Fabricating Interconnects
The fabrication process of interconnects is visualized in Figure 1 and explained in steps (a) to (i) as follows:
(a) Substrate Core: The foundation of the board, often referred to as the carrier substrate, is initially coated with a layer of copper.
(b) Photoresist Application: A layer of photoresist is applied to the copper-coated substrate core.
(c) Special Photoresist Property: The photoresist used has a unique property. It can be dissolved with a developer after being exposed to light.
(d) Mask Placement: A mask, typically a glass plate, is used. The image of the desired interconnect pattern is applied as an opaque layer on the bottom surface of the glass plate (shown in black).
(e) Mask Exposure: The mask is positioned over the printed circuit board (PCB) and exposed to light. The areas that receive light (illustrated in yellow) and those that remain shaded (in gray) create an image of the desired interconnect pattern on the PCB.
(f) Development of Photoresist: The exposed areas of the photoresist become dissolvable (pale blue in color) due to the exposure. These areas can then be dissolved and washed away using the developer. The unexposed areas, still covered with photoresist, protect the underlying copper layer from etching.
(g) Etching Process: An etching agent is applied. It only removes the copper in the unprotected areas where the photoresist has been removed. The resistance effectively “masks” the etching process.
(h) Cleanup: After etching, only the copper remains in the areas where the photoresist protected it. The remaining photoresist is washed away using an appropriate fluid.
(i) Interconnect Structure: Through this process, the desired interconnect structure, as patterned on the mask, is created in the copper layer.
In cases where only a few PCBs are needed, such as for prototypes, the interconnects may not be formed through etching but by mechanically milling the metal layer.
What is the purpose of the substrate core in PCB fabrication?
The substrate core serves as the foundation for the PCB and is often called the carrier substrate because it “carries” and holds in place the electronic devices and interconnects.
How does a photoresist play a role in the PCB fabrication process?
The photoresist is applied to the substrate core after it’s coated with copper. It has the property that, after exposure to light, it can be dissolved using a fluid called the developer.
What is the function of the mask in PCB fabrication?
Answer: The mask is a transparent glass plate with the desired interconnect pattern applied as an opaque layer on its bottom surface. It is used to transfer the interconnect pattern onto the PCB.
How does the exposure and development process affect the photoresist during PCB fabrication?
When the mask is positioned over the PCB and exposed to light, it creates an image of the desired interconnect on the PCB. The exposed areas of the photoresist become dissolvable, allowing them to be dissolved and washed away with the developer.
Why is etching used in the PCB fabrication process?
Answer: Etching is used to shape the copper layer into the desired interconnect pattern. The photoresist acts as a protective mask during etching by preventing the etching agent from removing copper in the areas covered by the photoresist.