Although a manufacturer may seem to use magic in the creation of a PCB, with parts and materials going in one door and completed boards coming out another, the reality is quite the opposite. The PCB contract manufacturer has a very detailed process that they go through in order to produce the highest quality circuit boards for their customers. Come along with us as we pull back the curtain of the PCB development process in order to show you how this magic is actually created.
The Magic Unmasked
After months of product development, your circuit board design is finally ready to be sent out for manufacturing. You’ve gone through the design concept, schematic capture, and circuit simulation. The mechanical designers have created the system enclosure, and the PCB layout team has finalized the circuit board design. Everything is ready, and all that remains now is to get the boards manufactured and back into your hands to complete the product development. Have you ever wondered though what exactly happens to your PCB design once you release it for manufacturing?
Your company probably already has a good relationship with your PCB contract manufacturer, and you are familiar with the work that they’ve done for you before. You also know that they will partner together with you to perfect the design for manufacturing, and they have a solid reputation in the industry for quality and reliability. But you may still not understand what actually happens once your design enters their manufacturing cycle.
In this article, we will outline the different steps that your contract manufacturer will take in the PCB development process. Starting with the preparations for manufacturing, your design will first go through several engineering evaluations in order to verify that it is ready for production. The next step will be to order the raw boards from a PCB fabricator, procure the components for the board, and inspect all materials before they are assembled together. Finally, the board will go through the different PCB assembly processes including component insertion, soldering, test, rework, and final inspection.
So, hang on. The magician is rolling up his sleeves in preparation for showing you everything behind the curtain of PCB manufacturing.
What Kind of Prototypes Will Best Serve Your NPI Needs?
The first step of circuit board manufacturing is for the PCB contract manufacturer to review the fabrication and assembly data that you have given them. Not only are they verifying that they have all the data they need to proceed, but they are also looking for potential problems in the design that could delay or stop manufacturing. To do this, they will examine the following areas of the design:
Bill of Materials (BOM) Review
The bill of materials is the master list for all the parts used on the circuit board, and the PCB CM will first verify that it matches the parts used in the design. If the BOM hasn’t been updated with the latest design information, it may be out of date. From there the next step is to verify that the components are available for manufacturing for an affordable price. If not, the engineering team will recommend part substitutions or even changes to the design in order to use preferred components.
Design for Manufacturing (DFM) Review
This next review will look at how the components on the board are placed and routed in reference to manufacturing. Often, PCB designs submitted for assembly are not optimized for the manufacturing processes, and those problems can slow down or even stop the assembly of the board. A prime example of this are parts that are not placed according to the requirements of the solder processes that will be used during assembly, and therefore may not solder correctly. The engineering team at the PCB CM will report back to the customer any DFM problems discovered in their review, along with their recommendations on how best to resolve the problems.
Design for Test (DFT) Review
The PCB design will also be reviewed for testability. This is a crucial process at the end of the manufacturing process to verify that the assembly has been completed correctly. It requires that the board have probable testpoints built into it, as well as being accompanied by test fixtures and software as required. The engineering team at the PCB CM will look for potential areas of improvement to the layout in order to enhance its testability and improve test coverage. They may also build or alter test fixtures and/or write test software as needed.
PCB Usability Review
In addition to the reviews listed above, the PCB design will also be looked at by the CMs engineering team for common problems that may cause usability problems. For instance, silkscreen reference designators and markings that are either unclear or incorrect, or component placement without sufficient space to work with. In either case, these types of problems can make working with the board difficult for test, debug, or field technicians. As with the other reviews, any problems discovered will be forwarded to the customer along with recommendations on potential solutions.
These reviews are essential to the success of the manufacturing operation as well as the operation and expected use of the completed circuit board. If any problems are found, the engineering team of the PCB contract manufacturer will present the customer with a complete report detailing the problem along with their recommended solutions. Depending on the alterations that are needed, the engineering team themselves are often available to do the work in order to get the board ready for manufacturing.
Preparing for Circuit Board Assembly: Component and Materials Procurement
Ordering the Bare Boards from a PCB Fabricator
PCB contract manufacturers usually have several different circuit board fabricators that they have a history of working with. Some of these fabricators will specialize in only certain PCB technology types, while others may be devoted to only producing prototype builds. Having a vetted list of vendors like this gives the PCB CM the resources they need to cover any type of circuit board that a customer may specify.
Once the best match for the board type is determined, the chosen fabricator will run their own DFM review on the design before proceeding. If their review doesn’t turn up any questions, they will begin their fabrication process. If there are questions though, the fabricator and the CM will work together to resolve the problems and send their findings and recommendations back to the customer for approval. Fabrication times depend on the complexity of the job and the customer’s urgency and can be anywhere from a few days to a couple of weeks, with the average being about five to seven working days.
