Letter 26016

PCB trainee asks difference between black oxide process and natural oxidation of copper  

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I am a trainee in a PCB manufacturing company. I will be having my presentation this Dec. +++ regarding the purpose of the black oxide (Cupric Oxide) in the lamination process. My question is "What's the difference of the having the natural oxidation of copper and the black oxide process?" "Why do we need a crystalline structure?"

Rodjel Cuebillas
Technical Specialist (trainee)in PCB industry - Manila, Natinal Capital Region, Philippines

The purpose of black oxide on Cu in the PCB process is to provide a rough surface in preparation for the lamination process. If you peel-back the foil copper on a pcb you will find that the underside of the copper foil is rougher than the top side. Under a microscope or SEM you will see that there is a rough or dendritic surface. Black oxide coatings create much the same roughness on a smooth copper surface. This enables the pcb layer to laminate more effectively. Black oxide also comes in brown and red.

Bill Watson
- Santa Clara, CA, USA

The response above is appropriate for normal or "Shiny" foil.

FR-4 Laminate is also sold with Reverse-Treat (RSTF) or Double-Treat (DSTF) foils which have the rough/tooth side out, making the black oxide finish unnecessary. Peel strengths are higher than when black oxide is used, and much higher than those obtained using the newer oxide replacements (which have a bad name among some OEMs).

If your management is open to it, suggest a side-by-side comparison using the reverse-treat vs the black oxide treated layers. You may be able to get rid of the black oxide finish altogether. Several PCB manufacturers have switched over from BO to RSTF/DSTF and have yield increased by 50% or more.

Douglas Corbett
PCBs - Salt Lake City, Utah, USA

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