Cutting Respins: Journey to the Single-spin PCB

Estimated reading time: 4 minutes

PCB design is more than a short sprint to the finish line; it is a journey best suited for the prepared adventurer. According to a study by Lifecycle Insights, the average PCB design project requires 2.9 respins. These respins can cost anywhere from tens of thousands to millions of dollars—each!

As an engineer/business owner, I find respins frustrating because I would rather spend my time and money applying scientific principles inventing, improving technology, and solving problems. I am not an advocate for perfectionism, but rather I focus on becoming a better adventurer. Sometimes I get to taste the sweet wine that is a single spin PCB. As fellow adventurers, let’s discuss some topics that influence unnecessary return trips on our PCB design journey: simulation, technical reviews, and interest in PCB design.

I like to say, “A sim in time will save dimes, especially on the assembly line.” Yes, it’s true: Circuit simulation guides PCB design decisions and helps avoid costly mistakes. A customer of mine ran into a simple problem of misapplying design reuse, which could have been easily caught if a simple simulation had been performed. They were using a simple NPN transistor to drive a 12V relay in a previous design and decided to use the circuit design for a new 5V design. The new product passed certification testing, and all design documents were locked down in configuration control with the regulatory authority.

The first assembly run immediately began to see failures in production testing—many more failures than were allowed by the regulatory authority. The transistor that worked well for the 12V design did not work for the new 5V design. A simple Spice simulation performed as part of the root cause analysis clearly showed the transistor did not have sufficient DC current gain to drive the 5V relay. Ultimately, a respin was needed to address the issue. The cost was in the millions of dollars, and the delay was over six months.

Most often, I see respins occur due to simple design mistakes rather than some complex multiple-variable problem that exists. The lesson is that simulating your designs will reduce the risk of respins due to simple mistakes or bad assumptions.

Peer reviews are great, but SME reviews are better. Early in my career, I fell victim to the fallacy of allowing only my peers to review my work. I was responsible for the redesign of a digital processor/IO control board in an airborne surveillance system. The control signals to the RF transmitter were TTL (5V), and my task was to interface them to an LVTTL (3.3V) FPGA. Simple enough, I thought; I will use a 3.3V-to-5V translation buffer. My peers in the preliminary design review agreed that this was a simple solution that was implemented correctly. But during system integration, an engineer noticed that when the system was powered on, a high-powered RF pulse was being transmitted from the system.

I did not understand what was happening and just happened to speak with the power supply expert on our staff. When he saw my schematics, he instantly discovered the cause of the problem. He pointed out to me that the 3.3V and 5V power supplies were not sequenced together in the design, and in fact, the 5V leads the 3.3V by several milliseconds.

During the time when the 5V is up and the 3.3V is off (~0V), the 5V transmitter control signals from the translation buffer are commanding the transmitter to transmit at full power. This resulted in a respin of the processor/IO control and power supply boards in the system. Including our power supply expert in my design review would have avoided two board respins in this project. Lesson learned: Always include subject matter experts in your design reviews. They have valuable knowledge and insights that can help keep you out of trouble.

Respins do not have to be a part of your company’s reality. If you enjoy PCB design, invest your time in becoming good at your job. Being interested in PCB design gives you the tenacity needed when facing difficult or complex design problems, and it provides the fuel for improving your design skills.

A litmus test for gauging your interest in PCB design is how you spend your free time. Do you find yourself daydreaming about PCB design in your off time and/or tinkering at home with your own designs?

If the answer is yes, then I suggest you find others who enjoy it as well, including SMEs from across the PCB design spectrum. Interacting with those who share your interest in PCB design helps strengthen your own abilities. An excellent resource for connecting to others with strong interest in PCB design is the Printed Circuit Engineering Association. PCEA’s core values of collaboration, inspiration, and education are derived from the founding members’ expertise and interest in PCB design. My inspiration for sharing my ideas and experiences in this article is derived from my interest in PCB design and encouragement from like-minded colleagues and friends.

Remember: A sim in time will save dimes. And always let the appropriate subject matter experts take part in your design reviews. You might save your company millions of dollars and months of lost time-to-market.

This article appeared in the December 2020 issue of Design007 Magazine.