Working for manufacturer engineering couple years. Sharing the manufacturing experience, skill, DFx, design, and information. Be care of this blog is personal share and content may not be 100% correctly.
Workingbear recently received questions from readers about electronic component soldering leads turning yellow or showing discoloration (purple, blue, etc.) after going through the reflow oven. Why do the leads of components discolor after reflow? Does this discoloration affect product functionality? And how should we handle discolored components or assembled PCBs? Let’s dive into these questions for this article.
The primary reason for the discoloration of component leads after high temperatures is the formation of a thin passivation layer on the surface treatment metal of the leads. You could think of this as an oxidation layer. Essentially, the surface metal of the leads oxidizes when exposed to high temperatures. The thickness of this oxidation layer directly affects how light is reflected and refracted on the surface. Remember those “prism experiments” from school? When light refracts, it creates different colors. The discoloration on component leads works on a similar principle, although with some slight differences.
Some readers have mentioned that flux residue could also cause discoloration. While that’s partially true, flux residue typically forms a transparent film on the solder surface after reflow. However, this flux film is generally thicker than an oxide film, so it doesn’t immediately cause discoloration. Instead, flux residue can turn milky white or yellow over time if left on the surface. That said, flux residue usually remains on the solder joints and can be softened and removed with alcohol-based solvents, like IPA. The discoloration we’re discussing here occurs in areas without solder and cannot be cleaned with alcohol-based solutions.
The discoloration on component leads happens because the oxidized passivation layer forms what is known as an “thin-film optics.” When light hits the thin film, different wavelengths of light interact with the film. Light waves are reflected and refracted at the top and bottom surfaces of the film, interfering with one another to create new wavelengths. This interference results in the different colors we see. Sounds a bit complicated, right? You probably learned about this in middle or high school, but maybe you’ve already forgotten it. In everyday life, you’ve likely seen similar effects—think of the rainbow colors on soap bubbles or the iridescent sheen of oil on water under sunlight. These are also examples of “optical thin films.” If you’re curious and want to dive deeper into the science, you can look up “thin film optics” or “thin film interference.”
Depending on how light is interfered with, the colors observed on component leads can vary. As the thickness of the oxide film increases on the leads, the colors shift from yellow to yellow-blue, blue, purple, and even black.
The Relationship Between Oxide Film Thickness and Colors on Component Leads
Based on shared experiences from others, here’s how the color changes based on the thickness of the oxide film on component leads:
(Note: The color of light observed may vary depending on the angle or position of observation. This is because the thickness of the oxide film might differ at certain angles, such as on curved lead sections or areas where the oxide film is uneven.)
Oxide film thickness: 2–8 nanometers (nm) → No discoloration
Oxide film thickness: 8–15 nm → Light yellow
Oxide film thickness: 15–20 nm → Yellow-blue
Oxide film thickness: 20–30 nm → Blue
Oxide film thickness: 30–50 nm → Purple
Oxide film thickness: 50+ nm → Black
(1 nanometer (nm) = 10 Ångströms (Å))
Does Discoloration on Component Leads Affect Product Functionality?
It depends on whether the discoloration occurs before or after soldering.
If oxidation occurs after soldering: Generally, this will not affect the functionality of the product. Once soldering is complete, the soldered areas of the leads are already coated with solder, preventing oxidation. Upon closer inspection, you’ll find that the discolored oxidation typically only appears on the unsoldered portions of the leads.
However, discoloration may create cosmetic issues. If the PCBA is delivered directly to customers, the discoloration may need to be addressed, or at the very least, the issue should be discussed with the customer before shipment. To ensure no soldering issues arise, you can perform a pull-strength test to compare the solder joints of discolored and non-discolored leads and confirm there is no difference in solder strength.
If oxidation occurs before soldering: This could lead to quality issues, as the oxidized leads may result in poor solderability. Problems such as insufficient solder wetting, voids, cold solder joints, or excessive solder encapsulation are more likely to occur.
Why Does Oxidation Discolor Component Leads?
Discoloration of component leads usually happens after exposure to high temperatures (e.g., during reflow soldering) or when components absorb moisture. High temperatures and humidity accelerate oxidation, increasing the thickness of the oxide film.
It has been observed that lead surface finishes made of tin (Sn) and nickel (Ni) are more prone to discoloration after high temperatures. This is because both metals tend to form oxide films and are commonly used for the surface treatment of component leads in electronics.
Why do some batches of the same component have leads that discolor (turn yellow) after reflow, while others don’t? Or why do components from different manufacturers behave differently? The issue likely lies in the surface plating formulation of the leads. Generally, leads with this type of problem are plated with pure tin or pure nickel, which could be applied via electroplating or electroless plating. Both tin and nickel are metals prone to passivation (oxidation), forming a protective film on their surface in atmospheric conditions. This can be summarized into two main reasons:
Poor plating process control: If the tin plating process isn’t well-controlled, an unclean plating bath may introduce impurities into the tin layer. This results in a less compact plating with loose gaps. After exposure to high temperatures during reflow, these flawed tin plating surfaces become even more uneven.
Oxidation during high-temperature reflow: Pure tin or nickel plating may oxidize under reflow conditions, forming a passivation layer on the metal surface, which creates an optical film.
When discoloration is observed on component leads, the first step is to confirm that the reflow oven settings meet the specifications provided by the component manufacturer. If the process strictly follows their guidelines and discoloration still occurs, the issue should be reported to the component supplier, and replacement materials should be requested.
Can Discoloration on Component Leads Be Fixed?
As previously mentioned, discoloration is caused by an oxide film forming on the surface plating of the leads. If the discoloration occurs before soldering, it means the leads were already oxidized before assembly. In this case, Workingbear strongly advises not to use these components, as the risk is too high.
For components that discolor after soldering, you can try increasing the amount of solder used. By allowing the solder to fully wet and cover the leads, you can prevent the passivation layer on nickel from forming, which helps to avoid discoloration.
What if adding more solder doesn’t fully cover the leads? Or what if the reflow process is already complete? For small quantities, you can manually add solder through hand soldering. If a hot air rework station is available, it’s an even better option, as it minimizes cosmetic issues. However, temperature control is critical to avoid damaging the component or PCB.
Other Possible Causes of Discoloration
In addition to the passivation layer creating an optical film, another major cause of discoloration is flux residue. Try using a solvent to clean off the flux residue to see if the discoloration can be fixed.
Another potential cause, though less common, is insufficient cleaning of the plating solution during the plating process. While this is unlikely, it’s worth considering if other causes have been ruled out.
Leave a Reply