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16 The PCB Magazine • October 2017 What you can expect back from the manu- facturer is a: • Complete stack-up, with copper and dielectric thicknesses, BOM (bill of materials) • Track widths that you shall use on your impedance traces • Space to use between the impedance traces (differential pairs) • Calculation of the results above You can now start your layout. Here's a tip: Make your impedance traces visible to the man- ufacturer. You can have most connections rout- ed with, for example, 100 µm, and if there are some single-end traces using the same width, the manufacturer cannot separate these from the others. What you can do, is to set the width of the critical single end traces to a slightly higher or lower value (e.g., 99 µm or 101 µm). This change is so small that it has no influence on the production, but it will now be possible for the manufacturer to separate these traces from the others. As a designer you should know that the stack-up you have received from your manufac- turer, with values used for your routing, is not final. At the stage where your layout is ready and you want your PCB produced, there will, in most cases, be some technical issues on im- pedance values asked by the PCB manufacturer. These are caused, for obvious reasons, by the fact that manufacturers do not have access to your production data (Gerber, ODB++, IPC- 2581…) at the stage when the stack-up is done. When manufacturers have received your production data, they will also calculate the percentage of copper you have used on your signal and plane layers. This will influence the types and prepregs to be used, and will affect impedances. The manufacturer will inform you via technical queries that some changes must be done for you to get your requested im- pedances. These changes will normally be very marginal changes on track widths, gaps or layer distances, and will in most cas- es be accepted without too much arguing. If there should be a change in the prepregs that will lead to a change in the resin content, this will also change the Dk value and will be taken into consideration when impedances are re-cal- culated. For the designer it is most important to get requested impedances, and when it comes to these minor adjustments, I consider the manu- facturer to know best what to do. I often compose start stack-ups for design- ers, and I often use the ICD stack-up planner from In-Circuit Design. What we've seen used by most PCB manufacturers is the software from Polar Instruments. My final tip to you is this: Get started. Com- municate with your PCB manufacturer and do not forget to include a netlist with your PCB production data. Enjoy the wonderful world of printed circuits! PCB John Steinar Johnsen (Josse) is Senior Technical Advisor with Elmatica. PLANNING A PCB: SIGNAL INTEGRITY AND CONTROLLED IMPEDANCE CONSIDERATIONS Figure 6: Example of a customer stack-up.