*Question T7D09:
What is the characteristic appearance of a cold tin-lead solder joint?*  

*Correct Answer is  C: A rough or lumpy surface*  

### AI Response: (may contain errors)  

To identify the characteristic appearance of a cold tin-lead solder joint, let's analyze the options provided:

### Correct Answer: C - A rough or lumpy surface

- **Explanation**: A cold solder joint occurs when the solder does not melt properly or is not heated sufficiently to create a good bond. This results in a joint that appears rough or lumpy, indicating poor adhesion and a weak electrical connection. Cold joints can lead to intermittent connections and are often unreliable.

### Why the Other Choices are Incorrect:

- **A: Dark black spots**: 
  - This appearance is typically associated with oxidation or contamination on the surface of the joint, not specifically indicative of a cold solder joint. It may suggest a problem with the soldering process but does not define a cold joint.

- **B: A bright or shiny surface**: 
  - A shiny surface is usually a sign of a good solder joint. Properly melted solder will have a smooth, shiny finish, indicating that it has flowed well and formed a solid connection.

- **D: Excessive solder**: 
  - While excessive solder can lead to a poor joint, it does not specifically indicate a cold solder joint. A joint can have too much solder and still be properly heated and formed, resulting in a good connection.

### Summary

- A cold solder joint is characterized by a rough or lumpy surface due to inadequate heating during the soldering process.
- Good solder joints are shiny and smooth, while dark spots indicate other issues like oxidation.

### Rule of Thumb

- Always ensure that both the component lead and the pad are heated adequately before applying solder to achieve a good joint. A well-made solder joint should be smooth and shiny, indicating proper melting and flow of the solder.