PCB manufacturing process | 16 steps to make a PCB board

Step 1: PCB Design - Design and Output

PCB Design

The board design is the initial stage of the etching process, while the CAM engineer stage is the first step in the PCB fabrication of a new printed circuit board,

Designers analyze requirements and select appropriate components such as processors, power supplies, etc. Create a blueprint that meets all needs.

You can also use any software of your choice with some popular PCB design software such as Altium Designer, OrCAD, Autodesk EAGLE, KiCad EDA, Pads, etc.

However, always keep in mind that the board should be strictly compatible with the PCB layout created by the designer using PCB design software. If you are a designer, you should inform the contract manufacturer of the version of the PCB design software used to design the circuit, as it avoids problems caused by discrepancies before the PCB is manufactured.

Once the design is ready, print it on transfer paper. Make sure the design fits on the glossy side of the paper.

There are also many PCB terms in PCB manufacturing, PCB design, etc. After reading some PCB terms from the next page, you may have a better understanding of printed circuit boards!

PCB design output

Typically, the data arrives in a file format called Extended Gerber (Gerber is also known as RX274x), which is the most commonly used program, although other formats and databases are also available.

Different PCB design software may require different Gerber file generation steps, all of which encode important information for synthesis, including copper trace layers, drill patterns, component symbols, and other parameters.

After the design layout of the PCB is imported into the Gerber Extended software, all the different aspects of the design are checked to ensure that there are no errors.

After a thorough inspection, the completed PCB design will be sent to the PCB manufacturing plant for production. Upon arrival, the manufacturer conducts a second inspection of the design, called a Design for Manufacture (DFM) inspection, to ensure:

●PCB design can be manufactured

●PCB design meets minimum tolerance requirements during manufacturing

STEP 2: PCB File Drawing - Film Generation of PCB Design

Once the PCB design is finalized, the next step is to print it. This is usually done in a dark room with controlled temperature and humidity. Align different layers of PCB photo film by punching precise positioning holes in each film. This film was made to help create the graphics of the copper paths.

Tip: As a PCB designer, after outputting the PCB schematic file, please don't forget to remind the manufacturer to do DFM inspection

A special printer called a laser photoplotter is usually used in PCB printing, and although it is a laser printer, it is not a standard Laserjet printer.

But for miniaturization and technological advancement, this shooting process is no longer sufficient. it's outdated in some ways.

A number of well-known manufacturers are now reducing or eliminating film use by using special Laser Direct Imaging (LDI) equipment that images directly on the wafer. With incredible LDI precision printing technology, highly detailed films for printed circuit board designs can be delivered at a reduced cost.

A laser plotter takes the plate data and converts it into a pixel image, which is then written by a laser onto film, and the exposed film is automatically developed and unloaded to the operator.

The final product will be a plastic sheet with a PCB negative with black ink on it. For the inner layers of the PCB, the black ink represents the conductive copper portion of the PCB. The remaining clear parts of the image represent areas of non-conductive material. The outer layers follow the opposite pattern: the copper layer is removed, but the black refers to the areas that will be etched away. The plotter automatically processes the film and stores it securely to prevent unwanted contact.

Each layer of the PCB and solder mask has its own sheet of clear black film. A two-layer PCB requires a total of four sheets: two for the multilayer and two for the solder mask. It is important that all films correspond perfectly to each other. When used harmoniously, they draw a PCB alignment diagram.

For perfect alignment of all films, alignment holes should be punched in all films. The accuracy of the holes can be achieved by adjusting the table on which the film is placed. Holes are punched when a small calibration of the table results in the best match. These holes will receive dowel pins in the next step of the imaging process.

Step 3: Inner Layer Imaging Transfer - Print Inner Layer

This step only applies to boards with more than two layers. Simple two-layer boards can skip drilling. Multilayer boards require more steps.

The film creation in the previous step was designed to graph the copper path. Now it's time to print the graphics from the film onto the copper foil.

The first step is to clean the copper.

In PCB construction, cleanliness is critical. The copper-faced laminate is cleaned and sent to a decontaminated environment. Remember to make sure that no dust settles on the surface to avoid shorting or opening the finished PCB.

The cleaning panel receives a photosensitive film layer called photoresist. The printer uses a powerful UV lamp that hardens the photoresist through the transparent film, defining the copper pattern.

This ensures an exact match from photographic film to photoresist.

