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Stay Alert When It Comes To DFM

All too often, the well regarded and highly respected design-for-manufacturing or DFM is inadvertently given only brief consideration. When that happens, things go wrong in manufacturing. In this one particular case, yield was at a remarkably below 60 percent when the OEM was anxiously anticipating better than 90 percent for its high-speed PCB design. Needless to say, the OEM in this case was none too happy.

The problem was the PCB layout design engineer (BTW not from NexLogic) reduced the pad size to match the trace width. The result was the design incurred several manufacturing issues and in particular, tombstoning, as shown in Fig. 1. The cause was solder following into the trace because it was the same size as the pad, and there was movement during reflow. The result was a mismatched pad size.

There were other DFM issues. Read more about the unusual DFM circumstances surrounding this particular PCB design in our EE Times/Embedded.com article.

In the meantime, there are some tips and hints to help you avoid or at least minimize these kinds of costly and time-consuming DFM issues:

  • Always work with in-house PCB design and assembly/manufacturing EMS Provider or contract manufacturer (CM).
  • Always submit complete CAD data files and not just Gerber files.
  • PCB layout/design experience plays a big hand in avoiding fabrication and assembly problems.
  • A savvy PCB designer assures there is the correct and uniform stack up to avoid board warping.
  • Get a basic understanding about fan out from your CM or EMS Provider. If not performed properly, acid or etch traps occur and cause trace damage.
  • Of great importance is the PCB layout designer’s experience because that plays a major role in eliminating opens, shorts, and tombstoning that results due to non-uniform pad thickness or incorrect registration for the assembly line.

The PCB layout design engineer has to move gingerly through various stages to avoid these fabrication and assembly problems. It’s important to note that 80 percent of PCB layout errors happen due to incorrect part geometry or creation, bad hole definition, inadequate spacing between through holes and surface mount components, and lack of rework ability around critical components.