With the role of providing electrical, mechanical and thermal connections between components and substrate, solder in electronics industry is the material that determines the reliability and durability of the PCB as well as the end product.
Solder can be in three common forms with various structrures and components: Solder bar is simply a solder alloy bar, used for wave soldering process with a separate flux, while, solder wire already includes flux in its core and is suitable for small quantities soldering as well as repairing requirements. And the other, solder paste is mainly used for the SMT process in the form of paste or cream which is usually printed onto the PCB by stencil.
The industry in general and the electronics industry in particular are constantly developing. SMT process for PCB assembly provides stability and better mechanical performance under vibration and shaking conditions. Printed circuit boards created with SMT process are more compact, providing higher circuit speeds. This is one of the main reasons most manufacturers opt for this method. This topic is related to lead-free solder paste- mainly used for SMT and its development trend to adapt to the increasing demands of the electronics industry.
So, What is solder paste?
Solder paste is a metal based material that is used to join two pieces of metal together. It is a suspension of fine metal alloy particles in a flux vehicle. Flux creates rheological properties to solder paste and aids in the soldering process by removing oxide films which form on the surface of metals being soldered. While flux technology is often the technology that makes certain differences in solder between manufactures, the composition, the size and shape of the metal alloy can be tailored to produce a paste with the desired melting range and strength.
The metals commonly used in solder paste are: Sn, Pb, Ag, Cu, Fe, Bi, Zn, Sb, Ni, In, ..which is responsible for forming strong connections between metal workpieces. The size of the metal alloy in the solder paste affects its rheological properties as well as the selection of components sizes and are divided into the following types:
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Type |
II |
III |
IV |
V |
VI |
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Powder Size |
45-75µ |
25-45µ |
20-38µ |
15-25µ |
5-15µ |
As the solder paste used in the semiconductor packaging process contains approximately 90 wt.% soldering metal powder, the physical and mechanical properties of the parent metal, such as tensile strength and electrical conductivity, greatly impact the performance of the soldering equipment.
Trend of metal alloy in the solder paste
1. Lead-based
The first solders used were lead-based, the most of which used for electrical/ electronic soldering is Sn-Pb37- eutectic mixture with the lowest melting point at 183°C (much lower than the melting points of either pure metal which are 232°C for tin and 327°C for lead). Lead-based solder paste offers the advantages of low cost, facilating wetting and good appearance of solder joint but also causes concern about harmful effects to human health as well as the environment. Therefore, lead solder was banned and restricted from use.
2. Lead free
As a necessity, the solder paste market witnessed many innovative technologies in lead-free solder paste. SAC alloy are the leading alloys replacing Tin-lead solders for electronic assembly application. SAC is an alloy of three elements: Tin (Sn), Silver (Ag) and Copper (Cu) and has been proven to perform well. However, the improvement does not stop here, solder materials have evolved to the 3rd generation:
The first generation of SAC alloy:
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Features |
Drawbacks |
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Generally speaking, adding silver to solder paste aims to improve wettability of soldering and strengthen soldering strength and fatigue resistance. Solder paste is able to pass cold-hot recycling test. However, adding too much silver (usually more than 4%), solder will become fragile instead. Besides, due to the steep rise of metal cost, the technology of soldering materials is aimed at reducing the silver content.
The 2nd generation of SAC alloy
However, the lower the silver content drops, especially the common SAC alloys: SAC0307, SAC1007, problems with durability and, more importantly, problems with resistance to thermal fatigue began to arise.
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Features |
Drawbacks |
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The 3rd generation of SAC alloy
To solve the 2nd generation’s problem, the lead-free low Ag soler paste is added with micro alloy such as: Bi, Sb, In, Fe, Ga,..
The presence of these metals has been shown to improve thermal fatigue of the solder paste. And this is the generation of Lead-free Low Silver high strength solder paste.
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Features |
Advantages |
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About the Lead-free Low Ag-high strength solder paste
Lowers silver content to keep cost down while providing excellent connection with good thermal fatigue resistance for the solder joint, the Lead-free Low Ag solder paste is the solution for both technical problems and environmental issues.
Additional Metals
The ability to enhance thermal fatigue resistance of this type of solder paste is based mainly on the presence of other metal particles in the alloy formulation. Some common metals and their effect on the solder joint properties are as follows:
Bi: Due to solution hardening phenomenon, harden alloy, improves thermal strength while lowers the melting point but makes the joint become brittle.
Fe: prevent from iron-tip erosion but lower wettability.
Ga: prevent oxidation
In: lowers the melting point and improves ductility,…
Sb: ensures an even higher longterm reliability
Additional metals improve the alloy strength by preventing the interfere dislocation in the solution hardening:
During the solder joint formation, the atom tends to relocate to stable position, causing transformation and destruction of the joint. Since the metal alloy and additional atoms are different sizes, they interrupt the regularity of the crystal lattice. Dislocations cannot easily move around this interruption. It will take a much higher stress level or temperature to enable the dislocation to move again. That is to say, the strength and thermal stability of the solder are better.
