The internal structure of a printed circuit board resembles a layered cake, with one layer stacked on top of another. Taking the simplest 4-layer board as an example, it consists of two outer layers sandwiching two inner layers. While the outer layers serve as protective layers, the inner layers are integral to multilayer PCB, composed of copper foil and dielectric material, used for signal transmission and power distribution, such as power and ground planes. Let’s delve into how PCB inner layers are made!
Before processing the PCB inner layers, several aspects need to be addressed regarding copper foil preprocessing, including cleaning, roughening, and environmental conditions.
For cleaning, because residues like oils, dirt, or debris can be left behind during handling or cutting processes, potentially causing issues such as poor adhesion or dry film delamination. Therefore, before manufacturing inner layer circuits, copper foils are immersed in a bath filled with deionized water or other cleaning solutions. They are gently wiped clean and then dried.
For roughening treatment, it involves creating tiny indentations and protrusions on the copper foil surface to increase surface area. This enhances the reactivity rate with chemical processing agents and improves adhesion to resins or polyimide films. Roughening can be achieved through various methods:
- Acid Etching: Immersed copper foils in an acidic solution to mildly corrode the surface, creating slight roughness.
- Electrolytic Roughening: Immersed copper foils in an electrolyte solution, and surface roughness is controlled by adjusting current density and time.
- Chemical Roughening: Involves using chemicals like ammonia water, hydrogen peroxide, or other substances to alter the copper surface chemically.
- Sanding/Blasting: A physical method that employs abrasives, grinding stones, or sand grains to impact the copper foil surface, increasing roughness.
For the processing environment, it’s imperative to maintain absolute cleanliness throughout the PCB manufacturing process. Typically, laminating and exposure work take place in a Class 10,000 cleanroom to avoid introducing contaminants at contact points, leading to manufacturing defects. Furthermore, the room temperature is usually set at the lower end, e.g., between 21 to 25 degrees Celsius, as the chemicals used in the manufacturing process are highly temperature-sensitive. If the substrate has a higher moisture absorption rate, relative humidity should be maintained between 55% to 60% to prevent size variations and distortion.
Before transferring the designed circuit pattern onto the inner layers, a layer of photosensitive dry film is applied to the substrate’s surface. This film is a material with photosensitive properties that hardens upon exposure to UV light. A laminator can be used to expedite the film application process. This film has a three-layer structure, and the laminator, at an appropriate temperature and pressure, attaches it to the substrate. Subsequently, it automatically peels off the plastic film on the side that bonds to the board.
It’s essential to note that the photosensitive dry film has a very short shelf life. Therefore, immediate exposure is necessary after lamination. This process uses an exposure machine, which emits high-intensity UV rays. Through image transfer, the image on the film is reversed and transferred onto the dry film. These images are typically negative, meaning the exposed areas become insoluble in the developing solution. Only the image part of the circuit pattern remains on the photosensitive dry film after immersion in an acidic or alkaline etching solution.
Inner Layer Inspection
Inner layer manufacturing employs a significant amount of chemical agents, and any deviations in concentration, time, or temperature can lead to defects in the inner layers. Therefore, meticulous inspection at this stage is a key factor in ensuring the effectiveness of subsequent work. Inspection tasks should be entrusted to AOI equipment, an optical inspection method.
Before initiating the inspection, operators input precise samples into the equipment. After inner layer manufacturing, PCB are transported to the inspection area. The entire PCB surface is scanned by the AOI inspection system using laser locators for precise positioning. It then abstracts the obtained pattern and compares it to the defect pattern to determine if there are any issues with the PCB’s line production.
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