The field of electronics heavily relies on Printed Circuit Boards (PCBs) to bring intricate designs to life. PCBs serve as the fundamental backbone of electronic devices, connecting and powering various components seamlessly. The journey from an initial design concept to a fully functional PCB involves a series of well-defined steps that ensure precision and reliability. In this article, we will delve into the general steps of the PCB manufacturing process, shedding light on the crucial stages that transform an idea into a tangible reality.
Before we dive into the detailed steps, it's essential to understand the role of photolithography in PCB manufacturing
Photolithography for PCB
Photolithography is an important technique employed in PCB manufacturing. It plays a fundamental role in creating the circuit patterns on both the inner and outer layers of the PCB.
Photolithography is an essential technique used in PCB manufacturing, responsible for creating circuit patterns on both inner and outer layers of the PCB. It involves transferring a circuit pattern from a digital design onto a copper-clad substrate using photosensitive materials and UV light. This process, also known as optical or UV lithography, facilitates the transfer of patterns onto the copper layer (whether inner or outer).
a. Refer to the below picture illustrating the copper-clad substrate, depicting the board with a base material and a copper layer affixed onto it. At the outset, the first step involves cleaning the surface of the copper. Please refer to Fig. 29 for the depiction of the copper-clad substrate.
Fig.29: Copper-Clad Substrate (Copper + Base Material)
b. After cleaning the Copper-Clad Substrate, the next step involves applying a layer of photoresist onto it. The photoresist is a material that alters its characteristics based on the light exposure it receives. Depending on the type of resist used, the photoresist can either harden or soften. Before proceeding, let's distinguish between Positive and Negative photoresist to understand their differences.
Positive Photoresist : With positive photoresist, the portions exposed to UV light undergo a softening process, while the unexposed areas retain their hardened state on the copper-clad surface. In essence, the masked regions, shielded from light exposure, maintain their hardness, while the unmasked areas soften the photoresist. Please refer to Fig. 30 for visual reference.
Fig.30: Positive Photoresist
Negative Photoresist:
In specific instances, negative photoresist is utilized. Unlike positive photoresist, negative photoresist retains its hardened state where exposed to UV light while staying softened where it's not exposed. This means that in this process, the masked area becomes softened, whereas the unmasked area remains hardened. For further clarification, please refer to Fig. 31.
Fig.31: Negative Photoresist
From this point forward, we'll proceed with utilizing the Negative Photoresist for all the subsequent steps mentioned in the PCB manufacturing process.
PCB manufacturing can employ either positive or negative photoresist based on specific applications and considering their respective advantages and disadvantages. Both types find utility in PCB production. Refer to Fig. 32, illustrating the application of negative photoresist layered onto the copper-clad substrate.
Fig.32: Negative Photoresist on Copper Clad
c. To prepare the masking layer for transferring the pattern onto the copper-clad, we use different approaches based on the type of photoresist employed. In our case, with the use of Negative photoresist, the masking layer represents the copper pattern in transparency. Alternatively, if employing Positive photoresist, the masking layer represents the negative of our copper pattern. For a visual representation, refer to Fig. 33, displaying the circuit layout as a photomask for Negative photoresist.
Fig.33: Circuit Layout as Photomask for Negative Photoresist
d. UV light exposure onto the Negative photoresist and Copper-Clad occurs through the masking layer. As the light reaches the mask layer, it selectively passes through the transparent areas while the darkened sections block the light rays. This results in the targeted exposure of the photoresist corresponding to the copper pattern, allowing the light to harden the desired areas. The unexposed regions of the photoresist remain softened.
Refer to Fig. 34 for the arrangement displaying the masking layer's positioning between the light source and the photoresist. Fig. 35 illustrates the outcome after exposing the light to the mask layer.
Fig.34: Arrangement of Photomask Patterns
e. The next step involves the chemical process of developing, where all the softened photoresist is removed, leaving behind the hardened photoresist intact. This process ensures that the desired pattern on the photoresist remains. Please refer to Fig. 36 for a visual representation of the developed photoresist on the copper-clad substrate.
