Issue link: https://iconnect007.uberflip.com/i/1533085
MARCH 2025 I DESIGN007 MAGAZINE 21 2. Designed impedance: e designer uses impedance calculators to determine the best line widths and dielectrics to meet the requirement. e design is built as is (with etch enhancements). e fabricator is responsible for using the designed line width and dielectrics; they do not verify the impedance results on the finished board. is can be a cost-saving way of achieving a specific impedance, especially if the requirements are not that critical. A Few Other Notes on Impedance Are the impedance traces identified? If you use a tool that offers an attribute for these, and then outputting Gerber and NC, the attribute information is lost when you output dumb data (Gerber files). It's also possible the fabricator's CAM tool does not support attributes. Do you really need ±5% controlled imped- ance? e difference in yield from ±5% vs ±10% vs ±20% on impedance can greatly affect your final cost. e tighter the tolerance, the more scrap the fabricator will incur, and that cost will be passed back to the designer. Do I Still Need to Send an IPC-D-356(A) Netlist? e fabricator can extract a netlist from the incoming data. Have you ever heard of "golden board" testing? You have just specified this. Golden board testing takes the data for one of the boards and compares it to all the others. So, if you start with bad data (no mat- ter what the source) and compare the finished boards to that bad data, guess what? All of your boards pass, and all of them are bad. But an IPC-D-356(A) netlist allows a netlist from the CAD system to be compared to the fin- ished boards. is is the true test of validity. Hitting the Right Notes Remember: e more print notes I specify, the greater the chance I will get a good board back. 1. Print notes should always start with applicable specifications called out. For example: a. Manufacture to comply with current revision of IPC-6012, Class 2. b. Inspect per IPC-A-600. 3. ose two notes remove the need for spec- ifying a lot of individual specifications. 4. Anything that is in a specification, does not need to be in the notes, and oen can lead to conflicts if you do have a specifica- tion called out. 5. From personal experience, I have seen prints that call out IPC, CM, and OEM specifications, which can all be in conflict. 6. Print notes should be concise, clear, and precise. 7. Don't be "that guy" and include "Use non-conductive copper on breakaways. Use V-groove at breakaways on edge, tabs on gold finger side" in notes. ese are just a few of the design errors that show up over and over in a fab shop. When in doubt, meet with the product engineer or application engineer of the board shop to clar- ify these issues. Get them involved early in the process. DESIGN007 Ray Fugitt is technical sales and marketing manager at DownStream Technologies. Figure 3: Asking customers to circle the via treatment they prefer is one way to make sure there are no miscommunication miscues here.