PCB design can become complicated very quickly. Here are seven PCB design tips for you to help you avoid some of the traps of PCB design that can easily sneak into a design.
1. Design Reviews are Essential
No matter how much time pressure a design is under, design reviews are an absolutely essential element of PCB design. Bring as many different eyes in to scrutinize the design as possible to force you to go through your design, the design choices, and give you new perspective on the design. Technicians are a great wealth of knowledge and can be key in finding where app notes, typical application circuits, and the overall design are not quite up to snuff and will not work the way it is intended. Remember, theory is not reality and experienced technicians often have a very finely honed sense of where the real life implementation of theory starts to fall apart. Even with multiple design reviews, errors often slip in to designs, but the vast majority of issues will be caught before production.
Remember - you have spent too much time looking at the design and will only rarely be able to spot your own errors and mistakes. Bring in other sets of eyes!
2. Backup Your Work!
Electronic Design Automation (EDA) software for PCB layout is a wonderful tool that lets designers rapidly create a very complex design. The flexibility and speed of EDA packages can let designers get lost in a design and forget to create regular backups and save points. Software crashes and EDA PCB layout software is no exception. Losing a day or even an hour of work can be maddening. Save regularly and create branching save points as well. Often as designs evolve, a previously saved design may have been a better option to build on than making tons of corrections to the current version of the PCB design. Make use of the easy saving options and save yourself from major design headaches.
Electronic Design Automation (EDA) software for PCB layout is a wonderful tool that lets designers rapidly create a very complex design. The flexibility and speed of EDA packages can let designers get lost in a design and forget to create regular backups and save points. Software crashes and EDA PCB layout software is no exception. Losing a day or even an hour of work can be maddening. Save regularly and create branching save points as well. Often as designs evolve, a previously saved design may have been a better option to build on than making tons of corrections to the current version of the PCB design. Make use of the easy saving options and save yourself from major design headaches.
3. Avoid Via-In-Pad
While they are often hard to avoid, putting vias in component pads is a major cause of problems in production and prototype PCBs. Some components, such as BGAs and some QFN ICs require vias to be placed on pads but they should be avoided where possible. The problem with vias in pads is that they can suck solder away from components through to the other side of the PCB. This results in small parts and components without any solder connecting them to their solder pads, bad mechanical connections on larger parts and BGA balls that get sucked off BGA ICs. If Vias must be used in a pad, make them as small as possible and cover them with soldermask if possible. If the opposite side of the via is capped with a soldermask, air can get trapped inside the via which will outgas during the soldering process and can result in unsoldered or poorly soldered components.
While they are often hard to avoid, putting vias in component pads is a major cause of problems in production and prototype PCBs. Some components, such as BGAs and some QFN ICs require vias to be placed on pads but they should be avoided where possible. The problem with vias in pads is that they can suck solder away from components through to the other side of the PCB. This results in small parts and components without any solder connecting them to their solder pads, bad mechanical connections on larger parts and BGA balls that get sucked off BGA ICs. If Vias must be used in a pad, make them as small as possible and cover them with soldermask if possible. If the opposite side of the via is capped with a soldermask, air can get trapped inside the via which will outgas during the soldering process and can result in unsoldered or poorly soldered components.
4. Keep ICs from Floating on Solder
One last critical aspect about a component's footprint is the soldermask. As mentioned above, when vias are used in the pad they should be covered with soldermask where possible to avoid component solderability issues. Another solderability issue is too much solder on the pad. When solder paste is used and a stencil is used to spread the solder paste, a solder mask with openings that are too large can result in excess solder on the board. This is not as big an issue for many surface mount components, but components with a large solder pad underneath the component (often used for heat sinking on QFN and other higher power ICs) can be pushed up by the excess solder which leaves their pins unconnected. To avoid this, a modified soldermask with only 50% coverage on these areas can be used.
One last critical aspect about a component's footprint is the soldermask. As mentioned above, when vias are used in the pad they should be covered with soldermask where possible to avoid component solderability issues. Another solderability issue is too much solder on the pad. When solder paste is used and a stencil is used to spread the solder paste, a solder mask with openings that are too large can result in excess solder on the board. This is not as big an issue for many surface mount components, but components with a large solder pad underneath the component (often used for heat sinking on QFN and other higher power ICs) can be pushed up by the excess solder which leaves their pins unconnected. To avoid this, a modified soldermask with only 50% coverage on these areas can be used.
5. Footprint Verification
Many EDA packages include component footprint libraries. While these make designing infinitely easier than creating your own package, PCB footprint, soldermask, and silkscreen from scratch libraries can sometimes contain errors. It is always a good policy to check to make sure the footprints for each component and the pinouts are correct. These errors can slip through very easily and can sometimes be fixed easily enough for prototypes. Always check the PCB footprints, especially if you defined your own component footprint or modified a footprint in your design. A few minutes checking a design is much better than several hours hand soldering tiny surface mount corrections on every board.
Many EDA packages include component footprint libraries. While these make designing infinitely easier than creating your own package, PCB footprint, soldermask, and silkscreen from scratch libraries can sometimes contain errors. It is always a good policy to check to make sure the footprints for each component and the pinouts are correct. These errors can slip through very easily and can sometimes be fixed easily enough for prototypes. Always check the PCB footprints, especially if you defined your own component footprint or modified a footprint in your design. A few minutes checking a design is much better than several hours hand soldering tiny surface mount corrections on every board.
6. Provide Readable Documentation with Good Silk-Screening
While the silkscreen is a non-critical part of a component footprint, ambiguous symbols and small text can significantly increase the difficulty of manual assembly and troubleshooting. Silkscreens are the primary way a person will be able to interact and read the PCB design and should be as well documented as possible. Components with a polarity, such as diodes, and electrolytic or tantalum capacitors, should be marked clearly, orientation should be marked for IC packages and components, and the first pin should be marked where possible. All components should be called out by the appropriate abbreviation and in a font size that is easily readable.
While the silkscreen is a non-critical part of a component footprint, ambiguous symbols and small text can significantly increase the difficulty of manual assembly and troubleshooting. Silkscreens are the primary way a person will be able to interact and read the PCB design and should be as well documented as possible. Components with a polarity, such as diodes, and electrolytic or tantalum capacitors, should be marked clearly, orientation should be marked for IC packages and components, and the first pin should be marked where possible. All components should be called out by the appropriate abbreviation and in a font size that is easily readable.
7. Be Careful with Part Substitutions
Part substitutions are often a requirement to get a design up and running. Some components are more tolerant of substations of their support components (resistors, ceramic capacitors, inductors, diodes, etc.), however some are very picky and require components that match a very specific range of values.
Part substitutions are often a requirement to get a design up and running. Some components are more tolerant of substations of their support components (resistors, ceramic capacitors, inductors, diodes, etc.), however some are very picky and require components that match a very specific range of values.
Selecting a component that is slightly outside the acceptable range can make the component oscillate, behave erratically, or worse. Troubleshooting these errors can be maddening since everything appears correct. Substituting ICs can also be a major problem, even with ICs that are advertised as drop in, pin for pin, replacements. These components demand careful study of the datasheet for the updates, changes, and improvements in the component that will have unforeseen impacts on the performance of the system.
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