Blog About PCB Manufacturing Knowledge

A PCB footprint refers to the physical layout or pattern on a printed circuit board that corresponds to the electrical and mechanical connections of specific electronic components. It defines the size, shape and location of pads, vias, holes, and other features required to mount and solder components to a PCB. Therefore, designers need to strictly adhere to some standards when designing footprints to ensure the accuracy of assembly.

How to Create a PCB Footprint

Designing custom PCB footprints becomes necessary in various scenarios, including instances where the component library is unavailable in the PCB design software database. These situations often require designers to create their own footprints to ensure accurate representation and connectivity of electronic components on the PCB.

Another common situation that calls for custom PCB footprint design arises when the manufacturer's datasheet does not provide the necessary land pattern information. In such cases, designers must rely on their expertise and experience to create the appropriate footprint that meets the specifications and requirements of the component.

Additionally, the need to design PCB footprints can arise when working with subassemblies that consist of multiple components, such as ICs and passive components, or when integrating multiple circuits into a single design. Custom footprints enable designers to accurately position and connect these components, ensuring optimal functionality and performance.

PCB Footprint Design Guidelines

Designing a PCB footprint involves careful consideration of various factors to ensure proper component placement and connectivity. Guidelines for PCB footprint design cover aspects such as pad arrangement, pad spacing, board edges, mounting holes, and more. Here are some commonly followed guidelines:

• Adhere to the specified pad spacing guidelines, considering surface elements, drill holes, other pads, and board edges.


• When creating the PCB footprint, include essential information like pad details, component outline, silk screen markings, reference designations, and STEP models.


• Ensure that the pads around the component edges are accurately sized.


• For through-hole components, indicate the location of the hole within the footprint.


• Obtain accurate component specifications and refer to industry standards like IPC-7351 for creating the footprint.


• For polarized components such as capacitors, clearly mark the polarity in the PCB footprint. Similarly, indicate pin details for diodes, SCRs, MOSFETs, and other components.


• Identify pin-1 in quad and DIP IC packages to ensure proper orientation.


• Include all the necessary data and attributes in the footprint to avoid outdated PCB boards.

Summarize

PCB footprint design accuracy is critical, and it is important for designers to follow the above criteria. If you want my content, you can read more interesting electronic circuit related knowledge on my private blog site.

As technology continues its relentless march forward, so too do the manufacturing processes for printed circuit boards (PCBs). A game-changer in PCB design is the emergence of semi-additive technology, enabling finer line widths as minuscule as 1.25 mil. This technological leap has had a profound impact on maximizing circuit assembly density, significantly influencing the integrated circuit industry's development.

Historically, the production of integrated circuits leaned heavily on the semiconductor IC photolithography process. Still, lately, the PCB manufacturing process has garnered considerable attention within the integrated circuit sector. Progress in this realm signifies more than just technological advancement; it represents a fundamental shift in our approach.

Traditional PCB manufacturing predominantly relies on subtractive manufacturing methods, typically accommodating minimum wiring design width tolerances within 0.5 mil. However, for circuits sporting narrow line width designs and relatively low signal edge rates, even a 0.5 mil variation can wield a significant impact. Here, the concept of impedance control takes center stage.

The PCB manufacturing process conventionally entails overlaying one or both sides of the core substrate material with copper foil, followed by applying a protective layer over the unexposed areas. Subsequently, an acid bath is employed to etch away the unexposed regions, thereby forming the wiring design. The goal here is to achieve a rectangular cross-section wiring design, although the vertical copper lines and portions of the horizontal wiring walls might experience etching during the process.

Subtractive manufacturing hinges on precise etching control, ensuring that the wiring design adopts an almost trapezoidal cross-section with an angle of 25 to 45 degrees. This trapezoidal cross-section plays a crucial role in predicting impedance, permitting nearly vertical repetitive routing and achieving the highest circuit packing density.

