How Do I Prep My PCB for Fabrication? Accelerating PCB Assembly with Design Best Practices

In this article, Parker Dillman, lead EE and co-founder of MacroFab, covers how to best prepare your PCB designs for fabrication and assembly.

It's a common situation: You're a hardware developer and deadlines are looming for your next product. A bad prototype PCB will delay the project for weeks and you need to reduce this risk. Getting the PCB assembled correctly the first time, quickly and without problems, is paramount.

To minimize these potential issues, I have compiled a list of some tips to help prepare your next prototype for manufacturing.

Double Check Footprints and Packages

Making sure the footprint matches the package for the component is the first way to avoid manufacturing hang-ups. The old-school way of printing out your PCB on paper with 1:1 scale and then overlaying your parts only goes so far nowadays, considering how small some components have become and under package contacts like BGA components.

Double-check that the dimensions on the footprint match the units of your design (mm or mil).

Some component manufacturers are unkind and draw the mechanical layout of the component as if looking through a clear, transparent PCB from the bottom. Be sure to watch out for this.

 

Figure 1. Silicon Labs EFM8UB10F8G in QFN20 package. Comparing the layout drawn in the EDA Tool to the landing pattern in the datasheet.

If your EDA tool can draw projection and dimension lines, it might be worth measuring your footprint in a way that matches the mechanical drawing in the component's datasheet. Verify the measurement units of the datasheet and your footprint.

This is also a good time to check the mapping between your schematic symbol and component footprint. Voltage regulator pinouts, discrete MOSFET, and transistors are commonly and easily flipped around.

Components that have polarity should have their footprints checked to make sure the polarity markings are clearly marked. This includes IC pin one markings, diode cathode marks, and polarized capacitor markings.

 

Figure 2. Marking of pin one on a CREE LED.

Go for a Large Selection of Tested Part Substitutions

A common delay in production occurs when key parts are unavailable and no substitutions have been pretested and approved. If a part has viable substitutions but is in a critical path of your circuit or product, I highly recommended you build prototypes and test with each substitution before going to production. This reduces the risk involved when switching to a substituted part in the future.

 

Figure 3. End of Life part on Mouser marked as NRND or Not Recommended for New Designs

 

If you have unique parts that do not have any direct substitutes (microcontrollers, specialized sensors, etc.) check on the part lifetime from the manufacture of the part. Manufacturers will mark components slated to be discontinued as “Not Recommended for New Designs”.

Typically, manufacturers guarantee a set life span for manufacturing the part and will notify users of the part when it will be End of Life'd (EoL). Make sure the part you need will be available until the end of your product’s production life span in order to help prevent costly product redesigns down the road.

Utilize Pre-Certified Radio Modules

If your product uses Bluetooth or WiFi, look into using a pre-certified radio module. These modules are pre-designed and packaged systems guaranteed to function correctly with an associated FCC identification number. Using a pre-certified radio module increases the chances of a properly functioning wireless system and will also reduce the possibility of failing FCC and CE radio emission compliance testing.

 

Figure 4. OSBeehives’ BuzzBox powered by a Particle Photon pre-certified radio module. Image Courtesy of OSBeehives.

Consider Your Wireless Antenna Layout

If you decide that the cost savings of rolling wireless connectivity onto your PCB is worth it, the PCB layout of the antenna is critical. For most wireless connectivity parts (a transceiver) there will be a recommended layout from the manufacturer's datasheet. Following the recommended layout will most likely be your quickest route to success.

There are some things to watch out for when doing the PCB layout. First, the impedance must be matched between the transceiver and the antenna. Second, the datasheet for the transceiver should have more details about selecting the proper antenna, designing a tuning filter, and the correct impedance needed for maximum performance.

I would highly recommend performing pre-compliance testing on your product if you design your own wireless connectivity. The pre-compliance testing will hopefully catch any obvious problems with your design. Look for frequency harmonics within what you're aiming for in the clocks, oscillators, and transmission spectrum.

Don't Forget the Decoupling Capacitors

Electrical components need stable voltage sources and decoupling capacitors should be included on your PCB near every single active component. Decoupling capacitors work best when they are as close to the power pins of the component as possible.

 

Figure 5. Decoupling capacitors ensure that this Texas Instruments LVDS converter has smooth power. 

 

For larger components that have multiple power pins, you may need decoupling capacitors at each power pin. Power sensitive parts like sensors, ADCs, and FPGAs you may want to include decoupling caps for the ground pins as well. The decoupling capacitor should be inline from the power source and the component as this improves the performance of the capacitor.

 

Figure 6. Bypass or decoupling capacitors should be placed inline from the power source.

