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The Top 10 Layout Traps

The Top 10 Layout Traps. #10 Ambiguous Part Substitutions. Some applications are tolerant of fuzzy part substitutions, others must conform exactly to spec. #9 Silk Screen Marking. Diodes are the most common problem Any polarized part Top vs. bottom layer.

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The Top 10 Layout Traps

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  1. The Top 10 Layout Traps

  2. #10 Ambiguous Part Substitutions • Some applications are tolerant of fuzzy part substitutions, others must conform exactly to spec

  3. #9 Silk Screen Marking • Diodes are the most common problem • Any polarized part • Top vs. bottom layer

  4. #8 PCB Storage and Handling Practices • Everything has a shelf life

  5. #7 Excessive Moisture • Greater issue with RoHS-compliant components • Can be an issue in some leaded components and PCBs

  6. #6 RoHS – Sort of • When is a RoHSpassive, not RoHS? • Multiple heat cycles may damage components • FR4 PCBs may also have this issue

  7. #5 Tombstoning • Possible layout causes: • Inner layers • Uneven traces • Large components close by Large part sinks heat Inner layer sinks heat Wide trace sinks heat

  8. # 4 Lead vs. No Lead • Lead vs. no lead • Assembly has to follow the BGA • Lead-free BGA is better on a leaded board than the converse • Reballing may be an option with more costly BGAs

  9. # 3 Parts Library Mismatch • CAD libraries may have land patterns that are close to what is needed but just off enough to cause issues • Metric vs. English

  10. #2 Floating QFNs • Solder paste stencil opening • No big openings Bad Good

  11. #2 Floating QFNs • Ideal if stencil is made correctly • Little chance for tilt

  12. #1 VIA IN PAD • Capillary action = bad • Sucking solder through a straw much worse than sipping cider through a straw • Worse with lead

  13. #1 VIA IN PAD

  14. #1 VIA IN PAD • Solder mask • Better than nothing • May break open

  15. Thank You Want more information? Email dbenson@screamingcircuits.com or visit our blog at www.screamingcircuits.com

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