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The Bare (Board) Truth: Tips for Getting the Boards You Need
Wednesday, September 05, 2012 | Mark Thompson, CID+, Prototron Circuits

This column is once again about understanding the customers’ needs. Some customers’ requirements are as simple as a specific definition for a fiducial size, rail tooling, or orientation feature, while other customers can require special processes.

Today, I will offer a few fabricator tips that can help designers get the boards they need, and not necessarily what they think they want. The last time I wrote about this topic, I got a good deal of feedback from our customers.

1. If you cannot provide image files such as Gerber or ODB++ with your quote and the part is impedance-controlled, many fabricators will send you an impedance checklist. This should include the very basics: Which traces are being controlled? On what layers do they reside? What is the threshold and tolerance associated with them?

Also, if we cannot see Gerber or ODB++ at the quote stage, we may ask, “What color and type will the mask be?” and “Will there be any coplanar coupling going on?” Believe it or not, at the smaller trace geometries, the color of solder mask can affect the impedance modeling. Any color other than the standard green masks are about half a mil thicker, and with traces and spaces less than .1mm, this can affect the impedance. Additionally, if we calculate all traces in “free space” because we have no image files to review (assuming no coplanar coupling) and the job comes in closely coupled, again the calculations/model will need to be redone.

2. We fabricators are a very literal group. If your drawing specifies the trace sizes that are impedance-controlled, make sure there are no rounding errors if converting from metric to Imperial, or from one output format to another. If you specify .0052” traces on X layers to be 50 ohms +/-10%, but there are no .0052” traces, you can expect a phone call from your fabricator. Remember, we will build it the way you tell us to build it.

3. Be sure you specify electrical connections that are feasible for your fabricator. For example, let’s say you have thermal ties on an inner plane layer, all inside a “fenced” or isolated area, and some are oriented in such a way that the hole with thermal ties is NOT escaping to the adjacent isolated plane because of the orientation of the spoke ties. The hole, however, is connected on another layer at the same spot or by association. Not everyone can build this type of construction.

4. Quotes from a fabricator based solely on preliminary drawings should be considered preliminary quotes, subject to change upon receipt of the final artwork files, drawing and drill files. Many times a customer will not include vias in the prelim data, and since vias can be 1/3 or more of the total hole count on any given job, they could easily affect the quote accuracy. Other items not on prelim drawings may include requirements for via fill or surface finish encapsulation, or even controlled impedances. Your chosen fabricator should be looking at the final data  and drawings carefully.

5. Beware material deviations. Most folks state on their drawings to use a specific type of material or equivalent based on the IPC-4101 number. This does NOT mean you can substitute any material the fab shop may have; it means you may use an EQUIVALENT. For instance, let’s says the drawing calls out P-370 or equivalent. In the 4101 family, the applicable options would be N-4000-29, IS-410 or IT-180 – all possess the survivability needed for the higher temps at assembly due to alternative surface finishes. G10 or standard FR-4 would NOT be an equivalent in this case.

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