 The operator loads the first sheet of film onto the pins and then loads the coated panel onto the second sheet of film. Dowels on the base of the printer mate with holes in the photo tool and panel to ensure precise alignment of the top and bottom layers.

The film and cardboard line up and receive a beam of UV light. Light passes through the transparent portion of the film, hardening the photoresist on the copper below. The plotter's black ink prevents light from reaching areas that are not suitable for hardening, so remove them.

Under the black area, the resistor is still not hardened. Cleanrooms use yellow lighting because the photoresist is sensitive to UV light.

When the plate is ready, it is washed with an alkaline solution to remove any unhardened photoresist. A final pressure wash will remove anything else left on the surface. The board is then dried.

As the product emerges, the resistors properly cover the copper areas to remain in the final form. Technicians inspect the board to make sure no errors have occurred at this stage. All the resist present at this point represents the copper that will be exposed in the finished PCB.

Step 4: Copper Etch - Remove Unwanted Copper

In PCB manufacturing, etching is the process of removing unwanted copper (Cu) from a circuit board. The excess copper is nothing but non-circuit copper removed from the board. As a result, a desired circuit pattern is realized. During this process, the base or starting copper is removed from the board.

The unhardened photoresist is removed, the hardened resist protects the desired copper, and the board undergoes undesired copper removal. We use an acid etchant to rinse off the excess copper. At the same time, we want the remaining copper to still be completely covered by the photoresist layer.

Before the etching process, the image of the circuit required by the designer is transferred to the PCB through a process called lithography. This forms a blueprint for deciding which part of the copper must be removed.

PCB manufacturers typically use a wet etching process. In wet etching, unwanted materials dissolve when immersed in a chemical solution.

There are two methods of wet etching:

● Acid etching (ferric chloride and cupric chloride).

● Alkaline etching (ammonia)

Acidic methods are used to etch the inner layers of the PCB. The method involves chemical solvents such as ferric chloride (FeCl3) OR copper chloride (CuCl2).

Alkaline methods are used to etch the outer layers of the PCB. Here, the chemicals used are copper chloride (CuCl2 Castle, 2H2O) hydrochloride (HCl) hydrogen peroxide (H2O2) water (H2O). Alkaline is a quick process and a bit expensive.

Important parameters to consider during the etching process are the speed at which the panel is moved, the spray of chemicals, and the amount of copper to be etched away. The whole process is carried out in a conveying high-pressure atomizing chamber.

The process is carefully controlled to ensure that the finished conductor width is exactly as designed. Designers should note, however, that thicker copper foils require more space between traces. The operator double-checks that all unwanted copper has been etched away

Once the unwanted copper is removed, the board is subjected to a strip process to remove tin or tin/poor gold or photoresist from the board.

Now, the unwanted copper is removed with the help of a chemical solution. This solution will remove excess copper without damaging the hardened photoresist.

STEP 5: Layer Alignment - Laminate the layers together

Coupled with a thin layer of copper foil covering the outer surfaces of the top and bottom sides of the board, the layers are stacked in pairs to form a PCB "sandwich". To facilitate bonding between the layers, a piece of "prepreg" will be inserted between each pair of layers. Prepreg is a fiberglass material impregnated with epoxy resin that melts under the heat and pressure of the lamination process. As the prepreg cools, it will bond the layers together.

To produce multilayer PCBs, alternating layers of epoxy-impregnated fiberglass sheets (called prepregs) and conductive cores are laminated together using a hydraulic press at high temperature and pressure. Pressure and heat melt the prepreg and bind it together. After cooling, the resulting material will use the same manufacturing process as a double-sided PCB. Here are some more details of the lamination process for a 4-layer PCB as an example:

For a 0.062-layer PCB with a finished thickness of 4 inches, we usually start with a 4-inch thick copper clad FR0.040 core material. The core layer has been processed by inner layer imaging, but now requires a prepreg and outer copper layer. Prepregs are called "B-staging" fiberglass. It is not rigid until heat and pressure are applied. Therefore, it is allowed to flow and bond the copper layers together as it cures. Copper is a very thin foil, usually 0.5 oz. (0.0007") or 1 oz (0.0014") thick, i.e. added to the outside of the prepreg. The stack is then placed between two thick steel plates and placed in a lamination press (press cycles vary by various factors, including material type and thickness). For example, 170Tg FR4 material commonly used in many parts is pressed at 375°F for 300 minutes at a pressure of 150 PSI. After cooling, the material is ready to move on to the next process.

