What You Need to Understand About PCB Stencils?

Understanding PCB stencil

The PCB printing phase is a critical step mostly in the PCB manufacturing process. Furthermore, it is the most difficult process since even little errors can have a substantial impact on the final product's functionality. The majority of printing phase mistakes are caused by the placement of solder joints on the boards.

PCB stenciling is the most effective way to eliminate these problems. You can virtually automate the installation of solder junctions with PCB stenciling. This reduces time and avoids human mistakes that occur with hand solder joint installation. This article will teach you all you need to know about solder paste PCB stenciling.

Definition of PCB stencil

A PCB stencil is a tiny layer of material having holes that alter the arrangement of surface mount components aboard. A stencil is a thin sheet of copper or stainless steel stencil foil with a circuitry stencil etched into it. Solder stencils are the most popular Printed circuit boards stencil that may be used to correctly determine the new quantity of solder paste in a single round across the PCB. After applying solder paste to a base of the PCB, it is ready to accept Surface Mount Devices (SMDs). After uninstalling the stencil, solder paste layers featuring aperture forms adhere to the surface.

The uniform deposition of the paste results in precise solder junctions with strong mechanical strength and excellent electrical connections. There are two kinds of stencils: framed stencils and frameless stencils. Although unframed stencils become less cheap and difficult to store, they are not suited for high PCB stencil manufacturing. Frameless solder paste stencils provide the best sound quality for prototype Printed circuit boards assembly. They are intended to be used in conjunction with a stencil tensioning method, which consists of reusable stencil frames.

The material of the stencils has a considerable impact on its capacity to deposit custom solder paste stencils through the holes. However, you may improve the glue discharge capabilities of the stencil by covering it.

Stencil printed circuit board layout

Circuit boards featuring copper pads that improve conductivity and heat dissipation necessitate custom-designed stencils. Copper lift and poor soldering of external leads occur in designs that lack pads. To avoid such problems, incorporate a 'window impact' into your hole layout. This avoids difficulties by restricting the amount of solder used.

If the copper pads have vias to improve thermal dissipation, another stencil design is required to prevent paste deposits from forming in the vias. Many pads also require stencils with varying thicknesses. Thin stencils, for instance, are required for sensitive pitch parts, whereas thicker stencils are required for components.

The good news is that using multilevel stencils with step-up and step-down portions, you can meet all of the aforementioned requirements. More material is applied to specified stencil spots to create step-up sections.

Other features of PCB stencil design are listed below.

Aperture Design

The holes' design has several effects on the joints. For starters, it has an impact on the likelihood of problems such as bridging and solder beads. Between the stencil and the board, it also serves as a liner cover.

Alignment

The accuracy of printing joints on the board is ultimately determined by this factor. Fortunately, super alignment can be achieved by etching registration holes – fiducial markings – on the stencil and board.

Thickness of stencil

Another important factor that influences the solder paste discharge capability of a PCB stencil is its thickness. To achieve the desired solder junctions, the thickness must fit into the size of the holes. Surface tension might cause the paste to cling to the internal aperture walls if the PCB thickness is wrong.

What to consider while choosing PCB thickness Aspect Ratio?

The quantity and depth of solder paste used to make solder joints are determined by the stencil thickness, as previously stated. The lower the quantity, the higher the possibilities of disconnection and the lower the odds of jumpers. The term 'aspect' refers to this relationship. Variations in forces interacting with the paste as it goes down the holes cause it.

For example, one force pushes the paste through the aperture, while another grasps the paste inside the perforations.

The aspect ratio describes the difference between these two forces (AR). The AR should be greater than one to achieve the best solder paste deposition. In general, the surface tension between the paste and the board must be greater than the

surface tension between the paste and the opening walls. In DIY projects, you can calculate the AR value by dividing the aperture ratio by the stencil thickness (W/T). There are, however, industry standards that describe the procedure of determining the optimum stencil thickness for a given aperture size

Area Ratio

A further stencil thickness component that impacts a stencil's paste discharge capacity is the area ratio. This is the proportion of the whole surface area to the hole wall surface area in general. 0.66 is the minimum permissible area ratio.

