Featured Case Study

Objective

Conduct material and process development of multilayer, heavy copper polyimide printed circuit board technologies with mixed copper weight surface and innerlayer conductors for use in integrated power electronics subassemblies. The major objective of this case study is to enable a cost effective manufacturing approach that supports a wide variety of design considerations utilizing a conductor thickness of 6oz copper or greater and will allow for insertion into DOD platforms.

Background

Unicircuit, Inc. in Littleton, CO is a leader in delivering technology driven, mission critical hardware. With that said Unicircuit, Inc. is interested in developing robust manufacturing approaches to enable higher current (power) handling capability in printed circuit board and other related technologies to allow for higher levels of circuit integration and lower manufacturing costs. The aforementioned activity will complement Unicircuit, Inc. existing thermal solutions, hence ultimately providing a deployable path of material development and process innovations towards cost effective products and platforms for DOD applications.

Technologies of Interest

• Materials that enable low cast manufacturing of heavy copper printed circuit boards
• Heavy Copper (> 6oz) on polyimide based materials
• Varied Cu weight intermixed on surface and or inner layers
• Plating and or conductive fill for large and small vias
• Sequential laminated multilayer and subsequent layer / via fill of  >6oz copper layers and high aspect ratio holes
• Fine line geometry on surface layers to allow for fine pitch part attachment
• Surface plating > than 25mils

Challenges / Process Specific

• Inner layer image
• Inner layer etch
• Automated optical inspection
• Final lamination
• Drill
• Conductive via fill
• Copper plate
• Outer layer image
• Soldermask

Final Results / Goal

• Establish manufacturing guidelines that allow for cost effective repeatable manufacturing

Inner layer Image

• The process challenge for the 6 oz and greater inner layers was photo resist adhesion through the etch process.
• The 6 oz and greater inner layers were processed consistently with no evidence of resist breakdown

Inner Layer Etch

• The process challenge was to define an etch factor that would consistently yield a .015mil spacing and trace width of .022mils at the foot and .015mils nominal at the top of the trace.
• Based upon the empirically complied data a .005mil etch factor was determined to be optimum and data was then uploaded into Unicircuit, Inc. CAM database.

Automatic Optical Inspection

• No issues identified

Final Lamination

• The process challenge was to mitigate lamination voids in low pressure areas as well as resin cracks in the fill areas of the 6 oz or greater copper clearances.
• As shown no lamination voids are present however resin cracks are evident. Upon re-evaluation of material set a new material stack up was deployed and results are positive.

Drill

• The process challenge was no nail heading, gauging or cracks due to mechanical drill.

Conductive via fill

• Updated manufacturing guidelines for minimum drill / finished diameter.

Copper Plate

• The process challenge was to selectively plate 3 oz and 6 oz coil areas of the board
• Multiple plating cycles have been defined and new chemistry deployed to decrease cycle time.

Outer layer image

• The process challenge was to define attributes to be deployed into the Unicircuit, Inc. manufacturing guidelines, of which was successfully completed and uploaded  for the following attributes:
• Minimum spacing copper to copper
• Minimum trace width
• Minimum drilled hole to copper
• Minimum pad size over drilled diameter

Soldermask

• Two different methodologies investigated to encapsulate > 6oz of which
• Multiple passes required due to extensive outer layer plating requirements and the necessity to thoroughly encapsulate copper features
• Two different methodologies investigated:
• Spray coat method
• Hand/Squeegee manual application
• Unicircuit recommends further investigation of soldermask types and methods of application as continued optimization of mask types and application would lead to cost reduction
• Strongest case for future Mantech projects

Results (Overall)

Unicircuit, Inc. has established manufacturing guidelines that demonstrate an excellent cost vs. performance curve, repeatability, and proven reliability for DOD products, platforms, and applications based upon the aforementioned case study. Please contact Lance P. Riley V.P. Business Development & Advanced Technology at LRiley@unicircuit.com or directly via phone at 303-548-6464.