1. What happens after the inner-layer circuit is etched?

After etching and stripping, the circuit pattern is formed on the inner layer of the PCB. However, the inner layer still needs to be inspected and treated before it can be laminated into a multilayer PCB.

2. What is AOI inspection?

AOI stands for Automated Optical Inspection. It is an automated inspection process that uses high-resolution cameras and image-processing software to check PCB circuits for defects.

3. What defects can AOI detect?

AOI can detect problems such as:

• Short circuits
• Open circuits
• Copper residues
• Nicks or damage in the circuit pattern

4. Why is AOI inspection important?

AOI finds defects before the inner layers are laminated together. Detecting problems at this stage helps prevent defective boards from continuing through production and reduces material waste and scrap.

5. What happens after the AOI inspection?

If the inner layer passes AOI inspection, it undergoes a surface treatment called brown oxide treatment or black oxide treatment.

6. What is brown oxide treatment?

Brown oxide treatment is a chemical surface treatment applied to the copper surface of an inner PCB layer. It creates a micro-roughened oxide layer on the copper.

7. Why is the copper surface roughened?

The roughened surface improves the ability of the resin to adhere to the copper during the multilayer lamination process. Better adhesion helps the layers bond together securely.

8. What could happen if the brown oxide treatment is insufficient?

If the oxidation treatment is insufficient, the copper surface may not bond properly with the resin. This can weaken the connection between PCB layers.

9. What could happen if the copper is over-oxidized?

Excessive oxidation can damage the copper surface. Therefore, the chemical composition and treatment time must be carefully controlled.

10. How do AOI and brown oxide treatment improve PCB reliability?

AOI helps ensure that the inner circuits are free from important defects before lamination. Brown oxide treatment improves bonding between the copper and resin. Together, these processes help reduce problems such as delamination and voids and contribute to reliable multilayer PCB construction.

11. Why are these processes important if they are near the end of inner-layer fabrication?

Although AOI and brown oxide treatment may seem like final steps, they are critical quality-control processes. Problems detected or prevented at this stage can have a major effect on the reliability of the finished multilayer PCB.

12. What is the main purpose of AOI and brown oxide treatment?

AOI: Checks the inner-layer circuit for defects.

Brown oxide treatment: Improves copper-to-resin adhesion during lamination.

Together, they help ensure that multilayer PCBs have accurate circuits, strong interlayer bonding, and good long-term reliability.