Post-Assembly: PCB Design Issues

After learning about every single tool, material and skill required in PCB assembly, as well as all of the different steps involved in the process, the next – and last – thing to understand is the possibility of post-assembly design issues.

Pad Size

The pad is the point of connection where components join the board’s conductor. For through-hole boards, the pad is either a rectangular or a circular space, and the end of a track that has a plated-through mounting hole for components at its center. Solder is placed to secure the component and finalize the connection. The mounting pin’s diameter determines the diameter of the hole, while the copper’s adhesion to the laminate determines the pad’s size.

PCB
For surface mount boards, the size of the pad is a compromise between the used area, the strength of the solder joint, and the accuracy of placement. If undersized, there will be a reduction in the joint strength. If oversized, however, no improvement in strength will be observed.

Similarly, the via is used to provide connections in between layers. The size of the via relies on the patterning and drilling techniques, and it is often smaller than “through” holes.

Quality

There are test coupons that are built into the design of the board itself. These are made with any spare material found outside the circuit area, allowing for a demonstration that every process has been finalized with proper alignment, as well as a testing of the mechanical and electrical characteristics of the standard pattern. Finally, these allow verification of the PCB assembly’s integrity – specifically for plated through-hole designs.

The appropriateness of the interconnection pattern is assessed using a “bare board” test. This employs a partial analysis to confirm the tracks found on every outer surface without having to check on the through-board connections or inner layers. However, for PCBs with multiple vias, it is recommended to perform a comprehensive test to exercise the complete structure of the PCB, mostly because of the high penalty cost that comes with a faulty PCB assembly. A lot of PCBs are also tested electrically by suppliers for any shorts or opens, often against the design data of the user.

Packaging

PCBs are packaged to be protected against moisture and other forms of ionic contaminants.
• The quality of the solder deteriorates when there is moisture present, or when copper surfaces get in contact with materials of high ionic content.
• Prolonged storage time can lead to the absorption of moisture by the PCBs. This can outgas during soldering processes, resulting to delamination and blow holes.

PCB Design

Note that FR-4 boards with multiple layers can be in stock for a maximum of six months, but only if in their original packaging. Any “exotic” material has to be thoroughly dried and sealed in metalized bags that have silica gel. This is a vital step for PCBs made of cyanate ester or have aramid reinforcements, particularly those that contain large and unbroken copper areas.

Since all PCBs are made using CAD, despite the modifications made on the details or layout of the board, it is important to always meet the standard requirements of the PCB production process. In higher frequencies of production where the PCB is merely a component, design verification and simulation can be tried without having to actually “cut metal” – however, programs that allow such practices are still in the early stages of development.

During the process of reflow, the entire PCB assembly should be evaluated, particularly on how to support the boards throughout the process to prevent warping, among other possible issues. With all things said, testing and constant retesting is very essential in PCB assembly – from the very first stages and all throughout the process.