Understanding the various phases of the PCB assembly process can help both the circuit designer and the contract manufacturer to build high-quality PCBs.
The worldwide usage of circuit boards in the electronics industry has emphasized the significance of the PCB assembly process. It is a simple yet lengthy sequence of steps involving extensive preparation, meticulous planning, and thorough monitoring to build a working PCB. If any of the steps are overlooked during the assembly process, then the impact on the PCB performance will cost time and money for the contract manufacturer. Understanding the various phases of the PCB assembly process can help both the circuit designer and the contract manufacturer to build high-quality PCBs.
Once the circuit design is finalized with expert reviews, the next step is to prepare for the PCB assembly. Usually, the contract manufacturers have a network of PCB fabricators who are specialized in different PCB technologies. Based on the end application, the right board type is suggested by a PCB fabricator. They also review the design for manufacturability. Once the design is cleared for production, it may take a few days to several weeks for PCB fabrication. The time duration depends on the circuit complexity and on the customer’s product-to-market timelines.
The contract manufacturer will evaluate the final BoM to be clean and in the agreed format. If any discrepancies are found in the BoM, then the CM will ask the PCB designer for clarifications. Also, all the components are checked for availability and a valid product life-cycle. CMs have several sources and distributor networks to procure the components. They can suggest alternate parts if available. Once the BoM is updated and approved by the designer, all components are procured in bulk at a competitive price.
Once the materials are received at the assembly unit, all the fabricated boards, components, and the associated mechanical parts are inspected to meet the acceptability standards. Optical imaging equipment is used to check the quality of the PCBs. Components are examined for any visible damages or incorrect markings. Usually, the components are sample-checked unless the source is from a trusted vendor. The received materials are labeled after the inspection and are packed for assembly.
There are several soldering techniques used in the PCB assembly process like wave soldering, reflow soldering, and selective soldering. Wave soldering is used in bulk productions wherein a wave of molten solder is pumped over the PCB to make the necessary connection. This is followed by a water spray to cool the components safely and to fix the parts in the right place. In this method, a controlled temperature is critical to avoid any soldering defects in the PCB.
Reflow soldering is often used to connect surface mount devices (SMD). In this method, a solder paste is applied to the component pins and the PCB is heated in a reflow oven. The solder paste melts to connect the component to the pad on the PCB. Usually, the solder paste is applied using a stencil as it can process multiple boards quickly. The amount of solder paste applied on each footprint should be optimum, else there will be connection failures on the PCB.
Selective soldering is usually used to solder through-hole leads, in cases where wave soldering is not effective. PCBs in which the components are tall enough to obstruct the wave or modules that could be damaged by the heat of the wave solder technique can consider using this method.
In a general assembly process, there are two types of assembly technologies. They are Through-Hole Technology (THT) and Surface Mount Technology (SMT). THT involves inserting components’ leads in the holes drilled on the PCB. Using the wave soldering technique these components are soldered to the pads on the board. It is a highly reliable mounting technique but difficult to automate whereas the SMT method is cost-effective and can be automated. The surface mount components are smaller in size and are soldered using the reflow solder technique. The SMT PCB assembly process involves the following steps:
The assembled boards are cleaned thoroughly and the non-washable parts are soldered at this stage. The complete assembled board is inspected using automatic optical equipment for any missing components, incorrect or damaged assembly. X-ray machines are used to check for any solder shorts or opens on the assembled PCB. If there are any defects then the board is reworked and inspected before sending for testing. Automated testing procedures like In-circuit testing (ICT) and flying-probe tests are done to qualify the board for shipment. After the final quality assurance checks are completed, conformal coating is done to protect the PCB from moisture, chemicals, corrosion, etc. With the completion of this PCB assembly process, the boards are packed and shipped to the customer.
To control the process quality, IPC-A-610 is a widely used industry standard defining the acceptability criteria of electronic assemblies. It describes how the electronic assemblies are to be handled with acceptable soldering results for THT and SMT boards. There are many IPC standards associated with the PCB assembly process. Some of the commonly used standards are listed below.
IPC J-STD-001G is a standard that outlines the requirements for soldered electrical and electronic assemblies to meet the IPC-A-610 standard.
IPC-A-600 primarily defines the incoming inspection requirements of bare boards from a PCB fabricator.
Cable and wire harness assemblies have to adhere to IPC/WHMA-A-620C standards.
IPC-A-630 is for acceptability standard for manufacture, inspection, and testing of electronic devices that has to be followed by the CM.
Rework, repair, and modification of electronic assemblies are covered under IPC 7711/7721C.
IPC-M-108 is the assembly cleaning guide and handbook covering all areas of cleaning from post solder solvent to the cleaning of PCB, assemblies, and surface insulation.
CMs who are certified in these IPC standards uphold the quality of the complete PCB assembly process. Collaborating with one such CMs will ensure the assembly of a high-performance PCB product.
About the Author
Suresh Patel has worked as a Sales Engineer and other management roles at Mer-Mar Electronics. He brings 25 years of experience in printed-circuit-board sales and technical client service and managing business. You can connect with him on Twitter.