Component Sourcing and Purchasing
Next, the PCB CM will work on sourcing and purchasing the components and other materials needed for the build. If the BOM is clean and the review hasn’t revealed any problems, the CM will start first with parts that they already have in stock, and then purchase the rest. As with the raw board fabricators, the PCB CM will have a vast network of component manufacturers and distributors that they work with. This gives them the ability to find the best prices and delivery times for the parts that are needed.
If, on the other hand, the BOM does have problems with components that are either unavailable, expensive, or projected to become obsolete, the CM will respond according to one or more of the following scenarios:
- Components that are a direct replacement and are readily available from the CMs network of part manufacturers and distributors are easy to implement. The CM can proceed with the substitution following a simple confirmation from the customer.
- Component substitutions that are more restrictive due to their function or availability will require the CM to do further research beyond their regular vendor and distributor networks. There are a number of part brokers that can be used in these circumstances, but the trade-off is that these parts can be more expensive or have shorter life cycle projections. Before proceeding, the CM will discuss any concerns with the customer.
- The components are not available from either the CM’s regular sources or the broker market due to obsolescence. This will result in discussions with the customer about the possible re-design options that are available.
Incoming Inspection of PCB Assembly Parts and Materials
Once the raw fabricated boards, components, and materials purchased have been received, they will be inspected before they are used in the assembly. The circuit boards will be examined for quality and acceptability before they are assembled, as will the different materials used during manufacturing. Any sub-assemblies that are ordered will also be inspected to make sure that they were manufactured according to specifications.
When the purchased components undergo inspection, they will be examined for cracked cases, visible damage, bent or broken leads, or other problems including incorrect markings or parts that are out of date. This testing is done with optical equipment to enlarge the parts, and the results are carefully recorded for tracking. Although it isn’t possible to inspect each and every component that will be used, the CM will conduct sample testing of new parts that come in until that component vendor has been verified as a trusted supplier.
Once all of the ordered parts and materials have been received and inspected, they will be assigned a barcode for tracking and documentation. These parts and materials are then all pulled together into a “kit” for the manufacturing floor, and the circuit board project is ready for assembly.
Manufacturing the Circuit Board
Solder Paste Application
Unlike wave solder, where the component leads are run through a molten wave of solder, PCBs that will be reflowed need their solder applied before they are processed. To do this, metal solder powder is combined with solder flux to form a sticky substance called solder paste. Not only does the solder paste act as a temporary adhesive to hold the component leads onto their circuit board footprints, but the flux also cleans the metal in preparation for a good solder joint. Then when the circuit board is run through the solder reflow oven, the metal particles in the paste will melt and form the solder joint.
The solder paste is usually applied by screen printing it through a stencil, or by using a jet printer. Although the jet printing method is easy to set up and run, the solder stencil is usually preferred as it can process a large number of boards quickly. Solder paste can also be applied manually by a technician for rework or unique parts that have to be hand soldered. One of the most important parts of this process is to inspect the board after the solder paste is applied to ensure that each footprint has the correct amount of paste.
SMT Component Pick and Place
Once the solder paste is applied, the components will then need to be installed onto their designated footprints. For this operation, automated pick and place machines are used. The components are loaded into the machines from reels or trays, and then picked up by a vacuum nozzle for placement. Using precision optical alignment equipment, the nozzles move and rotate the components in order to land them on their footprints.
Automated pick and place equipment like this is less expensive to use and much faster than manually placing the parts. In addition, some parts, such as smaller resistor and capacitor packages, can not be reliably placed manually and require the precision of the pick and place machine.
Manual Component Insertion
One of the challenges of building a PCB prototype is that it also has to fit into the system enclosure prototype that often is built by a different manufacturer. A PCB CM that also has box build capabilities can manufacture both projects at the same time, ensuring that they will work together. This can save an enormous amount of time and expense from having to wait for multiple vendors to synchronize with each other, only to find out that one of the pieces didn’t match the other. Add to that the ability to build cable assemblies and wire harnesses, and you can get your entire system prototype built under one roof by the same PCB contract manufacturer.
Detailed Documentation Generation and Control Processes
In addition to the surface mount components that have been installed, most circuit boards will also have some thru-hole parts as well. Although surface mount parts are usually preferred because of their size, cost, performance, and ease of assembly, there are still some very critical needs that can only be supplied by thru-hole parts. These can include parts that are used in high power applications, and where part strength is required such as connectors. In both cases, the size and strength of the lead soldered directly into a hole gives that part the advantage.
While there are some automated insertion machines that can be used for thru-hole parts, many manufacturers will install these instead by hand. With the greater percentage of circuit board components being SMT, this saves the manufacturers the cost of operating and maintaining thru-hole insertion machines.
