Stencil

Mar 28, 2024

Imagineering Inc., Elk Grove Village, Illinois

For many years, stencil printing has been the standard method of depositing solder paste on surface mount assembly printed circuit boards (PCBs). It has provided a durable method of applying solder paste, but there were always difficulties that significantly slowed down a change from one product to another in the assembly operation, and added cost. A significant challenge in newer, smaller electronics assembly is the huge difference in size among components. Therefore, trying to apply the right amount of solder paste for each component with one stencil is difficult. The biggest problem is how to produce quick-turn prototypes without disrupting series production that is already running in the line. Product changeover requires time-consuming tweaks to the stencil printing process, while unnecessarily shutting down an expensive assembly line to change the product. The inability of the stencil’s technology to vary solder paste volume by part, on the run, remains the biggest impact on the soldering quality.

High-speed solder paste depositing by jet printing is made possible by a unique ejection method. The technique ejects tiny droplets of solder paste from a cartridge through an ejector mechanism onto the PCB at the positions required by the Gerber file. The ejector system operates at speeds up to 500 droplets per second, which enables solder paste printing on the fly. The different types of solder paste used for jet printing are delivered in standard cartridges. The cartridge is swiftly snapped into the machine in seconds, so one can switch from tin/lead to lead-free solder paste in a matter of minutes. A barcode label on the solder paste cartridge and identification chip in the cassette ensure that the wrong type of solder paste or solder paste that is past its due date is never loaded by mistake.

The electronic identification barcode label and cassette memory make machine settings automatic. Once the paste code type is entered, the printing can start. Touch-less jet printing technology applies no force to the PCB, hence no support pins are needed, further improving start-to-finish speed. In addition, the print program automatically aligns and adjusts to the PCB board stretch based on the fiducial marks. Accurate temperature control ensures that proper viscosity of the solder paste can be maintained at all times, which in turn leads to higher levels of application accuracy.

The process is completely controlled by software. The solder paste volumes can be altered on-demand. The inkjet printing process allows control of solder paste deposits with precision, in three dimensions. It is possible to fine-tune the volume, area coverage, height, and layers of solder paste that need to be applied for every individual pad, component, and package. Inkjet can print pads for components with pitches as small as 0.4 mm (16 mil). With this level of control, small deposits can be printed next to large ones, something stencils had a hard time doing, which resulted in time-consuming hand-solder operations. To maintain a high degree of flexibility, printing programs can easily be adjusted on the fly if revisions are needed.

The absence of stencils means other time-reduction benefits such as no need for special paper for underside wiping, no need for stencil cleaning machines, no need for cleanup and storage of stencils, and no risk of damaging the stencils during handling.

This work was done by Khurrum Dhanji of Imagineering, Inc. For more information, Click Here .

This article first appeared in the January, 2016 issue of NASA Tech Briefs Magazine.