Direct Laser Drilling in Copper Clad Printed Wiring Boards : Suppression of Overhang Length on Drilled Hole Using PWBs Mixed with Silica Fillers in Build-Up Layer(Machine Elements, Design and Manufacturing)

This report describes the quality assessment of Blind Via Holes (BVHs) of Printed Wiring Boards (PWBs) drilled by a CO2 laser using Cu-direct drilling. In the Cu-direct drilling method, the copper foil and the build-up layer are melted at the same time, and the surface is treated to increase the laser energy absorbed by the copper foil since an untreated copper surface reflects most of the 10.6-μm-wavelength CO2 laser beam. However, there are few reports dealing with Cu-direct laser drilling of PWBs. In addition, when copper and resin with different processing thresholds are drilled at the same time, occurrences of a defect called overhang have been observed. So, in this report, first we propose a new method using thermography to measure the absorptance of a PWB surface for a CO2 laser. Moreover, we investigate how surface treatment of the outer copper foil influences the quality of a laser-drilled hole. Then, we observe the circumference of a point irradiated with the CO2 laser and explain how melting processes are different from surface treatment. Finally, based on the research we establish a method in order to cut down the overhang length as a parameter of drilled-hole quality. We also show that a high absorptance improves BVH quality.

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Microvia Formation for Multi-Layer PWB by Laser Direct Drilling: Improvement of Hole Quality by Silica Fillers in Build-Up Layer

ASME 2011 International Manufacturing Science and Engineering Conference, Volume 2 ◽

10.1115/msec2011-50222 ◽

2011 ◽

Author(s):

Keiji Ogawa ◽

Toshiki Hirogaki ◽

Eiichi Aoyama ◽

Kuniyoshi Obata ◽

Tsukasa Ayuzawa

Keyword(s):

Copper Foil ◽

Fem Analysis ◽

Printed Wiring Boards ◽

The Finite Element Method ◽

Silica Filler ◽

Silica Fillers ◽

Via Holes ◽

Overhang Length ◽

Novel Method ◽

Direct Drilling

Microvia formation technology using lasers has become the dominant method for drilling microvia called blind via-holes (BVHs) in printed wiring boards (PWBs). Laser direct drilling (LDD), drilling directly outer copper foil by laser, has attracted attention as a novel method. In particular, when copper and resin with different processing thresholds are drilled at the same time, an overhang defect occurs on the drilled hole. However, the overhang generation mechanism has not been clarified. Therefore, we investigated it by detailed observation of the drilled-hole section. Moreover, the overhang length was estimated using the finite element method (FEM). Influences of surface treatment of outer copper foil and thermal properties of the build-up layer were evaluated experimentally and analytically. Consequently, an experiment with a prototype PWB with silica filler added in the build-up layer was carried out. Using the prototype PWBs, the overhang was reduced as shown in FEM analysis results.

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