Engineering Spotlight: More Standard Manufacturing edits revealed...

By Mark Thompson | Published on November 19, 2009 - 10:00 AM PST

In This month’s column I will once again talk about some of the edits are made by a typical CAM department for Fabrication.

Previously, I talked about etch compensations for copper features based on the known loss at the Etch process, about Inner layer scale factors based on core thickness to achieve optimum layer-to layer registration. I have also talked about altering dielectrics to achieve desired Impedances staying as close to the intended original line sizes and edits or tweaks to line and space values to achieve Impedances if altering dielectrics is not enough.

This month, I will elaborate more on Inner layer feature edits including addition of flow and starburst patterns and constraints for scored jobs and how we fabricate edge plated type features (holes, slots, and part edge)

Let’s start with the addition of flow patterns.

The lamination process bonds the cores and foils together with pre-preg or “b-stage” materials under Heat and pressure. Once the Pre preg material reaches its Tg (glass transition temperature) and the Pre -preg becomes gelatinous it needs a pathway to flow off the edges of the panel and distribute evenly across the surface of the layer.

Traditionally, fabricator’s such as Prototron add a “dot pattern” around the parts on the panel which is a series of offset dots in metal that allows a web path for the pre-preg to flow. This worked well when dielectrics of cores were .008 or thicker as they are not dependant on the configuration of the layer itself (Plane, split plane or signal) to add Inherent stability to the layer. On thinner cores (.005 and below) we now add what is better known as a “starburst” type flow pattern. This still provides a path to move the preg to the panel edges but also adds additional metal rigidity for the thinner cores that tend to move more through lamination. Each subsequent layer needs to have either the dot pattern or starburst pattern offset so that the metal “dots” do not reside over the top of each other when stacked.

Other Inner layer feature edits at CAM:

Many times the Gerber files as received are done on CAD systems with certain limitations for thermal relief sizes on inner plane layers. An example of this would be thermals drawn so large that the arc’s intersect and overlap on a series of holes creating a disconnect to the adjacent plane for the inner-most holes. In cases such as this we contact the customer and ask to reduce the thermal size to ensure all the holes intended to connect to the plane actually do. This is where an IPC Net-list is invaluable as it would show up in a net-list compare at cam as broken or open nets.

Constraints for Scored Jobs:

Generally the optimum depth for a score line is 1/3 the thickness of the material being used (the overall dielectric of the parts) this leaves approx 1/3 the material thickness as a web remaining. Typically the top surface width of the V groove of the score line on a .062 thick part is .027-.030 wide due to the depth of the score line itself. This means that a .062 thick part set up for score can have NO copper features any closer to the part edge than .015 or the features will get clipped by the score blade. For a .031 thick part, the distance is less (approximately .009-.010) from copper feature to part edge. For a .125 thick part the minimum copper feature to part edge distance would be approximately .022.

Plated edge features:

Plated features at the board edges such as the edge itself (less tab locations to hold it in the panel) and plated half holes or slots require unique processing so that when finally routed the metal at the part edges does not tear out or burr. Plated half holes are drilled, and then the panels go through an electroless copper process (cuposit) then back to drill/rout to have the edge where the plated half hole resides routed. This way no electro less copper is present along the interface edge where the plated half hole meets the un-plated edge so that no electro-plating will “stick” to it eliminating tearing or burring of the plated metal at final rout. Plated edges are accomplished by pre routing the to-be plated edge prior to electroless. Additional metal slightly larger than the router track width is added outside the plated edge at CAM to reflect the rout before cuposit. (To expose the to-be plated edge) after all plating and processing is done the tabs that held the parts in place on the panel during the edge plating process are routed. These tabs should ideally be negotiated with the customer so that a tab is not placed when an edge must be plated.

Thank you for your time.
Please feel free to call me with any questions or comments at:

(425) 823-7000 extension 239
or E-Mail me at MarkT@Prototron.com