Issue link: https://iconnect007.uberflip.com/i/1348195
30 DESIGN007 MAGAZINE I MARCH 2021 is 50% away from requested nominal value. Inexperienced designers, or SI engineers, will have good impedance simulation tools but are loaded with bad material specifications. Or the model assumes that there was a reference plane for a microstrip line that was one layer away when the actual design has sections of the trace referencing a plane three or more layers away. Or the trace width needs to be reduced so far that it puts the required width outside of the fabricator's capability. One design was received that required the trace impedance to match the free space imped- ance of 377 ohms. We notified the customer that all PCBs are made with dielectric materi- als of which none had a dielectric constant that matched a vacuum. We could not get a reason for the specification, so this was a no-bid. 5G IoT designs have increased the quantity of first-time flex and rigid-flex designs. Rigid board designers who attempt their first flex or rigid-flex design oen violate flexure rules, which will result in damaged boards during use. Many of the flex rules are different than those for rigid boards and must be learned. Regional Quick-turn vs. Asian High-Volume Capability Inexperienced designers oen have only used local quick-turn production companies to get their boards back to engineering quickly, for example, in one to three days. e quick- turn shop does not have sufficient time to run an extensive DFM review and may change materials and design features without notify- ing the designer of these changes. ey may also have a wider process capability to ensure that boards do not scrap out. A significant amount of DFM issues may be raised when the design is transferred to a volume shop because of narrower technical capability, tighter qual- ity requirements, and small modifications that will significantly improve the yield. Common yield enhancements will not be included. ese include edits such as non- functional pad removal, thieving open areas, filling low-pressure areas, and tear-dropping pads. Experienced designers will generally modify their design rules to match the volume capability from prior builds. 'Don't Change Anything' Instructions included with a design may indicate that nothing can be changed at all. All requests for manufacturability improvements and design issues fixes are rejected. Much of the time, this response is because the layout person and their team does not fully under- stand how the changes may impact the design performance. A new engineer is commonly assigned the task of adding additional sources prior to or just aer the product is released for volume manufacturing. Manufacturing facilities oen have differing capabilities and design rules, and may request slight modifications to enhance the yield and/or meet product performance requirements. An experienced engineer will have a good understanding of what variations may affect the design and what will not. e inexperienced engineer will generally require that nothing be changed. 'Build It Exactly the Same as Our Current Supplier' is is a quite common guidance from com- panies that design mobile phones. e cell- phone NPI cycle is measured in weeks and the product lifetime is measured in months. Some of the PCB sections are copied from other designs and merged without spending a lot of time in design verification. ey will 5G IoT designs have increased the quantity of first-time flex and rigid-flex designs.