Effect of UV Laser Cut Parameters on Thin FR4 PCB ENIG Pad Oxidation or Discoloration

Mechanical cutting is a common method used for singulation of thick PCB. Examples of mechanical cutting are the wafer saw and package saw machine [1]. For thin PCB substrate with thickness of 0.3mm and below, mechanical cutting methods will induce delamination, deformation and other PCB defects that will lead to yield loss during SMT manufacturing. Therefore, a non-contact singulation process, UV laser cut is a preferable method to cut thin PCB panel. However, there are challenges on UV laser cut process mainly induced by the heat generated during the singulation process. The heat generated during UV laser cut process is higher if compared to mechanical cutting process. One of the reasons is that there is no DI water spray during the singulation process to reduce the temperature at the cutting area if compared to mechanical cutting process such as wafer saw and package saw with DI water cleaning and cooling. The heat generated from UV laser beam will cause oxidation/discoloration at the surrounding area of the laser beam and also burn mark at the cutting surface. As for PCB design with ENIG pad edge that close to the PCB edge(<150um) for singulation, the heat generated is high enough to oxidize the pad surface which will cause solderability issue during 2^nd level interconnect. Hence, the optimization of the UV laser cut parameters is critical/essential to reduce the risk of oxidation on the adjacent pad by the UV laser beam. Experiment has been conducted to assess the effect of UV laser cut parameters on ENIG pad discoloration/oxidation for thin FR4 PCB with thickness of 0.3mm. The UV laser cut parameters under this study are power, frequency and cutting speed of the UV laser beam. The responses/output parameters of this assessment are the HAZ(Heat Affected Zone) and visual comparison on the severity of the oxidation. From the experiment, it was concluded that the contribution factor of laser cut parameters of power, frequency and speed on ENIG pad oxidation in the sequence from high to low is: Speed>Frequency>Power. This paper will reveal in detail the design of the experiment, results, conclusion as well as recommendation on the next experiment to further understand the laser beam characteristic for advanced packaging industry application.