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Adventures in Engineering: A Purpose-Driven Prototype

Prototyping, simulation’s fraternal twin born from a desire for a finished product, is a common methodology employed in PCB/PCBA development. Aside from merely being a means to an end, what is your prototype meant to accomplish? Well, that depends on what you want it to do, but a clearly scoped prototype purpose is essential for making the effort truly worthwhile. Tasks defined with a clear target in mind are most often highly productive, measurable, and justifiable in the face of scrutiny. A mindful approach to making a preliminary model of a design is purpose driven prototyping.

What drives you to create a prototype? Most often it is exposure to risk from something that is unknown. Having a tangible artifact in hand that can be measured is a time proven method to eliminating the unknown. Unambiguous scope is where purpose driven prototyping in PCB/PCBA development begins. Writing a well scoped prototype purpose is akin to writing a good requirement.  As I have written before, “A well written requirement is something that is needed, can be verified, and is reasonably attainable.” A well scoped prototype purpose is something that addresses identified risks in the design, is traceable, and fits within the budget. 

An approach I like to take when developing a prototype purpose is to start with high level categories then work my way down to more specific areas and items. Is this a new product design, product upgrade, product sustaining, or switchover to mass production? What are the form, fit, and functional aspects of the design that are presenting risks? Are the risks associated with technology, skillsets, logistics, processes, regulatory certifications, etc.? Organization of design risks is a method I use to outline my prototype purpose.

Here are some examples I have encountered in my career: 

Example 1: New Design—32 Channel Tx/Rx ARINC 429 PCI Mezzanine Board.

• Risk 1: Fit—Technology, current PCI card design does not fit mezzanine envelope.
• Risk 2: Function—Technology, anticipated thermal dissipation exceeds mezzanine specification under heavy loading and high usage scenarios. 
• Prototype Purpose: Develop a 32 channel Tx/Rx ARINC 429 PCI Mezzanine prototype that conforms to PCI Mezzanine interface and envelope specifications and can be thermally characterized while running performance-based scenarios. 

Example 2: Product Upgrade—Replace old microcontroller with new next generation microcontroller.

• Risk 1: Fit (Technology)—New microcontroller package is fine pitch BGA vs Flat Pac.
• Risk 2: Fit (Logistics)—Pinout changes in the redesign cannot adversely affect existing product installations. 
• Prototype Purpose: Design a prototype that demonstrates manufacturability with new fine pitch BGA and that pinout changes are compatible with existing product installations. 

Example 3: Sustaining power supply redesign to avoid thermal failures in field.

• Risk 1: Function—Regulatory, design may not meet emission standards.
• Risk 2: Function—Technology, power supply may not be stable over temperature and loading profiles.
• Prototype Purpose: Develop a prototype to demonstrate compliance with regulatory emissions specifications and to show power supply stability over temperature and loading profiles. 

Now that we have a clear prototype purpose, tasks and effort can be estimated with a reasonable sense of confidence. The key is to have the tasks traceable to the prototype purpose and that are in turn traceable to design risks. Let’s create some tasks for the prototype purpose described in Example 2.

Prototype Purpose: Design a prototype that demonstrates manufacturability with new fine pitch BGA and that pinout changes are compatible with existing product installations. 

1. Create new microcontroller component in CAD library.
2. Replace existing microcontroller in board design schematic with new microcontroller.
3. Make identified pinout changes in schematic. 
4. Replace existing microcontroller in board layout with new microcontroller. 
5. Complete board layout and produce PCB fabrication and PCBA assembly artifacts.
6. Fabricate and assemble prototype boards.
7. The list goes on…

The tasks for developing the prototype are intentionally derived from the prototype purpose so that traceability back to risks are maintained. The tasks should be clear and concise so that they can be understood by competent resources and be verifiable by inspection, test, or review (technical consensus). Tasks should also be able to stand up against scrutiny by peers and higher authorities. I did not understand the relevance of this until I had to defend my projects in the wake of companywide budget cuts. I experienced some of my projects in my early career suffer horrendously because I did not clearly trace my project tasks back to design requirements and risks, especially risks. Prototypes were canceled, boards were spun with the expectation of a single spin success, which did not happen. You get the idea. It was bad. Simply having tasks clearly traceable to design risks would have saved me a lot of headaches. 

Time for everybody’s favorite topic, the budget. What I have not covered thus far is the impact of your project budget on any potential prototype. This is where projects typically get dicey. I routinely encounter these three scenarios: There is no money, there is money, and there is money but someone does not want to spend it.

• In the case where there is no money, it’s not going to happen. It is best to adopt a “You get what you get, and you don’t throw a fit” mentality.
• When there is money, I find it best to follow the purpose driven prototype principles outlined above.
• When there is someone not wanting to spend the money necessary for risk mitigation with the expectation of a successful project it is best to follow the purpose driven prototype principles outlined above as well.

On a closing note, a lot of personal and professional satisfaction can be gained by achieving the goals we set. I am looking forward to my next purpose driven prototype and I hope you are too.

Chris Young is owner/lead engineer at Young Engineering Services LLC.