The Soldering Processes
There are three automated soldering processes used in the assembly of printed circuit boards:
With the surface mount components held in place on their circuit board footprints by solder paste, the circuit board is run through the reflow oven on a conveyor belt. These industrial ovens use multiple zones of heat to solder the board according to a predetermined thermal profile. The profile is programmed into the system allowing control of the soldering process by both temperature and duration in the different heat zones of the oven.
The first zone is the preheat stage, and this brings the board up to its first designated temperature. The second zone soaks the board at that temperature, activating the flux in the solder paste and giving it time to remove any metal oxides. The third zone heats the board up again until the solder reflows or melts, to form the solder joints. The final zone is a controlled cooling cycle to help the newly formed solder joints solidify as they should.
The thru-hole parts on a circuit board will be soldered using a wave soldering machine. This is a tried and true system of soldering that has been in use for a long time for the production of circuit boards. The boards are run through a molten wave of solder using a conveyor system, and the wave forces solder up into the holes and around the inserted pins to form a good solder joint. Even some SMT packages can be soldered this way if they are held in place by glue.
Wave soldering can be set up quickly and gives more consistent results than manually soldering can. The caveat, however, is that if the board has areas that cannot travel through the wave, then those areas must be prepared in advance. This usually requires masking off areas or creating a fixture to protect components or features on the bottom of the board from the wave.
This is an automated system that uses molten solder pumped through a nozzle to solder thru-hole leads. The system moves in every direction to position the leads over the nozzle according to how it has been programmed by the operator. While not as fast, the selective soldering system can be used in circumstances where wave soldering isn’t effective. This can include components or other board objects that are too tall and will block the wave, or components that are already soldered and are too tight for a protective fixture to fit around them.
Selective solder gives the PCB CM the ability to quickly solder hard-to-reach areas of the board that used to only be accessible by manual soldering techniques. Where manual soldering can yield different results from pin to pin, the selective soldering system will give the same consistent results each time.
Through each step of the assembly process, the circuit board has been inspected before proceeding onto the next step. After the board has gone through automated soldering, the board is inspected once again and any defects in its assembly will be corrected by PCBA rework technicians. In addition to assembly defects, the technicians can also replace parts that were incorrectly specified, or parts that were damaged while reworking other components.
Circuit Board Test
To verify that the assembly process is complete and that the circuit board is operating as intended, the board will go through automated testing. There are two main methods of conducting this test, In-Circuit Test (ICT) and Flying Probe, and they both rely on testpoints that are designated during PCB design. Using the test data generated from the design files, these tests will verify that each component pin is solidly connected to the board with a good solder joint:
- ICT: This is commonly referred to as a bed-of-nails test, and it involves building a text fixture with probes to match each testpoint on the board. ICT fixtures can test either the top, the bottom, or both sides of the board at once. The test data from the design files are used to program the test machine, which is connected to the text fixture, allowing all connections to be probed. This test can be used to determine the functional capability of the board as well. While ICT test fixtures are expensive to produce and maintain, their advantage is that they can thoroughly test a board in a very short period of time.
- Flying Probe: This is an automated system that uses between two and six probes which will fly around the top and bottom of the board contacting the PCB testpoints. This system is much simpler and less expensive to set up and run than developing an ICT fixture, which makes it ideal for prototypes or large boards that don’t fit on an ICT machine. The downside, however, is that it takes much longer to run as the flying probes have to contact each testpoint individually, and it does not run functional tests.
Before the circuit board is completed, it will go through one last level of inspection for quality assurance. These checks ensure that the board is ready to be shipped back to the customer, and all the proper markings and labels are affixed as expected. In the case of the first circuit board (i.e. the first board of a production run), it will also be inspected as a first article and documented as the standard for future builds of the same board.
With the completion of the circuit board assembly process, the board will be carefully packaged in static controlled containers and shipped out to the customer. Here is where using a local PCB contract manufacturer can really pay off, as there aren’t any international issues to contend with. Where shipping PCBAs from an off-shore manufacturer can cost extra money in fees and tariffs, local shipping can sometimes be as simple as a same-day delivery by truck.
VSE, the Best Choice for a PCB Manufacturing Partner
Now that we’ve revealed the magic of the PCB development process, it’s time to introduce those who do the hard work behind the curtain. At VSE, we have been building circuit boards for over 30 years now, and we have perfected the assembly process described above. We can take your project all the way from design input to completed circuit boards quickly and at the highest levels of quality in the industry. In addition to the regular production of circuit boards in our factory, we also excel at the following:
Experienced and Skilled Engineers Committed to Working Together with You
At VSE, you will find that our engineering teams have the skill and precision that your projects need, and our manufacturing teams will exceed your expectations as they expertly build your circuit boards. We have the capabilities that you need for success on your next PCB design project. Let us show you the benefits that you will realize by partnering with us.