However, inadequate control can lead to excessive etching on the top half, detrimentally impacting circuit performance. Thus, the etching factor has become a vital metric for gauging the quality of etching process control. A larger etch factor implies that the wiring design cross-section more closely resembles a rectangle.

In contrast, semi-additive manufacturing presents an entirely different approach. Here, copper foil is added to the substrate, laminated, drilled, and electroless copper plated to create the wiring design. This method represents a divergence from the chemistry of subtractive manufacturing. Some PCB additive manufacturing techniques employ photolithography, resulting in wiring designs more closely aligned with the original design.

Under stringent tolerance requirements, partial additive manufacturing can achieve trace design widths as narrow as 1.25 mils while maintaining acceptable impedance control. Actual measurements have shown that the impedance variation across the entire PCB board will not exceed 0.5 ohms, merely one-fifth of the variation observed in subtractive manufacturing.

High-speed digital systems and microwave applications necessitate stringent impedance control, a feat readily achievable through partial additive manufacturing methods. Moreover, partial additive manufacturing facilitates further innovations in electronic design, thanks to its characteristic of enabling nearly vertical wiring designs, thereby optimizing circuit assembly density.

In summary, the landscape of printed circuit board (PCB) manufacturing is undergoing a revolutionary transformation. The advent of semi-additive technology brings forth a plethora of possibilities for electronic design. Whether opting for subtractive manufacturing or partial additive manufacturing, each approach boasts unique advantages and applications. Electronic engineers and manufacturers can now choose the most suitable process based on project requirements, ensuring more efficient and reliable circuit design. Continuous innovation in this domain will undoubtedly propel the electronics industry into an era of exciting technological breakthroughs.

PCB Manufacturing Process Overview

(1) Introduction to PCB Manufacturing Technologies

① Subtractive Method

The subtractive method involves creating circuit patterns by selectively removing unwanted portions of copper foil from the entire surface of a printed circuit board. In this process, a mask is applied to cover the desired circuit pattern, and then the unnecessary copper is etched away. This method is considered the industry standard due to its cost-effectiveness in the PCB manufacturing process.

② Additive Method

The additive method, on the other hand, builds circuit patterns by electroplating conductive material onto an insulating substrate. This is achieved by first applying a resist (protective film) to the areas where the circuit pattern is not needed and then plating the exposed areas with the desired conductive material.

(2) Inner Layer Manufacturing

Since the subtractive method is the prevailing approach in modern PCB manufacturing, let's delve into the manufacturing process of this technique as an example.

PCBs come in various layer configurations, including single-sided boards with circuit patterns on one side, double-sided boards with patterns on both sides, and multi-layer boards with multiple layers stacked on top of each other. Inner layer manufacturing is the initial step in the fabrication of all multi-layer PCBs. Here's how it is carried out:

1. Material Preparation: The base material is cut to the required size for further processing.
2. Dry Film Lamination: A dry film is laminated onto the inner base material using rollers or similar equipment.
3. Photolithography: A mask with the printed circuit pattern is placed on the dry film-covered substrate, and ultraviolet light is applied. The areas exposed to UV light react to the light (photosensitivity) and are transferred onto the substrate.
4. Development: After the circuit pattern is printed, the excess dry film is dissolved and removed through a process known as development.
5. Copper Etching: Chemical solutions are used to dissolve the exposed copper foil, leaving only the desired circuit pattern intact.
6. Dry Film Removal: The dry film that served as a protective layer for the circuit pattern is removed.
7. Blackening: A blackening process is employed to create an uneven surface texture, enhancing adhesion.
8. Lamination: The multi-layer board is formed by applying heat and pressure using a laminator.

