The importance of templates for PCB assembly

The surface mount assembly process uses templates as a pathway to accurate, repeatable solder paste deposition. A template refers to a thin or thin sheet of brass or stainless steel with a circuit pattern cut on it to match the position pattern of the surface mount device (SMD) on the printed circuit board (PCB) where the template is to be used. After the template is accurately positioned and matched to the PCB, the metal squeegee forces the solder paste through the holes of the template, thereby forming deposits on the PCB to fix the SMD in place. The solder paste deposits melt when passing through the reflow oven and fix the SMD on the PCB.


The design of the template, especially its composition and thickness, as well as the shape and size of the holes, determines the size, shape and location of the solder paste deposits, which is essential to ensure a high-throughput assembly process. For example, the thickness of the foil and the opening size of the holes define the volume of slurry deposited on the board. Excessive solder paste can lead to the formation of balls, bridges and tombstones. A small amount of solder paste will cause the solder joints to dry out. Both will damage the electrical function of the circuit board.

Optimum foil thickness

The type of SMD on the board defines the optimal foil thickness. For example, component packaging such as 0603 or 0.020″ pitch SOIC requires a relatively thin solder paste template, while a thicker template is more suitable for components such as 1206 or 0.050″ pitch SOIC. Although the thickness of the template used for solder paste deposition ranges from 0.001″ to 0.030″, the typical foil thickness used on most circuit boards ranges from 0.004″ to 0.007″.

Template making technology

Currently, the industry uses five technologies to make stencils-laser cutting, electroforming, chemical etching and mixing. Although the hybrid technology is a combination of chemical etching and laser cutting, chemical etching is very useful for manufacturing stepped stencils and hybrid stencils.

Chemical etching of templates

Chemical milling etches the metal mask and flexible metal mask template from both sides. Since this corrodes not only in the vertical direction but also in the lateral direction, it will cause undercuts and make the opening larger than the required size. As the etching progresses from both sides, the tapering on the straight wall will result in the formation of an hourglass shape, which will result in excess solder deposits.

Since the etching stencil opening does not produce smooth results, the industry uses two methods to smooth the walls. One of them is electro-polishing and micro-etching process, and the other is nickel plating.

Although a smooth or polished surface aids the release of the paste, it may also cause the paste to skip the surface of the template instead of rolling with the squeegee. The template manufacturer solves this problem by selectively polishing the hole walls instead of the template surface. Although nickel plating can improve the smoothness and printing performance of the template, it can reduce openings, which requires adjustment of the artwork.

Template laser cutting

Laser cutting is a subtractive process that inputs Gerber data into a CNC machine that controls the laser beam. The laser beam starts inside the boundary of the hole and traverses its perimeter while completely removing the metal to form the hole, only one hole at a time.

Several parameters define the smoothness of laser cutting. This includes cutting speed, beam spot size, laser power and beam focus. Generally, the industry uses a beam spot of about 1.25 mils, which can cut very precise apertures in a variety of shapes and size requirements. However, laser-cut holes also require post-processing, just like chemically etched holes. Laser cutting molds need electrolytic polishing and nickel plating to make the inner wall of the hole smooth. As the aperture size is reduced in the subsequent process, the aperture size of laser cutting must be properly compensated.

Aspects of using stencil printing

Printing with stencils involves three different processes. The first is the hole filling process, in which solder paste fills the holes. The second is the solder paste transfer process, in which the solder paste accumulated in the hole is transferred to the PCB surface, and the third is the location of the deposited solder paste. These three processes are essential for obtaining the desired result-depositing a precise volume of solder paste (also called a brick) in the right place on the PCB.

Filling the template holes with solder paste requires a metal scraper to press the solder paste into the holes. The orientation of the hole relative to the squeegee strip affects the filling process. For example, a hole with its long axis oriented on the stroke of the blade fills better than a hole with its short axis oriented in the direction of the blade stroke. In addition, since the speed of the squeegee affects the filling of the holes, a lower squeegee speed can make the holes whose long axis is parallel to the stroke of the squeegee better fill the holes.

The edge of the squeegee strip also affects how the solder paste fills the stencil holes. The usual practice is to print while applying the minimum squeegee pressure while maintaining a clean wipe of the solder paste on the surface of the stencil. Increasing the pressure of the squeegee may damage the squeegee and the template, and also cause the paste to be smeared under the surface of the template.

On the other hand, the lower squeegee pressure may not allow the solder paste to be released through the small holes, resulting in insufficient solder on the PCB pads. In addition, the solder paste left on the side of the squeegee near the large hole may be pulled down by gravity, resulting in excess solder deposition. Therefore, minimum pressure is required, which will achieve a clean wipe of the paste.

The amount of pressure applied also depends on the type of solder paste used. For example, compared to using tin/lead paste, when using lead-free solder paste, the PTFE/nickel-plated squeegee requires about 25-40% more pressure.

Performance issues of solder paste and stencils

Some performance issues related to solder paste and stencils are:

The thickness and aperture size of the stencil foil determine the potential volume of solder paste deposited on the PCB pad

Ability to release solder paste from the template hole wall

Position accuracy of solder bricks printed on PCB pads

During the printing cycle, when the squeegee strip passes through the stencil, the solder paste fills the stencil hole. During the board/template separation cycle, solder paste will be released onto the pads on the board. Ideally, all the solder paste that fills the hole during the printing process should be released from the hole wall and transferred to the pad on the board to form a complete solder brick. However, the transfer amount depends on the aspect ratio and area ratio of the opening.

For example, in the case where the area of ​​the pad is greater than two-thirds of the area of ​​the inner pore wall, the paste can achieve a release of better than 80%. This means that reducing the template thickness or increasing the hole size can better release the solder paste under the same area ratio.

The ability of solder paste to release from the template hole wall also depends on the finish of the hole wall. Laser cutting holes by electropolishing and/or electroplating can improve the efficiency of slurry transfer. However, the transfer of solder paste from the template to the PCB also depends on the adhesion of the solder paste to the template hole wall and the adhesion of the solder paste to the PCB pad. In order to obtain a good transfer effect, the latter should be larger, which means that the printability depends on the ratio of the template wall area to the opening area, while ignoring minor effects such as the draft angle of the wall and its roughness. .

The position and dimensional accuracy of the solder bricks printed on the PCB pads depend on the quality of the transmitted CAD data, the technology and method used to make the template, and the temperature of the template during use. In addition, the position accuracy also depends on the alignment method used.

Framed template or glued template

The framed template is currently the most powerful laser cutting template, designed for mass screen printing in the production process. They are permanently installed in the formwork frame, and the mesh frame tightly tightens the formwork foil in the formwork. For micro BGA and components with a pitch of 16 mil and below, it is recommended to use a framed template with a smooth hole wall. When used under controlled temperature conditions, framed molds provide the best position and dimensional accuracy.

For short-term production or prototype PCB assembly, frameless templates can provide the best solder paste volume control. They are designed for use with formwork tensioning systems, which are reusable formwork frames, such as universal frames. Since molds are not permanently glued to the frame, they are much cheaper than frame-type molds and take up much less storage space.