I’m not saying that I was a bad kid, but like a lot of children, I didn’t have the greatest respect for the efforts of my grade school teachers. That changed the day they put a couple of us in charge of working with a group of younger kids, and I quickly realized just how difficult the job of a teacher really was. It is one of those unwritten rules which states that people will be more invested in a process once they have a better understanding of what actually goes into making it work. Years later, when I started laying out printed circuit boards, I didn’t have much of an appreciation about how my designs could affect the assembly process either. That perception also changed very quickly on my first visit to a PCB contract manufacturer.
Circuit board assembly is a precise operation, and how we design the printed circuit boards that are being manufactured can have an enormous impact on this process. It is not uncommon for PCB designers to do things that make sense from the perspective of how the board operates, but in turn, unnecessarily complicate its assembly. One of the best things that you can do to ensure the success of your design is to have a good understanding of its manufacturing process. It will help you to make design decisions that are in the best interest of both the board’s functionality and its assembly. Here then are the manufacturing steps that your CM would like you to know about how to assemble a circuit board.
How to Assemble a Circuit Board: Preparing for the Build
The first step that your contract manufacturer will take in assembling your printed circuit board is to review all of your design data. They need to make sure that the manufacturing information is ready for use, and they will examine your design for its ability to be manufactured. This will include design for assembly (DFA) areas of concern such as component placement clearances and adequate thermal reliefs, and other areas of the design that could potentially cause problems. If they find any issues that need correction, they will work together with you on recommendations to make the board more manufacturable.
The next step will be to organize the design for the build. This will include:
- Having the raw PCBs fabricated, and ordering required components.
- Pull stock components and materials together for the build kit.
- Determine and plan the assembly and test processes that will be needed. This will include programming automated placement (Pick & Place) machines, and configuring and programming test fixtures.
Another part of preparing the board for assembly is to apply solder paste on the PCBs for those surface mount components that will be going through solder reflow. Traditionally this is done by creating solder paste stencils from the design CAD data, and screen printing the solder paste on the boards. Creating stencils can be an expensive process depending on how many stencil revisions are needed to get the best assembly yields, and your CM may use a solder jet process instead. This is a programmable machine that allows the precise amount of solder to be jet printed on the boards improving accuracy and eliminating the cost of stencils. Knowing how your board is prepared for assembly and why will you can easily why applying good DFA is so important.
Assembling the Board by Placing and Soldering Components
To assemble your board, your CM will first place the components. Thru-hole parts such as resistors or capacitors, can be inserted with automated placement machines or manually. In the case of high-density pin count components such as connectors, or components that are unique in their size and shape, manual insertion by a technician may be the only assembly option. For surface mount parts including passive and integrated circuits, automated pick and place machines will be used to place them. These parts are then held in place by the solder paste that had been previously applied to the board. As surface mount component sizes continue to get smaller with finer pitch spacing, it is more difficult to accurately place them manually. Because of this, contract manufacturers typically rely on automated assembly tools for their placement.
Once the board has been placed, it will next be run through different soldering processes. For boards that have a large population of thru-hole components, the CM will use a traditional wave soldering system. The boards will run on a conveyor belt through a molten wave of solder which will wick up through the thru-holes and solder the leads in place. For boards that are primarily composed of surface mount parts, the boards will be run through the solder reflow oven. This will heat the solder paste that is holding the parts in place until it reflows and creates a good solder joint. For boards that have a mixture of parts, such as a surface mount board with thru-hole connectors, a mixture of soldering technologies will be used. An example would be a board that has its connectors manually soldered in place after going through reflow. The quality of solder joints made by your CM during assembly relies upon your furnishing accurate BOM data that matches precisely with the component footprints on your board.
Testing and Inspecting the Completed PCB Assembly
Throughout the assembly process, and again upon its completion, the board will be inspected using a variety of methods. Included in this list are automated optical inspection tools, X-ray machines, and traditional manual inspection by a technician. These inspections are looking for incorrectly placed components, bad solder joints, or even wrong parts numbers on components. The final step is circuit board testing. There are a number of tests that your CM will run, including in-circuit test (ICT) and functional test. These procedures are looking for different issues from intermittent problems due to bad solder joints, to functional problems when the board is powered up and running. At this point, the board is ready to be packaged up, and shipped back to you. Knowing which tests you CM performs allows you to design for testing (DFT) and specify specific inspections or measurements that can aid in your bring up.
|Tempo‘s Custom PCB Manufacturing Service
At Tempo Automation, we have a detailed step by step process to ensure success on your prototype and/or low-volume PCB manufacturing. We have many years of experience as well as the resources you need to make sure that your design is manufactured to the highest levels of quality.
And to help you get started on the best path, we furnish information for your DFM checks and enable you to easily view and download DRC files. If you’re an Altium Designer or Cadence Allegro user, you can simply add these files to your PCB design software. For Mentor Pads or other design packages, we furnish DRC information in other CAD formats and Excel.
If you are ready to have your design manufactured, try our quote tool to upload your CAD and BOM files. If you want more information on assembling a circuit board and how a CM can help you optimize the entire PCB development process, contact us.