PCB Fabrication Testing: Ensuring Quality and Reliability
In this blog, we'll explore the various aspects of fab testing and why it's essential for producing reliable electronic devices.
Learn the essential steps in the PCB Manufacturing Process from design file conversion to electrical testing and more.
PCB manufacturing process involves transforming design files, such as Gerbers and net lists, into a tangible circuit board. This board serves as the foundation upon which electronic components can be mounted and soldered.
Highlights:
- The initial step in the fabrication process involves the utilization of design output files that include Gerber, net lists, and drill files.
- Output files are created as part of the design phase along with developing a product concept, schematic, and layout.
- The next steps in the process involve the manufacturing and assembly of the board.
Steps involved in PCB manufacturing process
Creation of design and manufacturing files
As soon as the design files are received from the designer, the manufacturing process begins. The designer prepares the comprehensive bill of materials (BOM) to facilitate seamless assembly process. Additionally, they generate the output files in either Gerber or ODB++ format, ensuring utmost precision and compatibility for the fabrication process.
The manufacturer diligently conducts DFM (design for manufacturability) checks to proactively identify any potential errors that may arise during the fabrication process. In the event of any errors, the designer or client is notified for immediate attention and resolution. Once the necessary adjustments have been made, the refined files are seamlessly integrated into the CAM system.
This system identifies the specific format of the design data and converts it into visual images. It also carries out many other tasks, such as design rule checks (DRC) and layer order verification.
After the completion of the design process, the CAM team will proceed to generate output files that cater to the specific needs of different manufacturing departments. These output files consist of drill programs (both sub-drill and main drill), imaging layers, solder mask file output, route file, and IPC netlist.
Imaging of inner layers
As a result of the remarkable advancements in miniaturization, fabricators have predominantly adopted the utilization of LDIs, also known as laser direct imaging. In addition, for printing circuit images, a specialized printer known as a plotter is employed to generate photographic films of the circuit layers, solder masks, and silkscreen layers.
The circuit panel used in this process has a layer of photosensitive film called a photoresist. The photoresist is composed of a coating of photo-reactive compounds that undergo polymerization upon exposure to UV light. The circuit panel with the photoresist layer is positioned under a computer-controlled laser. This laser seamlessly scans the board surface, subsequently transforming it into a digital representation.
This digital image is a representation of the patterns and connections required for the PCB. This aligns with the pre-loaded CAD/CAM design file. Similarly, a negative image is developed on the inner layers.
The image below illustrates the sequential steps involved in the LDI process flow:
Once the image has been developed, the next step involves the careful removal of the unhardened photoresist that serves as a protective layer for the desired copper. This crucial process is carried out using an alkaline solution, which effectively eliminates the photoresist while ensuring the integrity of the copper remains intact.
Copper etching
During the PCB manufacturing process, etching plays a crucial role in eliminating undesired copper from the board and attain the required circuitry.
When it comes to the production of PCBs, manufacturers typically opt for the utilization of a wet etching process. It involves immersing the substrate in a chemical solution, which facilitates the dissolution of the undesired material. Through the chemical reaction that takes place, the targeted material is effectively dissolved, leaving behind a precisely etched surface.
During the etching process, it is crucial to take into account several key parameters. These parameters include the rate at which the panel is moved, the dispersion of chemicals used in the process, and the quantity of copper that needs to be etched off. The entire process is executed within a conveyorized, high-pressure spray chamber.
Stripping of photoresist
During the photoresist stripping procedure, the residual photoresist is removed from the copper substrate. The procedure entails the dissolution of caustic pellets, which are chemical agents, in water through the application of a high-pressure water rinse. This rinse effectively breaks down the photoresist.
Post-etch punch and inspection
After all the layers are cleaned and prepared, the next crucial step in the PCB manufacturing process is to punch alignment holes. Manufacturers ensure precise alignment of the layers by punching alignment holes using targets provided on the inner layer, thereby improving layer-to-layer registration. The layers are then positioned in an optical punch machine to achieve exact alignment of the inner and outer layers.
Inspection at this stage is primarily conducted through a visual scan of the board's surface. Multiple light sources are used to illuminate the circuit board, while one or more high-definition cameras are employed for this inspection task. Moreover, an automated optical inspection (AOI) system constructs a comprehensive image of the board to verify its integrity and quality.
Coating of brown oxide
To prevent oxidation and corrosion of interior layers, the copper circuit pattern is coated in this step with brown oxide. Additionally, it offers superior adhesion properties for bonding with prepregs.
