Does Pin Discoloration Affect Soldering?
To address this question, we need to know five key aspects:
1. Meaning of Pin Discoloration
Due to the good chemical stability of tin, it does not easily react with air, and normally, there should be no discoloration.
However, in practice, pin tin plating discoloration is a common abnormality. Whether it is the tin plating on lead frames or connectors, discoloration can occur. Pin discoloration is usually caused by oxidation or other chemical changes.
2. Types of Pin Discoloration
Pin discoloration mainly manifests as discoloration due to poor storage conditions (primarily yellow) and high-temperature discoloration (primarily purple).
3. Extent of Pin Discoloration
When the tin plating is exposed to air for a long time, it reacts with the oxygen in the air to form oxides, causing the tin plating to turn yellow, which may affect the chip's solderability. The extent of tin plating oxidation depends on storage time and temperature; the longer the storage time, the greater the reaction with oxygen. Similarly, in high-temperature environments, the reaction between tin and oxygen is faster, resulting in a thicker surface oxide film. As the oxidation reaction intensifies, the discoloration of the tin plating deepens, progressing from light yellow to dark yellow, then to blue, purple, and brown.
For example, the pin discoloration is shown in the following image:
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4. How to Determine if Discoloration Affects Soldering
Usually, we can determine whether discoloration affects soldering by observing the location of the discoloration on the pins.
For Surface-Mounted chips, the pin soldering ends are at the bottom. Discoloration in the plating area near the plastic encapsulation does not affect soldering.
For Thru-Hole chips, the pin soldering ends are away from the plastic encapsulation. Discoloration near the plastic encapsulation is not in the soldering area and does not affect soldering.
5. Solderability Testing
Assessing whether usage is affected merely by observing the appearance is just a preliminary method. To determine if the oxidation condition impacts usage, further validation is required. This can be confirmed through solderability testing.
(1) Solder Pot Method (2) Wetting Balance Method