Browse Prior Art Database

ALTERNATING ANODE ELECTROPLATING

IP.com Disclosure Number: IPCOM000008832D
Original Publication Date: 1998-Sep-01
Included in the Prior Art Database: 2002-Jul-17
Document File: 2 page(s) / 130K

Publishing Venue

Motorola

Related People

Manes Eliacin: AUTHOR [+2]

Abstract

The circuits on a simple two layer print and etch board are made in three main steps. First, an epoxy/fiberglass panel, with 0.5 to 0.7 mils of copper foil on each side, is drilled to allow top to bottom access. Second, the drilled holes (and the original cop- per foil) are electroless plated and then electroplated with copper; this forms the vertical electrical connec- tions. Third, a polymer etch resist is applied and photo defined into the pattern of circuits, and the exposed copper is etched away; at this point all the copper cir- cuits are complete. After these circuit forming processes are completed, the remaining etch resist is dissolved away, and solder mask can be applied to pro- tect selected areas of the panel.

This text was extracted from a PDF file.
At least one non-text object (such as an image or picture) has been suppressed.
This is the abbreviated version, containing approximately 50% of the total text.

Page 1 of 2

0 M

MOTOROLA Technical Developments

ALTERNATING ANODE ELECTROPLATING

Manes Eliacin and Everett Simons

customer. Commercially available electroplating sys- tems typically deposit significantly more copper thick- ness on the surface foil (where plating is unnecessary) than in the holes where it is needed.

  Finer lines and spaces can be etched by reducing the total thickness of the surface copper (foil plus plat- ing); this can be done by reducing the ratio of plating on the copper foil to the plating in the drilled holes. PRIOR ART

  Chemical etching of the surface copper foil prior to electroless copper plating has been used to reduce the foil thickness. However, uneven etching of the foil leaves variations in copper thickness that cause varia- tions in etched line widths. This method is not widely used to significantly reduce foil thickness, although a mild etch is often used to clean and micro texture the

BACKGROUND

  The circuits on a simple two layer print and etch board are made in three main steps. First, an epoxy/fiberglass panel, with 0.5 to 0.7 mils of copper foil on each side, is drilled to allow top to bottom access. Second, the drilled holes (and the original cop- per foil) are electroless plated and then electroplated with copper; this forms the vertical electrical connec- tions. Third, a polymer etch resist is applied and photo defined into the pattern of circuits, and the exposed copper is etched away; at this point all the copper cir- cuits are complete. After these circuit forming processes are completed, the remaining etch resist is dissolved away, and solder mask can be applied to pro- tect selected areas of the panel.

  Increasing the interconnection density of a printed circuit layer may be accomplished three ways: using finer copper lines, narrower clearances (spaces) between copper features, or smaller via pads with smaller and/or better registered vias. It becomes diffi- cult to reliably etch copper features smaller than twice the copper thickness: etching the copper undercuts the etch resist, as shown in Figure I, until the resist sepa- rates from the ever narrowing tops of the etched lines. The resolution of the photo defined etch resist is much better than the resolution capability of etching lines with a conventional copper thickness of 0.0012 to
0.0018 inches.

  The copper thickness during etch comes from the original copper foil plus the thickness added while plating the holes. Minimum copper foil thickness is often limited by how fine the copper foil can be with- out getting damaged during the manufacturing process- es. Minimum hole wall plating thickness, typically >0.0005 inches, is specified by the customer for relia- bility reasons. The additional copper thickness plated onto the surface foil is thus an artifact of copper plating the holes, and often has no lower limit specified by the

surface.

  Another method to reduce the surface copper thickness is to start with an ultra-thin foil. However, ultra-thin fo...