Board lamination
Lamination involves applying heat and pressure to a stacked sandwich of prepreg, copper foil, and inner layer cores to bond the layers together. The procedure consists of two stages: stack-up preparation as per project requirements and bonding.
During the lamination process, a mechanical press equipped for both hot and cold pressing is employed. Throughout this procedure, a computerized bonding press monitors the controlled sequence of events, including the heating of the stack, the application of pressure, and the subsequent facilitation of a progressive cooling process.
The image below summarizes the LDI process:
Hole drilling
Holes are created during the drilling process to accommodate vias and leaded components. An X-ray drill is used to identify targets within the inner layer. It accurately drills registration holes with diameters as small as 100 microns.
Furthermore, this drilling machine is equipped with computer controls that enable it to accurately determine the X-Y coordinates of the registration hole and allow operators to choose drills of appropriate sizes or specific drill programs as per their needs.
The drilling operation produces protruding bits of metal, also known as burrs. Through a deburring procedure, these burrs and any other surface impurities are removed. This ensures the final board is smooth, clean, and free of obstructions.
Electroless copper deposition
Electroless copper deposition involves plating a thin layer of copper on the inner walls of drilled holes to make them electrically conductive. Prior to electroless copper deposition, the drilled hole is thoroughly cleaned to remove any contaminants or impurities from it to ensure effective deposition of copper.
The electroless copper deposition process begins with the introduction of a catalyst. The catalyst plays a vital role in triggering the chemical reaction that leads to the deposition of copper. Subsequently, the panel is submerged in a series of chemical treatments. These baths are meticulously designed to facilitate the controlled deposition of copper. Typically, a thin layer of copper, measuring approximately 0.08 to 0.1 microns in thickness, is deposited onto both the hole barrel and the surface of the panel as a result of the electroless copper deposition process.
Imaging of the outer layer
Just as the photoresist is used for the imaging of the inner layers, the imaging process will be performed on the external layers of the panel, utilizing a positive image. The process utilizes the print-plate-etch method. The initial step entails the cleaning of the panels in order to prevent the adherence of contaminants and dust particles. Subsequently, a coating of photoresist is applied to the panel. Following this step, laser direct imaging (LDI) technology is employed to facilitate the printing of the image.
Copper plating the holes and the surface of the board
During this stage, both the holes and board surface are electroplated with copper. The operator starts the procedure by loading the panels onto flight bars. The panels serve as cathodes and play a crucial role in facilitating the electroplating process. This is primarily due to the presence of a pre-deposited thin layer of conductive copper within the holes.
Prior to the electroplating process, the panels undergo a sequence of cleansing and activation baths to ensure optimal adhesion and uniform plating. The immersion period of each batch of panels is constantly monitored by a computerized system, which ensures exact control over the process. Typically, a 1-mil-thick deposition of copper builds up inside the hole barrel, increasing the conductivity of the holes.
After copper plating, tin plating is performed. Tin is commonly used as an etch resist to protect important surface features, such as copper pads, hole pads, and hole walls, during the etching process. This additional plating step is highly beneficial in preserving the integrity and functionality of the surface features on the PCB.
Stripping of photoresist
After plating, the photo-resist on a panel is no longer desirable and must be removed to reveal the copper. Here, the resist covering the undesirable copper is dissolved and washed away in a single, continuous process line.
In this state, as you can see in the image above, resin partially covers unbroken woven cloth fibers on the surface. In all classes (class 1, 2, and 3 boards), weave exposure is permissible as long as the remaining distance between the conductors satisfies the minimal spacing requirement.
Etching of undesired copper
During this stage, the undesired exposed copper is eliminated through the utilization of an ammoniacal etchant. Meanwhile, the tin effectively secures the necessary copper. This process is essential for establishing the required conducting areas and connections
During this stage, the undesired exposed copper is eliminated through the utilization of an ammoniacal etchant. Meanwhile, the tin effectively secures the necessary copper. This process is essential for establishing the required conducting areas and connections
Stripping of the tin layer
After the etching process, the layer of tin that covers the copper tracks is removed. This is achieved by using concentrated nitric acid, which effectively removes the tin layer without causing any damage to the underlying copper.
Application of solder mask
The LPI (liquid photo imageable) mask is created by blending solvents and polymers together to form a thin coating that effectively adheres to various circuit board surfaces. The coated panel is imaged by a printer. The ink in the transparent areas is hardened by a UV lamp in the machine. Subsequently, all of the unhardened resist is carefully removed from the imaged panel, leaving behind a precise and well-defined pattern.