Composing the planks At this stage, a lot of attention to detail is needed to keep the circuit aligned correctly on the different layers. After the stacking is complete, the interlayers are laminated, and the heat and pressure of the lamination process fuse the layers together into a single circuit board.

Step 6: Drilling holes - for connecting components

Through holes, mounting holes, and other holes are drilled in the PCB (usually in a panel stack, depending on how deep the holes are drilled). Accurate and clean hole walls are critical, and sophisticated optics provide this.

In order to find the location of the drilling target, the X-ray locator can identify the appropriate drilling target point. Then, punch holes in the appropriate pilot holes to secure the stack for a series of more specific holes.

Before drilling, the technician places a plate of buffer material under the drilling target to ensure a clean drill hole. The exit material prevents any unwanted tearing of the drill exit.

A computer controls every tiny movement of the drill bit - products that determine machine performance naturally depend on computers. This computer-driven machine uses the drill hole files from the original design to determine the appropriate places to drill.

The drill uses a pneumatic spindle at 150,000 rpm. At this rate, you might think drilling is instantaneous, but there are a lot of holes to drill. An ordinary PCB contains more than one hundred complete points. During drilling, each bit needs its own special moment, so it takes time. Afterwards, these holes will accommodate the PCB's vias and mechanical mounting holes. The final fixation of these parts takes place after electroplating.

After drilling, it is cleaned using chemical and mechanical processes to remove resin smudges and debris from drilling. Then, a thin layer of copper is chemically covered over the entire exposed surface of the board, including the inside of the holes. This will create a metal base for plating additional copper into the holes and onto the surface in the next step.

After drilling is complete, the extra copper lining the edges of the production panels is removed by a profiling tool.

Step 7: Automated Optical Inspection (Multilayer PCB Only)

After lamination, it is impossible to find errors in the inner layers. Therefore, the panels are subjected to automated optical inspection before bonding and lamination. The machine scans the layer using a laser sensor and compares it to the original Gerber file to list the differences, if any.

Once all the layers are clean and ready, they need to be checked for alignment. Both the inner and outer layers will line up with the help of the holes drilled earlier. An optical punch drills a pin through the hole to keep the layers aligned. After this, the inspection process begins to ensure that there are no defects.

Automated Optical Inspection (AOI) can be used to inspect multiple layers of a multilayer PCB before laminating it together. The optics examine these layers by comparing the actual image on the panel with the PCB design data. Any difference in excess or missing copper can result in a short or open circuit. Once the inner layers are laminated together, the manufacturer can spot any defects that might prevent the problem. As you might imagine, shorts or opens found at this stage are easier to correct than after the layers have been laminated together. In fact, if an open or short is not found at this stage, it may not be discovered until it is too late during electrical testing at the end of the manufacturing process.

The most common events that occur during layer image processing are those that lead to brief or open-ended related questions:

● Improper exposure of the image, resulting in an increase/decrease in the size of the functional parts.

● Poor dry film will not adhere and may create scratches, cuts or pinholes in the etched pattern.

● Copper is under etched, leaving excess copper or causing feature size increases or shorts.

● Copper is overetched, removing necessary copper features, thereby reducing feature size or reducing kerfs.

Ultimately, AOI is an important part of the manufacturing process, helping to ensure PCB accuracy, quality, and on-time delivery.

STEP 8: Oxide (Multilayer PCB only)

Oxide (referred to as black oxide or brown oxide depending on the process) is a chemical treatment of the inner layers of a multilayer PCB prior to lamination to increase the roughness of the clad copper to improve the bond strength of the laminate. Once the manufacturing process is complete, the process helps prevent delamination or separation between any layers of the substrate or between the laminate and the conductive foil.

STEP 9: Outer layer etching and final stripping

photoresist stripping

Once the panel is plated, the photoresist becomes undesirable and needs to be stripped from the panel. This is done in a lateral process containing a soda ash solution that effectively removes the photoresist, leaving the panel's base copper exposed to the underlying etch process for removal.

final etch

At this stage, tin protects the ideal copper. Remove unwanted exposed copper and copper under the rest of the resist layer. In this etch we use an ammonia etchant to etch away the unwanted copper. At the same time, tin ensures the required copper at this stage.

