Mylar Boards – Mastering Solder Stencil Design for Precise Surface‑Mount Assembly

Last modified on May 12, 2025
by Hommer Zhao

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Introduction

Working with surface‑mount components can be challenging, especially when it comes to soldering the smallest parts. Instead of hand‑soldering each component, the industry standard is to apply solder paste to a stencil, place all components simultaneously, and then reflow the board. The stencil is a thin sheet—commonly made from Mylar—with precisely measured holes that align with the board’s solder pads. Creating a high‑quality Mylar stencil is essential for reliable assembly.

One efficient method for producing these stencils is laser cutting. By using CAD software such as EagleCAD and ExpressPCB, you can export the top and bottom layers of your board design. The laser then carves the Mylar sheets to match the pad outlines.

During the cutting process, the pad outlines are slightly reduced in size to compensate for the thermal expansion that occurs when the laser melts the Mylar. This adjustment prevents the final stencil from being too large, which could cause solder bridging.

The typical workflow involves cutting two 7‑mil Mylar sheets. The laser is set to fully cut through the top sheet while only partially penetrating the base sheet. This technique softens the edges of the cut‑out squares and allows the two layers to separate cleanly. After cutting, you peel the layers apart, leaving a pristine stencil ready for use and a scrap piece.