1 Adhesion and Etching of Nickel-Chromium Tiecoat on Adhesiveless Flexible Laminate by Bergstresser, T., Hilburn, R. and Wang, J. Gould Electronics Gould Electronics Inc. | Curtis Boulevard | Eastlake, Ohio U.S.A. Tel: (440) | Fax: (440) |

2 Copper on Polyimide Substrate Polyimide Film Vacuum Deposited Tiecoat Vacuum Deposited Seedcoat Electroplated Copper Vacuum Deposited Tiecoat Vacuum Deposited Seedcoat Electroplated Copper Gould Electronics Inc. | Curtis Boulevard | Eastlake, Ohio U.S.A. Tel: (440) | Fax: (440) |

3 Test Samples Materials Single-sided construction Polyimide film – 1 mil Kapton*-E Tiecoat –Nickel – chromium (80:20) 50 Å 100 Å 200 Å –Chromium – 170 Å –Nickel – copper (67:33): 70 Å –No tiecoat Copper seedcoat : 2000 Å Electrodeposited copper : 18 µm * Kapton is a registered trademark of E.I. DuPont de Nemours & Co. Peel Strength Coupons Process: Print and etch 1/8th inch (0.32 cm) L & S Ten 8 mil (200 µm) L & S Gould Electronics Inc. | Curtis Boulevard | Eastlake, Ohio U.S.A. Tel: (440) | Fax: (440) |

4 Peel Strength - Initial Results Copper thickness has an effect on peel strength All Meet IPC specifications NiCr peels are comparable to chromium No significant effect of NiCr thickness Gould Electronics Inc. | Curtis Boulevard | Eastlake, Ohio U.S.A. Tel: (440) | Fax: (440) |

5 Peel Strength – Thermal Aging In Air at 150 °C Results All tiecoats improve peel strength retention NiCr comparable to chromium, marginally less effective NiCr improvement over monel No effect of NiCr thickness (168 hrs?) Gould Electronics Inc. | Curtis Boulevard | Eastlake, Ohio U.S.A. Tel: (440) | Fax: (440) |

6 Peel Strength – Thermal Aging In Air at 150 °C Peelback Analyses NiCr samples - 50 and 200 Å, no exposure and after 168 hours No exposure – Failure in PI Thermal aged –Failure primarily in PI –Tiecoat metals detected on trace-side surface, suggests small degree localized interfacial failure –Copper detected on trace and PI side surfaces of 50 Å sample only, indicates small amount copper diffusion NiCr acts as diffusion barrier preventing interface destabilization. Thicker NiCr may provide slight improvement XPS Copper PI Gould Electronics Inc. | Curtis Boulevard | Eastlake, Ohio U.S.A. Tel: (440) | Fax: (440) |

7 Peel Strength – Pressure Cooker Exposure Results PCT conditions: 121 °C, 2 atm, 100% RH All tiecoats improve peel NiCr marginally less improvement than chromium NiCr comparable to monel No effect of NiCr thickness Gould Electronics Inc. | Curtis Boulevard | Eastlake, Ohio U.S.A. Tel: (440) | Fax: (440) |

8 Peel Strength – Pressure Cooker Exposure Peelback Analyses NiCr samples - 50 and 200 Å after 168 hours PCT Exposed –Failure primarily in PI –Nickel on trace-side surfaces, suggests small degree localized interfacial failure –No copper detected Primary failure mode thought to be PI degradation. NiCr acts as diffusion barrier to prevent acceleration XPS Copper PI Gould Electronics Inc. | Curtis Boulevard | Eastlake, Ohio U.S.A. Tel: (440) | Fax: (440) |

9 Peel Strength – After Gold Electroplating Results Very aggressive – no adhesion without tiecoat. 1/8 th inch trace sample had 37% loss All tiecoats improve adhesion retention Results of NiCr comparable to chromium NiCr superior to monel Notes - (10) 8 mil L&S pattern - 30 µin nickel - neutral potassium gold cyanide - 30 µin gold applied in 13 min Gould Electronics Inc. | Curtis Boulevard | Eastlake, Ohio U.S.A. Tel: (440) | Fax: (440) |

