Forming the backbone of almost all electronic devices, from simple to the most complex, Printed Circuit Boards (PCBs) is an electro-mechanical structure for holding the electronic components together, while providing them electrical connections through a unique network of copper traces.
Each PCB requires a carefully thought-out process of design to create it. The PCB designer typically uses a CAD software package to create a schematic of the circuit describing its electrical functioning, a Bill of Materials (BOM), an optimized layout of the components, and copper trace designs of the various layers. On satisfactory completion against DRC or Design Rule Checks, the designer exports the design into a format their contract manufacturer can understand and support. In most cases, this program is the extended Gerber.
At this stage, the designer and the PCB manufacturer conduct multiple checks on the design files of the PCB including Design for Manufacturing (DFM) and Design for Assembly (DFA). This ensures weeding out most of the potential bottlenecks that could hold up the PCB manufacturing process and the design of the PCB may undergo one or two iterations to satisfy the checks. On satisfactory completion of the checking process, the PCB design files are ready for the manufacturing process involving the following steps:
- Film Printing
- Alignment and Registration
- Photo-Lithography / Laser Direct Imaging
- Automated Optical Inspection
- Chemical Etching
- Layer Bonding
- Outermost Layers
- Solder Mask Application
- Surface Finish
- Legend Printing
- Electrical Testing
The manufacturer uses special plotters to create films with extreme precision from the Gerber files. The plotter creates highly detailed films of the PCB design for each intermediate layer, the two extreme outer layers, the solder masks, and the silk screens.
Fig 1: Film for a PCB Layer
The films are all on transparent plastic with black ink representing a photo of the specific layer. Films for the two outer layers have the clear parts referring to areas without copper and the black parts referring to the copper traces. It is reverse for the inner layers, where the clear parts refer to the copper traces, and the black parts refer to areas without copper. It is very important to have all films match and align together.
Alignment and Registration
For perfect alignment, the manufacturer lines up each film carefully to an optical match and punches registration holes through them. In all subsequent processes, the films will fit onto registration pins for automatic alignment.
Pinless Automatic Registration: As the number of layers in a PCB goes up, using pins for aligning the films adds tolerances causing accuracy degradation in alignment and registration. Therefore, manufacturers prefer using Pinless Automatic Registration, where cameras attached to vision systems and computers align the films automatically without requiring holes and pins. An added advantage is the operator can program the computer to check the proper sequence of the films as well, preventing expensive mistakes.
The manufacturer fabricates a multi-layer board from the inside out. The basic PCB comprises a laminate board. The core or substrate material is glass fiber and epoxy resin with a layer of copper bonded on both sides. This structure forms a sturdy starting point for the PCB. On this, the manufacturer bonds additional layers in pairs, as necessary, to build the stack.
The manufacturer must clean and decontaminate the copper bonded laminate in a clean room, ensuring there are no dust particles on it. A layer of photo resist or photo-sensitive film then goes on to both sides of the clean panel.
The manufacturer then places the correct film on the photo resist, taking care to register it properly by fitting it onto the registration pins on the laminate panel. Exposing the assembly to UV light hardens the photo resist under the clear areas of the film through which the UV light can pass. The black portion of the film blocks the passage of UV light and these areas do not harden.
Washing with an alkaline solution removes the unhardened photo resist, and after drying, the resist properly covers the copper areas meant to remain in the finished PCB.
Laser Direct Imaging (LDI)
Rather than printing films from the Gerber files and using them as above, manufacturers often use a more accurate method known as Laser Direct Imaging, or LDI. Here, they use data from the Gerber files in a CNC laser printer to control a highly focused laser bean, which then digitally creates the circuit pattern directly onto the board covered with photo resist.
The laser beam hardens the photo resist it falls on, while the rest remains unhardened. An alkaline solution removes the unhardened photo resist, as in the Photo-Lithography process.
LDI offers several advantages. As a computer controls the entire process, it eliminates alignment and registration issues, and imperfections related to environment variations. The computer’s enhanced optical alignment also compensates for distortions on the materials automatically, offering higher precision and resolution when printing the image.
Whereas introducing a change in the PCB layout would require printing a new set of films in the photolithography method, involving time and money, with LDI, the changes are only a matter of updating the new data.
Automated Optical Inspection: At this stage an Automated Optical Inspection (AOI) of the board is necessary to ensure there are no errors. The manufacturer uses computers fitted with video cameras for checking the boards. The computer compares the captured image of the board with the stored image of a good board, highlighting the differences. The detection is fast, and operator independent.
If the board is a simple two-layer one, it can proceed for drilling. Multi-layer boards require additional steps before drilling.
The PCB manufacturer now removes the excess copper on the board. Excess copper is the exposed copper on the board, not hidden under the hardened resist cover. The use of a copper solvent solution bath for removing the exposed copper is necessary, while the hardened layer of photo resist protects the wanted copper underneath it.
Fig 2: Copper Exposed After Etching
After removing the unwanted copper completely, washing in another solvent removes the hardened layer of photo resist, leaving only the wanted copper on the substrate. Completing this process requires a stage of AOI.
Manufacturers now prepare additional layers for a multi-layered PCB. These are usually sheets of woven glass cloth, pre-impregnated with epoxy resin, commonly known as prepreg. The manufacturer then adds sheets of copper foil, aligning them using tooling holes in each sheet, as they stack up the layers.
As an example, a four-layer board will have a sheet of copper foil at the bottom, on which there will be a specified number of prepreg sheets, followed by the inner core. Another specific number of prepreg sheets follow on top of the inner core, ending with a final top sheet of copper foil.
