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JULY 2019 I PCB007 MAGAZINE 13 reinforced dielectric material. After the UV la- ser opens the copper window, the CO 2 laser then removes the remaining laminate material to expose the target pad. The copper target pad reflects the CO 2 laser beam, and the reflected photons collide with the incident photons at the copper surface. The photon interaction and reflection leaves a very thin resinous skin [2] on the copper surface of the target pad that must be removed before the microvia is plated for a reliable connection. Many manufacturers today add a UV target pad cleaning step to remove the residual film. The intensity of the UV laser cleaning step may adversely modify the target pad copper surface. It was observed that melted and recast copper features were produced around the rim and edge of the copper foil at the outer layer of the microvia, but this damage did not pro- duce reflow-induced failures. The entry copper thickness is typically less than 0.5-oz. copper. 3. CO 2 with Chemically Etched Copper Window A low-power CO 2 laser beam cannot ab- late through reflective copper but can ablate glass-reinforced dielectric material. An open- ing must first be created in the copper foil to allow the CO 2 laser beam to drill a microvia. The window is created by imaging and etch- ing the copper foil. The diameter of the copper window may be either less than or greater than the CO 2 beam diameter. The process practiced depends on the ability to register the CO 2 beam within the copper window and target pad, tak- ing into account cumulative registration toler- ance for material stretch and shrink. The CO2 beam reflects off the target pad and leaves the characteristic resinous film that must be re- moved. Since copper windows are formed by a print-and-etch process, there is no restriction on entry copper thickness. The low-power CO 2 laser does not melt or recast target pad copper. There are two etched window processes practiced. The conformal etched copper win- dow process consists of an etched copper win- dow diameter that is less than the CO 2 beam diameter, so the copper window defines the microvia diameter. The oversized etch copper when thermally stressed during reflow assem- bly. The rest of this article presents SEM pho- tographs of microvias after laser drill and after electroless copper to show the condition of the copper surface of the target pad produced by the commonly practiced laser processes. Results and Discussion The PWB manufacturing base uses different laser types, plating chemistries, and laminate materials in the manufacture of PWBs with mi- crovias. A reliable stacked microvia depends on the condition of the target pad after the laser process. Laser drilling is the accepted method of forming microvias. UV Nd:YAG and CO 2 la- sers were developed to drill microvias and are commercially available in four different con- figurations to the PWB industry. 1. UV Nd:YAG Laser The UV laser can micro-machine a variety of materials, including copper and glass-rein- forced dielectric laminate to produce unique features. The ability to ablate copper can po- tentially damage copper surfaces, such as the target pad of a microvia. SEM photographs of target pads touched by the UV beam show evi- dence of melted copper and copper particles that were explosively ejected from the melted copper surface. Copper flow patterns were vis- ible on the target pad along with porosity and inclusions that were characteristic of the sput- tering and recasting of molten copper. Molten copper flow patterns were visible over the sur- face of the target pad and the rim of the out- er layer copper foil surrounding the microvia opening—basically any copper surface the UV beam contacted. Constructions require surface copper thickness thinner than target copper thickness to prevent the UV beam from pierc- ing the target pad copper. In most construc- tions, UV copper ablation requires entry cop- per to be less than 0.5-oz. copper weight. 2. UV-CO 2 Laser Referred to as a combination laser drill ma- chine, it consists of a UV laser and a CO 2 laser. The UV laser first creates a window through the copper foil and slightly bites into the glass-

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