SMT Component Placement: Part 2 ― Accuracy

As a parent, there have been many times when I have told my children that they are “out of line.” This is usually because their behavior had been intolerable, and a correction was needed. To help them grow into mature adults as well as keeping them safe, parents typically define a range of acceptable behavior for our children which could also be considered as their performance tolerance. I can tell you from personal experience, however, that trying to administer discipline by telling my children that they are “out of tolerance” is pretty useless.

An area where tolerance concerns are very important though is in printed circuit board manufacturing. If the parts on the board aren’t placed accurately, there will usually be problems in how the board is assembled. There are many factors that can contribute to this including whether the physical parts themselves are within their manufacturer’s tolerances, and how they are used on the circuit board. Let’s look at some of these issues, and some recommendations on how you can design for better SMT component placement accuracy.

Component Manufacturing Tolerances and PCB Design

Components that are out of tolerance can have a huge impact on the manufacturing of a circuit board. If the physical part does not match its published specifications, it may not fit on the PCB land patterns that have been designed for it resulting in assembly problems. On the flip side, however, is if the PCB land patterns are out of tolerance because they were designed incorrectly for the published specifications. This may create a situation where the parts don’t fit on the board, or poor solder joints are created during assembly.

To check that the physical components are in compliance, the PCB contract manufacturer will run inspections of the parts both before and after the assembly of the board. The inspections prior to assembly are to verify the compliance of the component to the tolerances specified in the part datasheets. This is usually done using a low power microscope to examine a few component samples from each batch of parts that are being worked with. Components that are non-compliant will either be corrected or returned to their manufacturer. After assembly, the components on the board are inspected both manually and with automated optical inspection equipment. This is to verify the assembly process and confirm that good solder joints have been created for the parts.

To ensure that the PCB land patterns are within tolerance, the component footprints in the CAD database will be checked against manufacturer and industry specifications. The key is to make sure that the footprint pads are both large enough to promote the formation of good solder joints, while not being so big that the component leads float off the pads. It is also important that the footprint is created at the maximum material width of the part to ensure that there is ample room for parts that are placed closely together on the board.

With the physical parts and the component footprints having been verified as being correct, the next step is to ensure the accuracy of the component placement on the circuit board.

Using Your PCB Design CAD Tools to Ensure SMT Component Placement Accuracy

There are many utilities within PCB design CAD tools that can help the designer to place their component footprints accurately on the circuit board:

• Library tools: Although we’ve already covered the importance of creating an accurate component footprint within the CAD tools, this step cannot be overstated. To put it simply, a bad component footprint can not only introduce unexpected manufacturing problems, but it can also cause problems for parts placement if the components are too close or too far apart. Therefore make use of the automated part generators and wizards if possible. These tools are usually based on IPC specifications and will do the heavy lifting of part creation for you. Another good resource is to use online library services that often have completed footprints already available for the design tools that you are using.
• Design rules: These often underused features can be a significant help to the PCB designer. Not only can they be set up for basic clearances such as pad to pad spacing, but they often have advanced capabilities as well. These can include defining different clearance values for component groups, areas, and sides of the board. In addition, make sure to use the cross-probing features of your design tools so that you can place the components in the correct order based on the signal paths defined in the schematic.
• Placement features: Many tools have utilities that allow you to automate many of the component placement tasks such as part alignment, or copying placement patterns. These features can save you a lot of time and effort and should be leveraged as much as possible.
• 3D: With the use of STEP models, many PCB design systems will allow you to see your design in full 3D. Although layout tools have the capability to check for clearance conflicts in the X, Y, and Z-axis, it is extremely helpful for the designer to be able to see the parts as they will actually be assembled. For example, the design rules may allow a manufacturing clearance between parts, but 3D will allow you to see if there is enough room to plug in connectors or to manually rework a part.

In addition, the PCB designer should ensure that the component placement is correct for the electrical, thermal, mechanical, and manufacturing specifications of the board. Using the schematic as a road map for component placement is a great help here, as is using various analysis tools to verify the accuracy of the final placement. Another helpful tip is to have a complete understanding of the component placement rules before the layout starts, and that information is available from your PCB contract manufacturer.

Partnering with Your CM to Define Component Placement Accuracy

When it comes to component placement on a printed circuit board, the parts need to be placed to satisfy both the performance and manufacturing requirements of the board. Here is where an experienced PCB contract manufacturer can help. With the different types and technologies of printed circuit boards that they have been building, the PCB CM will understand what the most optimum placement strategy will be for your circuit board. They will be able to advise you on design for manufacturing (DFM) rules that will ensure the best production yields of the boards, as well as how to layout the board for its maximum performance.

At VSE, our engineering and manufacturing teams have that experience. With over 30 years of building printed circuit boards for electronic innovators both locally and around the world, they can give you the PCB design answers that you need.