Lead-Free Solder Versus Leaded Solder in Assembly

Two families of solder alloy dominate electronics assembly, and the choice between them affects the thermal profile, the joint appearance, the reliability model and the mix of materials the supply chain will accept. Since lead is toxic and its use in electronics is restricted, lead-free assembly has become the default, but leaded alloys remain in use for high-reliability and military work, and mixed assemblies are common enough in repair and rework that every engineer should understand both.

Alloy Composition

The classic leaded alloy is the eutectic tin-lead mix at 63 percent tin and 37 percent lead. It melts at 183 degrees Celsius, wets readily, forms a bright and ductile joint and has been characterised so thoroughly that its behaviour is predicted rather than tested in most applications.

The most common lead-free alloy is SAC305, containing approximately 96.5 percent tin, 3 percent silver and 0.5 percent copper. Its melting range begins around 217 degrees Celsius, roughly 34 degrees higher than the eutectic tin-lead alloy, and it wets less readily, which is why the assembly process has to change rather than simply use a different paste.

It is worth noting that lead-free does not mean lead absent. The standards permit a small residue of lead, typically below 500 parts per million, so a lead-free process is defined by a threshold rather than by absolute purity.

Reflow Profile and Peak Temperature

The higher melting point of SAC305 drives the reflow profile. The soak and ramp portions must bring the assembly to a temperature at which the paste activates and the components reach thermal equilibrium, and the peak must exceed the liquidus by enough to form a proper joint without exceeding the damage threshold of the components or the laminate.

The window is narrower than it is for leaded assembly. On the low side, an insufficient peak produces incomplete melting and a grainy joint; on the high side, the laminate can degrade, the intermetallic layer grows faster and moisture-sensitive packages can crack from internal vapour pressure. The acceptable peak for SAC305 is typically in the region of 235 to 250 degrees Celsius, which is close enough to the limits of many plastic packages that the profile must be verified with a thermocouple on the actual assembly rather than assumed.

The higher temperature also affects the board itself. The laminate must be rated for the process, which is why the glass transition temperature of the material becomes a purchasing criterion in lead-free work, and why designs with tight vias and thin dielectrics need to be checked for their ability to survive the additional thermal excursion.

Wetting, Appearance and Inspection

Lead-free alloys wet less readily than tin-lead. The contact angle is larger, the fillet is less concave and the surface appearance is duller and more grainy. The appearance is normal, not a defect, and inspection criteria must be written accordingly, because an operator trained on leaded assemblies will reject perfectly good lead-free joints.

Because wetting is weaker, the surface finish of the board matters more. A finish that oxidises or that has a rough surface will produce more non-wetting and dewetting on a lead-free process than on a leaded one. The plating chemistry that produces a clean, solderable surface is discussed in this article on electroplating additives, since the same deposition control that determines plating quality also determines solderability, and the pad geometry that sets the wetting area is covered in this discussion of PCB pad design standards.

Intermetallic Growth and Long-Term Reliability

Between the solder and the copper pad, a layer of intermetallic compound forms during soldering. It is essential for a good joint, because it provides the metallurgical bond, and it is also brittle. The layer grows with time and temperature, and beyond a certain thickness it becomes the weakest part of the joint, particularly under thermal cycling or mechanical shock.

The growth rate depends on the alloy and on the temperature the joint experiences in service. SAC alloys with a higher silver content tend to form stronger joints but also have a higher melting range, while the copper content affects the growth of the intermetallic layer at the interface. For a product that will operate at elevated temperature for years, the choice of alloy and the control of the reflow profile both influence how the joint ages.

Tin Whiskers

Pure tin and high-tin finishes can grow conductive whiskers: thin crystalline filaments that emerge from the plating surface over months or years. A whisker that bridges two adjacent pads creates a permanent short, and because the growth is unpredictable, the mitigation is usually a material choice rather than a layout change.

Whisker risk is managed by avoiding pure tin finishes where the product is critical or where the service life is long, by using a matte tin with controlled grain structure, by annealing the deposit, or by choosing a finish such as a nickel-gold combination that does not grow whiskers. The layout can help by providing adequate spacing between pads and between pads and nearby metal, and the clearance rules that govern those gaps are the ones described in this article on via to trace clearance.

Mixed Alloys and Rework

When a leaded component or a leaded solder finish is used on a lead-free assembly, the joint becomes a mixture. The melting behaviour of the mixture is not the same as either alloy, and the joint may not fully reflow at the profile used for the rest of the board. This is the origin of the requirement to identify leaded parts in the bill of materials and to handle them explicitly.

Rework is the other place where mixing occurs, and it is where the largest number of mixed-alloy joints are created in practice. A repair technician using leaded wire solder on a lead-free board raises the local lead concentration and lowers the local melting point, which is acceptable for a single repair but changes the long-term behaviour of that joint. Best practice is to match the alloy used in rework to the alloy used in production, and to record the alloy used so that a later repair does not unknowingly build up a third composition in the same joint.

FAQ

Is a dull, grainy solder joint a defect in lead-free assembly? Not by itself. SAC alloys naturally produce a duller, more granular surface than tin-lead and wet with a larger contact angle. Judgment should be based on the fillet geometry and the wetting angle to the pad, not on the brightness of the surface.

Why is the reflow peak temperature higher for lead-free solder? Because SAC305 begins to melt around 217 degrees Celsius, about 34 degrees above eutectic tin-lead. The peak must exceed the liquidus to form a sound joint, which is why lead-free profiles typically peak between 235 and 250 degrees and why laminate ratings matter more.

What causes tin whiskers and how are they controlled? Whiskers are crystalline filaments that grow from high-tin finishes over time, driven by internal stress in the deposit. They are controlled by avoiding pure tin on critical products, by using matte tin with controlled grain structure or by selecting a finish that does not grow whiskers, rather than by layout alone.