Formal Stackups are designed by Fabricators, NOT PCB designers.
PCB designers create requirements that the fabricators use to design the stackup.
Fabricators can select from a wide variety of materials, like . . .
Cores, PrePregs, Glass Weaves and Resin Contents
Fabricators make material choices that meet their manufacturing processes needs.
Here's what PCB Designers can and SHOULD do:
When requesting a stackup let the fabricator know if the design is using VIPPO or heavy copper.
Let the fabricator know if you have CAF concerns or if you need high speed glass.
For High-speed and RF you may want to specify materials that have low Df specs.
Specify the Desired Overall Board Thickness +/- 10%
PCB Designers Create Fab Drawings with Controlled Requirements, like . . .
50 OHM 0.005" TRACES L1 REF TO L2 +/- 10%
50 OHM 0.004" TRACES L3 REF TO L2 AND L4 +/- 10%
100 DIFF 0.004" TRACES, PITCH 12 (GAP + WIDTH = PITCH)
RF TRACES
50 RB/CPW 0.020" TRACES, PITCH 30 L1 REF TO L4 +/- 10%
Designers can add notes to the project Fab Drawing, like . . .
Reference Only: Refer to Fabricator Stackup.
Customer Approval is Required for Stackup Changes.The requirements in the fab drawing will drive the stackup designed by the fabricator.
PCB Designers can use various calculators to approximate the stackup, however the final stackup needs to come from the fabricator that is fabricating the PCB and performing TDR tests for the controlled impedances.
To expedite designs and minimize layout rework a formal stackup should be obtained from the preferred fabricator to optimize controlled impedances.
Length matching may also need to be reworked if it is done before a formal (fabricator) stackup is obtained.
Summary: Working with a fabricator sooner, rather than later is highly recommended.
Abbreviations:
CAF is Cathodic Anodic Filament
Df is Dissipation Factor ( signal attention at higher frequencies )
VIPPO is Via In Pad Plated Over
That's it !