Fig.36: Developed Photoresist on Copper-Clad
f. In Fig. 36, you'll notice exposed unwanted copper after the development process. To remove this, acid liquids are used for etching. Refer to Fig. 37, depicting the etching process of copper and its resultant effect.
Fig.37 : Pattern Transfer Post Etching
g. The final step involves stripping off the hardened photoresist from the copper surface, revealing the transferred copper pattern. Fig. 38 illustrates the resulting transferred copper pattern after this process.
Fig.38 : Final Copper Pattern After Photoresist Stripping on Board
Up to this point, we've covered the process of Photolithography, which is essential knowledge before delving into the steps of PCB manufacturing. I trust you now have a good understanding of the fundamentals of photolithography. Let's proceed to explore the general steps involved in transforming design concepts into fully functional printed circuit boards (PCBs).
1. PCB Design
The initial stage is PCB design, which commences using PCB CAD (Computer-Aided Design) software. This specialized software meticulously plans the layout, arrangement, and interconnections of components. Essentially, this step involves transforming conceptual ideas into an electronic circuit layout, ensuring readiness for further development.
2. PCB CAM (Computer-Aided Manufacturing)
The next stage after PCB design is PCB CAM (Computer-Aided Manufacturing), which occurs at the PCB manufacturing company. Here, specialized software generates instructions for manufacturing equipment, ensuring an accurate replication of the digital design. The focus is on checking the design for errors concerning production criteria. A CAM engineer oversees this process, conducting various checks such as Track to Track, Track to Pad, Pad to Pad, Pad to drill, Copper to drill, and more, known as DRC (design rule check).
Essentially, the PCB CAM engineer meticulously reviews the customer's design before the manufacturing process begins. They are responsible for identifying any critical changes required and can generate queries for the customer if modifications are necessary. This stage involves creating necessary instructions and programs that will be utilized during the PCB manufacturing process.
Fig.40 : Examples of Data Correction in CAM (Computer-Aided Manufacturing)
3. Inner Layer Process :
In this stage, the core of a multi-layer PCB is fabricated. Thin copper sheets are laminated onto a substrate, determining the layer stack-up. Negative Photolithography is employed to create the inner layer circuit pattern. As discussed earlier, Negative Photolithography involves patterning the inner layer onto the copper sheet, adhering to the copper's surface to establish the electrical layout. For visual reference, please consult Fig. 41, which illustrates the transfer of the electrical layout onto the core using Photolithography.
Fig.41 : Inner Layer Post Pattern Transfer
4. Build-Up/Stack-Up
The Build-Up or Stack-Up phase involves adding supplementary layers, including prepreg and core/copper foil layers, to achieve the required layer count for the PCB. These layers are meticulously aligned and bonded together to construct a multi-layered PCB. Fig. 42 illustrates the stacking and bonding of the inner layers in a 4-layer PCB, where the inner layers are bonded with the outer copper foil. This process is crucial for establishing the structural integrity and functionality of the multi-layer PCB.
Fig.42 : Example of Build-Up/Stack-Up Process
5. Drilling
The subsequent step after bonding the copper layers involves drilling precise holes into the PCB. These holes are essential for facilitating component placement and establishing electrical connections between the layers. Fig. 43 illustrates the drilling process of the PCB, depicting the meticulous creation of these necessary holes.
Fig.43 : PCB Drilling Process
6. Electroless Plating
The electroless plating process is employed to deposit a precise amount of copper onto the walls of the holes, transforming them into plated through holes (PTH), while holes designated as non-plated (NPTH) are left unaffected. This plating ensures sufficient conductivity between different layers via these holes. Fig. 44 provides a visual depiction of the PTH Holes post-electroless plating, illustrating the outcome of this process.
