When we first considered purchasing our house, we were attracted by the promise of RV parking in the listing. The ad wasn’t wrong. The property does have room on one side of the house to park an RV; you just can’t access it due to the curve of the driveway. If the layout of the lot had been designed with a straighter driveway, it would work. As it is, the most we can get back in there is a small utility trailer. We bought the house anyway, but I wish I could use that parking area as intended.
This unusable RV parking has always served as a gentle reminder about the importance of careful design, specifically the placement of parts on a circuit board. When creating an effective parts placement, many concerns have to be considered, including signal performance, power integrity, accessibility, and manufacturability. And although you may have to make compromises between these different requirements, some rules must be followed, especially when placing surface mount parts. Here are some SMT component placement guidelines that can help.
PCB Design Preparation for Placing Parts
Before parts can begin to be placed, it is necessary to set up your layout database. You will want to configure your board stackup and assign routing and plane layers as needed. Also, establish any keep-out zones or height-restricted areas. During PCB layout, the last thing you want is to find out you need to rip everything up and start over because of an overlooked keep-out zone.
You will also want to ensure your library parts for the PCB layout are approved and ready to be used. PCB component footprints are often downloaded from exterior websites, but those who build their own parts make sure that you correctly reproduce the manufacturers’ specifications. Designers should create the component outlines at their maximum material width, and the land patterns should be built to accommodate the minimum and maximum pin dimensions. Now is also a good time to ensure that the PCB footprints have all of their properties and attributes assigned and include 3D STEP models if they are available.
PCB design preparation also includes setting up your design rules and constraints, which are essential to ensure the proper spacing between parts while placing them on the board. Don’t reuse other rules and constraints as is, and take the time to verify that the rules are correctly set up for your design. And with those chores completed, it’s time to start placing your parts.
SMT Component Placement Guidelines for PCB Layout
Most circuit boards have a mixture of surface mount and thru-hole parts, but in these guidelines, we will focus primarily on the SMT part placement for manufacturability and signal performance.
Solder reflow
Most surface mount parts will be assembled on the board using the solder reflow process. Here are some considerations to keep in mind when placing these parts:
- Different sized pads can lead to thermal imbalances between pins, which can cause the solder on one pin to melt and reflow faster than the other. This scenario could result in the part pulling up off the unmelted pad and standing up in a condition known as “tombstoning.”
- Placing smaller two-pin parts where one pad is embedded in a metal fill can also create the same tombstoning effect. Each pin should have approximately the same width of connecting traces to balance the heat between them.
- One nice thing about solder reflow is that the spacing between SMT parts can be tighter than those going through a wave solder process.
Wave soldering
While solder reflow relies on an oven to melt solder paste applied to the metal SMT pads, wave soldering employs a molten solder wave that the circuit board passes through on a conveyor belt. Wave soldering is intended mainly for thru-hole parts as the solder will wick up through the holes to create a connection. However, many surface mount parts can be wave soldered too. They need to be held in place with a spot of glue, and you should consider the following guidelines as you place SMT parts for the wave:
- Be careful not to place larger parts in front of smaller parts in relation to the direction the board will travel through the wave. This condition could create a shadow effect robbing the smaller parts of the solder they need for a good connection.
- Small two-pin discrete parts should be aligned perpendicular to the board’s direction of travel through the wave to ensure both pins will be equally soldered.
- Wave soldering does have a limit to how high it can reach, and taller components should not be used on the side of the board that will be wave soldered.
- Integrated circuits with multiple pins can also be wave-soldered and should be aligned in the direction of travel, so one row of pins doesn’t trail the other.
PCB testing
Circuit boards are usually tested with automated systems that probe test points once assembly is completed. The following testing guidelines should be observed while laying out the test points in the PCB design to ensure the accuracy of the automated testing:
- Maintain your PCB manufacturer’s minimum spacing requirements for test points to SMT components.
- The standard spacing used between test points and components may increase depending on the component’s height.
- Keep test points back 0.100 inches from the edge of the board to give the ICT test fixture enough room for a good vacuum seal to the circuit board.
Circuit board performance
Although manufacturability is important, it won’t matter if the board doesn’t operate correctly once it is assembled. High-speed circuit designs require diligence in their component placement to minimize any chance that the board creates or is victimized by noise and interference problems. Here are some SMT component placement guidelines to keep in mind for signal and power integrity:
- Keep parts as close together as possible to minimize routing distances.
- Follow the signal path in the schematic when placing your parts.
- Don’t place components where sensitive high-speed routing will have to cross breaks or splits in the reference plane for the signal’s return paths.
- Bypass capacitors should be as close as possible to their assigned supply pins on high-speed devices to provide immediate power and reduce parasitic inductance between them.
- Power supply circuits should have their SMT parts laid out tightly together to promote short and wide trace routing, reducing inductance in the connections.
In general, keep SMT parts on one side of the board if possible, as this will reduce expenses attributed to stencils and other assembly challenges. And even though signal performance would prefer the SMT components be as close together as possible, manufacturing needs some minimum spacing for automated assembly processes. And if your board is going to be produced in an assembly panel, be aware that the panel requirements may force a larger clearance to the edge of the board.
These guidelines will help you stay on track when placing SMT and other parts on your circuit board design, but as you can see, there are many rules to keep in mind. Fortunately, your PCB contract manufacturer can help you through this process.
Check Specific Component Placement Details with Your Manufacturer
It is essential to begin working with your PCB CM as soon as possible when designing a new printed circuit board. Not only will they be able to help you with component selection questions and board layer stackup configurations for impedance control and signal integrity, but they can help you with specific spacing rules for their manufacturing processes. You can then input these values into your design rules and constraints to ensure that the SMT parts you are placing on your design will be manufacturable without any problems.