Fig 3: Four-Layer PCB Stackup
Manufacturers build the stack of panels on a heavy metal plate, adding another top plate to create a book. The book then moves to a heated hydraulic press, where pressure, heat, and vacuum over a twohour period forces the resin from the prepreg to become more elastic and flow across the core and foil surfaces. On cooling, the resin bonds with the glass sheets, the core, and the copper foils to form the multi-layer PCB panel.
The manufacturer now uses a CNC drilling machine and the drill information from the Gerber files to drill holes in the panel. These holes are necessary for mounting through-hole components and creating through vias. After drilling is over, the panel undergoes cleaning, scrubbing, and decontamination. Cleanliness is very important in PCB manufacturing, as one speck of dirt may create a short circuit in a dense board.
Fig 4: Different Types of Via
A multi-layer PCB may have buried vias that go only partially through the stack. For instance, a fourlayer board may have vias connecting layer two to layer three. For such buried vias, it is necessary to drill the respective layers and plate them before laminating all the layers together. Therefore, a PCB with blind and buried vias may require multiple steps of drilling, plating, and lamination processes.
In plating, a chemical deposition process adds a one-micron thick layer of copper over the entire surface of the panel, including the walls of the drilled holes. This process interconnects the copper of the inner layers through the holes, thereby creating vias.
The manufacturer now applies photo-resist to the outermost layers of the panel inside a clean room. On this, they place the films for the outermost layers, taking care to align them properly. The entire panel and film combination then undergoes exposure to UV light.
As earlier, the UV light passing through the clear parts of the film hardens the photo-resist. A chemical process then removes the unhardened resist, which the opaque part of the film had protected. An operator inspects the board to ensure a complete removal of all the undesired photo-resist.
The panel then returns to the electro-plating process, which deposits another thin layer of copper on the panel. However, this time, only the exposed sections of the copper surface receive the additional plating of copper. The manufacturer follows this up with a plating of tin on the plated copper. The tin plating guards the copper meant to remain on the panel during the next etching process. After removal of the photo-resist layer, the etching process removes the unwanted copper now exposed. The board is now ready for solder mask application, surface finish, and silk screening.
Solder Mask Application
Solder mask is the green ink covering the entire board, except the pads meant for soldering. Its function is to protect the scopper traces from tarnishing, and to act as an insulating coat. The manufacturer may use one of four methods to apply the solder mask to the board:
Epoxy Liquid: This is the cheapest method, and requires the creation of a woven screen mesh that supports ink-blocking patterns based on Gerber information. The manufacturer aligns the screen with the board, and draws the ink over the mesh with a squeegee. The epoxy liquid passes through the open spaces in the mesh and deposits on the board. The method is slow, not very accurate, and is not effective for high-density boards.
Liquid Photo Imageable Solder Mask (LPSM): The manufacturer applies a layer of liquid epoxy solder mask ink to both sides of the panel. On this they place the solder mask photo film, align it properly, and expose the combination to UV light, which hardens the exposed film. After removing the unhardened part, only those copper parts that will undergo soldering remain exposed and ready for surface finish.
Dry Film Photo Imageable Solder Mask (DPSM): Rather than apply a liquid onto the board, manufacturers can also vacuum-laminate a dry photo-imageable film onto the PCB to avoid trapping of air bubbles. The rest of the process remains the same as for the LPSM process. The dry film solder mask offers a uniform thickness across the surface, but works the best only for exceptionally flat board surfaces.
Inkjet Printing: Using an inkjet printer for applying solder mask on the PCB is a relatively new method and a viable alternative to the older methods. This method does not require printing any films, as the printer utilizes the information in Gerber files to directly print the ink on the board only in the areas where the cover is necessary. Inkjet printing offers several advantages over other conventional methods, including automatic registration, selective thickness, improved resolution and accuracy, and capability of fine pitch printing of reliefs, solder dams, and BGAs.
Irrespective of the process manufacturers use for solder mask printing, the ink requires a curing process at high temperatures for good adhesion to the board.
Surface finish adds extra solderability to the exposed copper parts of the PCB while preventing them from tarnishing due to exposure to the elements. Depending on the application, this can be simply hot air leveling, or a chemical plating of gold, silver, tin, or other combinations.
Manufacturers use a layer of ink traces on the PCB to identify components, their polarity, test points, warning symbols, logos, and marks. They use a non-conductive epoxy ink, with colors varying from black, to white, and yellow. In the traditional method of application, the manufacturer uses a polyester film stretched across a frame to act as a printing stencil, hence the name silk screening.
Manual Silk Screen Printing: Just as in printing the solder mask, the manufacturer pushes the ink through the stencil onto the PCB. The ink on the PCB requires curing in a baking oven to become sstable. Although the manual printing method is simple, it lacks precision.
Liquid Photo Imaging (LPI): This method is also similar to the LPSM method manufacturers use for solder mask printing, LPI requires a film through which UV rays fall on selective areas and hardens them. The manufacturer must wash off the unwanted and unhardened ink. They use this method only for line widths greater than 4 mil.
Direct Legend Printing (DLP): Ink-jet writing on the surface of the completed board offers the best resolution. The manufacturer uses data from the Gerber files directly without the need for printing a film. Although DLP is an expensive method, it saves the cost for printing a film. It also saves time, as UV light can cure the acrylic ink as the printing progresses, and does not require extra curing time.
The PCB now receives its last preservative coating, and after a curing stage, enters the testing stage.
Depending on the complexity of the board, this A technician conducts electrical tests on the PCB to ensure there are no shorts or unwanted openings in the copper traces. may be a manual test, or an automated test with flying probes. After testing is over and successful, the board may have to undergo scoring so that the assembler can remove individual PCBs easily from the panel after assembly.
After successful testing and scoring, the PCBs move towards either the assembly line or packing for dispatch to the customer.