The PCB printing stage is an essential part of the PCB manufacturing process. Additionally, it is the most challenging step because even a slight mistake can significantly affect the end product’s functioning. Most of the printing phase errors arise from the mounting of solder joints on the boards.
PCB stenciling is the best solution to eliminating these errors. With PCB stenciling, you literally automate the mounting of solder joints. This saves you time and eliminates human errors that arise during the manual mounting of solder joints. This article discusses everything you should know about solder paste PCB stenciling.
What is a PCB Stencil?
A PCB stencil is a small piece of material with holes that affect the layout of parts on a board. Solder stencils are the most common PCB stencils that you can use to apply the right amount of solder paste accurately in a single round across the PCB. Your PCB becomes ready to fix Surface Mount Devices (SMDs) after using solder paste on its surface enclosed with a stencil. After removing the stencil, the solder paste deposits remain on the surface with formations of the stencil’s apertures.
The smooth deposition of the paste creates accurate solder joints, ensuring high mechanical strength and excellent electrical connections.
There are two main types of stencils: framed and frameless stencils. Frameless stencils are cost-effective and easier to store, but they are not ideal for high-volume PCB printing. The stencil material significantly affects its ability to apply solder paste via the holes. Nevertheless, you can increase your stencil’s paste discharge capability by coating it.
PCB Stencil Design
Circuit boards with copper pads that boost transmission and heat dissipation require stencils with custom designs. Designs lacking pads experience copper lifting and low soldering of external leads. To prevent such issues, you should introduce a ‘window effect’ to your hole design. This prevents the problems by controlling the solder amount.
If the copper pads contain vias to boost heat dissipation, you require another stencil design to inhibit the formation of paste deposits in the vias. Some pads also need stencils with fluctuating thickness. For example, delicate pitch components call for thin stencils, while large components require thicker stencils.
The good thing is that you can achieve the above requirements by applying multilevel stencils with step-up and step-down parts. Step-up parts are formed by putting more material to specific stencil points.
Below are other aspects of PCB stencil design.
Aperture Design
The design of the holes affects the joints in several ways. First, it influences the chances of faults occurring, like bridging and solder beads. It also acts as a lining cover between the stencil and the board.
Alignment
This is the final determinant of the accuracy of printing joints on the board. Luckily, you can attain super alignment by etching registration holes –fiducial marks – on the stencil and board.
Stencil Thickness
The thickness of a PCB stencil is another major element that affects its solder paste discharge capacity. The thickness must fit into the size of the holes to attain the desired solder joints. An incorrect PCB thickness can make the paste stick to the internal aperture walls because of surface tension.
Factors to Consider When Choosing Stencil Thickness
Aspect Ratio
As mentioned earlier, the stencil thickness determines the amount and height of the paste for forming solder joints. The less the amount, the greater the disconnection chances and the minimal the chances of jumpers. This correlation is known as ‘aspect.’ It is caused by the variances in forces interacting with the paste as it moves down the holes. For instance, a force thrusts the paste past the aperture and another one that grasps the paste inside the holes.
The difference between these two forces is described by the aspect ratio (AR). To attain the highest solder paste deposition, the AR should be higher than one. Basically, the surface tension acting on the paste and the board must be more than the one acting between the paste and the walls of the openings. You can establish the AR value by dividing the aperture ratio with the stencil thickness (W/T) in DIY projects.
However, there are industry standards describing the process of finding the stencil thickness appropriate to an opening size.
Area Ratio
The area ratio is another stencil thickness factor that affects the paste discharge capacity of a stencil. Generally, this is the ratio of the hole surface area to the hole wall surface area. The least acceptable area ratio is 0.66.
QFP and BGA Pitch
When establishing the correct stencil thickness, you should also consider the fine-pitch Quad Flat Surface (QFP), Ball Grid Array (BGA), and the least chip size. A QFP with a ≤ 0.5mm pitch, your stencil thickness should range between 0.12mm-0.13mm. A QFP with a more than 0.5mm pitch should have a stencil thickness ranging from 0.15mm-0.20mm.