10 Peel Strength – Peelback, After Gold Electroplating Observations Disrupted PI visible in upper areas – good adhesion No adhesion in lower areas Undercut attack – about 200 µm Features finer than 400 µm will show complete delamination 400X SEM – Peelback, no tiecoat, 1/8 th inch line Polyimide SurfaceBackside of Copper Trace Gould Electronics Inc. | Curtis Boulevard | Eastlake, Ohio U.S.A. Tel: (440) | Fax: (440) |

11 Peel Strength – Peelbacks, After Gold Electroplating Observations Disrupted PI visible in central areas – good adhesion Undercut attack is about 10 µm each side for monel Undercut attack is about 3 µm each side for NiCr Significantly narrows the line width that can maintain adhesion to the board after gold plating Polyimide SurfaceBackside of Copper Trace 700X 70 Å monel 8 mil line Polyimide SurfaceBackside of Copper Trace 700X 50 Å NiCr 8 mil line Gould Electronics Inc. | Curtis Boulevard | Eastlake, Ohio U.S.A. Tel: (440) | Fax: (440) |

12 Etching – Cupric Chloride Results NiCr does not etch at low HCl concentration NiCr etches readily at HCl concentrations greater than 30 g/L. Single-step etching of copper and NiCr accomplished readily with recommended HCl concentration of 60 g/L Gould Electronics Inc. | Curtis Boulevard | Eastlake, Ohio U.S.A. Tel: (440) | Fax: (440) |

13 Etching – Acidic Potassium Permanganate Results NiCr etch rate is fast initially, then drops to low values with time KMnO 4 concentration not critical for NiCr etch rate, but Cu begins to etch around 50 – 75 g/L. 90 g/L KMnO 4 is recommended. pH less than ~ 2.2 required for good etch rates Gould Electronics Inc. | Curtis Boulevard | Eastlake, Ohio U.S.A. Tel: (440) | Fax: (440) |

14 Etching – Acidic Potassium Permanganate Passivation NiCr etch rate slows due to passivation by MnO 2 reaction product ‘Neutralization’ with oxalic acid or ascorbic acid removes MnO 2 Neutralization with 80 g/L oxalic acid is recommended after acidic potassium permanganate etching For NiCr > 150 Å, multiple pass etching including neutralization is recommended to maintain good etch rates Gould Electronics Inc. | Curtis Boulevard | Eastlake, Ohio U.S.A. Tel: (440) | Fax: (440) |

15 Summary and Conclusions Adhesion and etching of copper on PI substrate with nickel-chromium tiecoat characterized Adhesion after thermal aging –NiCr acts as barrier and provides adhesion retention –Cr => NiCr > Monel >> No tiecoat Adhesion after pressure cooker exposure –NiCr acts as barrier and provides adhesion retention –Cr => NiCr = Monel >> No tiecoat Adhesion after gold electroplating –NiCr minimizes copper undercut at PI interface –Cr = NiCr > Monel >> No tiecoat NiCr Etching –Single step etch with copper in cupric chloride, HCl = 60 g/L –Acid KMnO 4 etches NiCr without etching copper when KMnO 4 concentration is the recommended 90 g/L, pH should be kept less than about 2.2, multiple pass process with oxalic acid neutralizer for NiCr thickness more than about 150 Å Gould Electronics Inc. | Curtis Boulevard | Eastlake, Ohio U.S.A. Tel: (440) | Fax: (440) |

16 Acknowledgements The authors wish to acknowledge Mr. Howard Kaplan and Mr. Richard Le of Siemens, AIM Facility, in Canoga Park, CA, for providing the samples for the adhesion studies as well as for helpful discussions. We would also like to acknowledge these Gould employees for their valuable contribution to this study: Mr. James Winchester - Adhesion Ms. Georgia Crosby - Adhesion Ms. Kathy Prokop - SEM Gould Electronics Inc. | Curtis Boulevard | Eastlake, Ohio U.S.A. Tel: (440) | Fax: (440) |