Issue link: https://iconnect007.uberflip.com/i/1507356
22 DESIGN007 MAGAZINE I SEPTEMBER 2023 Designing with common, mainstream mate- rials will usually ensure that the materials are either on the shelf or can be procured with minimal lead time. If a manufacturer has to order exotic materials that require a two- to three-month lead time (which happens more than you would think), that extra time is going to have to be added to the processing lead time. Most fabricators like to have the materials physically in their facility before they release a job to production, so the material lead time and processing lead time run consecutively, not concurrently. is can lead to a long wait for finished product." What can designers do to optimize their rigid-flex stackups for easier fabrication? Joe Fjelstad, author of Flexible Circuit Tech- nology, says, "Don't design for manufacturing. Design with manufacturing." In other words, he's telling us not to go it alone. Where there's a will and enough time and money, a lot of things can be built that maybe shouldn't have been built, or that you could have built with a less expensive alternative. An experienced rigid-flex fabricator is a great resource for this, and if you have enough lead time, you can even get a second opinion from another fabricator. Mark added that a reputable fabricator will have learned from thousands of successful (and maybe a few not-so-successful) designs over many years. Fabricators can coach you on what to do, and just as importantly, what not to do to get to a successful design. As I've noted, an entire book was written on the subject, so the list of dos and don'ts when optimizing rigid-flex designs could be quite exhaustive. Mark listed a few of his dos and don'ts for rigid-flex design: • Use standard materials whenever possible. • Optimize trace/space sizes where you can. Don't default the entire board to minimum feature sizes if they are only needed in a few small areas. is will increase etch yields. • Optimize pad sizes. Increasing pad size— even an extra mil or two—can make the fabrication process a lot easier. • Whenever possible, try to make blind vias 100 µm or larger. is will give you the most reliable plated via connections. • Use a symmetrical material stackup to minimize warping during reflow. • Always make each rigid area with the same construction and number of layers, even if you don't need them. It will not save you money to eliminate layers in a single area or even a few rigid areas. On the contrary, it will add significantly to the cost. Nick contributed his own short list: • e best rigid-flex stackups are symmetrical to minimize warpage. • e most successful designs don't have flex layers as the outer-most layers (top or bottom) in the rigid stackup. • Rigid-flex designs require larger annular rings to account for more variation in material movement. • Stacking-up dissimilar materials will result in more misregistration than you might be used to seeing in a rigid PCB. Very good. Is there anything else you'd like to add? Take a PCB fabricator with experience in rigid- flex manufacturing out to lunch and visit their facility. Better yet, offer to buy them lunch and see if they make a counteroffer to buy you lunch. I tried it with Nick and Mark aer their class at IPC APEX EXPO and got a pretzel out of the deal. It seemed fitting since we were talking about rigid-flex. You're making me hungry! Thanks, Bill. Any time. Please download my free eBook on stackup design; there's a whole chapter on rigid-flex. DESIGN007