A BGA with a pitch of more than 1.0mm should have a stencil thickness of 0.15mm, while one with a pitch ranging from 0.5mm-1.0mm should have a thickness of 0.13mm. Remember, it is important to factor in the BGA or the minor parts when using multiple integrated circuits simultaneously.
Surface Mount Technology Stencil Sizes
The guidelines for determining the ideal stencil thickness for a Surface Mount Technology (SMT) assembly are more complex. Nevertheless, it is essential to consider the AR and area ratio when establishing SMT stencil sizes.
You can use an AR of 1:1.5 for an SMT assembly that applies chemical etching. The recommended AR for laser-cut stencils is 1:1.12.
PCB Stencil Maker
After acquiring a PCB stencil with the right thickness, you are again challenged with creating apertures of suitable sizes in them. Considering that a stencil contains multiple holes aligned with high accuracy, it is not ideal for creating apertures manually.
The common methods of creating stencils with perfect holes are chemical etching, laser cutting, and electroforming. All these methods lead the surfaces of the pore wall to various finishes. Basically, the more refined the finish, the more effective the paste discharge.
In this subtopic, we will discuss the process of making PCB stencils using a laser cutter as it is the most popular method. You need a laser cutter, Mylar sheets, EagleCAD software, ViewMate software, PDF printing software, and AutoCAD to carry out this process. Below are the steps to follow.
Step 1: Stacking the Mylar Sheet
Mount two Mylar sheets and heat them until the first sheet separates from the other. Subsequently, the second sheet will absorb the molten pad of the first sheet, allowing it to be pulled off easily.
Step 2: Exporting the Design File from EagleCAD or ExpressPCB
If you use EagleCAD, export the upper and lower cream layers via cam file, like you do when exporting Gerber files for PCB manufacturing. In ExpressPCB, go to the File menu, and click the “Export DXF Mechanical Drawing” option.
Step 3: Swelling Pads in ViewMate
If your software exports Gerbers, you must compensate for the melting caused by the cuts. Melting enlarges the pad size; therefore, you must minimize it proactively.
Go to the File section, scrawl down to the Import option, and click on Gerber to import the layers into the ViewMate software. After that, click on Setup and select D Codes. Remember to pick all columns before visiting the Operations and Swell tabs. Enter the value for pad size adjustments according to the requirements. After that, convert your Gerber file into PDF and save it.
Step 4: Swelling Pads in AutoCAD
If you use ExpressPCB, you need to scale down your DXF contents using AutoCAD or any other software. Open the AutoCAD software, select all the drawings, and click the Scale button.
Enter the correct figure in the Scale Factor window that appears. It will adjust the drawings to their suitable scales. After that, print the graphics on a PDF.
Step 5: Cutting the Stencils
Modify the laser cutter settings appropriately to cut through the Mylar sheets. As mentioned earlier, the heat generated by the laser will melt the first sheet, but the second sheet will absorb the molten pad, letting you withdraw the first sheet efficiently. That is all!
Stencil Coating
There are several coatings for stencils that solve some PCB stenciling problems. For beginners, it is a bit hectic to clean PCBs after a high-volume manufacturing round. You can manually remove extra solder from the bottom layer to inhibit it from contacting other PCBs. But it is exhausting to do this in high-volume production. Some coating minimizes the residual solder in the hole walls that alter a smooth flux formation.
Conclusion
From the above discussion, stencils play an essential role in PCB manufacturing as they determine the proper performance of the final product. PCB stencils make solder paste applications easier, faster, and precise by ensuring that every pad receives the suitable solder paste quantity.
Kindly contact us for any PCB stencil products and services in your next project. We have rich experience in PCB manufacturing and state-of-the-art equipment that guarantee high-quality PCB stencils and services.
