueline]

By assimilation I meant it'll dissolve into the solder, forming an alloy. I'm not worried about oxidation, the flux takes care of that until it's wetted & protected by the solder.
A mix is what you're after. The only question I have is how much copper will dissolve and how fast, and how much the resulting alloy melting temp will change.
I'd try different mixes, even trying higher than 80-90% copper. You only need/want sufficient solder to wet and fill in the gaps between the copper particles.

 

[Fletch781]

I thought I'd give an update on my results.

- The more copper is added, the harder it is for the mixture to flow. At 80-90% it really doesn't "flow" like solder anymore, it just fuses and generally stays in the same form as when wet -- assuming that you ramp the temp gradually like most reflow profiles.

- adding the copper powder to solder paste seems to increase the solder melting point.

- at ration of about 80% copper if I only heat to 183c, (the melting point if the tin/lead solder paste), then the mixture will harden and have the appearance of sandstone or grout, very rock-like.

- but looking at this mixture under a 40X scope, you can see thousand of small solder spheres with clumps of copper in between. So not much fusion or melting at this temp.

- raising the temp to about 215c will cause the solder to begin to flow. Under 40x scope it looks like irregular solder clumps that have mostly fused together. Only a very few spots of copper are visible.

- I found that I didn't need the copper to increase conductivity, the tin/lead solder was about the same level of conductivity.

- Lower ratio's - about 20 copper - didn't seem to affect the melting point or the physical characteristics, it still flowed like solder at 183c.

One concern I ad was that so much fine copper powder would disrupt the flux ratio and therefore the oxidation protection, and in fact when you mix 90% copper, the resulting paste is very dry, kind of like copper mud.

I talked to someone who does reflow for a living and he said that today's fluxes don't do much to prevent oxidation anyway as they are mostly rosin or tree sap. He said in the old days, fluxes used to contain acid and the soldered parts would come out clean and shiny as the acid killed the oxidation.

As a side experiment I got some Naval Jelly which I understand is mostly phosphric acid and used that as "flux" with about 10% solder paste and the rest copper powder. At about 215c this creates a dull, silver, rock-like result, but is still conductive. It's a more toxic process though because of the fumes from heating the acid. It's best to do this with good ventilation.

Thanks again to all for the inputs.