Tutorial: A step-by-step guide to the PCB assembly process

Author : Philip King | Managing Director | Newbury Electronics

01 September 2021

Newbury Electronics_Step-by-step Guide to PCB Assembly Process_AOI inspection
Newbury Electronics_Step-by-step Guide to PCB Assembly Process_AOI inspection

Printed circuit boards (PCBs) are the foundational building blocks of most modern electronic designs, mechanically supporting & electrically connecting the electronic components. While PCB refers to the blank board, a PCB assembly, or PCBA, is the completed board after all the components & parts have been soldered & installed – and it now can accomplish the electronic function it was designed for.

This tutorial was originally featured in EPDT's H2 2021 Electronics Outsourcing supplement, included in the September 2021 issue of EPDT magazine [read the digital issue]. And sign up to receive your own copy each month.

PCBA can also refer to the process of assembling the board with the necessary components. In this tutorial, Philip King, Managing Director at PCB manufacturing & assembly specialist, Newbury Electronics outlines the SMT (surface-mount technology), pick-and-place and inspection techniques utilised during the electronics assembly process…

Step 1: Request for quote

The PCBA process begins with a quote, estimating the cost of parts and assembly according to the customer’s request. The team check the order requirements against our quotation carefully to ensure that everything that is quoted is on the order and that everything ordered is included in the quote.

Step 2: Buying & bill of materials (BOM)

Uploading a bill of materials (BOM) to the production control system is the first step in the purchasing process for the kit of parts. Again, engineers check that that all the necessary data is included, and then prepare documentation for production. In addition to allowing for extra quantities for spares as necessary, the packaging of components is also considered at this stage; most need to be on reels for machine assembly, while some parts may only be available loose and will have to be hand-placed.

Step 3: ESD (electro static discharge) precautions

During the electronics assembly process, components may be susceptible to damage through electrostatic discharge (ESD). To avoid this risk, Newbury Electronics take extensive ESD precautions. The floor of the production area is conductive and earthed, and all staff are required to wear conductive wrist straps and footwear. They also regularly check that their ESD protection is working satisfactorily on a test set.

Newbury Electronics_Step-by-step Guide to PCB Assembly Process_Kitting
Newbury Electronics_Step-by-step Guide to PCB Assembly Process_Kitting

Step 4: Electronic component kitting

Kitting is at the heart of the electronics assembly process, and involves producing production-ready kits, ensuring all parts are delivered to the right production personnel.

This process also checks that quantities are correct, to avoid shortages later down the line. Every part that arrives is checked against the Bill of Materials (BOM), and often parts will need to be examined within their protective packaging, making note of any moisture sensitive parts. Any that are identified as problematic are repacked in vacuum resealing, moisture resistant bags to exclude air and reseal them against the ingress of humidity. Once kitting is satisfactorily completed, all components are barcoded and labelled.

Step 5: Dry cabinet storage

Many electrical components are susceptible to the ingress of moisture, so to prevent this, many are stored in a humidity-controlled dry cabinet. This keeps the components at a relative humidity of around 3%, which is sufficient to keep them dry enough to solder later in the PCBA process.

Step 6: CAD/CAM programming

Nearly all component assemblies require the preparation of the SMD (surface mount device) placement programs which will guide the operator as to which components go on the feeder and in which position. CAD/CAM programming takes the client data and converts it into computer programs that are readable by the SMD and AOI (automated optical inspection) machines.  At this point, components within the kit are individually and physically checked for correct fitment on the PCB. Especial care is taken to ensure PCB surface mount pads match component footprints.

Newbury Electronics_Step-by-step Guide to PCB Assembly Process_Laser cut solder paste stencils
Newbury Electronics_Step-by-step Guide to PCB Assembly Process_Laser cut solder paste stencils

Step 7: Laser cut solder paste stencils

Laser cut stencils are used to aid the application of solder paste to surface mount component pads during the PCBA process. They allow assemblers to apply solder paste only where components will sit in the finished circuit board. Newbury Electronics is the only UK CEM (contract electronic manufacturer) to in-house manufacture its own solder-paste stencils. This enables them to design and cut laser stencils very quickly, without having to wait on a third party, which also saves customers time and money.

Step 8: Solder paste printing

Before the surface mount parts are placed by the SMD machines, the solder paste is screen printed onto the surface of the PCB pads. It’s a crucial step in PCB production, as the quality of solder paste can affect the performance and reliability of the surface mount assembly. Solder paste has the consistency of butter and is just thick enough so that when the surface mount parts are placed on the surface of the pad they will stay in position.

