Stencils Speed Soldering

By Joe Gorse

New challenges face printed circuit board (PCB) prototyping due to lower cost and higher performance driving electronic part manufacturers away from bulky through-hole components to higher contact density surface-mount form factors. Without dedicated manufacturing capabilities the in-house prototyper has two options for PCB assembly: Continue as before by adapting the hand-soldering technique to the shrinking surface-mount parts or integrating reflow soldering with stencil-applied solder paste.

The process of creating a prototype PCB requires multiple phases and steps. The process starts with design specification, proceeds through electrical design into the physical design (layout), manufacture of the printed circuit boards themselves, and culminates in the assembly of these boards. The whole procedure may be iterated through phases to provide flexibility in tracking and meeting evolving system requirements. Errors can be introduced at any one of these steps.

With the increase in the number of components available in surface-mount packages, assembly will continue to move away from the realm of hand-assembly. If you’ve tried, then you know the challenges of to hand-soldering an 0805 (pronounced “oh-eight-oh-five”) component. Trying to work a soldering iron in and around anything smaller is difficult indeed; especially when the component can be so light that a mis-directed exhale clears it from the board.

As small and leadless parts become the exclusive standard in form factor, alternative methods to the hand-soldering style for assembly are needed. The trouble is that those used in manufacturing, such as pick-and-place machinery, can be cost prohibitive in capital equipment and expertise.

In the traditional hand-solder workflow, you can estimate the time for picking, placing, and soldering at 2:20 per part. This is an average. For passives, it’ll be less; for large discrete, or connectors, it’ll be significantly more. But when you factor in the steps of priming the footprints with solder and an iron, then placing the component with tweezers, then soldering the ends, 2:20 per part is a reasonable estimate. Because the technician is preparing and soldering each component individually, the prep time and reflow time are built into the per-piece time estimate.

Stencils are customized to the board you are assembling. Generally speaking, the stencil is made out of a solid sheet such as aluminum, copper, and mylar. In general, there are two ways to obtain one: make it yourself with a laser cutter or have a PCB company make one for you.

With a stencil, a technician will align to the PCB and then apply a thin coat of solder paste across the whole board. The time needed to apply solder paste becomes fixed – the same length of time for a larger, complex board as for a smaller, simpler board. For estimating, we’ll assume it takes 2:30 per board to apply solder paste using a stencil. Also, the time it takes to place the components onto the PCB with tweezers is probably about the same except that you’ll be placing them all at once in a big batch. This part of the estimate scales up and down with the number of parts you need to place, but a time of 20 seconds per part to pick, place and nudge onto the proper footprint is reasonable. Remember, this 20 seconds is built in to the 2:20 of the hand-soldering estimate.

PCB development process

The stencil technique also means you use a reflow oven process to heat the solder paste and attach all components at one time. Again, this is a fixed-time step. Because you can be assembling the next board while you reflow the previous board, the reflow time has been left out of the estimate.

Now if we assume it costs $25/hr in labor for assembly and $120 for the stencil, a simple trade analysis can be made. It only takes a handful of parts, on even a single board, before the stencil starts paying for itself; first in time saved, then also in overall quality of work and increased yields from assembly.

Sunstone Circuits has added stencils to their product line to help make the in-house board assembly process faster, easier, and more accurate. The use of stencils will become even more common with the migration to surface-mount and leadless packaging for an ever-increasing number of components.

Apart from the variety of materials, stencils come in two classes: framed and prototype. Framed stencils are built with specific hardware, which the solder paste screen-printing machine uses for mounting.

Prototype stencils are not usually mounted to a frame. Instead, three edges of the stencil are bent at a right angle to create a “sink” for the solder paste, to stiffen the stencil, and to provide handles for the user. Prototype stencils are optimized for use with pre-release and prototype work.

Sunstone Circuits
www.sunstone.com

::Design World::