Thick Copper Design

A CTE Mismatch Between Copper And FR- – 4 Exists

• Copper = 18ppm
• FR-4 = 15-16ppm
• As Copper Gets Thicker, It Has Greater And Greater Influence Over Post Lamination Dimensions Of The Resulting Substrate

Rolled , Annealed Copper Reacts Differently Than ED Copper

• Design Influences On Net Shrinkage Become Far More Significant Standard Scaling Factors (That Compensate Net Shrinkage Effects) Do
Not Apply!

More Resin Is Required To “Fill” Etched Out Areas

• “In-plane” CTE Increases With % Resin Content
• “Another Complication To Net Shrinkage Prediction”
• Resin Rich Package = Higher Z-axis CTE = Greater Sensitivity To Thermal
Excursions, Assembly In Particular.
• Heavy Copper Substrates Generally Require More Heat Input To Assemble
• Reliability!

• Heavy Copper, Embedded In Glass Epoxy, Can Result
In Nonplanar Conditions On The Surface Of The Substrate
• The Heavier The Copper, The Worse The Effect
– – • In Multi- – layer Applications, Local Areas With “Stacked”
Copper, Beside Areas That Are Etched Out, Can Result In Extreme Non- – planarities
• PCB Fabrication Industry Typically Refers To This As “Image Transfer”

Lack Of Planarity Reduces The Effective Resolution Of The Fabrication Process, Limiting Feature Size

Heavy Copper Lamination

• Imaging Resist Thickness Is 1.3 – – 2mils. At Some Point, Image Transfer Interferes With The Imaging Resists Ability To “Conform” To The Lack Of Planarity On The Surface Of The Substrate.
• This Lack Of Planarity Also Reduces The Effective Resolution Of The System, Limiting Feature

Copper Etch Process

• H = Height of Copper Trace
• Etch Factor = 2H / ( W max – – W min)
• Etch Factor Should Be Larger Than 1.0

Etched Trace Design Considerations

1. Thinner Copper Produces More Precise Geometries on
2. Lines Less Than 0.005” in Width Actual Conductor Shape is Close to a Trapazoid
3. Copper Thickness is Slightly Reduced After PCB Processing

Pad Size Reduction

Current Procedures For Applying Uniform Etch Compensation Values Across All Surface Features Are Inadequate.
Below Illustrate A Near Exponential Reduction In Pad Size As The Pad Gets Smaller, For Both Round And Square Pads In
Either Orientation.

Leave a Reply

Your email address will not be published. Required fields are marked *