1 – Use of hybrid techniques
The general rule is to minimize the use of mixed assembly techniques and limit them to specific situations. For example, the benefits of inserting a single through-hole (PTH) component are almost never compensated by the additional cost and time required for assembly. Instead, using multiple PTH components or eliminating them entirely from the design is preferable and more efficient. If PTH technology is required, it is recommended to place all component vias on the same side of the printed circuit, thus reducing the time required for assembly.
2 – Component size
During the PCB design stage, it is important to select the correct package size for each component. In general, you should only choose a smaller package if you have a valid reason; otherwise, move to a larger package. In fact, electronic designers often select components with unnecessarily small packages, creating possible problems during the assembly phase and possible circuit modifications. Depending on the extent of the changes required, in some cases it may be more convenient to reassemble the entire board rather than removing and soldering the required components.
3 – Component space occupied
Component footprint is another important aspect of assembly. Therefore, PCB designers must ensure that each package is created accurately according to the land pattern specified in each integrated component’s data sheet. The main problem caused by incorrect footprints is the occurrence of the so-called “tombstone effect”, also known as the Manhattan effect or the alligator effect. This problem occurs when the integrated component receives uneven heat during the soldering process, causing the integrated component to stick to the PCB on only one side instead of both. The tombstone phenomenon mainly affects passive SMD components such as resistors, capacitors, and inductors. The reason for its occurrence is uneven heating. The reasons are as follows:
Land pattern dimensions associated with component are incorrect Different amplitudes of the tracks connected to the two pads of the component Very wide track width, acting as a heat sink.
4 – Spacing between components
One of the main causes of PCB failure is insufficient space between components leading to overheating. Space is a critical resource, especially in the case of highly complex circuits that must meet very challenging requirements. Placing one component too close to other components can create different types of problems, the severity of which may require changes to the PCB design or manufacturing process, wasting time and increasing costs.
When using automated assembly and test machines, make sure each component is far enough away from mechanical parts, circuit board edges, and all other components. Components that are too close together or rotated incorrectly are the source of problems during wave soldering. For example, if a higher component precedes a lower height component along the path followed by the wave, this can create a “shadow” effect that weakens the weld. Integrated circuits rotated perpendicular to each other will have the same effect.
5 – Component list updated
The bill of parts (BOM) is a critical factor in the PCB design and assembly stages. In fact, if the BOM contains errors or inaccuracies, the manufacturer may suspend the assembly phase until these issues are resolved. One way to ensure that the BOM is always correct and up to date is to conduct a thorough review of the BOM every time the PCB design is updated. For example, if a new component was added to the original project, you need to verify that the BOM is updated and consistent by entering the correct component number, description, and value.
6 – Use of datum points
Fiducial points, also known as fiducial marks, are round copper shapes used as landmarks on pick-and-place assembly machines. Fiducials enable these automated machines to recognize board orientation and correctly assemble small pitch surface mount components such as Quad Flat Pack (QFP), Ball Grid Array (BGA) or Quad Flat No-Lead (QFN).
Fiducials are divided into two categories: global fiducial markers and local fiducial markers. Global fiducial marks are placed on the edges of the PCB, allowing pick and place machines to detect the board’s orientation in the XY plane. Local fiducial marks placed near the corners of square SMD components are used by the placement machine to precisely position the component’s footprint, thereby reducing relative positioning errors during assembly. Datum points play an important role when a project contains many components that are close to each other. Figure 2 shows the assembled Arduino Uno board with the two global reference points highlighted in red.