The ability to do a 3D mock-up within your design software is an invaluable tool that no doubt has prevented countless re-designs and revision changes. This is particularly powerful in the world of flex and rigid-flex. When the circuit is intended to be bent, folded, or formed it is very easy to make a mistake with pinouts and orientation.
Before the power to model this within the design tool, paper-dolls and mylar cut-outs were often used to model the flex or rigid-flex. These techniques are still important today and often used for complex, high stakes designs with multiple bends and folds. There are a few examples I would like to share in this blog.
Can you relate to this scenario? You are working with a complex design involving a series of five stacked chassis, each connected with a three-layer rigid-flex that when installed will exceed the recommended bend radius for the materials. Not only is the design itself complex, but there are also additional challenges. First, there is limited headspace, making installation difficult, and in addition to that, this is a space application, requiring long term reliability. This is definitely not an ideal situation. Once the rigid flexes are connected there will not be an opportunity to go back and troubleshoot. So, what do experienced industry veterans do? They first do 3D modeling within their design tool, and then as an added level of insurance, they factor in the time and added expense of having their fabricator provide a mock-up of the rigid-flex to be absolutely sure that the pinouts and functionality are correct before final installation.
There are a few ways to approach this. In this particular situation, the rigid-flex fabricator made mock-ups with the exact material set, to be sure that even with the bend radius violations the design would be successful. When the final product was received, all were reasonably confident this would be successful in a high-reliability environment and there would not be a need to dis-assemble the stacked chassis connection to troubleshoot. Time and money well spent!
Not all examples are quite this complex. One example recently shared with me was actually somewhat of a “war story”. You know, those stories that seem ridiculously stressful when you are in the midst of them and then provide hours of entertainment when later recalling the events and the heroic efforts needed to resolve them? In this case, there was a series of 10 flex’s in the unit and all were bending, folding, and in some way conforming to the packaging. Yes, you might be thinking, this is EXACTLY why flex is an important tool in the design toolbox. But, managing a series of 10 of these proved to be a complex design effort. Pinouts, bend radius, service loops, etc. are all the more difficult with this level of complexity. One poor program manager finally threw up his hands and asked that all the flexes be increased by 10” to be sure that everything fit and formed as needed.
I know, another common attribute of a “war story” is that hindsight is always perfectly clear—errors of the past seem so easy to prevent. But it’s an entirely different story in the heat of the moment. The reality of that decision was that there were flex “loops” everywhere during installation, and it was incredibly difficult if not impossible to understand. Hindsight being clear, adding 10” to each flex without doing modeling was a rash decision that added quite a bit of cost to each design and also added significant complexity to the assembly, which, at the end of the day, still didn’t interconnect well.
Recently, I was researching flex “war stories”, and I found that the power and value of a mock-up was a clear and common theme. In many instances, a 3D simulation of the flex in use was invaluable for understanding how the pinout, bend radius, etc. were going to be realized in final use. In addition, it was interesting to hear that the good old-fashioned mylar or paper-doll mock-up is still being used for complex and high stakes designs. In certain circumstances, spending the time and money to have your fabricator create a mock-up creates a much greater return than the time and money spent. In other cases, simply creating a paper mock-up and simulating how you are intended that flex to perform provides a sanity check and confidence prior to releasing a design to fabrication.
Throughout my research of simulations and mock-ups, one other piece of advice clearly stood out. No matter what type of mock-up or simulation you choose, where you begin your design is critical. When there are multiple flex or rigid-flex in the unit, do not start with the rigid board design, start with the flex designs so you are incredibly clear on the pinouts and how they connect to the rigid board. Starting with the rigid board only adds additional complication. Good advice, I am sure often learned the “hard way”!