PCB + ASSEMBLY Source:
PCB + ASSEMBLY Source: Indium Source: Indium Soldering profile 1. Soldering profile 2. area increases significantly. This increased powder surface area will require more flux activation, or flux with a better oxidation barrier to protect the powder from surface oxidation 2) . Hence, it is important to both choose a correctly formulated flux that works with finer powder, as well as a solder powder with a low oxide level. These factors will enhance the stability of the fine-pitch solder paste in terms of shelf life, stencil life, and wetting or solderability performance. Excessive activator levels can also destabilize the paste, so flux formulation is complex. Flux system Different types of flux systems are typically available: water-soluble flux, standard no-clean flux, and more recently, the ultra-low residue (ULR) no-clean flux have seen widespread use, especially in semiconductor assembly applications 4) . Currently, most SiP applications use the water-soluble flux system, where the residue is cleaned away using water after reflow, followed by the drying and overmolding steps. As the miniaturization trend continues, some SiP applications will soon reach a point where effective cleaning with a very tight gap and standoff becomes challenging, and using an ultra-low residue no-clean paste will be necessary. This new class of solder paste enables all cleaning and chemicals process costs to be eliminated, while reducing both component warpage and cycle time. Acknowledgement Special thanks to Indium Corporation’s Suzhou Simulation Lab team – Ms. Wisdom Qu, Dr. Fiona Chen, and Leon Rao – for their help and support in performing the solder paste printing tests in their lab. First presented at IWLPC, October 2016, San Jose, California. Paste rheology Decreasing the powder size of the solder paste increases the viscosity, though, since the flux vehicle itself is thixotropic. The paste used in stencil printing is a shear-thinning thixotropic material 5) with a low yield stress, the rheology being governed by both the flux system and the solder powder. The paste rheology controls a variety of factors: paste aperture filling ability, transfer efficiency of paste from stencil onto substrate, and the shape of the paste deposit after printing. These are keys for good printing performance of a solder paste. Paste slump The term “slump” describes an x-y direction expansion of the solder paste deposit caused by gravity. A yield stress in the paste can prevent slump. A metal load that is too high can give the paste a very high yield stress and prevent the paste from filling and releasing from stencil apertures; therefore, optimizing the metal load for the printing process is important and is driven by several factors. Paste stencil life A solder paste with long stencil life is important to reduce print-toprint variability over time. Typically, a stencil life of at least 4 to 8 hours will be required for a high-volume manufacturing environment. Printing experiment Several combinations of paste, tooling setup, and stencil aperture sizes were investigated, together with three solder paste specimens featuring different flux chemistries and different powder sizes. Also, a board support system of pallet and vacuum support were compared to investigate whether the use of a pallet alone could allow the printing process to achieve a comparable consistency. Finally, laser-cut and electroformed stencils were compared by printing different apertures sizes. 52 EPP EUROPE April 2019
PCB + ASSEMBLY Source: Indium Source: Indium Soldering profile 3. Soldering profile 4. Test vehicle A test vehicle was specially designed to mimic a typical substrate size of 237 mm in length, 62 mm in width, and 0.5 mm thick. There are two arrays of pads on the test vehicle. Each row has different pad sizes: 150 μm x 125 μm on row 1; 150 μm x 100 μm on row 2, and 150 μm x 112.5 μm on row 3. Each column has a different gap distance between pads, i.e., 50 μm, 80 μm, 100 μm, 130 μm, and 150 μm. Pads are arranged in horizontal and vertical positions such that different squeegee wiping directions could be simulated. The pad surface finishing is NiAu (ENIG, electroless nickel immersion gold ) and are non-solder mask defined (NSMD). The test vehicle also consists of 01005 pads, but it is not the focus. Paste specimens Three flux vehicles were chosen in this printing test: two water-soluble fluxes and one no-clean flux. These three fluxes were mixed with type 6 and 7 powders. As finer powders yield higher viscosity, a slight tweak in the metal load for type 7 pastes is needed to reduce the viscosity to about the same viscosity as the type 6 solder pastes. Stencil design Three stencils were prepared for this study: one 50 μm-thick lasercut stencil and two electroformed stencils with thicknesses of 35 μm and 50 μm. The laser cut stencil has a 1-to-1 opening with the pad size on the test vehicle, while the left array of the electroformed stencil follows a 1-to-1 opening, and the right array has a smaller aperture. The two electroformed stencils have the same aperture designs but only differ in the stencil thicknesses. Equipment and tooling setup • DEK Horizon printer • Koh Young SPI machine • 12 “ squeegee at 60 ° wiping angle • Vacuum support • Carrier pallet Printing parameters Print speed was fixed in all experiments while the squeegee pressure was adjusted so that minimum pressure was used in each experiment, i.e., enough pressure to just wipe the stencil surface clean, see table. A minimum pressure is crucial in achieving consistent prints as excessive pressure could cause movement of the printing surface during the squeegee stroke. Cleaning frequency was set to three pieces, with a cleaning mode of wipe/vacuum/dry (W/V/D). Bridging between pads The comparison demonstrates a clear difference between tooling combinations. By using for the board support a carrier pallet, no bridging was observed down to an 80 μm gap distance when using a laser cut stencil. By using a vacuum support, the laser-cut stencil was able to achieve non-bridging at the 50 μm gap distance column. This suggests that the vacuum support provides a much better functionality with minimal gap between the stencil and PCB during printing. References 1) Lim, Thum and Mackie: Meeting Solder Paste Printing Challenges for SiP in ‘Smart’ IoT Devices, Chip Scale Review magazine, Jul-Aug 2016. 2) Ed Briggs: Meeting Future Stencil Printing Challenges with Ultrafine Powder Solder Pastes, International Conference on Soldering and Reliability, Toronto, Canada, May 2014. 3) Rita Mohanty Ph.D., S. Manian Ramkumar Ph.D., CEMA, Chris Anglin, Toshitake Oda: Effect of Nano-Coated Stencil on 01005 Printing, APEX 2011. 4) Sze Pei Lim, Maria Durham, A. Mackie: No Clean Material For Advance Packaging Assembly, Semicon China, Feb 2016. 5) Kravcik and Vehec: Study of the Rheological Behavior of Solder Pastes, Proc. Scientific Conference of Young Researchers 2010, FEI TU of Kovice. EPP EUROPE April 2019 53
