Six Sigma® Electrolytic Solder

The influence of the solder quality and purity is often a neglected parameter in the soldering process.  There is a lot of hypothesis and mind-sets in what bar solder should be. The bottom line is that the solder used in your solder pot will make the connection between component and circuit.  The characteristics of the solder and the alloy used will have a major impact in the soldering results i.e. reliability of the solder joint, process efficiency, maintenance and overall cost.

The eutecticum is the point where the melting point and solid point is the same and melting point lower then the melting point of the individual metals used in the alloy.  The eutectic point in tin/lead alloy is 61.9% tin and 38.1% lead. (if metals are 100% pure)

With non eutectic alloys a paste-phase develops in the zone between the liquid and solid line.  in theory this could result in soldering failures if the solder joint in this paste-phase undergoes mechanical stress or gets serious vibration.  Small tolerances from the eutectic point have little influence in the soldering process because of fast cooling.  When a board leaves the wave the temperature drop 200°F in a matter of seconds.  The phase diagrams were measured at a very slow, controlled heating/cooling (2°C/min.).  Non eutectic alloys most frequently used are Sn55/Pb45, Sn60/Pb40 and Sn63/Pb37.

Tin (Sn) is the metal in the Sn/Pb alloy that forms the intermetallic connection.  Therefore tin is the most important component in the alloy and will be consumed at higher rates then lead.  This effect will be most noticeable with bare copper or OSP coated boards and less with pre-tinned or HAL boards.  This explains why the tin percentage always drops in comparison with the original alloy added to the solder bath.  The method to correct the tin drop, is adding pure tin to keep the tin percentage within specs.  This proves that the conventional alloy is unstable and a corrective action is required.

Characteristics of Six Sigma® 65/35 bar solder

A higher tin content added to prevent tin depletion in the solderpot, but improves several other interesting characteristics

Surface tension and viscosity

Surface tension and viscosity directly affects the solder fluidity characteristic and ability to wet a surface.  The higher tin content (65%) improves wetting characteristics of the solder due to the lower viscosity.  The improved mobility and increase in surface tension reduces the potential for shorts compared to conventional solder alloys.

The surface tension and viscosity are very closely related to temperature.  We can interpret the benefit from a higher tin percentage with a better flow as a possibility to lower the temperature of the solderpot without loosing critical flow characteristics.  This results in the advantages of reduced oxides, longer lasting solderpot, less energy use but also less stress (temperature shocks) for sensitive components.

Tensile Strength

The tensile strength of solder increases depending on tin content.  At room temperature the tensile strength reaches its peak at 65% (7,900 psi).

Thermal Conductivity

the thermal conductivity diagram shows the linear relationship between the tin content and the thermal conduction capabilities of the solder alloy.  the capability of the alloy to effectively conduct heat to the PC boards becomes an important parameter in residue-free soldering because of the improved capability of the solder to eliminate excess flux. (see No-Residue technology/chemistry and physics)

Electrical Conductivity

there is a straightforward linear correlation between the tin content and the electrical conductivity of the alloy.  the higher the tin content in the alloy, the better the electrical conductivity.

Impurities and effects

Impurities in the alloy used can adversely affect the characteristics of solder.

Where do impurities come from?

   A. Present in the ore

   B. By chemicals used in refining the ore

   C. Mixing of ore with recycled alloy

   D. Impurities from boards and components in contact with wave.

The maximum values of commonly found impurities are determined in different standards: J-STD-006, DIN-29453, QQ-S-571-F, JIS-Z-3282, BA-219, NF-C-90550, MIL-STD-2000A.

Six Sigma® electrolytic solder.

Electrolytic purification is a process that separates the target alloy ion by ion from its impurities resulting in six-sigma purity.  Electrolytic purification is an environment friendly process and is more effective then pyro-metallurgical (thermal) methods commonly used.  The electrolytic purification allows narrow impurity tolerance and eliminates the manufacturer's dependency of ore impurities.

 Six Sigma® maximum impurity levels compared to standards.

Conclusion

Six Sigma® 65/35 bar solder is the purest solder available because of the electrolytic process, and is completely free of oxides.

Six Sigma® alloy consists of 65% tin.  this improves electrical and physical properties: lower viscosity, higher surface tension, higher reactivity, improved tensile strength and improved thermal and electrical properties.

Six Sigma® is a preventive action against tin depletion

Six Sigma® is a must in OSP soldering because the increased tin content together with the high purity will provide a buffer against tin depletion.  The electrolytic purity and higher tin content will reduce oxidation rate with 50%.

Patrick O. Bruneel

Technical Advisor

Interflux USA, Inc.

Tel: 214-350-5565  Fax: 214-350-9713