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Engineered Flux for Low Temperature Solders

Engineered Flux for Low Temperature Solders
The advantage of engineered flux for low temperature solders in achieving critical properties of the solder paste is summarized in this paper.
Materials Tech


Authored By:

Ramakrishna H V, Ph.D., Harish H S, Manjuvani J, Vangapandu Bhaskar, Ramesh Kumar , Vikas Patil and Siuli Sarkar, Ph.D.
Alpha Assembly Solutions, Alpha India Research Center
Bangalore, India

Traian C. Cucu, Ph.D.
Alpha Assembly Solutions


Low temperature solders are becoming more common in surface mount assembly. Critical SMT components cannot handle the temperatures used for lead-free soldering which typically reaches 240 – 250 °C. Sensitive critical components might be damaged by these high temperatures. High temperatures can also cause warpage and other damage to the assembly. Low temperature solder alloys can be used in many forms including solder paste, wire solder, pre-forms, and bar solder. The main motivation of low temperature soldering is driven by faster technology scaling, reduced energy and emissions. Increasing demand in electronics assembly materials like low temperature solder paste to meet and fulfil the requirements of today’s rapidly changing technology requires major innovation in developing new chemistry platform.

A new and innovative chemistry platform needs versatile, multifunction capable chemical molecules as its building blocks to achieve desired solder paste processing properties. To achieve desired properties, a carefully engineered paste flux is used to study high speed printing and reflow properties of low temperature solders. The effect of viscosity and thixotropic index on high speed printability, reflow properties and other warpage defects like non- wet open (NWO) and head in pillow (HIP) were also compared to conventional flux system for low temperature solders. Fine feature printability is evaluated in both low (50mm/sec) and high speed (150mm/sec) printing methods to demonstrate the capability of newly engineered flux system.

Further, low temperature solders are finding its importance in fine feature application for advance packaging applications. The newly engineered flux is studied with finer particle size (Type 5) for printability, reflow ability and solder ability along with warpage defects and large area pad voids.


In the current work, we have summarized the advantage of engineered flux for low temperature solders in achieving critical properties of the solder paste. The conclusions of this work is as follows:

  • Warpage defect ( Non wet open) of solder pastes (SP1, SP2 and SP3) with engineered paste flux is minimum to zero compared to conventional chemistry based solder paste.
  • Solder pastes with advanced chemistry platform showed good stability in terms of viscosity. Solder pastes SP1, SP2 and SP3 showed good stability at room temperature compared to conventional solder paste.
  • Based on the pin testability data, it was evident that the new chemistry platform is found to be better than the conventional chemistry platform. It was observed that the residue generated by new chemistry platform is found to be non tacky and flexible which allows good yield in circuit test.
  • On comparing the data of non wet open and ICT, It was demonstrated that the pastes with different viscosity with same metal % by maintaining same thixotropic index of around 0.6 has no effect on the performance of the solder paste.
  • Based on the printability data, it was confirmed that the lower viscosity solder paste (SP1) can be able to achieve excellent printability and transfer efficiency by having right thixotropic index.
  • Printability and paste transfer efficiency studies of the solder paste with engineered paste flux showed that the paste with any viscosity range can be printable by maintaining a proper thixotropic index of the solder paste.
  • Newly developed chemistry platforms for low temperature solder alloy enable better assembly with very minimum defects compared to conventional chemistry platform. From our study, it is very evident that the new chemistry platforms with advanced chemical molecules are very much required to meet the requirements of SMT industry.

Initially Published in the SMTA Proceedings


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