BGA and QFP Pitches

Fine-pitch Quad Flat Surface (QFP) should be also considered by you, Ball Grid Array (BGA), and the smallest chip size when determining the optimal stencil thickness. Your stencil thickness should be between 0.12mm and 0.13mm if you're using a QFP with a 0.5mm pitch. A stencil thickness of 0.15mm-0.20mm should be used on a QFP with a pitch greater than 0.5mm.

A stencil thickness of 0.15mm should be used on a BGA with a pitch of more than 1.0mm, while a thickness of 0.13mm should be used on a BGA with a pitch between 0.5mm and 1.0mm. Metal squeegee blades are commonly composed of stainless steel in a solid thickness (0.150mm) arrangement.

SMT stencil size

For a Surface Mount Technology (SMT) assembly, the requirements for calculating the optimal stencil thickness are more complicated. When determining Smt stencil sizes, however, the AR and area ratio must be taken into account. A Surface mount assembly process that uses chemical etching can have an AR of 1:1.5. For laser-cut stencils, the recommended AR is 1:1.12.

Printed circuit board Stencil producer

After you've found a PCB stencil that's the proper thickness, the next step is to cut apertures in it that is the right size. A stencil is not good for manually generating apertures since it comprises many precisely aligned holes.

Chemical etching, laser cutting, and electroforming are all standard ways to make stencils with precise holes. All of these processes result in a variety of pore wall surface finishes. In general, the more refined the finish, the better the paste discharge.

We'll talk about how PCB stencil manufacturers make stencils with a laser cutter in this subtopic because it's the most common way. To complete this technique, you'll need a laser cutter, Mylar sheets, EagleCAD software, ViewMate software, PDF printing software, and AutoCAD. The steps are outlined below.

Step 1: Mylar Sheet Stacking

Place two Mylar sheets on top of each other and heat them until the first sheet splits from the second. The second layer will then absorb the molten pad of the first sheet, allowing it to be easily lifted away.

Step 2: taking the File from EagleCAD

Generate the top and lower cream layers through a cam file, just like you would when exporting Gerber files for PCB manufacturing, if you're using EagleCAD. Go to the File menu in ExpressPCB and select "Export DXF Mechanical Drawing".

Step 3: ViewMate swelling Pads

You must correct for the melting induced by the cuts if your software exports Gerbers. Because melting expands the pad's size, you should try to keep it as little as possible.

To import the layers into the ViewMate software, go to the File area, scroll down to the Import option, and select Gerber. After that, pick D Codes from the Setup menu. Before going to the Operations and Swell tabs, make sure you've selected all of the columns. According to the specifications, enter the value for pad size modifications. Convert your Gerber file to PDF and save it after that.

Step 4: AutoCAD Swelling Pads

If you're using Express PCB, you'll need to reduce the size of your DXF files using AutoCAD or another program. Select all of the drawings in the AutoCAD software and then click the Scale option. In the Scale Factor window that displays, type the right figure. It will scale the drawings to their proper proportions. The visuals should then be printed as a PDF.

Step 5: Stencils cutting

To cut through the Mylar sheets, adjust the laser cutter settings correctly. As previously said, the laser's heat will melt the first sheet, but the second layer will absorb the molten pad, allowing you to quickly remove the first sheet. That is all there is to it

Step 6: Coating of stencils

There are several stencil coatings available that can help with PCB stenciling issues. Cleaning PCBs after a high-volume production round might be a bit of a hassle for beginners. To prevent the bottom layer from contacting other PCBs, physically remove excess solder. However, doing so in a high-volume setting is taxing. Some coatings reduce leftover solder in hole walls, which can disrupt a uniform flux formation.

Conclusion

As mentioned previously, stencils are critical in PCB production since they determine the end product's performance. PCB stencils make solder paste applications quicker, faster, and more exact by guaranteeing that the correct amount of solder paste is applied to each pad.