Issue link: https://iconnect007.uberflip.com/i/1085192
42 DESIGN007 MAGAZINE I FEBRUARY 2019 the spaces. It may even require the fabricator to ask for a lighter starting copper weight to al- low for a 0.5-mil change in trace width to still make the part producible. This is where the etch compensation based on starting copper weight is key to remember when designing the part to allow for slight changes to either dielectric distance, start- ing copper weight, or trace width changes to meet your impedances. Having gone through all this, if the fabrication house does not state additional deviations due to the color of solder mask, material finish type, or even the mate- rial UL rating required, this is also frustrating for the designer. You should communicate with an engineering person at the fabrication house ahead of time to preclude these deviations from slowing down the part. The second thing is being clear on what you need. If you initially ask for half-an-ounce starting copper and the copper weight needs to be increased due to voltage or EMI issues, this loses time in laying out the design again to accommodate a larger space to deal with the higher copper weight. If you've stated one copper weight only to change it, expect you may have to change trace widths or spaces to accommodate these changes. Likewise, chang- ing a mask color form from green to red may require a complete re-layout due to the slight- ly thicker red or other colored mask change. Thus, what you ask for should not change from your original request, or you should expect to have to re-lay out the design to increase space or trace widths. Again, with the example used previously of a 0.1-mm trace and space, if all the variables don't remain the same as what was asked for, the fabricator only has a few deviations that can be made on their end. All of these negotia- tions should be made before the release of the dataset. Frustrations can occur on both sides (the end user and the fabricator) when things change based on the information they have re- ceived versus what they get once the dataset has been released. The two main delays we see at the fabrica- tion level are: 1. Deviations that require re-lay outs due to changes being added to the drawing or RE- ADME file that were not previously negotiated and need a new layout from the designer 2. If your chosen fabricator does not identify all of the deviations at the quote stage, which requires more back-and-forth communication between the board designer and the fabricator, delaying the parts Much of this was never an issue 0.008"/0.008", and designers know that those numbers cor- respond roughly to the dielectrics needed to meet said impedances. However, with trace and space routinely at 0.004"/0.004" today, the deviations from copper weights or mask colors can and will affect the ability to produce your part as you wish it. Remember the things that a fabricator can change to meet your desired impedances, such as: • Trace widths if space allows based on copper weights. Remember, all copper is typically compensated at the CAM stage to deal with the known loss at the fabrica- tor's etcher • Dielectrics if overall thickness can be met by adjusting the dielectrics and copper weights can be reduced on interface layers. For example, if all plane layers are called out as one ounce but the trace widths on the surface layers are 0.004" or below to meet the impedance, you may have some pushback asking to reduce the copper If you've stated one copper weight only to change it, expect you may have to change trace widths or spaces to accommodate these changes.