Depending on the method of component integration to the bare board, manufacturers use one (or a combination) of three different soldering methods: reflow for surface mount devices (SMDs), wave soldering for through-hole (TH) components, and manual soldering for larger components when automated placement is not an option (size, shape, weight, etc.) For SMDs, the only solder present during reflow requires deposition before the process (unlike wave soldering, which occurs in-process); manufacturers need a method to reliably deposit solder onto the pads pre-placement that is repeatable, high-quality, and cost-effective. A PCB stencil, custom to each board design, ensures even application of solder paste to the intended land patterns of the board.
| Soldering With and Without PCB Stencils | |
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| Stencil | No Stencil |
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The Manufacturing of a PCB Stencil
The PCB stencil is necessary for boards that rely on reflow soldering to bond SMD components to the board. As designed by the manufacturer, the stencil is 1:1 with the pad openings where solder paste (solder particles suspended in flux) fills while a squeegee moves across the stencil. This process limits the solder deposition to an exact, repeatable volume per board within the same lot while preventing potential shorts where the solder is outside its intended areas during reflow. The solder paste eventually forms the joint and acts as an adhesive pre-reflow to keep SMD components attached to the board as placed.
Since the stencil must match the PCB design, stencil construction occurs after completing the board layout and following any compensation from the CAM department. The stencil is a thick metal foil that can withstand the pressure of the squeegee moving across its frame. To create the apertures in the stencil that align with the SMD pad areas, manufacturers have a few options available:
- Chemically etched – Similar to the PCB etching process of the laminate foil. The process is cheap (cost will increase with thicker foils due to greater consumption of etchant solution) with relatively low-pitch precision (suitable for component pitches >25 mils / 0.635 mm). The etchant attack on the stencil produces an hourglass shape in the apeture opening that is narrower at the center of the z-axis than the edges. Poor etching processes can produce undercutting, edges, and misalignment in the aperture openings.
- Laser cut – Laser cutting is more expensive than chemical etching but has two primary advantages. First, laser cutting is suitable for pitch precision no less than 14.75 mils / 0.4 mm; this is considerable due to the proliferation of 0.5 mm pitch components seen in many high-density interconnect (HDI) designs. Second, the laser cutting process forms a trapezoidal opening in the stencil foil, allowing for a less impeded flow of solder deposition with a reduced chance of defects in the aperture openings.
- Electroformed – Unlike chemical etching and laser cutting, electroformed stencils are an additive, not subtractive, process. Though typically much more expensive than subtractive processes, additive processes are much more controllable and precise. This precision supports a pitch precision no less than 7.87 mils / 0.2 mm, the highest among the standard options. Like laser cutting, electroformed stencils create trapezoidal openings in the aperture that support solder flow and deposition during application.
How Solder and Stencil Attributes Affect Assembly
Regardless of the construction method, stencil design has to account for the distribution of solder on the pad to ensure a quality solder joint. Lead-free restrictions (AKA RoHS) cause solder deposition, flow, and joint formation changes. It is possible to reduce the aperture with leaded solder alloys, but lead-free solders require surface finishes with excellent wettability characteristics. For example, immersion silver and lead-free HASL can reduce apertures by approximately 10%, while immersion tin and an organic solderability preservative (OSP) do not support aperture reductions.
Another stencil design aspect is the transfer efficiency or the deposited paste’s ratio against the aperture’s volume. There are three factors affecting the transfer efficiency:
- Stencil technology – Depending on the fabrication method, the aperture openings (and thus their volume) are shrinkable.
- Aspect ratio – The length of the aperture’s shorter side divided by the thickness of the foil; this value should be greater than 1.5.
- Area ratio – The area of the aperture opening divided by the total area of the surfaces of the aperture’s walls should be greater than 0.66.
Your Contract Manufacturer Covers (And Uncovers) All Aspects of PCB Production
Ultimately, PCB stencil design is far beyond the purview of the PCB layout designer, but its construction significantly impacts the assembly’s quality and reliability. Designers will want to partner with a manufacturer who has experience with different stencil fabrication methods to optimize the automated assembly processes. Lucky for PCB designers, VSE offers turnkey solutions for PCB manufacturing. Our engineers are committed to building electronics for our customers, including a full design review for cost, quality, and manufacturability. We’ve been realizing life-saving and life-changing devices alongside our valued manufacturing partners for over forty years.
