Abstract
The technique of sandblasting was utilized in this study on polyimide (PI) substrate. Alumina particles, F280, F240, F220, F150 (FEPA nomenclature) under air pressure 0.15x106 Pa to 0.50x106 Pa was used here. It was found that the average surface roughness of PI substrate increased with particle size and air pressure. The range of roughness was between 118 – 1189 nm, while the standard deviation was within 10% of average roughness suggesting reasonable uniformity of the process. After sandblasting, the PI substrate was coated by silver ink by the doctor blade method. Solvent of the ink was water. The sample was then sintered at 220℃ and 280℃ for 20 minutes and then tested for adhesion by ASTM D3359 and D4541, with the intention to establish definite correlation between these two methods. Our results showed that the adhesion between silver colloids and PI substrate increased with increasing surface roughness and could reach the highest 5B level for rough PI substrate. Nevertheless the increase in adhesion was accommodated by simultaneous increase in electrical resistivity of silver film after sintering. The optimal condition after compromise was: using F220 particle, 0.4 MPa air pressure to increase surface roughness to 792 nm. The adhesion level could reach 5B level and the electrical resistivity was 1.48x10-5 Ω-cm (about 9.4 times bulk value). If we substitute 12 wt% silver colloid by 3 wt% silver nanowire (11 micron in length), we could improve about 13% in electrical resistivity while the total quantity of silver was reduced by 22.5%. On the study of drying behavior of ink droplet, it was found that coffee ring would not form when the solid content was below 0.08 wt%. It was also found that the ring width increased with droplet volume. Adding n-propanol would reduce the contact angle between ink and glass substrate. When the solid content was 0.08 wt%, droplet volume of 1 μl, 10% propanol in the solvent, one would obtain a good quality ring with width of 6.7 μm and almost no particles were observed within 300 μm from the ring.