Step 9: SMD pick & place

After the solder paste has been applied to a board, the PCBA process moves onto the pick-and-place SMD assembly machines. Newbury Electronics has ten such machines, all being either Yamaha iPulse M20 or S20 Pick-and-Place machines. They are programmed to pick the parts from reels and place them onto the surface of the PCB in pre-programmed locations. These robots can operate at a rate of 20,000 parts an hour. The board is then moved onto a conveyor belt to process through a solder reflow machine, a large oven that fuses the solder paste into a permanent solder joint to connect the SMDs to the PCB.  All feeders, programs and feeder trolleys are completely compatible and interchangeable among all the pick & place machines.

Step 10: First article inspection

While the SMD assembly machines are extremely productive, it’s essential that the first-off boards out are tested for errors or misalignments. Alongside manual checks, the PCBs are inspected on small AOI (automated optical inspection) machines that check the correct part is placed in the right orientation at the right coordinate by each pick & place machine.

Newbury Electronics_Step-by-step Guide to PCB Assembly Process_SMD Pick & Place line
Newbury Electronics_Step-by-step Guide to PCB Assembly Process_SMD Pick & Place line

Step 11: Surface-mount technology (SMT)

Larger batch sizes, which may be quantities of 25 or 50 pieces upwards (and certainly if manufacturing 1,000 or more), will be run on our high capacity SMD assembly lines for quality and efficiency. The first SMD station in the line can place 20,000 parts an hour. The lines are reconfigured with one or two pick & place machines as required, allowing up to 40,000 placements per hour, with up to 288 different components.

Step 12: Automated optical inspection

3D AOI (automated optical inspection) is a quality control method essential for large batches of PCBAs. AOI scans the circuits under inspection, being automatically loaded and unloaded. All inspection programming and inspection itself is performed on up to five offline workstations. The AOI machine takes an optical image of the circuit board under examination using high-powered cameras, placed in different angles. In addition, it makes a 3D “Z” axis height map across the entire assembled circuit. It compares the image of the components with those in its memory, and it can then detect any misplacement of parts or missing components. The 3D data allows the volume of solder to be precisely measured on every visible solder pad. Any lifted SMD legs, or tilted or misplaced components are immediately recognised and reported. This is done at high speed, ensuring that a high quantity of PCBs can be processed quickly and efficiently.

Step 13: Hand placement of SMD parts

Most components these days are placed by service mount assembly machines, but there are always some hand-placed components during PCBA. Hand-placed components could be parts that have to be inserted down holes and the wires cropped off, or components that have arrived loose in a bag.

Step 14: Wave soldering

Wave soldering is a bulk process used to solder printed circuit boards quickly during PCB assembly. Using a wave soldering machine, PCBs are treated with flux, preheated and immersed in liquid solder. Circuit boards pass over a wave solder, so that all the joints on the underside of the board are soldered at the same time.

Newbury Electronics_Step-by-step Guide to PCB Assembly Process_Laser Stencil Cutting
Newbury Electronics_Step-by-step Guide to PCB Assembly Process_Laser Stencil Cutting

If circuits are mixed technology, in other words, part SMD and part leaded components, as is most common, then leaded parts will be selectively soldered after SMD placement. Selective soldering machines are automated robotic solder machines, which solder each component leg individually. The solder point is shrouded in nitrogen to ensure the integrity of the solder joint and to extend the life of the solder nozzle. Selective soldering gives superb quality of individual joints, as well as consistency and a speed many times that of manual soldering.

Step 15: X-ray inspection

Many components have their solder joints under the body of the part, for instance ball grid array (BGA) or QFN (quad-flat no-leads) devices. If that’s the case, then AOI or visual inspection cannot check the quality of the joint. Therefore, an X-ray inspection machine is required for quality control.

Step 16: Flying probe test

The flying probe test machine is a sophisticated test instrument which measures the value of components in circuit. It can test a circuit board at the rate of about 5 tests a second, and the machine will measure resistance, capacitance and inductance, and various voltages and frequencies. This way, the value of every part on the circuit board can be checked, if required.

Step 17: Functional test (FCT)

At the end of the PCBA process, a range of final tests are performed. A functional test is carried out, where we power up an electronic assembly and test whether it functions in accordance with the customer specification. Some parts on a circuit board can’t be inspected by earlier AOI or X-ray inspections, and so a final inspection by eye is also required to ensure the quality of the final product.

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