 

Protect Your Board with Proper Trace Width and Spacing

High current traces must be properly sized to ensure they do not burn up your PCB. I recommend using an online trace width calculator to do the calculations. Traces on the outside of the board can handle more current than internal since it is easier for the external trace to dissipate the heat generated. To keep the heat down, try to specify the temperature rise on the trace width calculator to be 10C. If you do not have room for a trace that wide, however, a 20ºC temperature rise should be fine for most applications.

If you cannot route a trace wide enough you may need to go to a thicker copper weight which will increase current capability. However, increasing the copper weight thickness can cause minimal trace width and spacing issues for the Design Rule Check (DRC) so make sure to take that into account. Typically going thicker with the copper weight will require larger trace widths and spaces and increase the price of your per PCB unit price.

 

Figure 7. Routes cut out between pads for increased voltage isolation. Image Courtesy of Scott Swaaley of MAKESafe Tools.

 

An often overlooked problem is making sure high voltage traces are sufficiently isolated from each other. If your product is connected to mains voltage you need to make sure the voltage can not jump the air gap and short out.

Choose the Right Power Supply Regulator Routing

There are two major types of voltage regulators in embedded systems: linear regulators and switching regulators. Each type has different guidelines for the PCB layout and routings.

 

Working with Linear Regulators

Linear regulators take the excess voltage and convert it to waste heat. This is inefficient, but linear regulators generally only need external capacitors to run correctly and can be less noisy than switching regulators. There are two things to make sure to get right with linear regulators:

1. Consider your capacitor selection. Follow the manufacturer's guidelines on which type, value, and location of the capacitors used to bypass the regulator. Typically the capacitors should be placed as close as possible to the input and output pins of the regulator.
2. Take care of the heat. Generally, this means making sure the package you selected for the regulator can handle the amount of heat you will be generating and that your layout can support it. Copper pours and via stitching will be your friend here. If a copper pour will not be large enough then a heatsink will be needed.

Figure 8. Linear voltage regulators with copper pours for heat dissipation.

 

Working with Switching Regulators

Switching regulators are more efficient than linear regulators but are more complicated to design for. Typically, heat is not an issue with switching regulators but you need to carefully select the components to ensure the switching regulator will work correctly. Switching regulators are also more susceptible to generating unwanted electromagnetic fields (EMF) and causing failures at the FCC/CE compliance stage of a product.

1. Follow the recommended layout from the manufacturer closely. These layouts have been tested to work correctly.
2. Keep the feedback loop for the switcher as small as possible. This will reduce EMF and parasitic resistance, inductance, and capacitance.
3. Pay close attention to your switcher regulators output capacitors ESR and ESL ratings. When looking for components, the datasheet for the switching regulator will usually tell you where to set the values.

For designing switchers, I like to use Texas Instruments Webench. It generates multiple designs for your desired specifications and gives you part numbers for the inductors and capacitors you will need to design the switcher correctly.

Include Thermal Relief for Large Copper Traces and Pours

A copper pad with thermal relief is created by connecting the pad to the copper trace or pour using smaller narrow tracks instead of connecting it directly. Thermal relief reduces the thermal load of soldering the component to the pad. This reduces the chance of cold solder joints due to the copper dissipating the heat too quickly.

 

Figure 9. Thermal relief on large SMT pads to promote proper soldering of the connections.

 

You should pay attention to the current load going through the thermal relief area. If these are designed too narrow, you can end up with a one-way fuse.

Optimize Your Design for SMT Assembly

Production costs and assembly time are both positively affected when you use as many SMT components as possible. SMT connectors can be created if the connector is only going to be interfaced during product assembly (like attaching an internal lithium battery during product assembly). 

 

Figure 10. Wave pallet tooling to enable faster through hole connector soldering.

 

Sometimes through-hole parts are necessary. Connectors interfaced with humans should almost always be through-hole to prevent the part from being forcibly removed during operation. When using through-hole parts, work with your contract manufacturer to find out how much space you need to leave around the parts to optimize for wave or selective soldering. If other components are too close to the through-hole contacts, the contract manufacturer may have to hand solder the connector, slowing down your assembly process and increasing costs.

Double Check Your Design Rule Checks

Double checking your design rule checks is probably the most important item on this list. Check with your manufacturer on their design rules. Most manufacturers have different levels of scaling design rules. If you can get away with the larger and more standard design rules you should.

Before sending your design files to your manufacturer I suggest you run your DRC one last time and check the following things:

• Run a design rule check (DRC)
• Check connections and routes
• Use your EDA tool's “airwires” or “rat lines” to visually show which part pads are connected to each other on a signal net
• Update any silkscreen text for date codes, PCB versioning, or metadata

Wrapping Up

I hope this article guides you on improving your PCB design process and helps you reduce your risk when ordering PCB assemblies and scaling up the production of your products. The more planning you do preassembly, the fewer hiccups in production. 

For additional information, check out my interview with the All About Circuits team and read the checklist I wrote for pre-FAB and production considerations.

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