One can compare leaded and lead-free solder joints on three levels—their physical properties, based on their mechanical strength, and from the reliability of the joint—with comprehensive interrelations between them.

With electronics trending more towards micro devices, solder joints too are diminishing rapidly. This is leading to questions on their mechanical, electrical, and dynamic strength. Additionally, there is concern about the reliability of solder joints with the advent of lead-free solder in the wake of directives like WEEE and RoHS.

Why Lead-Free Solder?
Traditionally, assemblers used leaded solder containing mainly Lead (Pb) and Tin (Sn) in certain ratios. However, Lead is a highly toxic substance, and long-term application of the element drastically affects people’s lives and the environment. Therefore, lead-free solder is fast replacing leaded solder. However, the two solders differ in their physical properties, warranting a change in the manufacturing process of printed circuit board or PCB assembly.

Major Difference between Leaded and Lead-Free Solder
The major difference is in their melting points, with the lead-free solder melting at a higher temperature than the leaded solder does:

• Traditional eutectic leaded solder (Sn63Pb37), melting point is 183 °C
• Lead-Free eutectic solder (SAC405), melting point is 218 °C

Consequences of the Melting Point Difference
With lead-free solder possessing a 35 °C higher melting point compared to that of leaded solder, there are consequences on PCB assemblies with lead-free solder usage:

• Easy oxidation of solder leading to lower wetting ability
• Quicker growth of chemical compounds between metals
• Harm to sensitive components like electrolytic capacitors and plastic packages
• Increased chances of failure for components with low dielectric constants
• Increased chances of tin whisker generation

Other Physical Differences between Leaded and Lead-Free Solder

Parameters	Leaded Solder	Lead-Free Solder	Units
Melting Point	183	218	°C
Density	8.5	7.44	g/cm3
Resistivity	15	11	MΩ-cm
Electrical Conductivity	11.5	15.6	IACS
Thermal Conductivity	50	73	W/m·1k·1s
TCE	23.9	21.4	micrometers / M / ºC
Fatigue Life	3	1
Surface Tension	481	548	mN/m
Shear Strength	23	27	MPa

A comparison of characteristics of leaded and lead-free solder demonstrates mechanical differences—absence of lead in lead-free solder makes it harder than its counterpart. Furthermore, higher melting point generates more surface oxides, dries off the flux sooner leading to flux contamination and alloy residue, increasing the contact resistance.

Higher surface tension in lead-free solder leads to poorer wetting ability. For PCB assembly, wetting ability decides how properly the molten solder bonds the copper on the PCB with the components. The poorer wetting ability of leaded solder makes the joint look dull gray. Insufficient wetting makes the joint mechanically weak, and produces a higher rejection of circuit boards.

Reliability of Leaded and Lead-Free Solder Joints
Extensive testing of joints with both leaded and lead-free solder shows that despite the physical differences between the two, lead-free solder joints can be as reliable as leaded solder joints. However, reflow soldering with lead-free solder requires a higher amount of process optimization to achieve these results.

Solder joints can fail due to several possible causes:

• Impact stress during socketing
• PCB bending during assembly
• PCB bending when actuating keypad
• PCB bending during a drop
• PCB bending during product assembly
• TCE mismatch during temperature cycling
• TCE mismatch during power cycling

However, no single lead-free solder has proven to be suitably good for all combinations of process conditions, pad metallization, and field environment. Therefore, depending on the application, the electronic industry uses several lead-free solder types:

Lead-Free Solder Type	Name
Sn1.0Ag0.5Cu	SAC105
Sn3.0Ag0.5Cu	SAC305
Sn4.0Ag0.5Cu	SAC405
Sn1.2Ag0.5Cu0.05Ni	SAC125Ni
Sn3.5Ag	Eutectic SnAg
Sn0.7Cu	Eutectic SnCu

For testing the reliability of solder joints, conditions include:

• Cyclic Drop Test: according to JEDEC Standard JESD22-B111
• Solder Joint Array Tensile Test
• Cyclic Bend Test: according to JEDEC Standard JESD22-B113
• Thermal Cycle Test

Although no single alloy performs the best under all tests, SAC405 consistently performs with the highest rank.

Conclusion

The industry has been using lead-free solders for several years now. There is no clear consensus on the alloy of choice. Manufacturers have been continuously at work on deriving new alloys to improve performance. However, from the point of reliability, both solders are equal.