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August 2017 • The PCB Magazine 47 the demonstrators are production borosilicate glass coverslips with low iron content. The 50 μm glass is a production alkali-free glass with a thermal expansion coefficient matching that of silicon for chip packaging applications. Typical glass ablation parameters are 1 MHz pulse rep- etition rate, 3 μJ/pulse and a feed rate of 500- 1000 mm/s for a spot size of 12-15 μm. When operating with the focus at the top of the glass substrate, trenches can be made in borosilicate glass using these parameters that are ~ 7 μm wide and 5 μm deep in a single pass; addition- al passes can be employed to deepen the trench without significantly affecting the width. Nar- rower and shallower features can be made by lowering the pulse energy or defocusing the beam. The same parameters can be used for pro- ducing pads and vias by utilizing a 7-μm pitch; cross-hatching the fill lines provides the best re- sults. Vias are produced by applying the cross- hatch pattern multiple times or by using a race- track pattern; scanning the Z range of the sub- strate while undertaking this process improves the shape of the via. Larger pulse energies are re- quired as the via diameter approaches the thick- ness of the glass piece (i.e., the aspect ratio nears 1:1). Blind vias can be drilled using either the company system described earlier, or the com- pany's CO 2 -based microvia platform, employ- ing a 9.3 μm CO 2 laser. Laser seeding for electroless plating was car- ried out as follows. A high-power diode-pump solid state green laser operating at 30 kHz and 50-200 μJ/pulse, with ~11 ns pulse width, was focused to a 30-40 μm diameter spot onto a thin copper foil laminated onto a glass slide, melt- ing the copper and directing the material to- ward the desired substrate. To prepare this do- nor substrate, a 4% aqueous solution of poly- vinyl alcohol (PVA) was spin-coated onto a 1 mm borosilicate glass slide and the film al- lowed to dry for several hours to produce a uni- form coating about 1 μm thick. The production foil, as received, consisted of a 10 μm copper foil bound to a 35 μm "carrier" layer of copper. The thin foil was laminated onto the PVA lay- er using a hot press operating near the melting point for PVA for several minutes. For this, the 10 μm copper layer was placed in contact with the PVA layer, and the carrier side is facing out. After lamination, the carrier layer can be easily peeled away leaving the thin layer adhered to the glass supporting substrate. Optimum laser processes utilize bite sizes (beam displacement between pulses) that are 50-75% of the focused spot diameter. Figure 1 shows deposited copper on a flat borosilicate glass surface using the method described above, with no offset between the forward transfer substrate and the receiving substrate. The pro- cess utilizes a single laser pulse with 30 μm fo- cused spot at the work surface with variable pulse energies. The scale bar is 100 μm. The Fig- ure shows that the method has a resolution (de- posited spot diameter) of ~ 50 μm at low pulse Figure 1: Copper deposits made on a smooth borosilicate glass surface using a 532 nm, 11 ns, 30 kHz Nd:YAG laser focused to a 30 μm spot on a 10 μm copper foil adhered with a 1 μm PVA layer to a 1 mm borosilicate glass slide. The pulse energy used to make each row of features is given in the figure. LASER PATTERNING AND METALLIZATION TO REDUCE PROCESS STEPS FOR PCB MANUFACTURING