Laser induced forward transfer of solder paste for microelectronics assembly applications

Currently the most widely used technique for solder paste deposition is Surface Mount Technology (SMT), which involves the printing of solder paste using a stencil, onto printed circuit board (PCB) interconnection pads. However, this process accounts for 50-70% of post-assembly defects. Here, we report the use of Laser Induced Forward Transfer (LIFT), for the reliable printing of commercially available solder pastes. LIFT is an environmentally friendly, mask-less technique and offers high resolution (down to 60 mu m) control over the printed volume with high throughput. LIFT has been previously employed for the reproducible and high throughput (speed up to 2 m/s) printing of metal nanoparticle inks, but the achievement of reproducible deposition of bumps comprising micro-particles (such as in type-5 or 6 solder pastes) stills poses severe challenges. By investigating the whole spectrum of LIFT process parameters - the donor film thickness, the donor - receiver gap, the effect of a sacrificial layer, the laser spot size and shape and the laser fluence - this paper reports on the digital and reproducible transfer of solder paste bumps at the designated pads of ultra-fine pitch PCBs. The process optimization is enabled by employing a side view set-up, consisting of a high-speed camera (up to 540 kfps) coupled with a lens system for 3x optical magnification of the ejection. The reported results highlight the advantages of a digital and high-resolution solder paste deposition method and validate the compatibility of LIFT with PCB assembly.