(3) Outer Layer Manufacturing

The outer layer manufacturing process is common to single-sided, double-sided, and multi-layer boards and follows the inner layer manufacturing. Here's an overview of how printed circuit boards are produced:

1. Material Preparation: Similar to inner layer manufacturing, the base material is cut to the required dimensions.
2. Drilling: Holes are drilled into the circuit board according to the design specifications.
3. Plated Through Holes: Plating is applied to the drilled holes to create plated through holes.
4. Dry Film Application: The outer base material is coated with a dry film.
5. Photolithography: Just like in the inner layer process, the outer substrate undergoes exposure and development to define the circuit pattern using the dry film.
6. Etching: Unwanted copper is etched away to form the circuit pattern, and the remaining dry film is removed.
7. Solder Resist Application: A protective solder resist layer is applied.
8. Silk Screen Printing: Information such as model names, manufacturer details, and component placement locations are printed onto the solder resist using silk screen printing.
9. Surface Treatment: Optional surface treatments like electroplating, solder coating, or gold plating can be performed to prevent copper oxidation and enhance solderability.
10. Contour Processing: Unnecessary portions of the circuit board are removed through contour processing.
11. Quality Assurance: Besides continuity testing, external inspections are conducted to check for defects such as scratches and contaminants.

Essential Equipment for PCB Manufacturing

(1) Etching Process Equipment

• Automatic Cutting and Laminating Machine: This machine precisely cuts the dry film to the required size for the substrate and affixes it in place.
• Exposure Device: It employs ultraviolet light to expose the dry film accurately, defining the circuit pattern.
• Dry Film Developer: This equipment removes the unwanted dry film, leaving only the circuit pattern intact.
• Etching Equipment: During the etching process, this machinery melts away excess copper, shaping the circuit patterns.

(2) Substrate Processing Equipment

• Multi-Notch Molding Machine: This machine is responsible for creating clean and well-prepared end faces after the lamination process.
• CNC Drilling Machine: It drills precise through-holes according to the design specifications.
• CNC Milling Machine: This equipment is used for processing the plate shapes and creating mounting holes.
• V-Cut Machine: It performs V-cut processing, which is crucial for certain applications.

(3) Solder Mask and Silk Screen Printing Equipment

• Coating Device: This machinery applies or sprays solder resist onto the substrate, ensuring the required coverage.
• Exposure Device: It exposes the solder resist accurately, following the defined pattern.
• Developer: Excess solder mask is removed through this process, leaving behind the intended pattern.
• Inkjet and Screen Printers: These printers are employed to print characters and other details on the substrate, adding essential information.

(4) Inspection Equipment

• Flight Checkers and Specialized Checker Equipment: These devices test continuity by making contact with circuit patterns using pins called probes.
• Hole Checker: It inspects through holes for any abnormalities or defects.
• Automatic Appearance Inspection Machine: This machine scrutinizes the surface for scratches, dirt, and overall appearance quality of the board.

Precautions During PCB Manufacturing

(1) Selection of the Most Appropriate Construction Method

In PCB manufacturing, the choice of construction method plays a pivotal role. As previously discussed, PCB fabrication methods can be broadly categorized as subtractive or additive, but within the subtractive approach, numerous variations exist. The optimal construction method hinges on factors such as the circuit pattern pitch and the volume of manufacturing batches. Therefore, it is crucial to select the construction method's scale carefully, aligning it with the specific requirements of in-house circuit board production and anticipated business needs.

(2) Embrace Manufacturing Process Automation

Printed circuit boards are critical components, directly impacting the functionality and quality of electronic devices. Consequently, there's a stringent requirement for high-quality PCBs, which has led to significant automation within the manufacturing process. Production lines often integrate specialized manufacturing equipment responsible for distinct processes.

Nevertheless, it's worth noting that certain manual tasks, such as equipment setup, material transportation, and visual inspections, still persist in many companies, contributing to reduced work efficiency. To address the challenges posed by low-volume, high-variety production demands and the potential labor shortages expected in the future, businesses must prioritize the automation of these manual tasks. This strategic move not only enhances productivity but also maintains competitiveness in the ever-evolving PCB manufacturing landscape.

Advanced reading: PCB Manufacturing: Subtractive vs. Additive Methods