While the global COVID-19 pandemic forced everyone to contend with many unexpected changes in their lives, it also opened the door for new advancements in technology and electronics. People needed new and improved computing and networking equipment for working and schooling from home. Simultaneously, there was significant demand for new entertainment, medical, and personal electronics. Makers of electronic devices have been rushing to push new products out into the marketplace to meet these needs. Unfortunately, with this push of new electronics, we also see some problems develop.
Some designs are being pushed out the door without enough attention, focusing on the basics of circuit board design to stay competitive and beat the crowd. This haste has been obvious in the placement of components on the circuit board. Carelessness in part placement can have a disastrous effect on the final product’s manufacturability, performance, and overall quality. Here we will look at surface mount components of a PCB and some of the more common SMT placement issues.
Component Placement, The Solid Foundation of PCB Layout
At one time a PCB designer could throw parts on a circuit board, and as long as the traces connected to the correct pins, the board would usually work. However, today’s high-speed, mixed-signal designs require meticulous attention to detail to ensure that the parts are correctly placed for the most optimum electrical performance. At the same time, the parts on a board must also be placed to facilitate efficiency in manufacturing. Meeting these needs can sometimes be a difficult challenge for the PCB layout designer, but one that must be done as a good parts placement serves as the foundation for the entire design.
Placement problems can be expensive in terms of both time and money. Sub-standard performance or manufacturing problems will force a lot of unexpected tests and debugging, which eventually lead to reworking the board. These additional steps will lead to manufacturing delays as the board is redesigned, resulting in missing the window of opportunity for the product being produced. Not only can a bad parts placement ruin a company’s reputation for quality, but it could even turn into a safety issue with high-powered designs. To avoid these problems, designers need to be aware of potential SMT placement issues from the beginning of their design work.
Eight SMT Placement Issues and How to Avoid Them
As we have seen, an incorrect PCB component placement can create problems for the completed circuit board ranging from performance to manufacturability. Because these SMT placement issues can manifest themselves in vastly different ways, we will generalize them in the following eight categories:
1. Signal integrity
SMT components that are not placed correctly for their circuits can create signal integrity problems leading to decreased performance or outright failure of the board. PCB layout designers must place their parts according to the signal paths depicted in the schematics and keep digital and analog parts isolated in their design partitions. It is also important to place parts in such a way as to allow for clear signal return paths on the reference plane.
2. Power integrity
Another important consideration for circuit board performance is its power integrity. Bypass capacitors must be placed close to their supply pins to reduce the chance of noise being created in the power delivery network. Power supply components need to be placed tightly together on the same board layer to keep their connecting traces short and reduce inductance in the line.
3. PCBA rework
Part of the PCB manufacturing process includes reworking a board during assembly, whether it is an inspection or actual repair or replacement of parts. SMT parts hidden in their placement by larger parts will make rework and inspection difficult. Repair and testing tools may not fit where they are needed, resulting in collateral damage to nearby parts.
SMT components placed too close to in-circuit test points can obstruct the test probes from making contact. Another problem occurs if an SMT part is placed over one or more test points, which completely obscures those points from being tested.
Small surface mount components on boards that are wave soldered may not receive the amount of solder they need if they are preceded by a larger part going into the wave. This effect is known as “shadowing.” Designers should try to place smaller parts in front of larger components to avoid it.
When small two-pin SMT parts go through the solder reflow oven, a thermal imbalance between their two pads can cause the part to pull up off one pad and stand straight up like a tombstone. Different sized pads can cause thermal imbalances in the PCB footprint, different sized traces connecting to the pads, or embedding one of the pads within a metal area fill. These conditions will act as a heat sink and cause the solder paste on the opposite pad to melt faster and pull the part towards it.
7. Insufficient solder
SMT parts can also be victims of insufficient solder during assembly resulting in a bad solder joint that can later break or create an intermittent connection. Insufficient solder can also happen during wave soldering if pins are not perpendicular to the direction of the wave. This problem can also happen during reflow if the pads of the part aren’t large enough to contain enough solder paste for a good joint to be formed.
8. Solder shorts and bridging
SMT parts can also have too much solder, which can bridge over to another pad creating an electrical short. Bridging can also happen if the PCB footprint pads or solder paste masks are created incorrectly or if parts are placed too close to each other.
Those are some areas that PCB designers need to keep an eye on during layout to avoid SMT placement issues. Next, we’ll look at some design techniques to help you catch these errors before they are created.
Taking it to the Next Level
When designing a new printed circuit board, resist the urge to cut corners and rush the job through the layout. Components need to be placed according to the needs of the circuitry for electrical performance, and according to industry standards for manufacturability. It’s a good idea to include your PCB assembly shop early in the layout phase to help you develop the best design for manufacturability (DFM) rules for your project. Once armed with this information, you can incorporate placement and spacing rules into your PCB CAD system as design constraints.
At VSE, we’ve been working with designers like you for over 35 years, and we understand the challenges you face in blending the electrical and manufacturing requirements of a circuit board. Our engineering team has a lot of experience in both areas and can help you with the SMT placement constraints and rules to ensure the highest quality in the circuit boards you are developing.