Prototron Soldermask

Impedance: Getting the Customer What They Need

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By Mark Thompson | Published on: November 25, 2013

I know I have been on my soapbox before about the importance of letting the fabricator know the desired end results if jobs are impedance controlled, and not merely indicating a material type or specifying dielectrics.

This is even more important now when many jobs are going through turn-key environments who are not the end users, communication about potential impedances takes valuable time out of the fabrication process and can delay the delivery of product making both the end-user and the turn-key assembler unhappy.

This can be easily avoided by sharing impedance information about the job up front on a drawing or Read Me file. The only four things a fabricator really needs are:

  1. What are the trace sizes that are controlled?
  2. Where do they reside? (On which layers)
  3. What is the threshold desired?
  4. What tolerance is associated with the threshold?

The material callout should express both the Tg and the Td desired. This allows the fabricator some alternatives in material selection to meet the customers’ needs. Avoid conflicting information about materials that require clarification and cause delays.

An example would be specific one number Dk callouts. Dk callouts should be expressed as an acceptable range as there are variances in Dk between cores and pregs even within the same material type.

It is understood that many fabricators charge a premium for impedance controlled jobs (Prototron does not charge a premium for impedance controlled jobs) and therefore the thought process from the customers’ side is to select a specific material and dielectric that has been pre-modeled and qualified so that they do not have to call out any impedance control. As I have said before this is a huge risk for a couple of reasons…if unique reference plane scenarios exist, like for 75-Ohm external lines where “pass through“ layers are common on the next adjacent plane and failure to estimate the potential deviation from different Dk values between cores and pregs. These small details can mean the difference of passing or failing impedance.

With environmental conditions, press parameters, differences in pressed dielectrics and differences in Dk don’t assume simply calling out a 4101/# and the dielectrics will get you what you need...Ask your fabricator!


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