Introduction to lead-free Carbon Solder alloy

1. Carbon Solder

Carbon Solder is a new type of lead-free solder created by adding carbon to the lead-free solder under certain conditions. There were failed attempts to mix carbon with solder in the past. However this time, we have successfully mixed the solder and carbon together evenly. The lead-free solder added with carbon is called "Carbon Solder."

Adding carbon to the solder reduces the crystals formed in the solder and increases the soldering's junction area. With this, we have successfully improved the solder's elasticity, melting point and conductivity rate.

By making the good use of the characteristics of Carbon Solder, the inexpensive production of a harmless solder with low melting point and high strength that will replace the tradional leaded and lead-free solders is now possible.

SEM images of as-soldered joint of (a) Sn-3.5Ag and (b) Sn-3.5Ag + 0.03wt.%C.

Metallic grains generally form in the solder / surface boundary of the board during soldering. Because of the fragility of intermetallic compounds, it is the cause for the decrease of the solder's mechanical strength when thickly formed in soldered parts, as well as the further detoriation of the solder's conductivity and corrosion resistance. Because of this, the growth of intermetallic compounds should be inhibited. The formation of metallic layers were confirmed from the results. The surface boundary of the solder without carbon is rough and every parts of it has a huge lump. On the other hand, the surface boundary of the solder added with carbon Sn-3.5wt.%Ag+0.03wt.%C is fine and even.

2. Carbon Solder strength measured results

A notable increase in the Vickers hardness scale were observed after adding carbon to the solder. The hardness of the solder added with 0.03wt% of carbon increased from 10.7Hv to 13.4Hv compared to Sn-3.5Ag. An increase of pulling strength by 50% was also showed.

Carbon Solder strength measured results

Alloy Vickers hardness (Hv)
Pulling strength (MPa)
Sn-3.5Ag 10.7 18.0
Sn-3.5Ag + 0.03 wt.%C 13.4 27.0

Factor analysis of Carbon Solder's mechanical strength improvement

  • Because Ag3Sn is 15 times harder than β-Sn and 12 times harder than the eutectic structure of the solder, it becomes a reinforcing agent for the solder.
  • The improvement in the mechanical strength of the lead-free solder added with carbon can be explained dispersion strengthening of the materials.
  • Adding carbon to the solder shrinks the Ag3Sn grains and prevents the rearrangement of solder alloys and strengthens the alloy even further as the carbon is evenly distributed to the solder.

Pulling strength and elasticity of the Carbon Solder

In mechanical strength, hardness and pulling strength are generally considered to be inversely proportional with each other. However, striking improvements in hardness and elasticity were seen at the experiments conducted to Carbon Solders. It was made clear that the Carbon Solder has characteristics far more superior than the leaded and unleaded solders that people still use today. It was found out that the strength and elasticity rate become higher in proportion to the amount of carbon in the solder.

3. The improvement in the wettability of the Carbon Solder

We discovered that adding carbon to Sn-3.5Ag decreases the angle of contact between the solder and the base material and improves the wettability of the solder.

Appearance of typical spread test specimen after spread test at 533K for 30 s and contact angle of solders before and after adding carbon

4. About the melting point of the Carbon Solder

Adding carbon to the solder made little changes to its onset and melting temperatures.
The onset temperature has dropped from 221.2℃ to 219.0℃ while the melting point moved from 222.0℃ to 221.0℃. By adding carbon to Sn-3.5Ag, the results of the experiment confirmed that the melting point only dropped a little, hence changes were barely made to the melting point. We are currently developing a Carbon Solder with even lower melting point.

DSC curves for (a) Sn-3.5Ag solder and (b) Sn-3.5Ag + 0.03wt.%C.