Fig.44 : Electroless Plating for Copper Deposition on Hole Walls
7. Outer Layer Process :
The outer layer undergoes a similar transfer process using photolithography, as previously discussed. However, a negative photomask is utilized in this instance to ensure appropriate copper distribution on the circuit layer in subsequent stages. An additional step in this process involves "tinning" the copper layer.
Fig. 45 exhibits the comprehensive process of transferring the outer layer pattern. This includes electrolyte copper plating, tinning, etching, and the subsequent removal of tin, illustrating the sequence of steps involved in completing the outer layer's transformation.
Fig.45 : Outer Layer Pattern Transfer via Negative Photolithography
8. Liquid Photo Imageable Solder Mask
The Liquid Photo Imageable (LPI) solder mask is applied to safeguard and insulate the copper traces, while leaving specific areas exposed for soldering components and external inputs through spraying or coating. Similar to photoresist, LPI exhibits hardening when exposed to light and remains softened in unexposed areas. This process includes photolithography, but here, the LPI solder mask is utilized instead of traditional photoresist. After light exposure, the softened solder mask is removed, leaving the hardened solder mask areas to cure. Fig. 46 portrays the comprehensive process of the solder mask ink application. For smaller quantities, solder mask can also be applied through screen printing
Fig.46 : Solder Mask Layer Process
9. Surface Finish
The Surface Finish phase involves the Hot Air Solder Leveling (HASL) process, which aims to apply a protective solder coating onto the exposed copper surfaces of a PCB. This procedure initiates with PCB cleaning and masking specific areas that shouldn't be soldered. Subsequently, flux is applied, and the board undergoes preheating. The PCB is then immersed in a molten solder bath, allowing excess solder to drip off upon extraction. Post-cooling, the board undergoes rinsing and meticulous inspection, followed by the removal of any masking materials. Final quality checks and testing are conducted to ensure compliance. To meet environmental regulations, lead-free HASL processes with alternative solder alloys are increasingly employed. Fig. 47 visually represents the Hot Air Solder Leveling process applied to the PCB.
10. Legends and Silkscreen
Legends and silkscreen printing involve the addition of component names, labels, and logos onto the outer layer of the PCB. This process utilizes legends and silkscreen printing, where character and symbol cutouts are prepared as film. The film is carefully placed on the PCB, and a squeegee is used to apply an adequate amount of colored ink, transferring the characters and symbols onto the PCB surface. Fig. 48 visually demonstrates the silkscreen printing process applied to the PCB for this purpose.
Fig.48 : Legends/Silkscreen Printing Process
11. Routing/V-groove
Routing or V-grooving in PCB manufacturing is the final stage that involves cutting or separating individual PCBs from a larger panel post-manufacturing. This step holds significant importance in mass production, as it converts the panel into distinct functional circuit boards. Methods like V-grooving, tab-routing, or milling are employed based on the board's design and the manufacturer's capabilities. The objective is to cleanly and precisely separate the PCBs while ensuring no damage occurs to components or traces. After cutting, the boards undergo inspection, testing, and preparation for assembly and integration into electronic devices. This stage marks the conclusion of the PCB manufacturing process.
Fig.49 : The Final Routing/V-Cut Process for PCB Cutting
This critical phase involves testing the integrity and functionality of printed circuit boards (PCBs) through various electrical tests and inspections. It includes checks for short circuits, open circuits, continuity, and other electrical parameters. Inspection involves visual examination to identify defects, and quality control ensures that the PCBs meet specified standards and tolerances.
13. Packaging and Shipping
In this phase, PCBs are carefully and securely packaged to protect them during transit. This includes using antistatic packaging, cushioning materials, and ensuring proper labeling for easy identification. After meticulous packaging, the PCBs are shipped to their respective destinations following established logistics and shipping protocols, ensuring that they reach customers or assembly facilities safely and on time.
In conclusion, the journey from PCB design to the final product involves a series of intricate steps, each contributing to the reliability and functionality of the printed circuit board. These steps ensure that the PCBs meet the highest standards of quality and precision, making them an indispensable part of modern electronics.
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