Do thinner boards require a different transport mode?

Just when we think we have reached the limit on shrinking substrate thicknesses, tighter pad spaces and higher component densities, the industry says, “Not so fast!” Today’s mobile phone boards average a remarkable 0.6mm thickness, with as many as 1,000 components packed into a 20mm x 80mm space. Over the past five years, advanced equipment sets have accelerated transport, tooling, vision systems, inspection capabilities and platform controls, all of which have certainly made producing high-quality products with ultra-small dimensions possible. However, in the stencil printing world, even more may be required to ensure maximum board stability during the print operation.

Traditionally, the mode of transport – bringing the PCB or pallet into the machine – has been achieved on some form of rubberized belt. This will no doubt continue as the solution for the assembly line. Inside the printer, however, not only is the board brought into the machine on the belt, but the substrate is clamped to the belt to hold it stationary, present it to the stencil and print. This has worked very well for years and is fine for multiple product builds. For mobile phones and other handheld products, however, current and future dimensions dictate a new paradigm. What are 600µm-thick phone PCBs today likely will continue to get thinner and, even at their current architectures, are susceptible to any type of undulation or extra pressure. Clamping thin, small boards or pallets to a rubber belt can result in movement, twisting or bowing at the substrate edges and potential print accuracy issues. There are flat belt options, which have been the interim solution for thin board printing, but the belts are still constructed from rubber and not completely rigid. Finally, belts are subject to wear; they eventually lose elasticity and require replacement. Without proper maintenance, even greater instability can occur.

To keep pushing the miniaturization envelope and remain high-accuracy print-capable, a novel approach to board stability during the print – i.e., the squeegee application of solder paste – has been developed. This off-belt printing solution has a radical new design. With this system, board transportation into the print platform is naturally managed with a belt system. However, when the substrate is raised to print height, instead of clamping to the belt, as in the conventional approach, the board is positioned on two flat metal surfaces engineered to extremely high degrees of flatness and coplanarity. The belt is removed from the material application operation, resulting in substrate rigidity and stability throughout the print cycle. This is designed to improve alignment and offer a highly stable presentation of the coplaned substrate to the stencil.

While clamping modification is one important element of ensuring substrate stability, the foundation for all system components – from cameras to understencil cleaning systems to the print head – must be robust to manage the multiple moving parts. The platform’s rail system and frame structure require engineering that balances the speed and complexity of the process with ruggedness and capability to mitigate vibration for optimized printing. Almost counterintuitively, a tough, vibration-resistant frame does not always mean using the heaviest materials available. Controlling movement and providing a solid, motionless foundation necessitates sophisticated designs that marry durable materials with advanced joining mechanisms to deliver maximum stability. Frame systems for extreme high-accuracy semiconductor wire bonding platforms, as well as those that effectively support the high-speed movement and sudden stops inherent with placement systems, are increasingly being leveraged for new stencil printer designs to ensure stability for miniaturized assemblies.

Printing capability has come a long way in the past five years. Still, chasing that extra 5 or 10µm needed for current and future designs – something that once seemed aspirational – is now necessary.

Clive Ashmore is global applied process engineering manager at ASM Assembly Systems, Printing Solutions Division (asmpt.com); clive.ashmore@asmpt.com. His column appears bimonthly.