A little while ago, we planted a new hydrangea to replace an old bush that had died in our yard. It is a beautiful plant with one large flower on it, but I am told that many more blooms will come as the years go by.
When you are gardening, however, you can’t just throw a plant in the ground and expect it to grow. We first had to dig a hole with enough clearance from the fence for the new hydrangea to grow. Then, we had to churn up the soil, remove the dead bush roots, and add lime and nutrients to the ground. Finally, we could set the new plant in place, fill in with dirt around it, and then set up the sprinkler next to it for watering. After all of that, I am really looking forward to seeing how it grows in the years to come.
Reflecting on the importance of understanding the correct gardening process, it occurred to me how important it is to know the PCB manufacturing process. Just as you can’t throw a plant on the ground and expect it to grow, you also can’t just throw components on PCB design and expect it to be easily assembled. There are a lot of steps to getting your PCB assembled, and understanding what your board will go through can help you when it comes time to make component placement decisions. Here is a brief outline of what happens with your PCB component assembly when you work with a quality contract manufacturer.
Getting Set Up for PCB Component Assembly
The first step in PCB manufacturing is to ready your board for assembly, and that starts with a full review of your design. There are many aspects of your design that need to be looked at by your CM, including:
Components: The component engineers at your CM should look over the components you are using on your board. They’ll need to check for what they have in stock so they know what needs to be ordered. They should also be looking at prices and availability, and recommend changes to you if it can help lower the manufacturing costs. If any parts are flagged as being not recommended for manufacturing (NRFM) or end-of-life (EOL), your CM should make recommendations to you for replacements or redesign.- Performance: Your CM’s engineering team should review your design for signal integrity or other performance issues. If they spot any areas of question, they should address these with you as well as provide recommendations to resolve the problem.
- Design for assembly (DFA): Your CM’s manufacturing engineers should look specifically at areas that could cause problems during assembly. This would include problems such as component spacing that is too close, or components that are not optimally placed for soldering. If any problems are found, they can either request design changes from you or make the changes themselves with your permission.
- Design for test (DFT): The test engineers should be looking at your design for how well it is set up to be tested. If changes are needed for testability, they should work with you on those changes. They should also be planning out any test fixtures or programming needed to test the board after assembly.
Components:In addition to these reviews, many other things are going on to get your design ready for manufacturing. The raw PCBs will be ordered from the fabricator, and any needed parts will be ordered as well. All incoming materials will be inspected and pulled together into a build kit, and all required documentation will be created and made available for use during manufacturing. At this point, your CM is ready to assemble the board.
Component Placement and Soldering: The Cornerstone of PCB Assembly
To start the assembly process, the parts will be placed on your board. For surface mount components, this is usually done with automated pick-and-place machines. Through-hole components can be placed by machine, but they often are placed by hand.
Component spacing is extremely important to the success of the placement process, which is one of the reasons why your design is reviewed for good DFA principles before assembly starts. Not only do you need to make sure the automation has room to work, but for any manual assembly or rework on your board, the technicians will need room to work as well.
Once the parts are in place, it is time to solder them to the board. Surface mount parts are held in place with solder paste, which will melt and reflow when going through the reflow oven. In some instances, surface mount parts may be wave soldered instead, in which case they will be held in place by a spot of glue until soldered by the wave. Through-hole parts will also go through the wave solder process, where molten solder will wick up through the holes, creating a good solder joint.
Designers should note that adhering to good DFA practices is what will determine the success of the soldering processes. Pad sizes and balanced metal on small passive devices is essential for even heating in the reflow oven, otherwise, those parts may stand up on one end. Likewise, component rotation and location in relation to the board direction through the solder wave can have a direct effect on how well those solder joints are formed.
There may be some components that are not a good candidate for either reflow or wave soldering. This can happen due to low-volume production runs when it is too expensive to create solder fixtures or with connectors and other through-hole components with high-density pin counts. Traditionally, the resolution for this is to hand-solder those parts, but some CMs now have a new tool to use instead—selective soldering.
The selective solder machine is an automated system that uses a nozzle to pump molten solder up into the leads on the back of the board it is programmed to solder. Once soldering is completed, the board is now ready for the final phase of manufacturing.
Completing the Process to a Fully Assembled Board
The final step in manufacturing your printed circuit board is inspection and test. Your CM should use a combination of different inspection processes, including visual examination, X-ray, and automated optical inspection systems. There are also different types of testing systems that your CM may use depending on the needs of the board:
- Flying probe: This system uses between two to six probes to “fly” around the top and the bottom of the board testing designated points. It is testing for connectivity between these points to check for good solder joints during assembly.
- In-circuit test (ICT): This system is much faster than flying probe in that it uses a bed of nails to contact all of the test points on the board at once. It, too, is testing connectivity for good solder joints.
- Functional test: This system conducts power signals through the board to test its functionality. This gives a more detailed look at how the board is working once assembled.
Any problems that are discovered during inspection and test will be corrected by rework technicians. Once their work is done and the board has passed its inspections and tests, it will be ready to be packaged and shipped back to you.
To get your board quickly through all of these assembly processes, it is in your best interest to use known good parts and to design your board with good DFA and DFT practices. One of the best ways to stay on top of all of this is to consult your CM early. At VSE, we encourage our customers to consult with us early so that we can answer any questions and give the best guidance possible to design a fully manufacturable PCB.