At this stage, conductive areas and connections are legally identified.

tin stripping

During the post-etch process, the copper on the PCB is covered by the resist, ie, tin, which is no longer needed. So, let's peel it off before we move on. You can use concentrated nitric acid to remove tin. Nitric acid is very effective at removing tin and will not damage copper circuit traces under the tin metal. So now you have a clear copper outline on the PCB.

After the plating on the panel is complete, the dry film will resist the residual residue that needs to be removed from the copper underneath. The panel will now go through the Strip Etch Tape (SES) process. Strip the resist off the panel, then etch away the copper that was exposed but not covered by the tin so that only the pads around the traces and holes and other copper patterns remain. Remove the dry film from the tinplate, then etch away the exposed copper (not protected by the tin), leaving the desired circuit pattern. At this point, the basic circuit of the circuit board is completed.

STEP 10: Solder mask

To protect the board during assembly, a UV exposure process similar to photoresist is used to apply the solder mask. This solder mask will cover the entire surface of the board except for the metal pads and functional parts to be soldered. In addition to the solder mask, component reference marks and other board markings are silkscreened on the board. Both solder mask and screen printing ink can be cured by baking the board in an oven.

The board will also have a finish on its exposed metal surface. This helps protect exposed metal and aids in soldering operations during assembly. An example of a surface finish is Hot Air Solder Leveling (HASL). Flux the board first to prepare it for solder, then dip it into a molten solder bath. After removing the board from the solder bath, high pressure hot air removes excess solder from the holes and smooths the solder on the surface metal.

Solder mask application

Solder resist is applied on both sides of the board, but before that, the panel is covered with epoxy solder resist ink. The UV light received by the board will pass through the solder mask. The covered portion remains unhardened and will be removed.

Finally, place the board in the oven to cure the solder mask.

Green was chosen as the standard solder mask color as it does not strain the eyes. Before a machine can inspect a PCB during manufacturing and assembly, all operations are hand-checked. The overhead lights that technicians use to inspect circuit boards do not reflect on the green solder mask, so are best for their eyes.

Step 11: Terminology (Silkscreen)

Screen printing or profiling is the process of printing all key information on a PCB such as manufacturer ID, company name, component number, debug points. This is useful when servicing and repairing.

This is a crucial step, as key information is printed on the board during the process. Once complete, the boards will go through the final coating and curing stages. Screen printing is the printing of readable identification data such as part numbers, 1-pin locators and other markings. These can be printed with inkjet printers.

It is also the most advanced PCB manufacturing process. Nearly finished boards are printed with easy-to-read letters that are often used to identify components, test points, PCB and PCBA part numbers, warning symbols, company logos, date codes, and manufacturer markings.

The PCB finally goes to the final coating and curing stage.

Gold or silver finish

The PCB is plated with gold or silver to increase the solderability of the board, which will increase the bonding force of the flux.

The application of each surface finish may vary slightly during this process, but involves dipping the panel in a chemical bath to coat any exposed copper with the desired finish.

The final chemical process used to manufacture PCBs is to apply a surface finish. Although the solder mask covers most of the circuit, its finish is designed to prevent oxidation of the remaining exposed copper. This is important because oxidized copper cannot be soldered. There are many different finishes that can be applied to circuit boards. The most common is the Hot Air Soldering Level (HASL), which is available in lead and lead-free. However, depending on the specification, application or assembly process of the PCB, suitable surface finishes can include Electroless Nickel Immersion Gold (ENIG), Soft Gold, Hard Gold, Immersion Silver, Immersion Tin, Organic Solderability Preservative (OSP), etc.

The PCB is then plated with gold, silver or lead free HASL or hot air solder leveler. This is done to be able to solder components to the pads formed and to protect the copper.

STEP 12: Electrical Test - Flying Probe Test

As a final precaution for inspection, the technician will perform a functional test of the board. At this point, they use an automated procedure to confirm the functionality of the PCB and its consistency with the original design.

Typically, an advanced version of electrical testing called flying probe testing will use moving probes to test the electrical performance of each net on an exposed circuit board.

The board will be tested to a netlist, which can be either provided by the customer with their data file or created by the PCB manufacturer from the customer data file. The tester uses multiple moving arms or probes to touch the spots on the copper circuit and send electrical signals between them.

Any shorts or shorts will be identified, allowing the operator to make repairs or treat the PCB as defective scrap. Depending on the complexity of the design and the number of test points, electrical testing can take anywhere from seconds to hours to complete.

