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Printed Circuit Boards: Right the First Time

There are so many factors to take into consideration when designing new circuit board prototypes. High complexity circuit boards in particular have massive potential for design errors.

New PCB designs rarely turn out perfectly on the first run, as some room for improvement always becomes evident after testing the first prototype. However we do expect new designs to be 'right the first time'. We consider prototypes right the first time when they have completed their demonstration and functional testing requiring only a few simple rework procedures.

We use the following checklist for catching the most common errors that come up during new board designs. Our PCB designers run through this checklist before sending out any new designs for fabrication. You are welcome to use this checklist as you are reviewing your designs.

PCB Design Checklist

Project Name:
Customer:
Project Number / Part Number:
PCB Revision:
Review Date:

1. Schematic Review

Item Description Yes/No? Comments
1.1 Do all the chips on the board have bypass capacitors, and are they placed as close as possible to the power pins?

1.2 Are unused input pins properly pulled or tied to their desired logic levels and not left floating?

1.3 Have the single source components in the design been identified? Where possible, have components been selected using the most common industry standard packages allowing for second source alternatives?

1.4 Has the bill of material been reviewed to minimize the number of unique components in the design?

1.5 Have the voltage ratings for capacitors and other passives been verified? We like to use capacitors with a voltage rating of at least 1.5x to 2x the operating voltage.

1.6 Has a power budget been completed for this board? Are the LDO regulators able to supply the load current without excessive heat loss?

1.7 Do connectors exposed to the outside world have adequate ESD/TVS protection?

2. Part Design

Item Description Yes/No? Comments
2.1 Have the pin assignments in the schematic been verified against the part datasheets?

2.2 Have the mechanical dimensions been verified for all the component footprints?

2.3 Are any component datasheets showing the the pin assignments from the bottom perspective? If so, is this accounted for?

2.4 Are the drill sizes for all the through hole components large enough, or too large, for the pin diameters?

2.5 Has the pin order been verified on all the connectors? Also take into consideration if the mating cable reverses the pin order.

2.6 Are the connectors adequately keyed to prevent backwards cable insertion?

3. PCB Mechanicals

Item Description Yes/No? Comments
3.1 Have the dimensions of the mating cables been considered when spacing the PCB connectors? Has finger access to cables been considered in the connector spacing?

3.2 If the circuit board is going into an enclosure, are the connectors positioned to be flush with the enclosure outer surfaces?

3.3 If the circuit board is going into a tight fitting enclosure, have component heights and other mechanical keepouts been accounted for?

3.4 Are the mounting holes in the correct locations, and are their diameters correct for the intended mounting screws? Are the mounting hole diameters also correct (not too large) when considering the outer diameter of the enclosure standoffs?

3.5 Do components and traces have adequate spacing away from mounting holes to account for the screw head diameter or potential damage due to a drill bit slipping during assembly?

3.6 Does the fabrication drawing show the PCB outer dimensions?

4. Silk Screen

Item Description Yes/No? Comments
4.1 Does the silk screen include company name, project number, part number, and PCB revision?

4.2 Are all the silk screen labels oriented uniformly?

4.3 Are all the silk screen labels clear of solder pads and via drills?

4.4 Do all the passive components with placement orientation (electrolytic capacitors, diodes, LEDs, etc) have adequate silk screen markings to indicate the install polarity?

4.5 Is pin 1 marked on all ICs where incorrect rotation is possible?

4.6 Do connectors have pin numbers or markings that show the pin or connector usage?

5. Copper Design

Item Description Yes/No? Comments
5.1 Does the PCB layout software confirm that all nets are 100% routed?

5.2 Does the PCB layout software Design Rule Check pass?

5.3 Have all the pads been verified as having good pad exits; no traces exiting at unusual acute angles?

5.4 Are all the high current traces accounted for? Do they use sufficient copper width to handle the current?

5.5 Do all component through holes and SMD solder pads have thermal isolation (wagon wheels) from ground planes and copper pours?

5.6 Do high power components have adequate heat sinking using PCB copper heatsink or mechanical heat sink?

5.7 Does the ground fill meet up with all of the desired grounded nets? No isolated islands of copper?

6. PCB Fabrication

Item Description Yes/No? Comments
6.1 Are all the layers labeled?

6.2 Are the manufacturing specifications and materials listed on the fabrication drawing?

6.3 Is the board outline indicated on the fabrication drawing and solder mask top (SMT) layer?

7. Design For Assembly

Item Description Yes/No? Comments
7.1 Does the PCB have at least 3 fiducials?

7.2 Are surface mount components used wherever possible instead of through holes? This improves assembly automation, quality, and manufacturing cost.

7.3 For all through hole components, are the bottom side copper rings nice and wide (copper diameter >= drill diameter + 40 mils) to allow for easy wave/hand soldering? Pins that are tight pitch can use ovals or rectangular annular rings?

7.4 Are bottom side SMT components appropriately spaced (> 250 mils) away from through hole pins?

7.5 Do all traces and components have sufficient clearance (>30 mils) away from the edge of the board?.

8. Design For Test

Item Description Yes/No? Comments
8.1 Does the circuit board have sufficient test points and locations for probing of critical signals and monitoring supply voltages? Are there convenient locations for attaching the probe leads?

8.2 Does the board include adequate structures or connectors for firmware load or functional test?

8.3 If the board is intended for high volume manufacturing, does the design include bottom side solder mask openings to allow for a bed of nails in-circuit test?

9. High Speed Signals

Item Description Yes/No? Comments
9.1 Are traces carrying high speed signals designed as transmission lines using appropriate PCB impedance?

9.2 Do high speed signals use the correct terminations as suggested by the part manufacturer?

9.3 Are high speed signals routed over solid unobstructed copper planes?

9.4 Are high speed differential pairs routed using proper symmetry and length matching?

9.5 Are traces spaced and routed to minimize crosstalk?



Congratulations!

If you've made it through this list, then you have maximized your chances for a board design that turns out right the first time.