The LPI curing process involves a drying step that facilitates the integration of the solder mask with the dielectric material for successful bonding. Furthermore, a final baking step is carried out in an oven or by utilizing infrared heat sources.
Application of PCB surface finish
PCB surface finishes play a vital role in facilitating intermetallic connections that occur between the exposed copper in the solderable regions. The copper surface of the board, in its untreated state, may be vulnerable to oxidation. Therefore, it is highly recommended to apply a protective surface finish. This coating serves two purposes: it protects the copper from oxidation and prepares the surface for soldering during assembly.
Furthermore, it plays a significant role in prolonging the board's shelf life, ensuring its long-term functionality and reliability. There are several different types of surface finishes available. However, lead-free surface finishes are commonly used due to the stringent RoHS regulations.
When selecting a surface finish, it is important to consider cost, environmental impact, shelf life, and production volume. Lead-free HASL, immersion gold, electroless nickel, immersion silver, immersion tin, and hard gold are some of the available metallic surface finishes. OSP and carbon ink are organic surface finishes available.
Application of Silkscreen
The silkscreen process employs a direct legend printing method that uses inkjet projectors to accurately reproduce the legends directly from the board's digital data. The ink is carefully applied to the surface of the panel using a jet printer. The panels are carefully baked to facilitate the curing process of the ink. This technique is utilized to mark various types of text, including part numbers, names, codes, logos, and more, providing important identification and labeling on the PCB.
Furthermore, the silkscreen can be applied using a manual screen-printing method that involves making use of a stencil or screen through which ink is pressed to produce the required designs and text on the board.
Electrical testing of the PCB
The term "E-test" refers to the process of conducting electrical test on a bare board. In this stage, electrical probes are utilized to examine each unpopulated board for potential issues such as shorts, opens, resistances, capacitances, and other essential electrical properties. This test assesses the electrical conductivity of the circuit board by utilizing the netlist file. A netlist comprises the data pertaining to the conductivity interconnection patterns of a PCB. Bed-of-nails and flying probe tests are commonly utilized to assess the functionality of a product during the PCB manufacturing process.
Bed of nails
The bed of nails is a commonly used technique in electrical testing to assess bare boards. The process involves designing a test template with pins that are properly aligned with the test locations on the PCB. The process is efficient and well-suited for large-scale production systems.
Flying probe testing
The process of flying probe testing involves the utilization of probes that are programmed to move between designated points based on instructions provided by specialized software. This test method does not require the use of fixtures. Initially, the generation of flying probe test programs (FPT) takes place, followed by the subsequent loading of these programs into the FPT tester. The tester applies electrical signals and power to the probe points, subsequently measuring them in accordance with the test program.
V-scoring and profiling
In the final stage of PCB manufacturing process, circuit boards are carefully profiled and separated from the production panel. This can involve either a router or a v-groove. The v-groove technique creates diagonal channels on both sides of the board, while the router method may result in small tabs along the borders. In any case, it is possible for the boards to be easily removed from the panel.
Key Takeaways
- The PCB manufacturing process begins with creating design files, such as Gerbers and netlists, which are converted into a physical circuit board.
- DFM review is performed by the manufacturers to identify and resolve potential manufacturing
- Laser direct imaging (LDI) technology is used for imaging the inner and outer layers of a circuit board.
- Copper etching eliminates unwanted copper, and holes are drilled and deburred.
- Electroless copper deposition makes drilled holes electrically conductive.
- The outer layer undergoes imaging, and copper plating is applied to preserve surface features.
- Tin plating acts as an etch resist to protect critical surface features during the etching process.
- Solder mask is applied to protect regions where soldering is not required.
- Surface finish is applied to protect exposed copper and facilitate soldering.
- Electrical testing (E-test) is conducted to check the circuit board's functionality and electrical properties.
- V-scoring and profiling is done to separate individual boards from the production panel in the final stage.
PCB manufacturing process begins with converting design files into a physical circuit board. The steps include laser direct imaging, copper etching, hole drilling, electroless copper deposition, and outer layer imaging. After all these steps, the solder mask, surface finish, and silkscreen are applied to the required board surfaces. Finally, electrical testing is carried out to ensure the board’s functionality.
DESIGN GUIDE
Check out our free PCB Design Rules Handbook
Everything you to need to know about designing your PCB in one convenient place.