Also, depending on various factors such as design complexity, number of layers and component risk factors, some customers choose to forego electrical testing to save some time and cost. This may not be a problem for a simple double-sided PCB, as there won't be many problems on a double-sided PCB, but regardless of the complexity, we always recommend electrical testing of multi-layer designs. (Tip: In addition to providing design files and manufacturing instructions, providing manufacturers with a "netlist" is one way to prevent unexpected errors.)

STEP 13: Production - Analysis and V Score

After the PCB panel has been electrically tested, the individual boards can be separated from the panel. This process is performed by a CNC machine or router to route each board from the panel to the desired shape and size required. Router bits typically used are 0.030-0.093 in size, and to speed up the process, multiple panels can be stacked two or three high, depending on the overall thickness of each panel. In the process, CNC machines are also able to create grooves, chamfers and bevels using a variety of different milling head sizes.

The routing process is a milling process in which a milling head is used to cut the contour of the desired board profile. Panels are "pinned and stacked," as was done earlier in the "drill-through" process. The usual stack is 1 to 4 panels.

To profile the PCB and cut it out of the production panel, we need to cut, i.e. cut out different boards from the original panel. The method either centers on using a notching machine or V-groove, or leaves small pieces along the edge of the board, and the V-groove cuts diagonal channels along the sides of the board. Either way the board can be easily ejected from the panel.

Instead of routing a single small board, the PCB can be routed as an array containing multiple boards with labels or score lines. This makes it easier to assemble multiple boards at the same time, while allowing the assembler to separate the boards after assembly is complete.

Finally, the board will be inspected for cleanliness, sharp edges, burrs, etc., and cleaned as needed.

STEP 14: Microsectioning - extra step

Micro-slicing (also known as cross-sectioning) is an optional step in the PCB manufacturing process, but it is a valuable tool for verifying the internal structure of a PCB for verification and failure analysis. To make a sample for microscopic inspection of the material, a cross-section of the PCB is cut and placed in a soft acrylic that hardens around it in the shape of a puck. Sections were then polished and viewed under a microscope. Detailed inspection can be done by examining many details such as plating thickness, drill bit quality and the quality of internal interconnects.

STEP 15: Final Inspection - PCB Quality Control

In the final step of the process, the inspector should perform a final close inspection of each PCB. Visually inspect the PCB for compliance with the acceptance criteria. Using Manual Visual Inspection and AVI – Compares PCBs to Gerber and is faster than human eye inspection, but still requires human verification. All orders are also fully inspected, including dimensions, solderability, etc. to ensure that the products meet our customer's standards, and a 100% quality audit of a batch of goods on board is carried out before being packaged for shipping.

Inspectors will then evaluate the PCBs to ensure they meet the customer's requirements and the standards outlined in the industry guidance document:

● IPC-A-600 – Acceptability of Printed Boards, which defines industry-wide quality standards for PCB acceptance.

● IPC-6012 – Qualification and Performance Specification for Rigid Panels, identifies the types of rigid panels and describes the requirements to be met during manufacture for three performance levels of panels (Types 1, 2, and 3).

Class 1 PCBs have a limited lifespan and are required to function only in end-use products such as garage door openers.

Class 2 PCBs will be critical, but not critical (like PC motherboards) if you want sustained performance, extended life, and uninterrupted service.

Class 3 PCBs include end uses where sustained high performance or on-demand performance is critical, failures cannot be tolerated, and the product must function when needed (eg, flight control or defense systems).

Step 16: Packaging - Meet Your Needs

Wrap the cardboard in a material that meets standard packaging requirements, then ship using the required shipping method prior to boxing.

As you might guess, the higher the grade, the more expensive the PCB. Often, the difference between categories is achieved by requiring tighter tolerances and controls, making the product more reliable.

Regardless of the assigned category, use a pin gauge to check the size of the holes, visually inspect the overall appearance of the solder mask and legend, inspect the solder mask to see if there is any penetration on the pads, and finish the inspection for the quality and coverage of the surface .

IPC inspection guidelines and their relationship to PCB design are very important for PCB designers to be familiar with, as is the ordering and manufacturing process.

Not all PCBs are created equal, and understanding these guidelines will help ensure that the product produced meets your aesthetic and performance expectations.

If you need help with PCB design or on PCB manufacturing steps, please don't hesitate to share with Tianweisheng, we are always listening!

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